MaterialsChemistryandPhysics243(2020)12259)Contents lists available at ScienceDirectMATERIALSCHEMISTRSICSANEMaterials Chemistry and Physics飞ELSEVIERjournalhomepage:www.elsevier.com/locate/matchemphys?opataCore/multi-shell particles based on TiH2, a high-performance thermallyactivated foaming agentM. Romero-Romeroab, C.Dominguez-Rios, R. Torres-Sanchez, A. Aguilar-ElguezabalaCentro de Imvesigacion en Materiales Avanzados S.C and Laboratorio Nacional de Nanotecnologia, Chihuahu, 31136, Mexico Instituto Tecnologico de Chihuchue, Tecnologico Nacional de Mexico, Chihuahue, 31310, MexicoHIGHLIGHTSGRAPHICALABSTRACT.TiH/Ti,O@TiO/SiO,core/multi-shellparticlesweresynthesisedviaLisothermal heating and sol-gel process.+V89.1nm4 It was possible to increase the tempera-72.1 nm89.1 nm72.1 nmture at which hydrogen is released theTiH2TTiH2core from 440to601°Cwhenmulti-shell is formed on TiH2 surface.OTi,o国C These core/multi-shell particles possessTiO2国suitable properties for use as potentialfoaming agents for aluminium alloys.SiO2国TiH2/Ti30@Ti02TiHz/SiO2TiH2/Ti30@TiOz/SiOARTICLEINFOABSTRACTKeywords:The key element in the production of metal foams is the availability of a gas provider, which must be able toTiH2 powderrelease thegas once the metal reaches the liquidus temperature,allowing the homogeneous formation of pores inCore/shell particlea narrow size distribution. TiH2 is by far the most widely used foaming agent for aluminium alloy foaming; Sol-gel processhowever, this compound starts to release hydrogen during heating below 450 °C, and the liquidus temperature ofFoaming agentaluminium alloys is above 600 °c. In this work we studied the formation of a core/shell structure based on TiH2Aluminiumfoamparticles.The best results were obtained with a TiH2/TigO@TiO2/SiO2 core/multi-shell structure, which starts torelease hydrogen around 600 °C.1.Introductiondeveloped to produce AAFs [1-4], the melting and powder metallurgyroutes being the most widely used processes. Both methods require theMetal foams are considered metal and gas composites and, due todecomposition of afoaming agent,which supplies thegas that inducestheir potential applications, are increasingly arousing worldwide inter-the foaming of the aluminium alloy. Powdered titanium hydride (TiH2)est. In particular, aluminium alloy foams (AAFs) have already positivelyis the most used foaming agent [5-11], due mainly to its high efficiencyimpacted the automotive, rail and naval industries, as well as theshown in the volumetric expansion of Al and AiSi, alloys [12].building industry; foam applications have also found uses in acousticOne of the challenges in the production of high quality AAFs is toisolation and industrial machinery[1].Several methods have beenproduce them with uniform pore size distribution and with desirable andCorresponding author.E-mail addresses:carlos.dominguez@cimav.edu.mx, carlos560405@yahoo.com.mx (C,Dominguez-Rios).https://doi.org/10.1016/j.matchemphys.2019.122591Received 21 May 2019; Received in revised form 2 December 2019; Accepted 30 December 2019Available online 2 January 20200254-0584/2020 Elsevier B.V.All rights reserved
Materials Chemistry and Physics 243 (2020) 122591 Available online 2 January 2020 0254-0584/© 2020 Elsevier B.V. All rights reserved. Core/multi-shell particles based on TiH2, a high-performance thermally activated foaming agent M. Romero-Romero a,b , C. Domínguez-Ríos a,* , R. Torres-Sanchez � a , A. Aguilar-Elguezabal a a Centro de Investigacion � en Materiales Avanzados S.C. and Laboratorio Nacional de Nanotecnología, Chihuahua, 31136, Mexico b Instituto Tecnol� ogico de Chihuahua, Tecnol� ogico Nacional de M�exico, Chihuahua, 31310, Mexico HIGHLIGHTS GRAPHICAL ABSTRACT � TiH2/Ti3O@TiO2/SiO2 core/multi-shell particles were synthesised via isothermal heating and sol-gel process. � It was possible to increase the temperature at which hydrogen is released the TiH2 core from 440 to 601 �C when multi-shell is formed on TiH2 surface. � These core/multi-shell particles possess suitable properties for use as potential foaming agents for aluminium alloys. ARTICLE INFO Keywords: TiH2 powder Core/shell particle Sol-gel process Foaming agent Aluminium foam ABSTRACT The key element in the production of metal foams is the availability of a gas provider, which must be able to release the gas once the metal reaches the liquidus temperature, allowing the homogeneous formation of pores in a narrow size distribution. TiH2 is by far the most widely used foaming agent for aluminium alloy foaming; however, this compound starts to release hydrogen during heating below 450 �C, and the liquidus temperature of aluminium alloys is above 600 �C. In this work we studied the formation of a core/shell structure based on TiH2 particles. The best results were obtained with a TiH2/Ti3O@TiO2/SiO2 core/multi-shell structure, which starts to release hydrogen around 600 �C. 1. Introduction Metal foams are considered metal and gas composites and, due to their potential applications, are increasingly arousing worldwide interest. In particular, aluminium alloy foams (AAFs) have already positively impacted the automotive, rail and naval industries, as well as the building industry; foam applications have also found uses in acoustic isolation and industrial machinery [1]. Several methods have been developed to produce AAFs [1–4], the melting and powder metallurgy routes being the most widely used processes. Both methods require the decomposition of a foaming agent, which supplies the gas that induces the foaming of the aluminium alloy. Powdered titanium hydride (TiH2) is the most used foaming agent [5–11], due mainly to its high efficiency shown in the volumetric expansion of Al and AlSi7 alloys [12]. One of the challenges in the production of high quality AAFs is to produce them with uniform pore size distribution and with desirable and * Corresponding author. E-mail addresses: carlos.dominguez@cimav.edu.mx, carlos560405@yahoo.com.mx (C. Domínguez-Ríos). Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys https://doi.org/10.1016/j.matchemphys.2019.122591 Received 21 May 2019; Received in revised form 2 December 2019; Accepted 30 December 2019
M. Romero-Romero et al.Materials Chemistry and Physics 243 (2020) 12259121] when compared to untreated TiH2.Table 1Synthesised foaming agents and their descriptionAs an alternative, multi-shell structured particles have been studiedas a way to increase the decomposition temperature of foaming agents.Method of synthesis of shell(s)Description and nomenclatureProa-Flores et al. [22] synthesised pre-oxidised TiH2 powders and theAs-received titanium hydride, TiH2TiH2/TiO2 structure obtained was then covered by a nickel layerCore/shell particles, TiH,/SiO2Sol-gel coatingdeposited by an electroless process. The resulting multi-shell powdersCore/double-shell particles, TiH/Heating of particles in airTigO@TiO2allowed an increaseintemperatureof hydrogenreleaseto525°C.TheCore/multi-shell particles, TiH2/Heating of particles in air and thereafterauthors reported that this multi-shell arrangement on TiH2 allowedTi30@TiO2/Si02sol-gel coatingobtaining a more homogeneous and reproducible pore structure whencompared to untreated TiH2Despite the improvement in pore structure and process controlreproducible properties such as low density, low thermal conductivity(allowing betterreproducibility of foam structure) by usingpre-oxidisedand high capacity to absorb energy by deformation or damping. Toand core/multi-shell TiH2 powders, the temperature difference betweenobtain a uniform porous structure, both melting and powder metallurgyfoaming agent gas release(decompositiontemperature)andliquidusprocesses face the problem of premature hydrogen release from TiH2stemperature of aluminium alloy still is large. Thus, it is fundamental toinasmuch asthere isa wide differencebetweentheTiH2decompositiondevelop foaming agents that are able to liberate the gas ata temperaturetemperature(450C[13])andtheliquidustemperatureofthetypicalasnearaspossibleto,oreveninsidetherangeof,theliquidustemper-aluminiumalloy (~600-650° [14]).In themeltingroute, onceTiH2isature of the aluminium alloy,to increase the opportunities for the suc-mixed with the molten aluminium alloy there is not enough time tocessful development of applications ofaluminium alloy-based foams.distribute the powder uniformly, due to its premature thermal decom-Taking into account the information mentioned above, this workposition.Ontheotherhand,inthepowdermetallurgyprocess,whenthefocuses on the development of a foaming agent structured as a core/compacted mix of aluminium alloy and TiH2 powders are heated themulti-shell based on TiH2 by the formation of three oxide layers. Thehydrogen gas is released when the aluminium alloy is barely melted,multi-shell was synthesised to increase substantiallytheTiH2decom-which leads to a heterogeneous pore distribution.position temperature up to the liquidus temperature range of typicalIn order to improve the foaming of aluminium alloys, the tempera-aluminium alloys, allowing these core/multi-shell particles to becometure of gas release from the TiH2 must be increased to near 600C; thus,better homogeneity of pore size distribution can be obtained duringfoaming.OnewaytodelaythereleaseofhydrogenfromTiH,istocover00the TiH2 particles with barriers to increase the temperature of thermal000.-000.0d)decomposition. A core/shell arrangement being necessary for this pur-pose, such that the shell is able to delay the heat transfer from thea*aluminium alloy to the TiH2 core. Another mechanism to delay gas*(e) aa*c)0xoxxoAXarelease isto have a sealedshell structure thatrestrains theprematureTiH,release of gas.Theeasiest way to obtain the core/shell structure isby thecontrolled oxidation of the surface of the TiH2 particles to form a TiO2oTiO,*tshell. For this purpose, TiH2 particles are heated in air, thus promotingxATi,ob) o山tothe formation of a surface layer of TiO2.The use of surface-oxidised TiH2 (TiH2/TiO2) in the melting routehas been reported for metal foaming, where the advantage of the delay4**in gas release is used to improve pore size homogeneity, based on大a)+increasing the mixing time [13,15-17]. A detailed study of the thermal儿八decomposition of TiH2/TiO2 and its impact on the structure and prop-erties of AAFswascarriedoutbyRomero-Romero etal.[16],wherean7020304050809060onset of release of hydrogen at540°C was achieved, instead of 440C,20 ()which was the temperature observed for the gas release from TiH2.Pre-oxidised TiH, has also been used in powder metallurgy, and severalFig. 2. X-ray diffraction patterns of a) TiH2, b) TiH2/TigO@TiO2, c) TiH/workshavereportedanimprovementinporehomogeneity[14,18,19]Tig0@TiO2/SiO2treated at 450°C and d)TiH:/Tig0@TiO2/SiO2treated at1100°℃.as well asanincrease inmechanical propertiesandprocesscontrol [20,b)8ra)T61(%) aunoA15I14王IANHHIIH*2-1fIHom0.151010010001μmParticle size (μm)Fig. 1. a) SEM micrograph of as-received TiH2 and b) particle size distribution of TiH2 powders2
Materials Chemistry and Physics 243 (2020) 122591 2 reproducible properties such as low density, low thermal conductivity and high capacity to absorb energy by deformation or damping. To obtain a uniform porous structure, both melting and powder metallurgy processes face the problem of premature hydrogen release from TiH2, inasmuch as there is a wide difference between the TiH2 decomposition temperature (~450 �C [13]) and the liquidus temperature of the typical aluminium alloy (~600–650 �C [14]). In the melting route, once TiH2 is mixed with the molten aluminium alloy there is not enough time to distribute the powder uniformly, due to its premature thermal decomposition. On the other hand, in the powder metallurgy process, when the compacted mix of aluminium alloy and TiH2 powders are heated the hydrogen gas is released when the aluminium alloy is barely melted, which leads to a heterogeneous pore distribution. In order to improve the foaming of aluminium alloys, the temperature of gas release from the TiH2 must be increased to near 600 �C; thus, better homogeneity of pore size distribution can be obtained during foaming. One way to delay the release of hydrogen from TiH2 is to cover the TiH2 particles with barriers to increase the temperature of thermal decomposition. A core/shell arrangement being necessary for this purpose, such that the shell is able to delay the heat transfer from the aluminium alloy to the TiH2 core. Another mechanism to delay gas release is to have a sealed shell structure that restrains the premature release of gas. The easiest way to obtain the core/shell structure is by the controlled oxidation of the surface of the TiH2 particles to form a TiO2 shell. For this purpose, TiH2 particles are heated in air, thus promoting the formation of a surface layer of TiO2. The use of surface-oxidised TiH2 (TiH2/TiO2) in the melting route has been reported for metal foaming, where the advantage of the delay in gas release is used to improve pore size homogeneity, based on increasing the mixing time [13,15–17]. A detailed study of the thermal decomposition of TiH2/TiO2 and its impact on the structure and properties of AAFs was carried out by Romero-Romero et al. [16], where an onset of release of hydrogen at 540 �C was achieved, instead of 440 �C, which was the temperature observed for the gas release from TiH2. Pre-oxidised TiH2 has also been used in powder metallurgy, and several works have reported an improvement in pore homogeneity [14,18,19], as well as an increase in mechanical properties and process control [20, 21] when compared to untreated TiH2. As an alternative, multi-shell structured particles have been studied as a way to increase the decomposition temperature of foaming agents. Proa-Flores et al. [22] synthesised pre-oxidised TiH2 powders and the TiH2/TiO2 structure obtained was then covered by a nickel layer deposited by an electroless process. The resulting multi-shell powders allowed an increase in temperature of hydrogen release to 525 �C. The authors reported that this multi-shell arrangement on TiH2 allowed obtaining a more homogeneous and reproducible pore structure when compared to untreated TiH2. Despite the improvement in pore structure and process control (allowing better reproducibility of foam structure) by using pre-oxidised and core/multi-shell TiH2 powders, the temperature difference between foaming agent gas release (decomposition temperature) and liquidus temperature of aluminium alloy still is large. Thus, it is fundamental to develop foaming agents that are able to liberate the gas at a temperature as near as possible to, or even inside the range of, the liquidus temperature of the aluminium alloy, to increase the opportunities for the successful development of applications of aluminium alloy-based foams. Taking into account the information mentioned above, this work focuses on the development of a foaming agent structured as a core/ multi-shell based on TiH2 by the formation of three oxide layers. The multi-shell was synthesised to increase substantially the TiH2 decomposition temperature up to the liquidus temperature range of typical aluminium alloys, allowing these core/multi-shell particles to become Table 1 Synthesised foaming agents and their description. Description and nomenclature Method of synthesis of shell(s) As-received titanium hydride, TiH2 – Core/shell particles, TiH2/SiO2 Sol-gel coating Core/double-shell particles, TiH2/ Ti3O@TiO2 Heating of particles in air Core/multi-shell particles, TiH2/ Ti3O@TiO2/SiO2 Heating of particles in air and thereafter sol-gel coating Fig. 1. a) SEM micrograph of as-received TiH2 and b) particle size distribution of TiH2 powders. Fig. 2. X-ray diffraction patterns of a) TiH2, b) TiH2/Ti3O@TiO2, c) TiH2/ Ti3O@TiO2/SiO2 treated at 450 �C and d) TiH2/Ti3O@TiO2/SiO2 treated at 1100 �C. M. Romero-Romero et al
M. Romero-Romero et al.Materials Chemistry and Physics 243 (2020) 122591b)Pa)727('ne)()xTi,oTiH,oTiO26052004006008001000120020040060080010001200Raman shift (cm-)Ramanshift(cm-l)c)d)40fman signal()(cne) isaTiH,OTiH,/TiO,@Ti,oTi,oTiO,2004001200200400600800100012006008001000Ramanshift (cm-l)Raman shift(cm-1)Fig. 3. Raman spectra of a) TiH2 powder, b) TigO@TiO2 shells, c) TiH2/TigO@TiO2 sample and d) deconvolution of TiH2/TigO@TiO2-a)440b)443TiH,/SiO,A[1070sio,1035()aoo(ne)aoeosq95218975-9358041200100080060040012001000800600400Wavenumber(cm")Wavenumber(cm-)Fig. 4. FTIR spectra of a) SiO2 particles and b) TiH2/SiO2 sample.promising foaming agents for aluminium alloys.A study of the thermal2.2.1.Synthesis ofTiH2/SiO2core/shell particlesperformance of foaming agents is presented, as well as a detailed studyThe TiH/SiO2 foaming agent was synthesised by the Stober processof the morphology and phase compositions of the layers on TiH2.[23] with modifications, in order to obtain a layer of SiO2 on the TiH2particles. For this purpose, 260 mL of a solution of ethanol (99.75%),2.Materials and methodsammoniumhydroxide(28.8wt%of NH3)and deionisedwater inavolume ratio 6.55:3.66:1, was prepared.The measured pH value was11-12,whichwascontrolled during the wholeprocess.Asuitable2.1.StartingmaterialsamountofSiO2precursor(TEOS)wasusedtoobtainashellofaround100 nm thickness on the TiH2 particles.Foaming agents were prepared using TiH2 powder (Sigma-Aldrich,Inthefirststage,2.5gofTiH2particlesweresonicatedfor30minin98% purity,<44μm)as core and TEOS (tetraethoxysilane,Alfa Aesar,ethanol,and1/6ofthetotalamountofammoniumhydroxidewas99%purity)toform Si02layers.added to the TiH2 suspension under vigorous conventional stirring. Thestirring was maintained for 15min.Then 1/6 of the total amount of2.2.Synthesis of foaming agents with core/shell structureTEOS was added ata rate of 0.05mLmin-1,thetemperature setat40Cand the solution kept under stirring for 24 h. In the second stage, theSeveral foaming agents were prepared using TiH2 starting particles,remaining ammonium hydroxide solution was added, followed by thewhich were treated to obtain particles surrounded by shells. Table 1addition of the deionised water. After 15 min stirring, the remainingsummarises the prepared materials and their description, as well as theamountof TEOSwas addedata rateof 0.05mLmin-and theresultantnomenclature used throughoutthis work.3
Materials Chemistry and Physics 243 (2020) 122591 3 promising foaming agents for aluminium alloys. A study of the thermal performance of foaming agents is presented, as well as a detailed study of the morphology and phase compositions of the layers on TiH2. 2. Materials and methods 2.1. Starting materials Foaming agents were prepared using TiH2 powder (Sigma-Aldrich, 98% purity, < 44 μm) as core and TEOS (tetraethoxysilane, Alfa Aesar, 99% purity) to form SiO2 layers. 2.2. Synthesis of foaming agents with core/shell structure Several foaming agents were prepared using TiH2 starting particles, which were treated to obtain particles surrounded by shells. Table 1 summarises the prepared materials and their description, as well as the nomenclature used throughout this work. 2.2.1. Synthesis of TiH2/SiO2 core/shell particles The TiH2/SiO2 foaming agent was synthesised by the Stober € process [23] with modifications, in order to obtain a layer of SiO2 on the TiH2 particles. For this purpose, 260 mL of a solution of ethanol (99.75%), ammonium hydroxide (28.8 wt % of NH3) and deionised water in a volume ratio 6.55:3.66:1, was prepared. The measured pH value was 11–12, which was controlled during the whole process. A suitable amount of SiO2 precursor (TEOS) was used to obtain a shell of around 100 nm thickness on the TiH2 particles. In the first stage, 2.5 g of TiH2 particles were sonicated for 30 min in ethanol, and 1/6 of the total amount of ammonium hydroxide was added to the TiH2 suspension under vigorous conventional stirring. The stirring was maintained for 15 min. Then 1/6 of the total amount of TEOS was added at a rate of 0.05 mL min 1 , the temperature set at 40 �C and the solution kept under stirring for 24 h. In the second stage, the remaining ammonium hydroxide solution was added, followed by the addition of the deionised water. After 15 min stirring, the remaining amount of TEOS was added at a rate of 0.05 mL min 1 and the resultant Fig. 3. Raman spectra of a) TiH2 powder, b) Ti3O@TiO2 shells, c) TiH2/Ti3O@TiO2 sample and d) deconvolution of TiH2/Ti3O@TiO2. Fig. 4. FTIR spectra of a) SiO2 particles and b) TiH2/SiO2 sample. M. Romero-Romero et al
Materials Chemistry and Physics 243 (2020) 122591M,Romero-Romero et al.TiKad)279248217OK186155TiL12493AIK62TiKCK2um4.026.030.000.671.342.012.683.354.695.36280.OKe)252224196168140112SiKTiL8456TiK28umO0.000.470.942.352.823.293.764.231.411.88TiKOKf)O261232203174145116SiKTIL8758AIKTiK292μm0.000.671.342.012.683.354.024.695.366.03Fig.5. SEM micrographs of a) TiH/Tig@TiO2, b) TiH2/SiO2, c) TiH2/Tig@TiO2/SiO2. Elemental EDS microanalysis for samples d) TiH2/TigO@TiO2, e) TiH2/SiO2 and ) TiH/TigO@TiO,/SiO2.Aluminium signal is due to the material of the holder used to support the samples.solution maintained under stirring for 24 h.2.2.2.SynthesisofTiH2/Tig0@TiO2core/double-shell particlesThe obtained TiH2/SiO2 particles were recovered by centrifugation.To synthesise the TiH2/TigO@TiO2foamingagent,TiH2particlesThe washing of the recovered particles was carried out with ethanolwere heated isothermally to oxidise the particle surfaces. The isothermalunder sonication. Particles were recovered by centrifugation andheating was carried out at 500 °C for 60 min in air.For this purpose,particles were spread on an alumina plate, which was previously heatedwashed by sonication once again.Finally, the solution was keptrestingovernight, the ethanol decanted and the particles dried at 100 °C for 60at 500 C. Synthesis conditions were defined based on results describedh.elsewhere [16], where it was demonstrated that TiH2 powders heatedisothermally at 500 c presented the best thermal decomposition
Materials Chemistry and Physics 243 (2020) 122591 4 solution maintained under stirring for 24 h. The obtained TiH2/SiO2 particles were recovered by centrifugation. The washing of the recovered particles was carried out with ethanol under sonication. Particles were recovered by centrifugation and washed by sonication once again. Finally, the solution was kept resting overnight, the ethanol decanted and the particles dried at 100 �C for 60 h. 2.2.2. Synthesis of TiH2/Ti3O@TiO2 core/double-shell particles To synthesise the TiH2/Ti3O@TiO2 foaming agent, TiH2 particles were heated isothermally to oxidise the particle surfaces. The isothermal heating was carried out at 500 �C for 60 min in air. For this purpose, particles were spread on an alumina plate, which was previously heated at 500 �C. Synthesis conditions were defined based on results described elsewhere [16], where it was demonstrated that TiH2 powders heated isothermally at 500 �C presented the best thermal decomposition Fig. 5. SEM micrographs of a) TiH2/Ti3O@TiO2, b) TiH2/SiO2, c) TiH2/Ti3O@TiO2/SiO2. Elemental EDS microanalysis for samples d) TiH2/Ti3O@TiO2, e) TiH2/ SiO2 and f) TiH2/Ti3O@TiO2/SiO2. Aluminium signal is due to the material of the holder used to support the samples. M. Romero-Romero et al
M. Romero-Romero et al.Materials Chemistry and Physics 243 (2020) 122591b)aTi2umc)d)si0Fig. 6. a) SEM micrograph of a TiH2/SiO2 particle. Elemental mapping obtained by SEM-EDS: b) Ti, e) O and d) Si signals.attributes compared to othertreatments.SiO2-TiH2, synthesised foaming agents and core free-shells were analysedby Raman spectroscopy using Horiba LabRam HR VIS-633equipment2.2.3.SynthesisofTiH2/Ti3O@TiO2/SiO2core/multi-shellparticlesThe foaming agent TiH2/TigO@TiO2/SiO2 was synthesised byequipped with a He-Ne laser (632.8 nm).isothermal heating as described above (Section 2.2.2) followed by theWhile synthesis ofTiH2/SiO2andTiH/Tig0@TiO2/SiO2foamingSiO2 sol-gel coating process described in Section 2.2.1.agentswas carried out,theSiO2particles suspended intheupper partofthe solution were recovered prior to the centrifugation step.Theseparticles and TiH2/SiO2 were analysed by FTIR spectroscopy using a2.3.SiO2andTigO@TiO,shellsPerkinElmerSpectrumGXFTIR spectrometer.In order to characterise the SiO2 and TigO@TiO2 shells on the TiH2/2.4.3.ScanningelectronmicroscopySiO2and TiH2/Tig0@TiO2particles,~6g ofcore/shell particleswereAnalyses by scanning electron microscopy (SEM) were made with akeptunderstirringinasolutionofHCl/H2Oina1/1volratiountil thefieldemission JEOLJSM-7401Fmicroscope to studythe morphology ofcore(TiH2)was dissolved.The obtained shells were recovered by vac-the TiH2 and the synthesised foaming agents, as well as SiO2 and theuum filtration and washed with ethanol.Ti30@TiO2 shells.TheTiH2/Tig0@TiO2/SiO2particles were encapsu-lated in a polymeric matrix and subsequently prepared using the con-2.4.Characterisation of thefoaming agents and shellsventional metallographic process with the purpose of transversallyexposing the particles for shell thickness measurement. The samples2.4.1.Particle size distributionwere analysed by energy dispersive x-ray spectroscopy (EDS)AMastersizer20o0laserparticlesizeanalvserfromMalvernInstruments was used to determine the particle size distribution of TiH22.4.4.TransmissionelectronmicroscopyA study of themorphology of SiO2and TigO@TiO2 shells wasmadepowders.Themeasurementofthesamplewasperformedthreetimes.bytransmissionelectronmicroscopy(TEM)usingaHitachi7700mi-2.4.2. Phase analysiscroscope.Thespecimensof TEMwerepreparedasfollows:eachsampleTiH2powders,as well as thesynthesised foamingagents,werewas dispersed in ethanol by sonication, afterwards two drops of theanalysed by X-ray diffraction (XRD) using a Panalytical Xpert'PROresulting suspensionweredeposited in aTEMgrid byusingacapillarydiffractometer. The XRD patterns were obtained under the followingtube, and finally, moderate heating was applied to evaporate theconditions: Cu Ka radiation, 20 scanning angle varied from 20° to 90°,ethanol.step size of 0.033° along with a 40 s step time. In other experiments,TiH2/Ti30@Ti02/Si02washeatedisothermallyat450Cfor120minand1100Cfor120min inordertoobtaincrystallinephasesofTi02and
Materials Chemistry and Physics 243 (2020) 122591 5 attributes compared to other treatments. 2.2.3. Synthesis of TiH2/Ti3O@TiO2/SiO2 core/multi-shell particles The foaming agent TiH2/Ti3O@TiO2/SiO2 was synthesised by isothermal heating as described above (Section 2.2.2) followed by the SiO2 sol-gel coating process described in Section 2.2.1. 2.3. SiO2 and Ti3O@TiO2 shells In order to characterise the SiO2 and Ti3O@TiO2 shells on the TiH2/ SiO2 and TiH2/Ti3O@TiO2 particles, ~6 g of core/shell particles were kept under stirring in a solution of HCl/H2O in a 1/1 vol ratio until the core (TiH2) was dissolved. The obtained shells were recovered by vacuum filtration and washed with ethanol. 2.4. Characterisation of the foaming agents and shells 2.4.1. Particle size distribution A Mastersizer 2000 laser particle size analyser from Malvern Instruments was used to determine the particle size distribution of TiH2 powders. The measurement of the sample was performed three times. 2.4.2. Phase analysis TiH2 powders, as well as the synthesised foaming agents, were analysed by X-ray diffraction (XRD) using a Panalytical Xpert’PRO diffractometer. The XRD patterns were obtained under the following conditions: Cu Kα radiation, 2θ scanning angle varied from 20� to 90�, step size of 0.033� along with a 40 s step time. In other experiments, TiH2/Ti3O@TiO2/SiO2 was heated isothermally at 450 �C for 120 min and 1100 �C for 120 min in order to obtain crystalline phases of TiO2 and SiO2. TiH2, synthesised foaming agents and core free-shells were analysed by Raman spectroscopy using Horiba LabRam HR VIS-633 equipment equipped with a He–Ne laser (632.8 nm). While synthesis of TiH2/SiO2 and TiH2/Ti3O@TiO2/SiO2 foaming agents was carried out, the SiO2 particles suspended in the upper part of the solution were recovered prior to the centrifugation step. These particles and TiH2/SiO2 were analysed by FTIR spectroscopy using a PerkinElmer Spectrum GX FTIR spectrometer. 2.4.3. Scanning electron microscopy Analyses by scanning electron microscopy (SEM) were made with a field emission JEOL JSM-7401F microscope to study the morphology of the TiH2 and the synthesised foaming agents, as well as SiO2 and the Ti3O@TiO2 shells. The TiH2/Ti3O@TiO2/SiO2 particles were encapsulated in a polymeric matrix and subsequently prepared using the conventional metallographic process with the purpose of transversally exposing the particles for shell thickness measurement. The samples were analysed by energy dispersive x-ray spectroscopy (EDS). 2.4.4. Transmission electron microscopy A study of the morphology of SiO2 and Ti3O@TiO2 shells was made by transmission electron microscopy (TEM) using a Hitachi 7700 microscope. The specimens of TEM were prepared as follows: each sample was dispersed in ethanol by sonication, afterwards two drops of the resulting suspension were deposited in a TEM grid by using a capillary tube, and finally, moderate heating was applied to evaporate the ethanol. Fig. 6. a) SEM micrograph of a TiH2/SiO2 particle. Elemental mapping obtained by SEM-EDS: b) Ti, c) O and d) Si signals. M. Romero-Romero et al
Materials Chemistry and Physics 243 (2020) 122591M, Romero-Romero et al.b)aT10μmc)d)si0Fig. 7. a) SEM micrograph of a TiH2/TigO@TiO2/SiO2 particle. Elemental mapping obtained by SEM-EDS: b) Ti, c) O and d) Si signals.2.4.5. Thermal analysiscrystalline SiO2 shell deposited on TiH2, the sample was treated at 1100The thermal stability of untreated TiH2 and synthesised foamingC for 120 min (Fig. 2d). As can be seen in Fig. 2d, only the TiO2 phaseagents was studied by thermogravimetric analysis (TGA) on a TA In-appears in the diffractogram and the expected signals from SiO2 did notstruments SDT Q600 simultaneous DSC-TGA thermal analyser.Eachappearanalysis was made with around 20 mg of sample, 150mL min-argonRaman spectroscopy was used to corroborate the information ob-gas flow and a temperaturerangefrom ambienttemperature to 900 CattainedbyXRD,andthespectracollectedforeachsampleareshownona heating rate of 10 C min-1.The analyses were carried out usingFig.3.Fig.3acorrespondstoas-receivedTiH2andsignalsat265,41599.998%high purityargon.and 601 cm-1are observed.Since TiH2 does not show active Ramansignals [24], these signals must be attributed to partial oxidation of TiH23.Resultsanddiscussionto TiO2. For Raman spectroscopy, the detection limit is around 1 wt %,and inasmuchastheobtained spectrumshowsnoisysignals,theTiO23.1. Materialscontent is around this level.As was mentioned before, shell sampleswere obtained by removing the TiH2 core from the core/shell particles,Fig.1 shows the typical morphology of TiH2 particles and their sizeand the Raman spectrum of Fig.3b corresponds to this sample, whereonly signals from rutile TiO2 and TigO phases are observed.It is knowndistribution. As seen in Fig. la, the particles present an irregular shapethat rutile Ti02 signals are located at 236, 448 and 610 cm-1 [25]. Thiswith sharp edges and some flat faces. According to Fig. 1b, the distri-information matches with the detected peaks at 443 and 605 cm-1bution of particle size indicates this powder has a mean size of 14.3 μm.(Fig.3b).Thesignalthatmustbepresentaround236cm-canbeassigned to signals between 225 and 245 cm-1.Additional Raman sig-3.2.TiH2/SiO2core/shell,TiH2/Ti30@TiO2core/double-shellandnals were clearly detected at 283, 381 and 727 cm-1, and can beTiH2/Tig0@TiO2/SiO2core/muilti-shell particlesattributed to TisO, which was previouslyidentified byXRD and has alsobeen reported by Kennedy and Lopez [26] when TiH2 is heated in air.3.2.1. Study of crystalline phases of particlesThe Raman spectrum of the TiH2/TigO@TiO2 sample is shown in Fig.3c,XRD patterns of TiH2, TiH2/Tis0@TiO2, TiH2/Tig0@TiO2/Si02where three main signals are seen at 265, 406 and 598 cm-1, besides aheatedat450Cfor120minandTiH2/Ti30@Ti0/Si02heatedat1100shoulder between 670 and 770 cm-1.On the other hand, deconvolutionoC for120min are showninFig.2.Twonewphases appear on TiH2/of the TiH2/Tig0@TiO2spectrum is presented in Fig.3d, anda98.5%ofTigO@TiO2 sample, which correspond to TiO2 (rutile) and TigO andconcordance was found with the Raman signal of TiH2/TigO@TiO2.which can clearly be seen in the corresponding diffractogram (Fig. 2b).Inordertovalidatetheformationof theSiO2layeronthesurfaceofFor the TiH2/TigO@TiO2/SiO2 foaming agent, which has an additionalTiH2 and on the surface of TiH2/TigO@TiO2 to obtain TiH2/SiO2 andSiO2 layer on the particles (Fig. 2c), there were no extra XRD signalsTiH2/TigO@TiO2/SiO2,respectively,samplesofSiO2nanoparticlesandrelated to the typical behaviour of amorphous SiO2. However, the SiO2TiHz/SiO2 were analysed by infrared spectroscopy. As can be seen inshell induced only slight changes to peaks intensity compared to theFig. 4a, in the FTIR spectrum of SiO2 particles an absorption signal forTiH2/TigO@TiO2 sample. In order to try to obtain signals of the6
Materials Chemistry and Physics 243 (2020) 122591 6 2.4.5. Thermal analysis The thermal stability of untreated TiH2 and synthesised foaming agents was studied by thermogravimetric analysis (TGA) on a TA Instruments SDT Q600 simultaneous DSC-TGA thermal analyser. Each analysis was made with around 20 mg of sample, 150 mL min 1 argon gas flow and a temperature range from ambient temperature to 900 �C at a heating rate of 10 �C min 1 . The analyses were carried out using 99.998% high purity argon. 3. Results and discussion 3.1. Materials Fig. 1 shows the typical morphology of TiH2 particles and their size distribution. As seen in Fig. 1a, the particles present an irregular shape with sharp edges and some flat faces. According to Fig. 1b, the distribution of particle size indicates this powder has a mean size of 14.3 μm. 3.2. TiH2/SiO2 core/shell, TiH2/Ti3O@TiO2 core/double-shell and TiH2/Ti3O@TiO2/SiO2 core/multi-shell particles 3.2.1. Study of crystalline phases of particles XRD patterns of TiH2, TiH2/Ti3O@TiO2, TiH2/Ti3O@TiO2/SiO2 heated at 450 �C for 120 min and TiH2/Ti3O@TiO2/SiO2 heated at 1100 �C for 120 min are shown in Fig. 2. Two new phases appear on TiH2/ Ti3O@TiO2 sample, which correspond to TiO2 (rutile) and Ti3O and which can clearly be seen in the corresponding diffractogram (Fig. 2b). For the TiH2/Ti3O@TiO2/SiO2 foaming agent, which has an additional SiO2 layer on the particles (Fig. 2c), there were no extra XRD signals related to the typical behaviour of amorphous SiO2. However, the SiO2 shell induced only slight changes to peaks intensity compared to the TiH2/Ti3O@TiO2 sample. In order to try to obtain signals of the crystalline SiO2 shell deposited on TiH2, the sample was treated at 1100 �C for 120 min (Fig. 2d). As can be seen in Fig. 2d, only the TiO2 phase appears in the diffractogram and the expected signals from SiO2 did not appear. Raman spectroscopy was used to corroborate the information obtained by XRD, and the spectra collected for each sample are shown on Fig. 3. Fig. 3a corresponds to as-received TiH2 and signals at 265, 415 and 601 cm 1 are observed. Since TiH2 does not show active Raman signals [24], these signals must be attributed to partial oxidation of TiH2 to TiO2. For Raman spectroscopy, the detection limit is around 1 wt %, and inasmuch as the obtained spectrum shows noisy signals, the TiO2 content is around this level. As was mentioned before, shell samples were obtained by removing the TiH2 core from the core/shell particles, and the Raman spectrum of Fig. 3b corresponds to this sample, where only signals from rutile TiO2 and Ti3O phases are observed. It is known that rutile TiO2 signals are located at 236, 448 and 610 cm 1 [25]. This information matches with the detected peaks at 443 and 605 cm 1 (Fig. 3b). The signal that must be present around 236 cm 1 can be assigned to signals between 225 and 245 cm 1 . Additional Raman signals were clearly detected at 283, 381 and 727 cm 1 , and can be attributed to Ti3O, which was previously identified by XRD and has also been reported by Kennedy and Lopez [26] when TiH2 is heated in air. The Raman spectrum of the TiH2/Ti3O@TiO2 sample is shown in Fig. 3c, where three main signals are seen at 265, 406 and 598 cm 1 , besides a shoulder between 670 and 770 cm 1 . On the other hand, deconvolution of the TiH2/Ti3O@TiO2 spectrum is presented in Fig. 3d, and a 98.5% of concordance was found with the Raman signal of TiH2/Ti3O@TiO2. In order to validate the formation of the SiO2 layer on the surface of TiH2 and on the surface of TiH2/Ti3O@TiO2 to obtain TiH2/SiO2 and TiH2/Ti3O@TiO2/SiO2, respectively, samples of SiO2 nanoparticles and TiH2/SiO2 were analysed by infrared spectroscopy. As can be seen in Fig. 4a, in the FTIR spectrum of SiO2 particles an absorption signal for Fig. 7. a) SEM micrograph of a TiH2/Ti3O@TiO2/SiO2 particle. Elemental mapping obtained by SEM-EDS: b) Ti, c) O and d) Si signals. M. Romero-Romero et al
Materials Chemistry and Physics 243 (2020) 122591M. Romero-Romero et al.outer layer; however,once silicon reachesthe lowest level towards the牌森生皇装学inb)lanecentre of the particle, oxygen remains high, which is due to the layer oftitanium oxides between the SiO2 shell and the TiH2 core.Fig.8c shows用another particle where the thickness of the SiO, layer was measured,being around 82 nm.Elemental analysis of zones 1 and 2, indicated inFig. 8c, are shown in Fig. 8d and e, respectively, where the high contentof siliconandoxygenintheoutershell(zone1)andthepredominantcontent of titanium in the core of the particle (zone 2) were observed,asexpected.As mentioned before, to study the SiO2 and TigO@TiO2 shells, TiH2d)was dissolved with HCI from TiH2/SiO2 and TiH2/TigO@TiO2 particles.Bn路Fig. 9a and b shows the morphology by TEM of the shells removed fromTiH2/SiO2 and TiH2/TigO@TiO2 samples,respectively.Theamorphousnature of SiO2allows abetter observation of the morphology of the shell,whereas the crystalline structure of the titanium oxides shell does notallow the electron beam to penetrate the shell and observe the substratesupport. On the other hand, Fig.9c and d correspond to the SiO2 andTigO@TiO2 shells that were intentionally broken into multiple pieces todetermine statistically the thickness of the shells, resulting in 89.1 ±的A16.5 nm and 72.1 ± 14.2 nm, respectively. The mean value of thee)determined thickness of the SiO2 shell agrees with the value measuredby SEM for the transversally cut sample (Fig.8c).These images obtainedby SEM and TEM corroborate that the shells are uniform and coverhomogeneouslytheTiH2coreThe characterisation results indicate that reduced titanium oxide isthefirstlayerto form during the isothermal heatingof TiH2; this layerevolves to TiO2 as oxygen diffuses from the surface to the inner layers[30]. At the end of the treatment, the particle structure becomesTiH2/TigO@TiO2 from core to outer shell. Otherwise, the method usedFig.8.SEM micrographs of cross sections of TiH>/TiO@TiO/SiO2particles (aand c). b) Content of oxygen (O), silicon (Si) and titanium (Ti), along linetoformtheSiO2layerallowsobtaininganamorphouslayerofSiO2withindicated in Fig. 8a d) EDS analysis of spot 1 indicated in Fig. 8c e) EDS analysisshell thickness around 89 nm. Fig. 10 shows a schematic representationof spot 2 indicated in Fig.8c.of the different developed core/shell particles based on TiH2SiO2 corresponding to O-Si-O deformation vibration is located around3.3.Thermal analysis of thefoaming agents440 cm-1, a signal at 804 cm- concerns the symmetric stretching vi-bration of the Si-0 bond, whereas the signal around 935-975 cm-1 isThe TGA pattern of TiH2,and TGA and DTG patterns of TiH2/related to in-plane stretching vibration of the Si-O bond of the Si-OHTigO@TiO2 particles are shown in Fig.11a and b,respectively,as well asgroup.The strong signal around 1035 cm-"is related to anti-symmetricthe TGAand DTGpatterns forTiH2/SiO2and TiH2/TigO@TiO2/SiO2stretching vibration of Si-O-Si. Other researchers have also attributed(Fig.11c and d, respectively). Table 2shows the specific information onthese bands to the characteristic vibration modes of the Si-O-Si groupthermal events that are useful for the performance of a foaming agent.[27] and tothevibrationmode oftheSi-OHbond [27-29].FortheAsseen in Fig.11a, TiH2powders present an initial decompositionsynthesised TiH2/SiO2 sample, the same bands, slightly displaced (i.e.temperature of 440 °C. On the other hand, a weight loss is evident for the443, 781, 952 and 1070 cm-1), are observed (Fig.4b), which corrobo-TiH2/TigO@TiO2 foaming agent (Fig.11b) that remains until aroundrate the presence of the SiO2 layer.500 C. Another weight loss is directly attributed to H2 release, whichbegins around 540 C, i.e.,100 C above the decomposition temperature3.2.2.Morphology and compositionobserved for untreated TiH2.ForTiH2/SiO2and TiH2/TigO@TiO2/SiO2,Morphologies and EDS analyses for the TiH2/Tig0@TiO2, TiH2/Si02Fig.11c and d, respectively, the loss of weight below 450 °C correspondsandTiH2/TigO@TiO2/SiO2samplesareshown in Fig.5.Theoxygento solventevaporation andlossof adsorbedmoisture.Forthesetwosignals detected (Fig. 5d, e and 5f) indicate that the samples are at leastsamples,the loss of weight above 450 °C consist of two steps: the firstpartially oxidised. Furthermore, the presence of silicon is corroboratedlossforsampleTiH2/SiO2isfrom450to550°C,asisobservedbytheforTiH2/SiO2andTiH2/Tig0@TiO2/SiO2,confirming theformationofchange of slope, whereas for TiH/TigO@TiO/SiO2 the loss reachesthe SiO2 layer. The presence of some spherical particles is due to the590 C. This step can be assigned to the dehydroxylation of OH groupsformationof SiO2spheresonthesurfaceof thefoamingagentsduetothelinked to the SiO2 structure [31]. The final weight loss, which is aroundsol-gel process.510°C for TiH2/SiO2 and 600 °C for TiH2/Tig0@TiO2/SiO2,can beIn order to observe the uniformity of the titanium and silicon oxideattributedtotheH,releasefromtheTiH2coreoftheparticles.Inordershells concerning the TiH2/SiO2 and TiH2/TigO@TiO2/SiO2 samples,to differentiate this weight loss from that due to Si-OH dehydroxylation,elemental mapping was carried out,as shown in Figs.6and 7.As can bethe derivative curve of weight change with respect to temperatureseen in Fig. 6 for TiH2/SiO2, the titanium, oxygen and silicon elementschange (dW/dT) is included in the corresponding graphs (Fig.11c andare distributed uniformly on the sample surface. The uniformity of dis-d). As can be seen, there are two thermal events in each case, the firsttribution of these elements can alsobe observed for theTiH2/TigO@-one being related tothedehydroxylationand the second toH2releaseTiOz/SiO2 sample in Fig.7.The increase in the hydrogen release temperature that core/shellTiH2/TigO@TiO/SiO2particles were cut transversallyand preparedfoaming agents present might involve two mechanisms: the reduction inby a conventional metallographic method before SEM characterisation.thermal conductivity from surface to core of particles, and diffusionalFig.8ashowsaparticleofTiH2/TiO@TiO2/SiO2alongwiththelineforbarriers, both due to the different oxide shells. The thermal conductivityEDS line scanning. The elements present along the line can be observedof TiO2 films between 80 and 150 nm goes from 1.07 to 5 W m-1 c-1in Fig. 8b, the presence of oxygen being clear, along with silicon as an[32,33], whereas for Si02 films of 100 nm of thickness and an7
Materials Chemistry and Physics 243 (2020) 122591 7 SiO2 corresponding to O–Si–O deformation vibration is located around 440 cm 1 , a signal at 804 cm 1 concerns the symmetric stretching vibration of the Si–O bond, whereas the signal around 935–975 cm 1 is related to in-plane stretching vibration of the Si–O bond of the Si–OH group. The strong signal around 1035 cm 1 is related to anti-symmetric stretching vibration of Si–O–Si. Other researchers have also attributed these bands to the characteristic vibration modes of the Si–O–Si group [27] and to the vibration mode of the Si–OH bond [27–29]. For the synthesised TiH2/SiO2 sample, the same bands, slightly displaced (i.e. 443, 781, 952 and 1070 cm 1 ), are observed (Fig. 4b), which corroborate the presence of the SiO2 layer. 3.2.2. Morphology and composition Morphologies and EDS analyses for the TiH2/Ti3O@TiO2, TiH2/SiO2 and TiH2/Ti3O@TiO2/SiO2 samples are shown in Fig. 5. The oxygen signals detected (Fig. 5d, e and 5f) indicate that the samples are at least partially oxidised. Furthermore, the presence of silicon is corroborated for TiH2/SiO2 and TiH2/Ti3O@TiO2/SiO2, confirming the formation of the SiO2 layer. The presence of some spherical particles is due to the formation of SiO2 spheres on the surface of the foaming agents due to the sol-gel process. In order to observe the uniformity of the titanium and silicon oxide shells concerning the TiH2/SiO2 and TiH2/Ti3O@TiO2/SiO2 samples, elemental mapping was carried out, as shown in Figs. 6 and 7. As can be seen in Fig. 6 for TiH2/SiO2, the titanium, oxygen and silicon elements are distributed uniformly on the sample surface. The uniformity of distribution of these elements can also be observed for the TiH2/Ti3O@TiO2/SiO2 sample in Fig. 7. TiH2/Ti3O@TiO2/SiO2 particles were cut transversally and prepared by a conventional metallographic method before SEM characterisation. Fig. 8a shows a particle of TiH2/Ti3O@TiO2/SiO2 along with the line for EDS line scanning. The elements present along the line can be observed in Fig. 8b, the presence of oxygen being clear, along with silicon as an outer layer; however, once silicon reaches the lowest level towards the centre of the particle, oxygen remains high, which is due to the layer of titanium oxides between the SiO2 shell and the TiH2 core. Fig. 8c shows another particle where the thickness of the SiO2 layer was measured, being around 82 nm. Elemental analysis of zones 1 and 2, indicated in Fig. 8c, are shown in Fig. 8d and e, respectively, where the high content of silicon and oxygen in the outer shell (zone 1) and the predominant content of titanium in the core of the particle (zone 2) were observed, as expected. As mentioned before, to study the SiO2 and Ti3O@TiO2 shells, TiH2 was dissolved with HCl from TiH2/SiO2 and TiH2/Ti3O@TiO2 particles. Fig. 9a and b shows the morphology by TEM of the shells removed from TiH2/SiO2 and TiH2/Ti3O@TiO2 samples, respectively. The amorphous nature of SiO2 allows a better observation of the morphology of the shell, whereas the crystalline structure of the titanium oxides shell does not allow the electron beam to penetrate the shell and observe the substrate support. On the other hand, Fig. 9c and d correspond to the SiO2 and Ti3O@TiO2 shells that were intentionally broken into multiple pieces to determine statistically the thickness of the shells, resulting in 89.1 � 16.5 nm and 72.1 � 14.2 nm, respectively. The mean value of the determined thickness of the SiO2 shell agrees with the value measured by SEM for the transversally cut sample (Fig. 8c). These images obtained by SEM and TEM corroborate that the shells are uniform and cover homogeneously the TiH2 core. The characterisation results indicate that reduced titanium oxide is the first layer to form during the isothermal heating of TiH2; this layer evolves to TiO2 as oxygen diffuses from the surface to the inner layers [30]. At the end of the treatment, the particle structure becomes TiH2/Ti3O@TiO2 from core to outer shell. Otherwise, the method used to form the SiO2 layer allows obtaining an amorphous layer of SiO2 with shell thickness around 89 nm. Fig. 10 shows a schematic representation of the different developed core/shell particles based on TiH2. 3.3. Thermal analysis of the foaming agents The TGA pattern of TiH2, and TGA and DTG patterns of TiH2/ Ti3O@TiO2 particles are shown in Fig. 11a and b, respectively, as well as the TGA and DTG patterns for TiH2/SiO2 and TiH2/Ti3O@TiO2/SiO2 (Fig. 11c and d, respectively). Table 2 shows the specific information on thermal events that are useful for the performance of a foaming agent. As seen in Fig. 11a, TiH2 powders present an initial decomposition temperature of 440 �C. On the other hand, a weight loss is evident for the TiH2/Ti3O@TiO2 foaming agent (Fig. 11b) that remains until around 500 �C. Another weight loss is directly attributed to H2 release, which begins around 540 �C, i.e., 100 �C above the decomposition temperature observed for untreated TiH2. For TiH2/SiO2 and TiH2/Ti3O@TiO2/SiO2, Fig. 11c and d, respectively, the loss of weight below 450 �C corresponds to solvent evaporation and loss of adsorbed moisture. For these two samples, the loss of weight above 450 �C consist of two steps: the first loss for sample TiH2/SiO2 is from 450 to 550 �C, as is observed by the change of slope, whereas for TiH2/Ti3O@TiO2/SiO2 the loss reaches 590 �C. This step can be assigned to the dehydroxylation of OH groups linked to the SiO2 structure [31]. The final weight loss, which is around 510 �C for TiH2/SiO2 and 600 �C for TiH2/Ti3O@TiO2/SiO2, can be attributed to the H2 release from the TiH2 core of the particles. In order to differentiate this weight loss from that due to Si–OH dehydroxylation, the derivative curve of weight change with respect to temperature change (dW/dT) is included in the corresponding graphs (Fig. 11c and d). As can be seen, there are two thermal events in each case, the first one being related to the dehydroxylation and the second to H2 release. The increase in the hydrogen release temperature that core/shell foaming agents present might involve two mechanisms: the reduction in thermal conductivity from surface to core of particles, and diffusional barriers, both due to the different oxide shells. The thermal conductivity of TiO2 films between 80 and 150 nm goes from 1.07 to 5 W m 1 �C 1 [32,33], whereas for SiO2 films of 100 nm of thickness and an Fig. 8. SEM micrographs of cross sections of TiH2/Ti3O@TiO2/SiO2 particles (a and c). b) Content of oxygen (O), silicon (Si) and titanium (Ti), along line indicated in Fig. 8a d) EDS analysis of spot 1 indicated in Fig. 8c e) EDS analysis of spot 2 indicated in Fig. 8c. M. Romero-Romero et al
Materials Chemistry and Physics 243 (2020) 122591M, Romero-Romero et al.b2μm500nmC1μm1μumFig. 9. TEM micrographs of a) SiO2 shell from TiH2/SiO2 and b) TigO@TiO2 shell from TiH2/Tig0@TiO2. SEM micrographs from shell fragments of c) SiO2 andd)Ti.0@TiO2amorphous SiO2shellsaswellasfor subsequentanalysesThermal and diffusional barriers give the foaming agent a delay tothe release of H2 from the TiH2 and TiH2/Ti3O@TiO2 particles and as a72.1 nm89.1 nmresult, the loss of H2 starts 100 °C above the temperaturefor pure TiH2.The temperature delay obtained from the incorporation of the SiO, shellon TiH2 particles is only 73 °C; i.e., despite the lower thermal conduc-tivityofthismaterial (SiO2)theTisO@TiO2shellpresentsbetterper-formance.whichcouldbeattributedtoamoreuniformformationoftheb)a)crystallinebarrierthrough the transformation of TiH2toTigO and TiO2,improving the diffusional restriction for H2 release. The poor perfor-mance of the SiO2 layer also can be due to the higher propensity of this89.1nmlayer to crack during heating,as a consequence of the loss of residuesfrom the sol-gel process.72.1nmTiH2Inordertoestimatethecontributionofeachbarrierinrestrainingtheheattransmissionacrossshells,thethermaldiffusivityofSiO2andTi,OTigO@TiO2 was estimated from thermal conductivity,density and spe-TiOcific heat. Thermal diffusivity is calculated from Ref. [37]:kSiO2a=Cpcwhere:Fig.10. Cross-section schemes of particles: a) core/double-shell TiHa/TigO@-TIO2; b) core/shell TiH2/SiO2: and c) core/multi-shell TiH2/TigO@TiO2/SiO2-a = Thermal diffusivity (m? s-l).k = Thermal conductivity (W m-1 oc-").amorphous structure it is between 0.76 and 1.08 W m-1 oc-1 [34,35];p = Density (kg m-3).these values are far from the -20 W m-1 C-1 value for TiH, [36] andc = Specific heat (J kg-1 oC-l),can be considered as representative data for crystalline TigO@TiO2 and
Materials Chemistry and Physics 243 (2020) 122591 8 amorphous structure it is between 0.76 and 1.08 W m 1 �C 1 [34,35]; these values are far from the ~20 W m 1 �C 1 value for TiHx [36] and can be considered as representative data for crystalline Ti3O@TiO2 and amorphous SiO2 shells as well as for subsequent analyses. Thermal and diffusional barriers give the foaming agent a delay to the release of H2 from the TiH2 and TiH2/Ti3O@TiO2 particles and as a result, the loss of H2 starts 100 �C above the temperature for pure TiH2. The temperature delay obtained from the incorporation of the SiO2 shell on TiH2 particles is only 73 �C; i.e., despite the lower thermal conductivity of this material (SiO2) the Ti3O@TiO2 shell presents better performance, which could be attributed to a more uniform formation of the crystalline barrier through the transformation of TiH2 to Ti3O and TiO2, improving the diffusional restriction for H2 release. The poor performance of the SiO2 layer also can be due to the higher propensity of this layer to crack during heating, as a consequence of the loss of residues from the sol-gel process. In order to estimate the contribution of each barrier in restraining the heat transmission across shells, the thermal diffusivity of SiO2 and Ti3O@TiO2 was estimated from thermal conductivity, density and specific heat. Thermal diffusivity is calculated from Ref. [37]: a ¼ k ρ c where: a ¼ Thermal diffusivity (m2 s 1 ). k ¼ Thermal conductivity (W m 1 �C 1 ). ρ ¼ Density (kg m 3 ). c ¼ Specific heat (J kg 1 �C 1 ). Fig. 9. TEM micrographs of a) SiO2 shell from TiH2/SiO2 and b) Ti3O@TiO2 shell from TiH2/Ti3O@TiO2. SEM micrographs from shell fragments of c) SiO2 and d) Ti3O@TiO2. Fig. 10. Cross-section schemes of particles: a) core/double-shell TiH2/Ti3O@TiO2; b) core/shell TiH2/SiO2; and c) core/multi-shell TiH2/Ti3O@TiO2/SiO2. M. Romero-Romero et al
M.Romero-Romero et al.Materials Chemistry and Physics 243 (2020) 1225911010.05b)440°Ca)540°℃oo100100:TGA(0/%)LP/MP99TiH2Couepn0.03(%)pM00.029840.01DTG97060.0TiH,/Ti,O@TiOb+-0.01to60070800500100200300400500600700800Temperature (°C)Temperature(C)1010.025c)601°℃d)513℃0.020100100TGATGA0.020.015(O/%) LP/MP(a/%) LP/MP99Coaps(%)0.01099DTG0.0080.00DTG980.00097TiH,/SiO,00e-0.05TiH,/TiO,@Ti,O@SiO,596-0.0109596N0.019100200300400500600700100200300500600700800800Temperature (°C)Temperature (°C)Fig.11. a) TGA pattern of TiH2 and b) TGA and DTG patterns of TiH2/TigO@TIO2 particles. TGA and DTG patterns of c) TIH2/SiO2 and d) TiH2/TigO@TiOz/SiO2particles. Analyses were made at a heating rate of 10 °C min-' under inert atmospherethatabetterlayeruniformityisobtainedwithaminimumnumberofTable 2cracks at the core/shell (TiH2/TigO@TiO2) interface.Thermal events of foaming agents core/double-shell TiH2/TigO@TiO2, core/Bowman Jret al.[41,42],studied thehydrogenmobilitythroughshell TiH2/SiO2 and core/multi-shell TiH2/Tig0@TiO2/SiO2amorphous andcrystallinestructuresusingprotonnuclearmagneticInitialFinalresonance (HNMR).In a comparison of hydrogen diffusion throughFoaming agentTemperaturetemperature° (C)temperature (°C)range (°C)amorphous α-TiCuHi,3±0.1 versus crystalline TiCuHo.94 and -TiHx, theyreported a higher diffusivity through the amorphous phase [41] and theTiH2440694254540667127TiH/Ti,0@TiO2sameconclusionwasreachedforavitreousα-Zr2PdH2.gversus crystal-513617104TiH2/SiO2line Zr2PdHx[42].In both cases, a lower activation energy of proton601714113TiHz/diffusionwasfoundfortheamorphousmaterial overcrystallinephases.TigO@TOz/According to our results, when the TiH2/TigO@TiO2/SiO2 foamingSiO2agent is under heating,SiO2 retards heat flow from the edge to the core Initial temperature was calculated applying the extrapolated onset temper-as the first thermal barrier, then heat is transferred to the core throughatureapproach onDTG curve.TisO@TiO2,whichalsoworks as athermalbarrier.Onceheatreachesthe TiH2 core, thermal activation ofthe latter allows H2 to start to diffusethroughthecrvstallineTi,O@TiO,shell,whichinthiscaseworksasaTable3diffusion barrier,and according to ourresults itsperformance asaRepresentative values of thermal conductivity k, density p, specific heat c (at adiffusion barrier is higher than that of the SiO2 shell. Then H2 diffusestemperature of 25°C), and thermal diffusivity a for Tig0@TiO2 and siO2 shells.through the amorphous SiO2 shell and leaves the particles. Thus, thek (Wm~1c(J kg-lMaterialpCkgα (m"s-l)multi-shell TigO@TiO2/SiO2 increases the H2 release temperature from"c-1)m-3)°C-1)440to601°C.3.041.07 ×TiO2 (rutile)4130 [38]690.37 [39]ThedevelopedparticlesofTiH2/TisO@TiO2fulfiltwoimportant10-6characteristics required for a suitable foaming agent: an increase in0.925.73 ×SiO22190 [34]733.36 [40]10-7(amorphous)decomposition temperature and a narrowing of the temperature range atwhich the gas is released. However, by covering these particles with anAverageofvalues rangeof thermal conductivity mentioned aboveextra shellof SiO2 (TiH2/TigO@TiO2/SiO2)theperformanceas foamingagent is improved. Since the H2 release temperature reached 601 C andTable 3 shows the data used for calculation and results.The thermalthetypical liquidustemperaturerangeofaluminiumalloys is600-650diffusivityof SiO2issignificantlylowerthanthatof TiO2,which con-°C [14], this modification of TiH2asfoaming agent is appropriate for thefirms that the amorphous SiO2 shell is a better thermal barrier than thepotentialfoamingofawiderangeofaluminiumalloysandimprovethecrystallineTigO@TiO2 shell.Taking into accountthat thelatter (TigO@homogeneityof theirporestructure(anarrowestporesizedistribution);TiO2) showed the best performance in retarding H2 release, it suggests9
Materials Chemistry and Physics 243 (2020) 122591 9 Table 3 shows the data used for calculation and results. The thermal diffusivity of SiO2 is significantly lower than that of TiO2, which confirms that the amorphous SiO2 shell is a better thermal barrier than the crystalline Ti3O@ TiO2 shell. Taking into account that the latter (Ti3O@ TiO2) showed the best performance in retarding H2 release, it suggests that a better layer uniformity is obtained with a minimum number of cracks at the core/shell (TiH2/Ti3O@TiO2) interface. Bowman Jr et al. [41,42], studied the hydrogen mobility through amorphous and crystalline structures using proton nuclear magnetic resonance (HNMR). In a comparison of hydrogen diffusion through amorphous α-TiCuH1.3�0.1 versus crystalline TiCuH0.94 and γ-TiHx, they reported a higher diffusivity through the amorphous phase [41] and the same conclusion was reached for a vitreous α-Zr2PdH2.9 versus crystalline Zr2PdHx [42]. In both cases, a lower activation energy of proton diffusion was found for the amorphous material over crystalline phases. According to our results, when the TiH2/Ti3O@TiO2/SiO2 foaming agent is under heating, SiO2 retards heat flow from the edge to the core as the first thermal barrier, then heat is transferred to the core through Ti3O@TiO2, which also works as a thermal barrier. Once heat reaches the TiH2 core, thermal activation of the latter allows H2 to start to diffuse through the crystalline Ti3O@TiO2 shell, which in this case works as a diffusion barrier, and according to our results its performance as a diffusion barrier is higher than that of the SiO2 shell. Then H2 diffuses through the amorphous SiO2 shell and leaves the particles. Thus, the multi-shell Ti3O@TiO2/SiO2 increases the H2 release temperature from 440 to 601 �C. The developed particles of TiH2/Ti3O@TiO2 fulfil two important characteristics required for a suitable foaming agent: an increase in decomposition temperature and a narrowing of the temperature range at which the gas is released. However, by covering these particles with an extra shell of SiO2 (TiH2/Ti3O@TiO2/SiO2) the performance as foaming agent is improved. Since the H2 release temperature reached 601 �C and the typical liquidus temperature range of aluminium alloys is 600–650 �C [14], this modification of TiH2 as foaming agent is appropriate for the potential foaming of a wide range of aluminium alloys and improve the homogeneity of their pore structure (a narrowest pore size distribution); Fig. 11. a) TGA pattern of TiH2 and b) TGA and DTG patterns of TiH2/Ti3O@TiO2 particles. TGA and DTG patterns of c) TiH2/SiO2 and d) TiH2/Ti3O@TiO2/SiO2 particles. Analyses were made at a heating rate of 10 �C min 1 under inert atmosphere. Table 2 Thermal events of foaming agents core/double-shell TiH2/Ti3O@TiO2, core/ shell TiH2/SiO2 and core/multi-shell TiH2/Ti3O@TiO2/SiO2. Foaming agent Initial temperaturea (�C) Final temperature (�C) Temperature range (�C) TiH2 440 694 254 TiH2/Ti3O@TiO2 540 667 127 TiH2/SiO2 513 617 104 TiH2/ Ti3O@TiO2/ SiO2 601 714 113 a Initial temperature was calculated applying the extrapolated onset temperature approach on DTG curve. Table 3 Representative values of thermal conductivity k, density ρ, specific heat c (at a temperature of 25 �C), and thermal diffusivity a for Ti3O@TiO2 and SiO2 shells. Material ka (W m 1 �C 1 ) ρ (kg m 3 ) c (J kg 1 �C 1 ) a (m2 s 1 ) TiO2 (rutile) 3.04 4130 [38] 690.37 [39] 1.07 � 10 6 SiO2 (amorphous) 0.92 2190 [34] 733.36 [40] 5.73 � 10 7 a Average of values range of thermal conductivity mentioned above. M. Romero-Romero et al
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Materials Chemistry and Physics 243 (2020) 122591 10 due to the release temperature of H2 increases the thermal stability leads to more mixing time of foaming agent powder into liquid aluminium, i.e. if melting route is used. 4. Conclusions TiH2/Ti3O@TiO2/SiO2 core/multi-shell particles were successfully synthesised where the thickness of the resulting shells was 72.1 � 14.2 nm and 89.1 � 16.6 nm, for TiO3@TiO2 and SiO2 respectively as SEM outcomes showed. By the formation of a diffusion barrier of Ti3O@TiO2 on the surface of TiH2 through simple isothermal heating of TiH2, and later covering the particles with an amorphous SiO2 shell by mean of a sol-gel process, it was possible to increase the temperature at which hydrogen is released from the TiH2 core from 440 to 601 �C. In addition, the temperature range of 254 �C of hydrogen release from TiH2 was reduced to 113 �C (core/multi-shell particles). Thus, the TiH2/Ti3O@TiO2/SiO2 core/multi-shell particles possess suitable properties for use as foaming agents for the aluminium alloys with the possibility of producing foams with a narrow pore size distribution. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors would like to thank D. Lardizabal, P. Piza, � C. Leyva, J. Holguín, E. Guerrero, L. de la Torre, W. Antúnez, R. Ochoa, A. Saenz � and C. Santill� an for the technical assistance provided during the realisation of this work. The help and advice provided by M. Roman � and K. Beltr� an during the execution in this work is greatly appreciated. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. References [1] F. 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