C 1998 Elsevier Science Limite Printed in Great Britain. All rights re PII:s0955·2219(98)000119 0955-22199851900+000 Stuijts Memorial Lecture 1997: Ceramic Science and Technology Facing Changing Paradigms Roman pampuch Department of Advanced Ceramics, AGH, al. Mickiewicz, Cracow 30-090, Poland Received 22 September 1997; accepted 19 December 1997 Abstract science and technology which seems to be a next to-impossible task. Already in 1919 the famous In the foreseeable future the main matter of concern chemist W. Ostwald wrote, To-day the production for applied research into ceramic materials will be, of new science has attained gigantic dimensions on the one hand, the need to ensure the sustainable while the conjoint development of its assimilation development by a reduction in the consumption of among all members of our society science has not nonrenewable energy and natural resources and, on been not a subject of serious and methodic con- the other, the challenge associated with the tendency siderations. At the time of Ostwald's writing the of the industry to move towards a sales-orientated number of published papers about materials and and just-in-time production of materials. The fo their technology was around 6000 per year while at ner concern should foster a growing utilisation of present it is now doubling within every 5 years and ceramic materials because these are, to an over- may now be many tens of thousands per year. constituted by light ele that are abundant in the earth s crust and air. This getting drowned in the information ocean endeavour emphasises a perfectioning of existing Nevertheless, some emerging trends in ceramic ceramic systems and their tailoring to a host of materials research may be discerned. Although sci advanced applications by improving the properties, ence cannot cease to regard the disinterested pur by using new or improved technologies, and by dis- suit of knowledge as its primary duty, the greater covering new properties of known compounds. This part of research in materials science has nowadays trend is illustrated in the paper by considering an applied character if not one that is concentrate applications of successively developed types of Sic- on development and improvement of marketable based materials and by the perspectives opened by products. In this perspective it is most probable newly revealed properties of earlier known materials, that in the foreseeable future the main matter like Ti3 SiC2. In addition to providing improved of concern for research into ceramics shall be the properties, newly developed ceramic technologies problem of ensuring sustainable development comply to an increasing degree with requirements for and the questions associated with changing produ sustainable development and for just-in-time cing practice in industry and, consequently, with production. To illustrate this tendency, the rBao the attitude to traditional research ways. 4 In order (reaction bonded aluminium oxide) method and to contribute to sustainable development, materials several variants of combustion methods are reviewed in research should look for and realise ways of redt the paper. C 1998 Elsevier Science Limited. All rights cing the consumption of nonrenewable energy eserved sources and of natural resources in the production and utilization of materials (Fig. 1 ). The change in producing practice which has the greatest bear 1 Introduction ing on applied research is the tendency toward sales-orientated production and, therefore, the At present paradigms are changing in all spheres of growing interest in versatile, rapid just-in-time life; and science and technology should not and are production methods, in other words in producing not an exception. Therefore, it is tempting to assess only what is going to be used immediately on the he changing paradigms taking shape in ceramic market
Journal of the European Ceramic Society 18 (1998) 993-1000 0 1998 Elsevier Science Limited PII: SO955-2219(98)00011-9 Printed in Great Britain. All rights reserved OSSS-2219/98/$19.00 + 0.00 Stuijts Memorial Lecture 1997: Ceramic Science and Changing Paradigms Technology Facing Roman Pampuch Department of Advanced Ceramics, AGH, al. Mickiewicza, Cracow 30-090, Poland (Received 22 September 1997; accepted 19 December 1997) Abstract In the foreseeable future the main matter of concern for applied research into ceramic materials will be, on the one hand, the need to ensure the sustainable development by a reduction in the consumption of nonrenewable energy and natural resources and, on the other, the challenge associated with the tendency of the industry to move towards a sales-orientated and @St-in-time production of materials. The former concern should foster a growing utilisation of ceramic materials because these are, to an overwhelming majority, constituted by light elements that are abundant in the earth’s crust and air. This endeavour emphasises a perfectioning of existing ceramic systems and their tailoring to a host of advanced applications by improving the properties, by using new or improved technologies, and by ‘discovering’ new properties of known compounds. This trend is illustrated in the paper by considering the applications of successively developed types of SiCbased materials and by the perspectives opened by newly revealed properties of earlier known materials, like Ti+SiCz. In addition to providing improved properties, newly developed ceramic technologies comply to an increasing degree with requirements for sustainable development and for ‘just-in-time ’ production. To illustrate this tendency, the RBAO (reaction bonded aluminium oxide) method and several variants of combustion methods are reviewed in the paper. 0 1998 Elsevier Science Limited. All rights reserved 1 Introduction At present paradigms are changing in all spheres of life; and science and technology should not and are not an exception. Therefore, it is tempting to assess the changing paradigms taking shape in ceramic science and technology which seems to be a nextto-impossible task. Already in 1919 the famous chemist W. Ostwald wrote, ‘To-day the production of new science has attained gigantic dimensions while the conjoint development of its assimilation among all members of our society science has not been not a subject of serious and methodic considerations’.’ At the time of Ostwald’s writing the number of published papers about materials and their technology was around 6000 per year while at present it is now doubling within every 5 years and may now be many tens of thousands per year. Therefore, for want of more knowledge we are getting drowned in the information ocean. Nevertheless, some emerging trends in ceramic materials research may be discerned. Although science cannot cease to regard the disinterested pursuit of knowledge as its primary duty, the greater part of research in materials science has nowadays an applied character if not one that is concentrated on development and improvement of marketable products. In this perspective it is most probable that in the foreseeable future the main matter of concern for research into ceramics shall be the problem of ensuring sustainable development2y3 and the questions associated with changing producing practice in industry and, consequently, with the attitude to traditional research ways.4 In order to contribute to sustainable development, materials research should look for and realise ways of reducing the consumption of nonrenewable energy sources and of natural resources in the production and utilization of materials (Fig. 1). The change in producing practice which has the greatest bearing on applied research is the tendency towards sales-orientated production and, therefore, the growing interest in versatile, rapid ‘just-in-time’ production methods, in other words in producing only what is going to be used immediately on the market. 993
R. Pampuch Goals Measures Driving fo of no of mate of process to functions Knowledge Roduction of consumption Fig. 2. Reserves of the most abundent elements in the earths of not renewable crust(Na, K, and H excluded, fossil carbon and N2 in air uded). The reserves of iron taken as equal to one compounds. Therefore, some authors speak of a yet unexplored mountain'of materials6 However, a search for new, yet unknown com- Fig1. Main goals and measures in material production which pounds seems not to be probable, and this in spite d special, fine) have appeared on the market later than metals and There is a growing body of published research polymers and are, accordingly, less explored. In all which responds to these requirements and chal- historically proven stages of materials develop- lenges. Some examples illustrating this tendency ment'the longest spans of time were characterised have been drawn subjectively from the field of by improvements of earlier known materials Such experience of the author and are quoted in the a development is assumed to be taking place now prescnt papcr with polymers and seems to be bcing followed also by Therefore, one can expect that at present and in 2 Reduction in the Consumption of Nonrenewable the foreseeable future the development of a broad Natural Resources and Advance in the Implemen- palette of new ceramic materials for a host of high tation of Ceramic materials performance applications will take place pre- dominantly on the basis of already known com When the requirement for reduction in the pounds and materials. This development can be consumption of nonrenewable natural resources is realised by revealing new and or not yet utilised considered the need for further development and properties, by improving fabrication methods or exploitation of ceramic materials is easy to under- introducing new technologies, by varying the stand because the overwhelming majority of ceramic materials microstructure, and by combining var materials are constituted by light elements such as ious components. Such a way should be especially silicon, aluminium, carbon, oxygen, and nitrogen successful with ceramic compounds and materials that are abundant in the earths crust and air because they have multifaceted properties. An ( Fig. 2) example of such a development is provided by the Owing to their manifold properties, the ceramic large family of Sic-based materials which have materials, and especially the advanced types, have appeared successively in response to emerging been already utilised to perform many functions. a technical requirements Table 1). Another well further expansion in the use of ceramic materials known example is furnished by ceramic composites may be realised in two ways. There is an enormous( fibre-reinforced, particulate, nanocomposites) numbcr of potcntial but still unknown compounds which earlier known compounds are combined to constituted by light elements because already the produce materials having new properties. There number of combinations (Cr=n /r!(n-r)9) has been spectacular progress with the known among the n= 12 most abundant elements in the multicomponent oxides having the perovskite- earths crust taken, respectively, r=2, 3, and 4 at a structure and appreciated earlier for their dielectric time is equal to 781. This number exceeds the properties. Owing to the discovery, by bednorz number of known binary, ternary and quaternary and Mueller, that some oxides with this structure refractory inorganic compounds, i.e. of ceramic are superconductors at around 80 K they are now
994 R. Pampuch GO& Measures Driving force Time- & Reduction of consumption of not renewable energy sources t Knowledge Reduction of consumDtlon of not reriwable _ raw materials SubstnuHon of abundant raw materials for defklent ones -I materlals Fig. 1. Main goals and measures in material production which permit to establish sustainable development. There is a growing body of published research which responds to these requirements and challenges. Some examples illustrating this tendency have been drawn subjectively from the field of experience of the author and are quoted in the present paper. 2 Reduction in the Consumption of Nonrenewable Natural Resources and Advance in the Implementation of Ceramic Materials When the requirement for reduction in the consumption of nonrenewable natural resources is considered the need for further development and exploitation of ceramic materials is easy to understand because the overwhelming majority of ceramic materials are constituted by light elements such as silicon, aluminium, carbon, oxygen, and nitrogen that are abundant in the earth’s crust and air (Fig. 2).5 Owing to their manifold properties, the ceramic materials, and especially the advanced types, have been already utilised to perform many functions. A further expansion in the use of ceramic materials may be realised in two ways. There is an enormous number of potential but still unknown compounds constituted by light elements because already the number of combinations (c; = n!/r!(n - r)!) among the n = 12 most abundant elements in the earth’s crust taken, respectively, r = 2, 3, and 4 at a time is equal to 781. This number exceeds the number of known binary, ternary and quaternary refractory inorganic compounds, i.e. of ceramic O.dOl O.iJi 0.1 1 Fig. 2. Reserves of the most abundent elements in the earths crust (Na, K, and H excluded, fossil carbon and N2 in air included). The reserves of iron taken as equal to one. compounds. Therefore, some authors speak of a yet unexplored ‘mountain’ of materials.6 However, a search for new, yet unknown compounds seems not to be probable, and this in spite of the fact that advanced (special, fine) ceramics have appeared on the market later than metals and polymers and are, accordingly, less explored. In all historically proven stages of materials development7 the longest spans of time were characterised by improvements of earlier known materials. Such a development is assumed to be taking place now with polymers* and seems to be being followed also by ceramics. Therefore, one can expect that at present and in the foreseeable future the development of a broad palette of new ceramic materials for a host of high performance applications will take place predominantly on the basis of already known compounds and materials. This development can be realised by revealing new and/or not yet utilised properties, by improving fabrication methods or introducing new technologies, by varying the materials microstructure, and by combining various components. Such a way should be especially successful with ceramic compounds and materials because they have multifaceted properties. An example of such a development is provided by the large family of Sic-based materials which have appeared successively in response to emerging technical requirements (Table 1). Another well known example is furnished by ceramic composites (fibre-reinforced, particulate, nanocomposites) in which earlier known compounds are combined to produce materials having new properties. There has been spectacular progress with the known multicomponent oxides having the perovskitestructure and appreciated earlier for their dielectric properties. Owing to the discovery, by Bednorz and Mueller, that some oxides with this structure are superconductors at around 80K they are now
Ceramic science and technology facing changing paradigms. A viewpoint Table 1. Utilisation of various properties of Sic-based materials in different applications Property Hardness Abrasive material High thermal stability and Refractories of universal applications in trength at elevated temperatures metallurgy and power industry (plates, retorts, recl rs, blast furnace linings, muffle kilns, etc. High resistance to irradiation and Fuel element cans in high-temperature nuclear reactors strength at elevated temperatures Semiconducting properties Non-linear resistors, varistors, elements of wave-guides, Resistance to acids and bases Mechanical seals, bearings, no Elevated thermal conductivity, Heat exchangers(tube and shell recuperators )in high theral stability, corrosion High strength and low weight Reinforcing components of composites of Sic whiskers, platelets and polycristalline fibres Pinning down of grain boundaries Ceramic matrix nanocomposites having a high creep resistance at elevated temperatures Fracture characteristics under Bullet-proof vests, rocket parts, etc dynamic condition Potential application regarded as the high temperature superconductors (HTC) par exo Another example of the'disclosure'of hitherto 1200 unknown properties of earlier known compounds is provided by the 312 and 'H compounds. Thesc compounds embrace among others Ti3SiC2, Ti3 Ge C2, Ti2AIC, Ti2AIN; they are nanolaminates in which the strength of the interplanar bonds is intermediate between that of the primary bond and the interlayer bonds in graphite. Their struc- ture and some of their properties have been synthesis of larger amounts, first by solid com bustion2-15 and then by arc melting and other techniques6, I permitted to produce polycrystal 200 line materials and to measure further properties of these materials. According to the measurements the materials show a set of properties not typical of any metallic, ceramic or polymeric material. 18, 19 In fact the electrical. thermal and mechanical Fig 3. the brittleness parameter, b, of some ceramic and properties of these compounds are intermediate metallic materials and of Ti]SiC2. Basic data used for calcula tion of B: for Ti3Sic2 after Refs 12, 14, 15, 17-19, for a Sic, between typical metals and ceramics. In order to AION, Al2O3, Si3N4, after Ref. 20, for rolled structural steel illustrate this the brittleness of Ti3 SiC is compared Ti alloy, and al alloy according to various catalogue sources in Fig. 3 with those of typical ceramic and metallic materials. The brittleness is expressed in terms of Of equal if not more interest is another unique the brittleness parameter proposed in Ref. 20, property of Ti]SiC2, namely its easy machinability namely:B=Hve E/KIc where: Hve is the Vickers resembling that of wood and graphite. 18, 19 Since hardness mcasured at a transition point from load machining constitutes an important part of fabri dependent to load independent values; E is the cation costs for ceramics (table 2), such a property Youngs modulus; and KIc is the critical stress should be of value under the conditions of a sus intensity for mode I loading tainable developmen
Property Ceramic science and technology facing changing paradigms. A viewpoint Table 1. Utilisation of various properties of Sic-based materials in different applications Application 995 Hardness Abrasive material High thermal stability and strength at elevated temperatures High resistance to irradiation and strength at elevated temperaturess Refractories of universal applications in metallurgy and power industry (plates, retorts, recuperators, blast furnace linings, muffle kilns, etc.) Fuel element cans in high-temperature nuclear reactors* Electrical resistivity Semiconducting properties Resistance to acids and bases, abrasion resistance Resistors, resistance heating elements Non-linear resistors, varistors, elements of wave-guides, ultraviolet receivers, cold cathodes, etc. Mechanical seals, bearings, nozzles Elevated thermal conductivity, high thermal stability, corrosion and erosion resistance Heat exchangers (tube and shell recuperators) in waste heat recovery and management High strength and low weight of BC whiskers, platelets and polycristalline fibres Reinforcing components of composites Pinning down of grain boundaries by nanometric (n) BC particles Ceramic matrix nanocomposites having a high creep resistance at elevated temperatures Fracture characteristics under dynamic conditions *Potential application. Bullet-proof vests, rocket parts, etc. regarded as the high temperature superconductors (HTC) par excellence. Another example of the ‘disclosure’ of hitherto unknown properties of earlier known compounds is provided by the ‘3 12’ and ‘H’ compounds. These compounds embrace among others TisSiCz, Ti3GeC2, Ti2AlC, Ti2AlN; they are nanolaminates in which the strength of the interplanar bonds is intermediate between that of the primary bonds and the interlayer bonds in graphite. Their structure and some of their properties have been known for many years s1 1 but only a successful synthesis of larger amounts, first by solid combustion’2-15 and then by arc melting and other techniques16y17 permitted to produce polycrystalline materials and to measure further properties of these materials, According to the measurements, the materials show a set of properties not typical of any metallic, ceramic or polymeric material.*8~19 In fact the electrical, thermal and mechanical properties of these compounds are intermediate between typical metals and ceramics. In order to illustrate this the brittleness of T&Sic2 is compared in Fig. 3 with those of typical ceramic and metallic materials. The brittleness is expressed in terms of the brittleness parameter proposed in Ref. 20, namely: B = Hvc E/K:, where: H,, is the Vickers hardness measured at a transition point from load dependent to load independent values; E is the Young’s modulus; and Klc is the critical stress intensity for mode I loading. 12345676 9 10 11 Fig. 3. The brittleness parameter, B, of some ceramic and metallic materials and of Ti3SiCz. Basic data used for calculation of B: for Ti3SiC2 after Refs 12, 14, 15, 17-19, for (Y SIC, AlON, AlzOs, Si3N4, after Ref. 20, for rolled structural steel, Ti alloy, and Al alloy according to various catalogue sources. Of equal if not more interest is another unique property of TisSiC2, namely its easy machinability resembling that of wood and graphite.‘*,19 Since machining constitutes an important part of fabrication costs for ceramics (Table 2), such a property should be of value under the conditions of a sustainable development
Table 2. Distribution of production costs of a silicon nitride ones various alumina-based materials can be pre cam follower(after Ref. 21) duced in a rather simple way. The Japanese tree Stage of Participation in the of this method, demonstrating its versatility, is production costs(%) shown in Fig. 4.22 Powder synthesis and processing 35 Still more external energy is saved in the com- Shaping, sintering and other operations 175 bustion methods for the synthesis of solids because leading to a densified material Maching to final product 47.5 here the energy derived from internal sources(heat of exothermic reactions) make art of he energy needed for the synthesis. Initiated by 3 Energy- Saving and Versatile Nonconventional external heat sources the exothermic reactions Routes to Ceramics bring about an avalanche-like local temperature increase to high levels(ignition) and a self-sustain There is a growing body of published research, ing propagation of the heat wave when a positi especially into nonconventional preparation routes feedback is established in the reactive system. This for ceramics, which not only demonstrates occurs when the flux density of heat losses to the improved properties in the products but also com- environment and colder parts of the system is less plies with the requirements of sustainable develop- than the flux density of heat passing to the material ment by increasing energy efficiency and by next to react. In addition to the energy efficiency, responding to the requirements of just in time many combustion methods feature additionally a production. Some illustrative examples of it are high versatility which is desirable for the develop now quoted ment of just-in-time' production methods. All It is well known that reaction bonding(sintering) these features of combustion methods can be illus- methods have a higher energy efficiency than typi- trated with reference to the production of sinter- cal sintering methods because of the partial utili- able ceramic powders sation of the heat of reaction for bonding and In the most developed method of combustion because of the ability to produce near-net shape synthesis, namely solid combustion,25-29 at least components with reduced machining costs. Some one of the reactants and the final products are solid of the reaction bonding methods are, in addition, and the production of heat occurs in the condensed versatile and, thercfore comply with the require- state only. The method utilises exothermic reac- ments of just-in-time' production. Such features tions between elements(solid and gaseous), such are shown, for instance, by the RBAO method as: 3Si(s, I )+ 2N2(g)=Si3N4(s)or Si(s, 1)+C(s) (reaction bonded aluminium oxide). 22-24 In this SiC(s), thermite reactions or combinations of both method, attrition-milled Al/Al2O3 powder com- like: 3Ti02+ 3C+(4+x)Al-3TiC+2Al2O3+xA pacts are heat treated in air such that Al oxidizes to The most known variant of solid combustion form small 'new'Al2O3 particles which sinter and involves a layer-by-layer self heating to high tem- thereby bond together the originally added Al2O3 peratures by an interplay of the positive and nega or other ceramic particles. By adding different tive feedback and is referred to as self-propagating starting components to the aluminium-bearing high-temperature synthesis (SHS). The simple experimental arrangement used to put SHs in action is shown in Fig. 5. Solid combustion has been most successful in the L知 a-Hgh strengh production of simple and composite nitride, car- bide and boride powders. It permits to access not only to the high temperatures needed for the synthesis of such compounds but also to high (Cr, Anzo, reaction rates. The latter are believed to be due, at least in Si- and Al-containing systems, to a 'migrating thin reaction layer'mechanism and to the precipitation of the final products from liquid 6) The occurence of this mechanism involving a 一A3 liquid phase and others involving transport through the gas phase(fig. 7)favours the forma 4A+30,=2ALO3 tion of composite nitride, carbide powders which are characterised by a homo- ig. 4. The Japanese tree'of the RBAO (reaction bonded geneous distribution of the components. When aluminium oxide)method; after Ref. 22 sintering aids are required for densification of the
996 R. Pampuch Table 2. Distribution of production costs of a silicon nitride cam follower (after Ref. 21) Stage of the technology Participation in the production costs (%) Powder synthesis and processing Shaping, sintering and other operations leading to a densified material Maching to final product 35 17.5 47.5 3 Energy-Saving and Versatile Nonconventional Routes to Ceramics There is a growing body of published research, especially into nonconventional preparation routes for ceramics, which not only demonstrates improved properties in the products but also complies with the requirements of sustainable development by increasing energy efficiency and by responding to the requirements of ‘just in time’ production. Some illustrative examples of it are now quoted. It is well known that reaction bonding (sintering) methods have a higher energy efficiency than typical sintering methods because of the partial utilisation of the heat of reaction for bonding and because of the ability to produce near-net shape components with reduced machining costs. Some of the reaction bonding methods are, in addition, versatile and, therefore, comply with the requirements of ‘just-in-time’ production. Such features are shown, for instance, by the RBAO method (reaction bonded aluminium oxide).22-24 In this method, attrition-milled Al/A120s powder compacts are heat treated in air such that Al oxidizes to form small ‘new’ A1203 particles which sinter and thereby bond together the originally added A1203 or other ceramic particles. By adding different starting components to the aluminium-bearing Fig. 4. The ‘Japanese tree’ of the RBAO (reaction bonded aluminium oxide) method; after Ref. 22. ones various alumina-based materials can be produced in a rather simple way. The ‘Japanese tree’ of this method, demonstrating its versatility, is shown in Fig. 4.22 Still more external energy is saved in the combustion methods for the synthesis of solids because here the energy derived from internal sources (heat of exothermic reactions) makes up a large part of the energy needed for the synthesis. Initiated by external heat sources, the exothermic reactions bring about an avalanche-like local temperature increase to high levels (ignition) and a self-sustaining propagation of the heat wave when a positive feedback is established in the reactive system. This occurs when the flux density of heat losses to the environment and colder parts of the system is less than the flux density of heat passing to the material next to react. In addition to the energy efficiency, many combustion methods feature additionally a high versatility which is desirable for the development of ‘just-in-time’ production methods. All these features of combustion methods can be illustrated with reference to the production of sinterable ceramic powders. In the most developed method of combustion synthesis, namely solid combustion,25-29 at least one of the reactants and the final products are solid and the production of heat occurs in the condensed state only. The method utilises exothermic reactions between elements (solid and gaseous), such as: 3Si(s,l) + 2N2(g) = Si3N4(s) or Si(s,l) + C(s) = Sic(s), thermite reactions or combinations of both, like: 3Ti02 + 3C + (4 + x)Al = 3TiC + 2A1203 + xA1. The most known variant of solid combustion involves a layer-by-layer self heating to high temperatures by an interplay of the positive and negative feedback and is referred to as self-propagating high-temperature synthesis (SHS). The simple experimental arrangement used to put SHS in action is shown in Fig. 5. Solid combustion has been most successful in the production of simple and composite nitride, carbide, and boride powders. It permits to access not only to the high temperatures needed for the synthesis of such compounds but also to high reaction rates. The latter are believed to be due, at least in Si- and Al- containing systems, to a ‘migrating thin reaction layer’ mechanism and to the precipitation of the final products from liquid solutions30 (Fig. 6). The occurence of this mechanism involving a liquid phase and others involving transport through the gas phase (Fig. 7) favours the formation of composite nitride, carbide, and boride powders which are characterised by a homogeneous distribution of the components. When sintering aids are required for densification of the
Ceramic science and technology facing changing paradigms. A viewpoint eaction front Products Reactants Time Fig. 5. Self-propagating high-temperature synthesis(SHS): (a)experimental arrangement; (b) temperature versus time at a given of the reactive mixture powders, their homogeneous distribution in the particulate product from the solid combustion is 19502150℃ attained by adding the aids to the reactant mixture before the combustion synthesis(Fig 8) C The features of the particulate products from reaction layer of primary SIC their processing to yield sinterable powders in the secondary SIC (crystallising from lIquld) case of SiC, Si3 N4, AIN, composite SiC-Si3 N4 Inttialposttionof SIC reactive layer Sic-B4C, and Cr2 C3 and a substantial reduction in Fig. 6. The 'moving thin reaction layer'mechanism of reac- the number of major processing steps of the in tions in the Si-C reactive system which occurs during solid comparison with conventional methods. 31-33 This combustion at 1950-2150 C. due to similar rates of growth applies also to the synthesis of ferrites because of C at the C-SiC interface and of dissolution of this primary the absence of the first firing into C leaving in its wake supersaturated C solutions in liquid tional methods: in solid combustion, ferritization yer moves Si; precipitation of the final product(secondary SiC)occurs already occurs in the course of the compound from the liquid phase: after ref. 30 synthesis. 34 The extent of simplification which is feasible by using solid combustion is indicated by 2000 the flowsheets given in Figs 8 and 9 where a com- parison is made between the production of sinter- able SiC+B.C and ain powders by using, SN3)+3C()=sC"()+2N respectively, solid combustion and other advanced methods. Together with the extremely short time needed for the synthesis itself, all the above mentioned features render powder producing methods using solid combustion synthesis especially favourable for the development of just-in-time Solid combustion has also been found to be very SI(c)+NI(g)=S,N, (s) SK(c)C(s)=SIC"(s) successful in the production of powders of intry metallic compounds. To show but one example, the interaction of powders of Ti with Co or Ni during 1300 the synthesis of, respectively, TiCo and TINi, by illustrated in Fig. 10.35 The sequence of interactions resembles the one in the Fig. 7. Reactions in the Si-N-C system under conditions of Al- and Si-containing ceramic systems, and mixtures at the resulting temperatures of solid combustion; involves consecutively: (partial)melting of one of (after Kata, personal communication) the reactants with fragmentation of the more
Ceramic science and technology facing changing paradigms. A viewpoint 997 TilTI Fig. 5. Self-propagating high-temperature synthesis (SHS): (a) experimental arrangement; (b) temperature versus time at a given point of the reactive mixture. q C reaction layer of primary SIC secondary SIC (crystallislng from llquld) initial position of SIC reactive layer Fig. 6. The ‘moving thin reaction layer’ mechanism of reactions in the Si-C reactive system which occurs during solid combustion at 195&215o”C. Due to similar rates of growth of Sic at the C-Sic interface and of dissolution of this primary Sic at the SiC/Si(l) interface a thin reaction Sic layer moves into C leaving in its wake supersaturated C solutions in liquid Si; precipitation of the final product (secondary Sic) occurs from the liquid phase; after Ref. 30. 1900 9 2 18W i 1700 + 1600 1500 1400 1300 Xc) +N&) = S&N. 6) sl(c)+c(s)_sIc’(s) +--F-J t Fig. 7. Reactions in the Si-N-C system under conditions of solid combustion at different carbon contents in the reactant mixtures at the resulting temperatures of solid combustion; (after Kata, personal communication). powders, their homogeneous distribution in the particulate product from the solid combustion is attained by adding the aids to the reactant mixture before the combustion synthesis (Fig. 8). The features of the particulate products from solid combustion have allowed a simplification of their processing to yield sinterable powders in the case of Sic, Si3N4, AlN, composite SiC-SisN4, Sic-B&, and Cr&s and a substantial reduction in the number of major processing steps of the in comparison with conventional methods.31-33 This applies also to the synthesis of ferrites because of the absence of the first firing necessary in conventional methods; in solid combustion, ferritization already occurs in the course of the compound synthesis.34 The extent of simplification which is feasible by using solid combustion is indicated by the flowsheets given in Figs 8 and 9 where a comparison is made between the production of sinterable Sic+ B4C and AlN powders by using, respectively, solid combustion and other advanced methods. Together with the extremely short time needed for the synthesis itself, all the above mentioned features render powder producing methods using solid combustion synthesis especially favourable for the development of ‘just-in-time’ production. Solid combustion has also been found to be very successful in the production of powders of intermetallic compounds. To show but one example, the interaction of powders of Ti with Co or Ni during the synthesis of, respectively, TiCo and TiNi, by solid combustion is illustrated in Fig. 1O.35 The sequence of interactions resembles the one in the Al- and Si-containing ceramic systems, and involves consecutively: (partial) melting of one of the reactants with fragmentation of the more
R. Pampuch 2h, Acheson Process)l th liquid pre 0°c) eutectics distance from ignition site Novolak Successive stages of interaction of titanium powders with nickel or cobalt on combustion synthesis of TiNi or TiCo. Legend: T, tcmpcraturc; n, degree of advancement of the reaction; heat production rate; after Ref. 35 refractory one, formation of liquid solutions, and crystalisation of the final products from the melts. This facilitates the synthesis of intermetallics pressiNg hot-pressing especially from reactants with widely different melting points stc· BC composite The value of other variants of combustion synthesis developed for the production of multi component oxide powders has also been demon Fig. 8. Flow charts duction strated. When such oxide powders are prepared by and magnesiothermic reduction; (B) - ising carbothermic solid state reaction of the components, they are often compositionally inhomogeneous and consist developed to produce more homogeneous and fine Al2 0, powde Carbon blac Al scrap powder Oxide additives] powders are complicated and still require calcina tion to obtain crystalline products. However, crys- talline multicomponent oxide powders may be prepared instantaneously by ignition and by self- sustaining combustion brought about by exothermic reactions in various redox mixtures such as citrate- nitrate gels 36, 37( Fig. 11), aqueous solutions of metal in nitrogen 1700C of nitrogen, 15 m nitrates urea(carbohydrazide)3841(Fig12),and hydrocarbon aerosol sprays containing inorganic Solution of Pb(NOJ,+Zo(NO ,+n-citTe Filtration Collold魏 persons(∞ lation tothermic anionIc AIN sinterable Combustion CO,+n,+HO FIg 9. Flow charts of production of sinterable AIN powders by:(A) carbothern fter Fig. 11. Synthesis of PZT powders by self-sustaining combus Ref 3 tion of citrate-nitrate gels; after Ref. 36
R. Pampuch A Solid combustion (0.3 hours) Sinterable powder PreSSUraleSS sintefing or hot-pressing SiC+&C composite 6 Fig. 8. Flow charts of production of sinterable SiC+B&J powders by: (A) an advanced method utilising carbothetmic and magnesiothermic reduction; (B) solid combustion. + Mixing A S Fig. 9. Flow charts of production of sinterable AlN powders by: (A) carbothermic reduction; (B) solid combustion after Ref. 31. eutkctics T /------“-.\ .I ’ / . . . . ! 1:’ ,“,,*,;. _. . . . . . . ..’ / /.i \ i.r \ $ \ :.I \ \ $,‘. ‘. /t/ ‘_l_. distance from ignition site Fig. 10. Successive stages of interaction of titanium powders with nickel or cobalt on combustion synthesis of TiNi or TiCo. Legend: T, temperature; 3, degree of advancement of the reaction; 4, heat production rate; after Ref. 35. refractory one, formation of liquid solutions, and crystalisation of the final products from the melts. This facilitates the synthesis of intermetallics, especially from reactants with widely different melting points. The value of other variants of combustion synthesis developed for the production of multicomponent oxide powders has also been demonstrated. When such oxide powders are prepared by solid state reaction of the components, they are often compositionally inhomogeneous and consist of hard agglomerates. Wet chemical methods developed to produce more homogeneous and fine powders are complicated and still require calcination to obtain crystalline products. However, crystalline multicomponent oxide powders may be prepared instantaneously by ignition and by selfsustaining combustion brought about by exothermic reactions in various redox mixtures such as citratenitrate gels36T37 (Fig. 1 I), aqueous solutions of metal nitrates-urea (carbohydrazide)3841 (Fig. 12), and hydrocarbon aerosol sprays containing inorganic Solution of FWNOJ, + KXNOJ, + Tl - ciirate 4 1 Colloidal suspensbns (sol) 1 4 ~Geiailon~ 4 ciflate-nltrute gel / lnitkMon of reactions by heatIngup using an external heat source Exothermlc anlonlc redox reaotlon between cHmte and nltrate Ions 1 Combustion I- C02+b+t$0 1 Ash=PZTpowderI Fig. 11. Synthesis of PZT powders by self-sustaining combustion of citrate-nitrate gels; after Ref. 36
Ceramic science and technology facing changing paradigms. A viewpoint Aqueous solutions of redox mature Method BoHing decomposition P0量 Purity gases theIr ignition SmaH size Compositional Combustion}→→CO2+N+HO Fig. 12. Synthesis of composite oxides powders by combus- ion in combustible gases evolving from heated redox mixtures of aqueous solutions of metals nitrates and urea or carbohy drazide: after Ref. 38 Low cost of Reaction Me(NO3)2+CnHm co2+№+H2O Combustible(C,H Fig. 14. Features of some powder producing methods and haracteristics of powders from these methods. The reaction Me(NOah +CnHm=MeO CO +N2+ H2o Fig 13. Synthesis of oxide powders by self-sustaining pyroly. tional configuration of the starting body. 43 How sis-combustion of atomised redox mixtures of metal nitrate ever, in the case of application of combustion for olutions(oxidiser)and hydrocarbons(fuel) after Ref. 42 producing near-net shape materials in one step combining combustion synthesis, shaping and precursors of the oxides, usually in the form of densification, positive results have only been metal nitrates(Fig. 13). 42 These methods have obtained with intermetallics and metal-ceramic yielded homogeneous powders of: Y 3AlsO12, composites; 4 ceramics have presented difficulties ZrO,/AlO3. MAlOa, MCr2O4 (M=transition owing to their insufficient plasticity at elevated and alkaline earth metals), Lal-xSrxMO3(M=Cr, temperatures. An interesting application for com Mn, Co, and Fe), YBa2Cu3Ox, mullite, and other bustion methods may arise in the cvd growth of multicomponent oxides. The combustion-derived diamond films because oriented diamond nuclei on powders are characterised by fine particles substrates grow with higher rates and the resulting (<1um) and by a narrow particle size distribution diamond films have a larger area uniformity when making the powders more sinterable than these treated in a combustion system than when pre- obtained from other processing routes. All these pared by more conventional hot-filament or variants of combustion synthesis are characterised plasma CVD processes y rapidity, simplicity, and versatility. The main features of the difTerent variants of the combustion method and the main characteristics of the pow- 4 Summary ders produced from this method are compared in Fig. 14 with the attributes of powders deriving Some examples have been presented to illustrate from other advanced methods the changes of paradigm that are believed by the Combustion methods have also been expected to author to be taking shape in ceramic materials sci- mprove and/or simplify many other technologies ence and technology in response to the require but only some applications have turned out to be ments of sustainable development(reduction in the effective, such as metal-ceramic joining and the consumption of nonrenewable energy and natural production of functional gradient materials resources) and in response to the challenges asso (FGMs)where the rapid reaction rates can limit ciated with the trend in industry towards just-in diffusional processes and maintain the composi- time'production
Ceramic science and technology facing changing paradigms. A viewpoint 999 I5 Ash=oxlde powder Fig. 12. Synthesis of composite oxides powders by combustion in combustible gases evolving from heated redox mixtures of aqueous solutions of metals nitrates and urea or carbohydrazide; after Ref. 38. Reaction Me(NO& + C&A,,, I t C02+ &+ H20 I Me0 powder The reaction Me(NO& + C,H, = Me0 + CO2 + b + H,O produces most of the energy for pyrolysis Fig. 13. Synthesis of oxide powders by self-sustaining pyrolysis-combustion of atomised redox mixtures of metal nitrate solutions (oxidiser) and hydrocarbons (fuel); after Ref. 42. precursors of the oxides, usually in the form of metal nitrates (Fig. 13).42 These methods have yielded homogeneous powders of: Y3A15012, tZr02/A1203, MA1204, MCr204 (M = transition and alkaline earth metals), Lar_-SrXM03 (M = Cr, Mn, Co, and Fe), YBa2CusOX, mullite, and other multicomponent oxides. The combustion-derived powders are characterised by fine particles (< 1 pm) and by a narrow particle size distribution making the powders more sinterable than these obtained from other processing routes. All these variants of combustion synthesis are characterised by rapidity, simplicity, and versatility. The main features of the different variants of the combustion method and the main characteristics of the powders produced from this method are compared in Fig. 14 with the attributes of powders deriving from other advanced methods. Combustion methods have also been expected to improve and/or simplify many other technologies but only some applications have turned out to be effective, such as metal-ceramic joining and the production of functional gradient materials (FGMs) where the rapid reaction rates can limit diffusional processes and maintain the composiMethod Positive features of powder & method Purity Small size & narrow size distribution of particles CornmT;ma No prolonged milling No calcination Low cost of reactants Short$rethesis Low cost of installations Fig. 14. Features of some powder producing methods and characteristics of powders from these methods. tional configuration of the starting body.43 However, in the case of application of combustion for producing near-net shape materials in one step, combining combustion synthesis, shaping and densification, positive results have only been obtained with intermetallics and metal-ceramic composites;34 ceramics have presented difficulties owing to their insufficient plasticity at elevated temperatures. An interesting application for combustion methods may arise in the CVD growth of diamond films because oriented diamond nuclei on substrates grow with higher rates and the resulting diamond films have a larger area uniformity when treated in a combustion system than when prepared by more conventional hot-filament or plasma CVD processes.a 4 Summary Some examples have been presented to illustrate the changes of paradigm that are believed by the author to be taking shape in ceramic materials science and technology in response to the requirements of sustainable development (reduction in the consumption of nonrenewable energy and natural resources) and in response to the challenges associated with the trend in industry towards ‘just-intime’ production
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