《复合材料 Composites》课程教学资源（学习资料）第二章 增强体_Al2O3-whisker Production of Chromium-Doped a-Al2O3 Whiskers Via Vapor Liquid Solid Deposition

J. A. Ceran. Soc., 89 [1]323-327( DOI:10.11111551-2916.200500 c 2005 The American Ceramic Society journal Production of Chromium-Doped a-AL2O3 Whiskers Via Vapor Liquid Solid deposition Carmen Cerecedo. Victor valcarcel t Monica Gomez and Francisco Guitian Instituto de Ceramica de galicia(lC G), Universidade de santiago de Compostela(U.SC ) S-15782 Santiago do The prime objective of this work is to demonstrate that chro- prompted us to further investigate the means of"doping"alu- mium-doped alumina fibers could, for the first time, be obtained mina whiskers. The prime objective of this work is to demon- r liquid solid (vls)deposition. various procedures are strate that doping with chromium(and feasibly other elements described and discussed in the text. The mechanism for effective can be performed through small variations of the basic VLs is also discussed and the resulting fibers are analyzed. a mechanism. as described below The reaction between Sio modification of the basic Vls deposition ss was investi Al in inert Ar atmospheres is now carried out in the presence of gated with the aim of producing doped a-Al2O3(a-alumina or gaseous Cr in its appropriate ionization state(Cr+). Here we corundum)whiskers. Chromium-doped(ruby)corundum whisk demonstrate that the adequate introduction of Cr't vapor with- ers were obtained by the introduction of Cr't in gas form within in such inert atmospheres produces doped alumina whiskers the argon flow used to attain inert furnace atmospheres. various Unfortunately, the production of Cr-doped fiber is limited by procedures are described and discussed in the text, using differ- the small dimensions of the furnace used, but we observed noth- ent ch mpounds, and the mechanism of effective dol g to impair the extrapolation of these results to a larger scale. ing is also discussed in each case To summarize, in this report we describe various methods of roducing Cr-doped alumina single crystals in both fiber and ribbon forms. The basic principle is the reaction between Sio . Introduction and Al in Ar atmospheres. with the novel incorporation of gas phases containing Cr in its appropriate onization E XTENSIVE research efforts over the past decades have led te state. To the best of our knowledge, this concept of doping any developments in the field of fiber-reinforced compos- in a VLS mechanism has not been previously reported in the ites. Whiskers are defect-free, very thin single-crystal fibers, literature, neither for alumina nor any other whisker chemical which are among the most resistant materials known. Long whiskers of a-Al2O3(a-alumina or corundum) could be part ularly appropriate" as strengtheners in composites because of gh elasti leir ther IL. Experimental Procedure bility. We have previously described a simple, novel method for the production of corundum whiskers at moderate temper The idea behind the following procedure was to incorporate the tures and low cost which could be used on an industrial scale Cr in gas form with the argon entering the furnace. The main Here, we report modifications of this method that allow the obstacle to such ing nto the VLs crystal generation of chromium-doped(ruby) corundum whiskers rowth mechanism of alumina fibers is the lack of gaseous spe- ntroducing small quantities of a doping element produces Cr+ at th es at which fibers grow color variations in crystalline alumina Doping not only causes (1300%-1600C), making it difficult to distribute the desired dop- changes in color. but in other f the alumina as well elements evenly to all fibers (table D) This makes the behavior of doped alumina an active field of We have performed experiments using two procedur research, as the kinetics of precipitation and phase stability in Procedure 1(Fig. la): A mixture of SiO, with 10% wt of Ni npurity-doped a-Al2O3 have significant economic impor powder (<63 um) was placed on an alumina crucible. Alumi nce. For example doped alumina particles are used in com- num wires(length 10 mm, diameter 4 mm) were deposited on the ites for the special features of the metah-ceramic interface wdered bed and the crucible was then introduced into a tube Ignace(70 mm diamet This means that doped whiskers could offer very attractive consisted of an initial temperature-increase rate of 10C/min to a properties for metal matrix composites, MMCS It can be said that each doping element will impart new prop- 1550C plateau temperature that was maintained for 3 erties to the fiber (or whisker), and therefore to the composites which the system was cooled to room temperature at a rate of For example, Cr-doped alumina can be used in lasers operatin 10.C/min Inert atmospheres of argon gas were used, and a 0.6 at a wavelength of 694 nm, in pressure sensors, in devices for L/min argon fux was maintained during both the temperature- honon detection, etc, and improves the metal-ceramic inter- increase and the temperature-decrease steps face because of charge-transference phenomena. This is the fea- A small glass tube loaded with 2 g of CrO, was immersed in ture that interests us the most. As another example, Ti-doped an oil bath at 250C and connected to the furnace gas input alumina shows enhanced strength of crystalline surfaces device in such a way that the CrO3 was melted, and such that the These potential properties and our previous work on the Venturi effect allows CrO3(g)to enter the furnace with the main vapor liquid solid (VLS) growth of alumina single-crystal fibers flow of A Procedure 2(Fig 1b): A mixture of Sio with 10 wt% of Ni powder(<63 um)was placed on an alumina crucible. Alumi J. Moya-contributing editor num wires (length 10 mm, diameter 4 mm)were deposited on the powdered bed, and the crucible was then introduced into a tube furnace (70 mm diameter) A small container with 10 cm of commercial Cr(NO3)3 Author to 9H2o(diluted in water at 65%)was heated to 130oC for 4 h. w

Journal J. Am. Ceram. Soc., 89 [1] 323–327 (2006) DOI: 10.1111/j.1551-2916.2005.00660.x r 2005 The American Ceramic Society Production of Chromium-Doped a-Al2O3 Whiskers Via Vapor Liquid Solid Deposition Carmen Cerecedo, Vı´ctor Valca´rcel,w Mo´nica Go´mez, and Francisco Guitia´n Instituto de Cera´mica de Galicia (I.C.G.), Universidade de Santiago de Compostela (U.S.C.), S-15782 Santiago de Compostela, Spain The prime objective of this work is to demonstrate that chro￾mium-doped alumina fibers could, for the first time, be obtained via vapor liquid solid (VLS) deposition. Various procedures are described and discussed in the text. The mechanism for effective doping is also discussed, and the resulting fibers are analyzed. A modification of the basic VLS deposition process was investi￾gated with the aim of producing doped a-Al2O3 (a-alumina or corundum) whiskers. Chromium-doped (ruby) corundum whisk￾ers were obtained by the introduction of Cr31 in gas form within the argon flow used to attain inert furnace atmospheres. Various procedures are described and discussed in the text, using differ￾ent chromium compounds, and the mechanism of effective dop￾ing is also discussed in each case. I. Introduction EXTENSIVE research efforts over the past decades have led to many developments in the field of fiber-reinforced compos￾ites.1 Whiskers are defect-free, very thin single-crystal fibers, which are among the most resistant materials known. Long whiskers of a-Al2O3 (a-alumina or corundum) could be partic￾ularly appropriate2 as strengtheners in composites because of their high elastic modulus and their thermal and chemical sta￾bility.3 We have previously described4–6 a simple, novel method for the production of corundum whiskers at moderate temper￾atures and low cost, which could be used on an industrial scale. Here, we report modifications of this method that allow the generation of chromium-doped (ruby) corundum whiskers. Introducing small quantities of a doping element produces color variations in crystalline alumina. Doping not only causes changes in color, but in other properties of the alumina as well. This makes the behavior of doped alumina an active field of research,7–9 as the kinetics of precipitation and phase stability in impurity-doped a-Al2O3 have significant economic impor￾tance.10 For example, doped alumina particles are used in com￾posites for the special features of the metal–ceramic interface. This means that doped whiskers could offer very attractive properties for metal matrix composites, MMCs. It can be said that each doping element will impart new prop￾erties to the fiber (or whisker), and therefore to the composites. For example, Cr-doped alumina can be used in lasers operating at a wavelength of 694 nm, in pressure sensors, in devices for phonon detection, etc., and improves the metal–ceramic inter￾face because of charge-transference phenomena. This is the fea￾ture that interests us the most. As another example, Ti-doped alumina shows enhanced strength of crystalline surfaces. These potential properties and our previous work on the vapor liquid solid (VLS) growth of alumina single-crystal fibers prompted us to further investigate the means of ‘‘doping’’ alu￾mina whiskers. The prime objective of this work is to demon￾strate that doping with chromium (and feasibly other elements) can be performed through small variations of the basic VLS mechanism, as described below. The reaction between SiO2 and Al in inert Ar atmospheres is now carried out in the presence of gaseous Cr in its appropriate ionization state (Cr31). Here we demonstrate that the adequate introduction of Cr31 vapor with￾in such inert atmospheres produces doped alumina whiskers. Unfortunately, the production of Cr-doped fiber is limited by the small dimensions of the furnace used, but we observed noth￾ing to impair the extrapolation of these results to a larger scale. To summarize, in this report we describe various methods of producing Cr-doped alumina single crystals in both fiber and ribbon forms. The basic principle is the reaction between SiO2 and Al in Ar atmospheres, with the novel incorporation of gas phases containing Cr in its appropriate Cr31 ionization state. To the best of our knowledge, this concept of doping in a VLS mechanism has not been previously reported in the literature, neither for alumina nor any other whisker chemical compositions. II. Experimental Procedure The idea behind the following procedure was to incorporate the Cr31 in gas form with the argon entering the furnace. The main obstacle to such incorporation of Cr31 into the VLS crystal growth mechanism of alumina fibers is the lack of gaseous spe￾cies containing Cr31 at the temperatures at which fibers grow (13001–16001C), making it difficult to distribute the desired dop￾ing elements evenly to all fibers (Table I). We have performed experiments using two procedures: Procedure 1 (Fig. 1a): A mixture of SiO2 with 10% wt of Ni powder (o63 mm) was placed on an alumina crucible. Alumi￾num wires (length 10 mm, diameter 4 mm) were deposited on the powdered bed, and the crucible was then introduced into a tube furnace (70 mm diameter). The programmed temperature cycles consisted of an initial temperature-increase rate of 101C/min to a 15501C plateau temperature that was maintained for 3 h, after which the system was cooled to room temperature at a rate of 101C/min. Inert atmospheres of argon gas were used, and a 0.6 L/min argon flux was maintained during both the temperature￾increase and the temperature-decrease steps. A small glass tube loaded with 2 g of CrO3 was immersed in an oil bath at 2501C and connected to the furnace gas input device in such a way that the CrO3 was melted, and such that the Venturi effect allows CrO3(g) to enter the furnace with the main flow of Ar. Procedure 2 (Fig. 1b): A mixture of SiO2 with 10 wt% of Ni powder (o63 mm) was placed on an alumina crucible. Alumi￾num wires (length 10 mm, diameter 4 mm) were deposited on the powdered bed, and the crucible was then introduced into a tube furnace (70 mm diameter). A small container with 10 cm3 of commercial Cr(NO3)3
9H2O (diluted in water at 65%) was heated to 1301C for 4 h. We 323 J. Moya—contributing editor w Author to whom correspondence should be addressed. e-mail: cevictor@usc.es Manuscript No. 20650. Received June 7, 2005; approved June 23, 2005

Communications of the American Ceramic Society Vol. 89. No. I Table I. Melting and Boiling Points of Cr and Cr2O3 are being investigated via CVD of chromium-doped alumina ruby) thin films, for which the desired Cr concentrationis Melting point(°O Boiling point (C below 0.3 at. % The strength of the crystalline field is directly related to the 2672 oxygen-metal (Ormetal) distance in the octahedric complex Cr2O3 435 400 and is determined by the following equation C chose this temperature because 130oC is the melting point of △5 pure salt, so then this allowed total evaporation of water. The d(Me-o) container was then connected to a spraying device(Spraying Systems, Wheaton, IL, 1/4J, PF1050 with PA64)with a valve, where C is a constant and Ao is a parameter of the crystalline nd this was attached to the ar entrance. The furnace was heat field. Because of its atomic radius, Cr can only substitute d up to 1550oC at 10C/min, with an Ar flux of 0.6 L/min Once Al atoms in alumina in octahedric positions, and therefore the furnace temperature had reached 1300oC, the valve wa d (Me-o)corresponds to the metal-oxygen distance in octa- pened every 5 min for lapses of 1 min, allowing concentrated hedric coordination. The reduction of such a distance gives Cr(NO3)3 to be sprayed into the furnace atmosphere. Two rise to a corresponding augmentation of the energy in the routes were used at this point crystalline field which affects Cr, causing the cited transition (a) When the plateau temperature of 1550.C was reached, from green to red no more doping agent was incorporated into the atmosphere. Before discussing how we can carry out the doping process This method allowed us to demonstrate the effect of the ex within a Vls deposition mechanism, it is worth considering haustion of the available Cr, as explained below some basic aspects of the Vls mechanism of fiber growth itself. (b) Alternatively, Cr solution was also added during the The key points of the aforementioned VLs procedure can entire plateau of the heating cycle(opening the valve for lapses summarized as follows: pieces of Al were placed on a 20 min). shallow bed of quartz in an inert furnace atmosphere of Ar at temperatures between 1300 and 1500.C. Corundum crystals were readily produced after at least I h of plateau tem- lI Results and discussion perature. These crystals grow into a white cotton-like mass around the al piece. Utilization of Ni allows the use of temper Procedure 1 resulted in the generation of ruby(Cr'+ atures of 1550C or even higher thus increasing the production ped)alumin (Figs. 2(a)and(b). Proce- of alumina fibers doped alumina whiskers Detailed scanning microscopy analysis revealed two mor- nowing the of the cr exhaustion durin phology classes: fibers and ribbons. Fibers have hexagonal sec- the vls deposition tions and form drops at one of their tips. Strikingly, fibers have Prior to discussing our results, it is worth mentioning here aspect ratios often over 1000, and sometimes up to 10000. Rib- how the chromium content can alter the properties of the bons(Figs. 3(a)and(c)have flat shapes(length -I cm,branch- es width 10 um, thickness 0.5 um), triangular tips with Red varieties of alumina containing <2.5% of Cr't are casual drops, and they branch out at fixed angles( 60), produc- called rubies. In rubies. an increase in chromium content ca us- ing large single crystals with elaborate dendritic structure. Drops es an increase in the lattice constants of the Al,Ox-Cr2O3 solid solution, and the crystal field strength around the Crt ion de- deposited on ribbons are used by fibers as a suitable crystalli- zation surface (Fig. 3(b)). The mechanism of fiber growth creases. Cr* can produce color variations"ranging from green, known as VLs deposition, 3 as is clearly seen through the drops at the tip of the fibers(Figs. 3(c)and(d) Using this property, decorative coatings on sapphire substrates Aluminum evaporation allows reaction with the quartz pow- der present in the crucible, generating new gaseous species such as Al and Si oxides, i.e. AlO(g), AlO(g), or Sio(g). Some of (a) Procedure 1 these gases react to produce Al2O3 and liquid silicon, the latter being a prerequisite for drop formation at the fiber tip. Gaseous drops. Inside these drops, the probability of a reaction produc- ing corundum is increased, and it can be deposited onto an ad- equate crystallization surface. The fiber tip itself provides such a Aluminum pieces crystallization surface, allowing epitaxial growth to continue with constant cross-section In summary, VLS is initiated by deposition of a drop. Gases Furnace dissolve into the drops and react to produce Al2O3(which pre- Oil bath(250°c) cipitates at the base of the drops)and Si. The excess of Si dif- fuses back to the atmosphere because of Si equilibrium vapor pressure. vlS deposition continues indefinitely as long as drops exist at the fiber tips. As mentioned earlier, the production of Cr-doped alumina fibers needs the deposition of small amounts of Cr. One could think that a simple approach could be to transfer Cr met vapors to Vls drops, but, because of the high melting and boil Ar ing points of Cr (Table D), vaporizing appreciable amoun Sprying system of this metal at the temperatures of fiber growth(1550.C)is Valve Aluminum pieces eventually impossible. And even if some Cr is vaporized, it will be in its metal ionization state Cr", so it is no use for doping Capillary Furnace lumina(we have remarked elsewhere that Cr must be in its 3+ )3 Next, the possibility of using metal oxides(Cr2O3)was con Fig 1. Schematic of the devices used for the production of ruby fibers. sidered, but, taking into account the values shown in Table l,at

chose this temperature because 1301C is the melting point of pure salt, so then this allowed total evaporation of water. The container was then connected to a spraying device (Spraying Systems, Wheaton, IL, 1/4J, PF1050 with PA64) with a valve, and this was attached to the Ar entrance. The furnace was heat￾ed up to 15501C at 101C/min, with an Ar flux of 0.6 L/min. Once the furnace temperature had reached 13001C, the valve was opened every 5 min for lapses of 1 min, allowing concentrated Cr(NO3)3 to be sprayed into the furnace atmosphere. Two routes were used at this point: (a) When the plateau temperature of 15501C was reached, no more doping agent was incorporated into the atmosphere. This method allowed us to demonstrate the effect of the ex￾haustion of the available Cr, as explained below. (b) Alternatively, Cr solution was also added during the entire plateau of the heating cycle (opening the valve for lapses of 1 min every 20 min). III. Results and Discussion Procedure 1 resulted in the consistent generation of ruby (Cr31- doped) alumina single-crystal fibers (Figs. 2(a) and (b)). Proce￾dure 2 also gave rise to uniformly doped alumina whiskers (Fig. 2(c)), also showing the effect of the Cr exhaustion during the VLS deposition. Prior to discussing our results, it is worth mentioning here how the chromium content can alter the properties of the alumina. Red varieties of alumina containing r2.5% of Cr31 are called rubies. In rubies, an increase in chromium content caus￾es an increase in the lattice constants of the Al2O3–Cr2O3 solid solution, and the crystal field strength around the Cr31 ion de￾creases. Cr31 can produce color variations11 ranging from green, when it is inside a weak field, to deep red when the field is strong. Using this property, decorative coatings on sapphire substrates are being investigated via CVD of chromium-doped alumina (ruby) thin films, for which the desired Cr concentration12 is below 0.3 at.%. The strength of the crystalline field is directly related to the oxygen–metal (O2–metal) distance in the octahedric complex and is determined by the following equation: A0 ¼ C dðMe OÞ 5 where C is a constant and A0 is a parameter of the crystalline field. Because of its atomic radius, Cr13 can only substitute Al atoms in alumina in octahedric positions, and therefore d(Me–O) corresponds to the metal–oxygen distance in octa￾hedric coordination. The reduction of such a distance gives rise to a corresponding augmentation of the energy in the crystalline field which affects Cr31, causing the cited transition from green to red. Before discussing how we can carry out the doping process within a VLS deposition mechanism, it is worth considering some basic aspects of the VLS mechanism of fiber growth itself. The key points of the aforementioned VLS procedure can be summarized as follows: pieces of Al were placed on a shallow bed of quartz in an inert furnace atmosphere of Ar at temperatures between 13001 and 15001C. Corundum crystals were readily produced after at least 1 h of plateau tem￾perature. These crystals grow into a white cotton-like mass around the Al piece. Utilization of Ni allows the use of temper￾atures of 15501C or even higher, thus increasing the production of alumina fibers. Detailed scanning microscopy analysis revealed two mor￾phology classes: fibers and ribbons. Fibers have hexagonal sec￾tions and form drops at one of their tips. Strikingly, fibers have aspect ratios often over 1000, and sometimes up to 10 000. Rib￾bons (Figs. 3(a) and (c)) have flat shapes (length B1 cm, branch￾es width B10 mm, thickness B0.5 mm), triangular tips with casual drops, and they branch out at fixed angles (601), produc￾ing large single crystals with elaborate dendritic structure. Drops deposited on ribbons are used by fibers as a suitable crystalli￾zation surface (Fig. 3(b)). The mechanism of fiber growth is known as VLS deposition,13 as is clearly seen through the drops at the tip of the fibers (Figs. 3(c) and (d)). Aluminum evaporation allows reaction with the quartz pow￾der present in the crucible, generating new gaseous species such as Al and Si oxides, i.e. AlO(g), Al2O(g), or SiO(g). Some of these gases react to produce Al2O3 and liquid silicon, the latter being a prerequisite for drop formation at the fiber tip. Gaseous species rich in Al and Si oxides are then slowly dissolved into the drops. Inside these drops, the probability of a reaction produc￾ing corundum is increased, and it can be deposited onto an ad￾equate crystallization surface. The fiber tip itself provides such a crystallization surface, allowing epitaxial growth to continue with constant cross-section. In summary, VLS is initiated by deposition of a drop. Gases dissolve into the drops and react to produce Al2O3 (which pre￾cipitates at the base of the drops) and Si. The excess of Si dif￾fuses back to the atmosphere because of Si equilibrium vapor pressure. VLS deposition continues indefinitely as long as drops exist at the fiber tips. As mentioned earlier, the production of Cr-doped alumina fibers needs the deposition of small amounts of Cr31. One could think that a simple approach could be to transfer Cr metal vapors to VLS drops, but, because of the high melting and boil￾ing points of Cr (Table I), vaporizing appreciable amounts of this metal at the temperatures of fiber growth (15501C) is eventually impossible. And even if some Cr is vaporized, it will be in its metal ionization state Cr0 , so it is no use for doping alumina (we have remarked elsewhere that Cr must be in its 31 ionization state). Next, the possibility of using metal oxides (Cr2O3) was con￾Fig. 1. Schematic of the devices used for the production of ruby fibers. sidered, but, taking into account the values shown in Table I, at Table I. Melting and Boiling Points of Cr and Cr2O3 Metals Melting point (1C) Boiling point (1C) Cr 1857 2672 Cr2O3 2435 4000 324 Communications of the American Ceramic Society Vol. 89, No. 1

January 2006 Communications of the American Ceramic Society 4 mm 100pm Fig. 2. (a) Using the procedure described in the text, ruby fibers are formed around aluminum pieces. (b) Details of such ruby fibers are shown.(c) Procedure 2(see text)also gives rise to Cr-doped fibers. The ending tips of some fibers are white(not doped) because of Cr exhaustion in the furnac atmosphere during the final part of some experiments, as described in the text. 1550 C there is still no vaporization of Cr,O3, and doping will and, in each case, Cr+ distribution was affected via different not take place. Additionally, all reactions between Al gases and nechanisms Cr,O3 have positive AG, and therefore this chromium oxide has Procedure 1: As described in the experimental section, CrO3 no effect at all within the system. as placed in a bath of oil at 250C. As CrO3 melts at 19 e observed that mixing Cr2O3 with the Sio2 powder some molecules will evaporate at 250C and will be incorporated uced a few individual doped fibers, which we can only attribute into the Ar current entering the furnace tube. Molecules passing to direct physical contact between a Vls drop and the Cr2O3 through the furnace tube rapidly reach 1550C in a fast tem present in the crucible. perature gradient. The following decomposition reaction is ther In order to overcome the aforementioned difficulties tw nodynamically favored above the CrO3 melting point, although procedures were devised to obtain ruby fibers consistently total dec sition is attained at approximately 400C

15501C there is still no vaporization of Cr2O3, and doping will not take place. Additionally, all reactions between Al gases and Cr2O3 have positive DG, and therefore this chromium oxide has no effect at all within the system. We observed that mixing Cr2O3 with the SiO2 powder pro￾duced a few individual doped fibers, which we can only attribute to direct physical contact between a VLS drop and the Cr2O3 present in the crucible. In order to overcome the aforementioned difficulties, two procedures were devised to obtain ruby fibers consistently, and, in each case, Cr31 distribution was affected via different mechanisms. Procedure 1: As described in the experimental section, CrO3 was placed in a bath of oil at 2501C. As CrO3 melts at 1961C, some molecules will evaporate at 2501C and will be incorporated into the Ar current entering the furnace tube. Molecules passing through the furnace tube rapidly reach 15501C in a fast tem￾perature gradient. The following decomposition reaction is ther￾modynamically favored above the CrO3 melting point, although total decomposition14–17 is attained at approximately 4001C, Fig. 2. (a) Using the procedure described in the text, ruby fibers are formed around aluminum pieces. (b) Details of such ruby fibers are shown. (c) Procedure 2 (see text) also gives rise to Cr-doped fibers. The ending tips of some fibers are white (not doped) because of Cr exhaustion in the furnace atmosphere during the final part of some experiments, as described in the text. January 2006 Communications of the American Ceramic Society 325

Communications of the American Ceramic Society Vol. 89. No. I 10um apor liquid solid (VLS) deposition: Four stages of fiber growth mechanism(VLS deposition the figure. Ribbons bstrate for the deposition of a silicon drop(b), allowing the epitaxial growth(c)of alumina fibers, d in the text. At the fina h, fibers can attain lengths of several hundreds of microns(d). This picture also shows how hex mina Bravais cell) are arr 1)and fibers(c). llowing the production of Cr+ that enters the furnace with the Procedure 2: The presence of Cr+ must be attributed to the Ar flow following reaction 4Cro3(L, g) △G2mPc=-48.9k Cr2O3+302(g) (1) Cr(NO3)3→Cr2++3NO eL This procedure results in the vast majority of the produced mina fibers showing a deep red coloration, as expected for Special attention must be paid to the evaporation process, be- cause any remnant H2O can hinder the vls process. Another To rule out the possibility that the coloration of alumina fib- disadvantage of this procedure is that an excess of NO3 in the ers was because of other causes, effective doping was confirmed tube atmosphere could impair the vls growth mechanism of via EDX analysis(Fig 4 alumina fibers In this procedure the available CrO, (g)can be easily control- As seen in Fig. 3 the tips of the alumina fibers do not present led by changing the temperature of the oil bath. One important ly apparent coloration, meaning that during the last part of the disadvantage of this process is that it requires handling of Cr heating cycle there is no more available Cr+, because the which is highly toxic. Cr(NO3) added before the beginning of the plateau tempera Fig 4. The typical EDX analysis of a drop reveals that Cr is available during the quid solid (VLS)mechan he rest of the elements detected are, as explained in the text, basically Al(the source of Al2O3 crystals), and a N e all loy that acts as the inter liquid for VLS growth

allowing the production of Cr31 that enters the furnace with the Ar flow. 4CrO3ðl; gÞ ! DG200C¼48:9 kJ 2Cr2O3 þ 3O2ðgÞ (1) This procedure results in the vast majority of the produced alumina fibers showing a deep red coloration, as expected for rubies. To rule out the possibility that the coloration of alumina fib￾ers was because of other causes, effective doping was confirmed via EDX analysis (Fig. 4). In this procedure the available CrO3(g) can be easily control￾led by changing the temperature of the oil bath. One important disadvantage of this process is that it requires handling of Cr61, which is highly toxic. Procedure 2: The presence of Cr31 must be attributed to the following reaction: CrðNO3Þ3 ! Cr3þ þ 3NO 3 (2) Special attention must be paid to the evaporation process, be￾cause any remnant H2O can hinder the VLS process. Another disadvantage of this procedure is that an excess of NO3 in the tube atmosphere could impair the VLS growth mechanism of alumina fibers. As seen in Fig. 3 the tips of the alumina fibers do not present any apparent coloration, meaning that during the last part of the heating cycle there is no more available Cr31, because the Cr(NO3)3 added before the beginning of the plateau tempera￾Fig. 3. Vapor liquid solid (VLS) deposition: Four stages of fiber growth mechanism (VLS deposition) are shown in the figure. Ribbons (a) are the perfect substrate for the deposition of a silicon drop (b), allowing the epitaxial growth (c) of alumina fibers, as explained in the text. At the final stage of their growth, fibers can attain lengths of several hundreds of microns (d). This picture also shows how hexagons (alumina Bravais cell) are arranged in ribbons (a) and fibers (c). Fig. 4. The typical EDX analysis of a drop reveals that Cr is available during the vapor liquid solid (VLS) mechanism. The rest of the elements detected are, as explained in the text, basically Al (the source of Al2O3 crystals), and a Ni–Si alloy that acts as the intermediate liquid for VLS growth. 326 Communications of the American Ceramic Society Vol. 89, No. 1

January 2006 Commumications of the American Ceramic Society 327 ture has been exhausted. We have chosen this picture because it Variations in the novel Vls deposition e described learly illustrates the stages of the whisker-doping process ere present intriguing possibilities, which merit further investi- When additional Cr(NO3)3 solution is sprayed into the sys- gation, for the generation of new materials with a wide range of n during the plateau ramp of the heating cycle, uniformly potential applicati ped whiskers are obtained The amount of solution sprayed must be determined by trial and error, depending on the dimensions of the furnace chamber References the Ar flow rate, etc. If too much Cr(NO3)3 solution is sprayed ITE into the furnace, an excess of NO3 will enter the system, and the Review,"74[2]295978(1991) J Y Chatellier. "On the mechanica VLS mechanism will be stopped because of passivation of the Al Aluminum Alloys Reinforced by Long or Short Alumina Fibers or SiC Whiskers. surface. If the amount of Cr solution sprayed is too low, the amount of Cr available will also be too low for the effective of Single Crystal and Polycrystalline Alumina of whiskers Differential heating or echniques could be con- Valcarcel, A. Souto and F. Guitian.""Development of Single-Crystal a- sidered for future research ormel example, a tem- perature gradient could be Powdered Silica. " Ady ping elements in the hot lcarcel, A. Perez, M. Cyrklaff, and F. Guitian, Novel Ribbon-Shaped An alternative would be to -Al 0, Fibers, "Adv. Mater, 10(16) 1370-3( 1998) putter the sample with CI etc, during the developmen bV. Valcarcel. C Cerecedo, and F. Guitian, "Method for Production of Alpha- f corundum fibers rs Via Vapor-Liquid-Solid Deposition, "J. Am. Ceram Soc.,86 may behind these methods may prove useful for the doping of wlg p pplied in a broad field of whisker-doping research. The ide M. Backhaus-Ricoult, S. Hagege, A. Peyrot, and P. Moreau, ""Internal Re- duction of Chromium-Doped a-Alumina, "J. Am. Ceram. Soc., 77[2]423-30 ers with different chemical compositions via VLS deposition R W. Grimes. "Solution of Mgo CaO and TiO, in -Al-O,, "J.Am. Ceram. so;T237884(939 Moon and M. R. Phillips, Defect Clustering and Color in Fe, Ti x-AlO3. J. Am. Ceram. Soc. 77[2]356-67(1994) Sheenan. ""Growth and Properties of Their high aspect ratios make single-crystal alumina fibers useful Single Crystal Oxide Fibers, Ceram. Eng. Sci. Proc. 12(9-10, Proc. Ann. Con. materials in the automobile and aerospace industries, in high Compos ads ishida N. Takeuchi, and K. Fujiyoshi, "Chromium-Based Ceramic temperature gas filters, etc. Here we have demonstrated that Colors, Ceran. Bull. 71 [5]759-64(1992) it is possible to obtain chromium-doped corundum single crystal fibers by introducing the doping element within a VLs ”1mh、dAR. Barron,"CVD of Chromium-Doped Alumina henn. Vap Deposition, 7[2]62-6(2001) echanisms of Oxide whisker Growth, Ceram. Eng. Sci. Proc., 15 4] 170-9(1994) Two methods were discussed for Cr doping of alumina fibers B Kubota. "" Decomposition of Higher Oxides of Chromium Under Various demonstrating that it is possible to obtain consistently doped n."J.Am. Ceran.So,4S23948(1961) e crystals, and other doping methods have Somiya, S. Yamaoka, and S Saito, ""Phase Relation Between CrO, and 20, by the dec Report, Bull. Tokyo Inst. Technol 66, 81-4(1965). The introduction of other impurities allows the generation of alumina fibers with new properties(chemical, electrical, etc. at Elevated Pressures up to 4 Kile Ribbons will be of particular interest because of their quasi L.M. Koizumi and s. Kume. "Chromium Diox ide-Chromium(Ill) Oxide Phase Boundary Under High Oxygen Pressures. "J. bidimensional behaviors(optical, electrical, etc. ) m. Ceran.Soe.,565248-9(1973)

ture has been exhausted. We have chosen this picture because it clearly illustrates the stages of the whisker-doping process. When additional Cr(NO3)3 solution is sprayed into the sys￾tem during the plateau ramp of the heating cycle, uniformly doped whiskers are obtained. The amount of solution sprayed must be determined by trial and error, depending on the dimensions of the furnace chamber, the Ar flow rate, etc. If too much Cr(NO3)3 solution is sprayed into the furnace, an excess of NO3 will enter the system, and the VLS mechanism will be stopped because of passivation of the Al surface. If the amount of Cr solution sprayed is too low, the amount of Cr available will also be too low for the effective doping of whiskers. Differential heating or sputtering techniques could be con￾sidered for future research. In the former, for example, a tem￾perature gradient could be achieved inside the furnace, with the doping elements in the hotter zones. An alternative would be to sputter the sample with Cr, Ti, Fe, etc., during the development of corundum fibers. In this report, we have presented two procedures that may be applied in a broad field of whisker-doping research. The ideas behind these methods may prove useful for the doping of whisk￾ers with different chemical compositions via VLS deposition. IV. Conclusions Their high aspect ratios make single-crystal alumina fibers useful materials in the automobile and aerospace industries, in high￾temperature gas filters, etc. Here we have demonstrated that it is possible to obtain chromium-doped corundum single￾crystal fibers by introducing the doping element within a VLS mechanism. Two methods were discussed for Cr doping of alumina fibers, demonstrating that it is possible to obtain consistently doped alumina VLS single crystals, and other doping methods have also been proposed. The introduction of other impurities allows the generation of alumina fibers with new properties (chemical, electrical, etc.). Ribbons will be of particular interest because of their quasi￾bidimensional behaviors (optical, electrical, etc.). Variations in the novel VLS deposition technique described here present intriguing possibilities, which merit further investi￾gation, for the generation of new materials with a wide range of potential applications. References 1 T. F. Cooke, ‘‘Inorganic Fibers, A Literature Review,’’ 74 [12] 2959–78 (1991). 2 M. Touratier, A. Be´akou, and J. Y. Chatellier, ‘‘On the Mechanical Behavior of Aluminum Alloys Reinforced by Long or Short Alumina Fibers or SiC Whiskers,’’ Compos. Sci. Technol., 44, 369–83 (1992). 3 G. Das, ‘‘Thermal Stability of Single Crystal and Polycrystalline Alumina Fibers and 85% Al2O3–15% SiO2 Fibers,’’ Ceram. Eng. Sci. Proc., 16 [5] 977–81 (1995). 4 V. Valca´rcel, A. Souto, and F. Guitia´n, ‘‘Development of Single-Crystal a￾Al2O3 Fibers by Vapor–Liquid–Solid Deposition (VLS) from Aluminum and Powdered Silica,’’ Adv. Mater., 10 [2] 138–40 (1998). 5 V. Valca´rcel, A. Pe´rez, M. Cyrklaff, and F. Guitia´n, ‘‘Novel Ribbon-Shaped a-Al2O3 Fibers,’’ Adv. Mater., 10 [16] 1370–3 (1998). 6 V. Valca´rcel, C. Cerecedo, and F. Guitia´n, ‘‘Method for Production of Alpha￾Alumina Whiskers Via Vapor–Liquid–Solid Deposition,’’ J. Am. Ceram. Soc., 86 [10] 1683–90 (2003). 7 M. Backhaus-Ricoult, S. Hagege, A. Peyrot, and P. Moreau, ‘‘Internal Re￾duction of Chromium-Doped a-Alumina,’’ J. Am. Ceram. Soc., 77 [2] 423–30 (1994). 8 R. W. Grimes, ‘‘Solution of MgO, CaO, and TiO2 in a-Al2O3,’’ J. Am. Ceram. Soc., 77 [2] 378–84 (1994). 9 A. R. Moon and M. R. Phillips, ‘‘Defect Clustering and Color in Fe,Ti: a-Al2O3,’’ J. Am. Ceram. Soc., 77 [2] 356–67 (1994). 10J. S. Haggerty, K. C. Wills, and J. E. Sheenan, ‘‘Growth and Properties of Single Crystal Oxide Fibers,’’ Ceram. Eng. Sci. Proc. 12(9–10, Proc. Ann. Conf. Compos. Adv. Ceram. Mater., 15th, 1991, Pt. 2), 1802–15 (1991). 11F. Ren, S. Ishida, N. Takeuchi, and K. Fujiyoshi, ‘‘Chromium-Based Ceramic Colors,’’ Ceram. Bull., 71 [5] 759–64 (1992). 12B. D. Fahlman and A. R. Barron, ‘‘CVD of Chromium-Doped Alumina ‘‘Ruby’’ Thin Films,’’ Chem. Vap. Deposition., 7 [2] 62–6 (2001). 13N.-W. Chen, D. W. Readey, and J. J. Moore, ‘‘Mechanisms of Oxide Whisker Growth,’’ Ceram. Eng. Sci. Proc., 15 [4] 170–9 (1994). 14B. Kubota, ‘‘Decomposition of Higher Oxides of Chromium Under Various Pressures of Oxygen,’’ J. Am. Ceram. Soc., 44 [5] 239–48 (1961). 15S. Somiya, S. Yamaoka, and S. Saito, ‘‘Phase Relation Between CrO2 and Cr2O3 by the Decomposition of CrO3 Under High Oxygen Pressure-Preliminary Report,’’ Bull. Tokyo Inst. Technol., 66, 81–4 (1965). 16K. A. Wilhelmi, ‘‘Formation of Chromium Oxides in the Cr2O3–CrO3 Region at Elevated Pressures up to 4 Kilobar,’’ Acta Chem. Scand., 22 [8] 2565–73 (1968). 17Y. Shibasaki, F. Kanamaru, M. Koizumi, and S. Kume, ‘‘Chromium Diox￾ide–Chromium(III) Oxide Phase Boundary Under High Oxygen Pressures,’’ J. Am. Ceram. Soc., 56 [5] 248–9 (1973). & January 2006 Communications of the American Ceramic Society 327

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