Glass ceramics: new compositions and uses

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Journalof Non-CrystallineSolids181 (1995) 1-15

Glass ceramics: new compositions and uses P.F. James

*

Department of EngineeringMaterials, University of Sheffield, PO Box 600, Mappin Stree~ Sheffield $1 4DU, UK

Received5 August1994

Abstract

Glass ceramics, materials prepared by the controlled crystallisation of glasses, have a variety of established uses dependent on their uniform reproducible fine-grain microstructures, absence of porosity and wide-ranging properties which can be tailored by changes in composition and heat treatment. Moreover, during manufacture complex shapes may be produced using standard glass-forming techniques prior to crystallisation and dimensional changes are small. In recent years, new compositions, processing methods and applications have begun to emerge. Among the recent developments considered are new phosphate-based compositions, the use of sol-gel processing, glass ceramic matrix composites, glass ceramics in microelectronics packaging, glass ceramics bonded to metals and as joining media, high-strength and high-toughness materials and machineable compositions. 1. Introduction

The first practical glass ceramics, materials prepared by the controlled crystallisation of special glasses, were developed nearly forty years ago. Since that time, a wide variety of applications of these versatile materials have developed as a result of their many outstanding properties and the distinct advantages of the glass ceramic method, in certain circumstances, over conventional ceramic processing routes. Of particular importance in many applications is the high uniformity of the microstructures of glass ceramics, the absence of porosity and the minor changes in volume during the conversion of glass into glass

Presented at the 12th UniversityConferenceon Glass Science, AlfredUniversity,Alfred,NY, USA, 25-29 July, 1993. * Correspondingauthor.Tel: +44-742 825 484. Telefax: +44742 754 325.

ceramic (usually only a few percent). Because the glass ceramic process begins with a glass, all the well established glass-forming techniques can be employed to manufacture components with a variety of complex shapes including blowing, casting, pressing and rolling. Subsequently the glass component is readily converted into a fine-grained polycrystalline ceramic by a controlled nucleation and crystal growth heat treatment schedule. The original glass ceramics were produced by inducing volume nucleation in melt-derived bulk silicate glasses, usually by the addition of nucleating agents. More recently, glass ceramic processing has been greatly extended to include non-silicates and even non-oxide compositions, and to include the preparation of the precursor glasses by sol-gel techniques. Also the powder processing route has developed in importance. In this method, fine glass powders (melt- or sol-gel-derived) are formed into bodies of desired shapes, densified and crystallised. Densification may be achieved by cold-pressing and

0022-3093/95/$09.50 © 1995 ElsevierScienceB.V. All rightsreserved SSDI 0022-3093(94)00515-X

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P.F. James./Journal of Non-Crystalline Solids 181 (1995) 1-15

sintering or by hot-pressing and must be largely complete prior to crystallisation if low porosity is required. Nucleation probably takes place at the surfaces of the glass particles during sintering but the major part of the crystal growth occurs at a later stage to produce a 'bulk' crystallised product. The established compositions and uses of glass ceramics are extensive and include the low-expansion, highly thermal shock resistant and chemically durable Li20-AI203-SiO 2 compositions based on 13-quartz or 13-spodumene solid solution phases used in both transparent and opaque cooking ware, cooker range tops, heat-resistant windows and telescope mirror blanks. Other notable glass ceramics are the photomachineable lithium silicates, high-strength aluminosilicates, mechanically machineable micabased materials and a variety of compositions with hardness and wear resistance, resistance to chemical attack and oxidation, and superior optical and electrical properties These aspects are covered at length in various textbooks and reviews [1-11]. Here a number of recent developments are highlighted. These include phosphate-based glass ceramics, the use of sol-gel processing methods, matrices for composites, materials for microelectronics packaging, sealing materials, high-strength and high-toughness materials, and machineable compositions. Because of the extensive literature available and the rapid developments in the last few years, the present discussion is of necessity highly selective.

2. Phosphate-containing glass ceramics 2.1. Calcium-phosphate-based systems

It is well known that additions of P205 (a few percent) to certain silicate glass compositions promote volume nucleation and glass ceramic formation. There is some evidence for precipitation of phosphate crystals which subsequently act as heterogeneous nucleation sites for the major phases, although the detailed role of P205 remains debatable [1,12,13]. However, glass ceramics may be produced in which P205 is the main glass-forming component or at least a major component. Until recently there have been relatively few studies of glass ceramic formation in phosphate systems.

Early work indicated that volume nucleation was difficult to promote. Abe and co-workers [14-16] produced surface crystallised CaO-P20 S glasses with an highly orientated microstructure. Kokubo et al. [17] prepared glass ceramics in the M g O - C a O SiO2-P205 system containing 16.3 wt% P205 from powdered glass compacts, in which wollastonite and apatite crystal phases appeared to nucleate near the surface of the glass particles. The preparation of glass ceramics from CaO-P205 glasses with up to 21 wt% A1203 and up to 25 wt% alkali oxide, containing apatite and aluminium phosphate (AIPO 4) as major phases were discussed by Vogel et al. [18]. CaO-P205 glass ceramics have applications arising from their biocompatibility, for example in orthopaedics for bone replacement or repair of prosthetic devices, as coatings for orthopaedic appliances, or for implant materials in dentistry. Additions of TiO 2, ZrO 2 and P205 (typically 2-20 wt%) are crucial in promoting volume nucleation in commercial aluminosilicate glass ceramics, but the action of these agents is specific to certain systems. Recently James and co-workers [19-25] studied the role of various oxide additions as nucleating agents in the CaO-P205 system. Heat treatment of binary CaO-P205 glasses yielded only surface nucleation but addition of A1203 in combination with TiO 2 and SiO 2 promoted fine-scale volume crystal nucleation [19,21]. More than fifty glasses were investigated but best glass ceramic formation occurred for a P205 content of about 40 mol% and a CaO:P205 ratio of slightly greater than one. An alumina content typically in the range 4.5-9.5 mol% was required to produce precipitation of fine AIPO 4 crystals, which acted as sites for heterogeneous nucleation of needle-shaped crystals of the major calcium phosphate crystal phase. A typical composition (mol%) was 39.1P205, 40.2CAO, 7.4A1203, 7.8SIO 2, 5.5TIO 2 (CPTSA 42). The silica aided glass formation and the TiO 2 was found to enhance glass ceramic formation, although its role was not fully understood [21]. The glass was melted at 1450°C, cast into transparent blocks and converted to a glass ceramic by a nucleation heat treatment in the range 670-735°C followed by an increase of 10°C/min to the growth temperature (850°C) with a short hold (Fig. 1). The resultant (CPTSA) glass ceramic was white and opaque (similar to ivory) with a crys-

P.F. James./Journal of Non-Crystalline Solids 181 (1995) 1-15

tallinity of 75% by volume, modulus of rupture and Vickers hardness values of about 90 MPa and 490 kg mm -2, respectively, and a thermal expansion coefficient of 13 × 1 0 - 6 ° C -1. Samples could be polished to a high degree, In addition to biocompatibility, a particularly interesting property of these alumina-nucleated calcium phosphate glass ceramics was their good machineability, which was far superior to that of ordinary glass or ceramics, in spite of the hard nature of the material. Samples could be easily drilled using conventional workshop equipment and tungsten carbide tipped drill bits cooled by water to produce fine holes from several mm down to only 0.5 mm in diameter. Also specimens could be turned and threaded and fine holes drilled or fine patterns engraved using a dental drill (Fig. 2). Few examples of mechanically machineable glass ceramic compositions have been reported, with the notable exception

Fig. 1. 39.1P205, 40.2CAO, 7.4A1203, 7.8SIO2, 5.5TIO 2 (tool%) glass heated at 700°C for 1 h and then at 850°C for 12 min; optical transmission mierograph in polarised fight. (After Ref [21].)

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Fig. 2. Samples of glass ceramic after machining, prepared from 39.3P205, 40.5CAO, 6.8A1203, 7.8SIO2, 5.6TiO 2 (mol%) glass heated at 700°C for 60 min, and then at 810°C for 25 rain. (After

Ref. [21].)

of the well known silicate materials developed by Coming, based on fluorophlogopite mica or tetrasilicic mica, which have a variety of engineering and dental applications [4,11]. In these glass ceramics, exceptional machineability results from the interlocking plate-like and easily cleavable mica crystals dispersed in a glassy matrix, so that fine particles can be removed from the surface by 'pulverisation'. The origin of machineability in the calcium phosphate glass ceramic is probably similar; cracks may propagate preferentially through the needle-like crystals but are diverted at crystal-crystal or crystal-glass interfaces so that small blocks are removed readily during machining [21]. In the context of machineability, Hrland et al. [26] have developed commercial glass ceramics which are silicate-based and contain bulk nucleated fluormica crystals, as in the Coming materials. However, some CaO and P205 are also added to the composition resulting in the additional precipitation of the fluoropatite phase. The materials have good strength with the combined advantages of biocompatibility and machineability. A subsequent study of the role of A1203 and TiO 2 in the formation of the CPTSA glass ceramics using transmission electron microscopy and analysis including energy dispersive X-ray spectroscopy (EDS) has revealed greater detail of the complex nucleation processes [22]. The early stages of crystallisation were captured in a sample of the glass composition (CPTSA 42) given earlier, heated at 680°C for 20 h and containing only small ( < 3 i~m) crystals in a glass matrix. The crystal nuclei consisted of several

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P.F. James./Journal of Non-Crystalline Solids 181 (1995) 1-15

phases. In their centre was a cubic phase containing all the elements present in the glass (CPTSA phase). Emanating from the nucleus centre were impure dendrites of the tridymite form of A I P O 4 and after longer heat treatments dendrites of an unidentified (probably metastable) Ca2P20 7 polymorph (the major phase) grew from these, later transforming to 13-Ca2P20 7. Also at a later stage small crystals of pure AIPO 4 and of TiP20 7 were observed to crystallise between the 13-Ca2P20 7 dendrite arms, although the TiP20 7 phase was not responsible for enhancing nucleation. Shi and James [23-25] have shown that addition of B20 3 also promotes volume nucleation in calcium phosphate glasses although it is not as effective as A120 3. For glasses in the series ( 1 0 0 - x ) C a O . P2Os-xB203, volume nucleation occurred for contents, x, of 15-25 mol% B203; only surface crystallisation occurred outside this range. The crystalli-

sation mechanism and nucleation kinetics in the 20 mol% B203 glass were studied using X-ray diffraction, electron microscopy and quantitative optical microscopy. Volume crystal nucleation occurred after heat treatments near the glass transformation temperature Tg (613°C) (Fig. 3). The first phase to precipitate was BPO 4 and subsequently the phase 4CaO.P205 appeared by heterogeneous nucleation on the BPO 4. The nucleation kinetics were determined by optical microscopy of samples given twostage nucleation and growth treatments. At lower nucleation temperatures, a non-linear relation between crystal number density, Nv, and nucleation time was found for short times indicating non-steady state nucleation behaviour, but at longer times the plots were linear, the nucleation rate reaching a constant steady-state value after a certain induction period (Fig. 4). The induction time decreased with increase in temperature and eventually became so

Fig. 3. 40CaO-40P2Os-20B203 glass nucleated at 620°C for 30 min followedby growth at 770°C for 15 rain. Optical reflection micrograph.

P.F. James./Journal of Non-Crystalline Solids 181 (1995) 1-15

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CaO-40P2Os-20B203 glass against nucleation time at different nucleation temperaturesfollowed by growth treatment at 770°C for 15 rain. Nucleation temperatures: O, 590°C; o, 600°C; zx, 610°C; m, 620°C; A, 630°C. (AfterRef. [25].)

5

Nucleation rates in the CaO-P2Os-B20 3 glasses were too low for practical glass ceramic formation [25], but addition of AI20 3 allowed fine-grain glass ceramics and a higher degree of crystallinity to be produced. In a 37CAO. 37P205 • 20B203 • 6A1203 (mol%) glass, the first phase to be detected after heat treatment near Ts and above was A1PO4 rather than BPO 4 [23]. After longer heat treatment times or two-stage treatments, the phase 2CaO.P205 appeared, probably heterogeneously nucleated on the AIPO 4. The larger maximum nucleation rate occurred at 680°C, a temperature higher than in the alumina-free 40CaO • 40P205 • 20B20 3 glass. 2.2. Other phosphate-containing systems

short that the Nv versus time plots were practically linear through the origin. The maximum nucleation rate (relatively low at only 5 X 107 m -3 s -1) occurred at 620°C, close to Tg (Fig. 5). These effects of time and temperature are very similar to observations of homogeneous nucleation kinetics in silicate glasses [12], and the results are of special interest as a study of nucleation kinetics in a system where the first phase to precipitate catalyses the heterogeneous nucleation of a major phase. There have been few if any such studies, whereas the nucleation kinetics in silicate systems without deliberate additions of nucleating agents have been widely investigated [13].

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600 6/,0 680 TEMPERATURE*C Fig. 5. Steady state nucleation rate, I (In-3 s-1) versus nucleation temperature in 40CaO-40P2Os-20B~O3 glass. (After Ref. [25]). 560

Wilder et al. [27] were able to promote bulk nucleation in Na20-CaO-P205 and N a 2 0 - B a O P205 glasses (containing about 50 mol% P205) by adding 0.01 wt% platinum as a nucleating agent. Glass ceramics in these systems had expansion coefficients in the range (140-225) × 10-7°C - i and were suitable for applications involving sealing to highthermal-expansion metals such as aluminium and copper. MacDowell and Wilson [28] and Klein et al. [29] have discussed glass ceramic formation in the ternary P 2 O s - A I 2 0 3 - B 2 0 3 system. In the former study, compositions with a minimum of 75 wt% (in combination) of P205, AI203 and B203 and 25 wt% other oxides were studied, yielding fine-grained glass ceramics containing AIPO 4 and BPO 4 solid solution phases and thermal expansion coefficients from 73 to 185 × 10-7°C -1. The use of these materials in high-temperature lighting applications and as seals for molybdenum or tungsten were described. Fine-grained transparent and chemically durable glass ceramics in the B203-P2Os-SiO 2 system containing BPO 4 with molar ratios between 1 : 1 : 1 and 1 : 1 : 3 have been reported [30,11]. Homogeneous nucleation is believed to be the dominant process in the formation of these materials, which have expansion coefficients between 45 and 55 X 10-7°C - 1 and high dc resistivities (up to 1016 1~ cm at 250°C). Boron phosphate glass ceramics prepared by sintering and surface crystallisation of glass powder compacts [31], with useful dielectric properties (a dielectric constant between 3.5 and 4) and a thermal

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P.F. James./Journal of Non-Crystalline Solids 181 (1995) 1-15

expansion coefficient of about 40 × 10-7°C -1 (a good match for silicon), have potential as a substrate for electronic packaging. MacDowell and BeaU [32] have recently reported glass-forming systems (including the psuedo-binary system BPO4-SiO 2) that nucleate spherical bubbles, in addition to crystals, to produce glass ceramic microforms of 'gas ceramics'. These can form when, for example, NH4H2PO 4 or (NHa)2HPO 4 are used as a source of P205, the glass is melted under reducing conditions, cooled and then fired at 800-1000°C. Hydrogen gas is generated during firing within the foam cells, which range from < 1 txm to > 100 I~m in diameter, depending on composition and firing schedule. These BPO4-SiO 2 microfoams have excellent dielectric properties and high resistivity, and also have potential application in electronic packaging. An interesting class of phosphate glass ceramics based on NaZr2(PO4) 3 has been discussed by Aitken [33]. These have a wide range of thermal expansion coefficients from - 2 0 to 60 × 10-7°C - 1, depending on composition, and excellent chemical durability. Recently novel porous ceramics have been prepared by methods analogous to the production of porous glass by acid leaching a phase-separated sodium borosilicate glass in the 'Vycor' process. Porous ceramics have a variety of applications including reactive filters, membranes for separation, catalyst supports and gas sensors. Kokubu and Yamane [34,35] prepared microporous TiO2-SiO 2 glass ceramics containing large amounts of TiO 2 by heat treatment and subsequent acid leaching of phase-separated glasses in the TiO2-SiO2-A1203-P205(or B203)-CaO-MgO systems. The presence of TiO 2 is important in applications involving photocatalytic activity, and oxygen-gas sensing properties and the catalytic properties of the material produced are improved by incorporation of transition metal oxides in the porous skeleton. Abe and co-workers [36-39] have used a similar method to prepare porous ceramics in the CaO-P2Os-TiO2-Na20 system using volume crystallisation followed by leaching. Heat treatment of 45 CaO-25TiOE.30P205 glasses containing a few mol% Na20 yielded glass ceramics with CaTi4(PO4) 6 and I~-Ca3(PO4)2 phases. The latter phase was leached with acid leaving a porous skeleton of the former phase with a surface area of 40 mZ/g and mean pore radius of 13 nm [36]. This

material has excellent chemical durability to both acid and alkaline solutions and has advantages over silica as a porous support for immobilisation of enzymes in biotechnology [37]. Porous Na2Ti2(PO4) 3 and TiO2-SiO 2 were produced by a similar process [38,39], the former materials having potential uses as porous intercalation electrodes for secondary batteries, the latter in catalysts and gas sensors.

3. Sol-gel.derived glass ceramics Preparation of certain highly refractory oxide compositions as glass ceramics is extremely difficult by the conventional route because very high temperatures may be required to melt these compositions to provide the parent glasses required in the first place and problems may occur with melt-container reactions, vapour loss of constituents and uncontrolled crystallisation on cooling. Sol-gel routes using colloidal or alkoxide precursors are therefore of considerable interest with the advantages of producing highly pure and homogeneous glasses as bulk samples, powders or coatings at relatively low temperatures, but the disadvantages of long processing times, large shrinkages during processing and the high cost of raw materials. In general, the crystaUisation of sol-gel-derived glasses and their conversion to glass ceramics is imperfectly understood and is an area of current intense interest [40]. Usually gel-derived glasses crystallise faster than melt-derived glasses and observable crystallisation appears at lower temperatures in the former. Such differences result in part from higher hydroxyl contents in the gels. In some compositions, such as alkali silicates containing mobile ions, crystallisation may precede densification of the gels. However, in compositions containing less mobile ions (such as Ca, Ba or Ti), homogeneous glasses that would be almost impossible to melt can be prepared at low processing temperatures without the occurrence of crystallisation or even amorphous phase separation. Many of the features of sol-gel processing may be conveniently illustrated by recent work on celsian, BaO .AI203 • 2SiO 2 (BAS2), glass ceramics. Ceramic matrix composites (CMCs) based on refractory silicates, such as cordierite (liquidus temperature Tm = 1470°C), mullite (Tm = 1830°C), barium osumilite

P.F. James./Journal of Non-Crystalline Solids 181 (1995) 1-15

(BaO • 2MgO.3A1203 • 9SIO2, Tm = 1580°C) and celsian (Tm = 1760°C), are currently under intense investigation as structural materials for operation at elevated temperatures ( > 1200°C) in severe oxidising conditions. There are potential aerospace applications and possible spin-off uses in automotive engines, cutting tools, bearings and refractories. Composites containing these refractory phases as matrices are often made by glass ceramic processing routes using melt-derived or sol-gel-derived glass powder precursors. In the case of celsian, the monoclinic form, with a low thermal expansion coefficient (TEC) of 23 × 10-7°C -1 and excellent phase stability to 1590°C is the desirable form for CMCs. However, another crystalline form, hexacelsian, metastable below 1590°C is frequently the first phase to crystallise from stoichiometric BAS 2 melt-derived glass, and is unsuitable for CMCs because it undergoes a polymorphic transition at around 300°C with an associated large volume change (4%) and it has a higher T E C ( 8 0 × 1 0 - 7 ° C - 1). A l s o the hexagonal to mono-

7

clinic transition is sluggish for the BAS 2 composition [41]. Because of the high temperature required to melt celsian glass and other difficulties mentioned above, sol-gel routes have been studied. To avoid problems experienced with precipitation of barium salts when using mixtures of alkoxides and metal salts as precursors, and to obtain improved homogeneity by molecular level mixing, an all-alkoxide (all-liquid) route to celsian glass was developed [42-44]. Stoichiometric celsian produced as small bulk samples or glass powder compacts densified at a much lower temperature than powdered compacts of the corresponding melt-prepared glass. In bulk gel samples, 97% of its theoretical density was achieved after only 2 min at 1000°C or only 30 min, at 900°C prior to the onset of crystallisation. The predominant crystallisation product was hexacelsian but monoclinic celsian was obtained by prolonged heating at high temperatures (e.g., 4 weeks at 1320°C). Cold-pressing t h e gel-derived glass powder prior to sintering also

Fig. 6. Transmission electron micrograph of thin section of stoichiometric celsian gel-derived glass after heating at 10°C/min, to 1000°C with no hold, showing volume nucleated crystals of hexacelsian and a few residual spherical pores (e.g., at A).

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P.F. James./Journal of Non-Crystalline Solids 181 (1995) 1-15

yielded significant monoclinic after heating at 1200°C, although substantial hexacelsian remained. However, completely monoclinic celsian was obtained most easily in gel samples containing an added 5 mol% Li20 (introduced using lithium alkoxide to the original solution) - typically after only a few minutes at ll00°C [44]. Moreover, almost full densification could be achieved at about 900°C prior to the onset of crystallisation. Scanning electron microscopy (SEM) revealed an average grain size of 3 p,m at ll00°C with gradual coarsening to 10 p,m at 1600°C after 24 h. Doping with Li20 therefore appears to be an effective way of forming monoclinic celsian. Differential thermal analysis studies of the stoichiometric celsian gel-derived glass suggest that the predominant crystallisation product, hexacelsian, is volume nucleated [43], by contrast with results indicating surface nucleation in the melted glass [45]. Recently, transmission electron microscopy (TEM) and SEM have confirmed the volume nucleation of

hexacelsian in gel samples heat treated at >t 900°C, although in the melted glass of the same composition only surface crystallisation was observed. In the partially densified gel, after heating at 880°C for 2 h many small pores of up to 30 nm in diameter were clearly visible but no volume crystal nucleation was evident at this temperature and no clear evidence of crystals initiated from internal pores [46]. After 1 h at 900°C, the gel was almost fully densified; volume nucleated crystals and only a few fine residual pores were observed, indicating that full densification preceded volume crystallisation in the gel. Crystals and some residual pores in a sample heated to 1000°C are shown in Fig. 6. The fine hexacelsian crystals are hexagonal disc-shaped with bottom faces (0001) and six side faces of (1120) type [46]. Measurements of nucleation rates at different temperatures using SEM and TEM revealed a 'bell-shaped' nucleation curve with a maximum nucleation rate of 1017 m - 3 s - 1 at 980°C [47]. The maximum nucleation temperature is somewhat above Tg (930°C). This behaviour is in

Fig. 7. Transmissionelectronmicrographof thin sectionof stoichiometficcelsian gel with 10 wt% of added monocliniccelsian seeds (3.2 ~m mean size), heatedat 10°C/rain, to 1018 °C with no hold, showingexpitaxialgrowthof monocliniccelsian(M) on monoclinicseed(s), and some volumenucleatedhexacelsiancrystals(h).

P.F. James./Journal of Non-Crystalline Solids 181 (1995) 1-15

accordance with classical nucleation theory and is similar to results on melt-derived silicate systems [10,12,13], although the maximum rate in the gel glass is much higher than previously measured values in other glasses. The high volume nucleation in the gel-derived celsian glass but its apparent absence in the corresponding melted glass, where surface nucleation is predominant, is partly attributed to the much higher OH content in the gel glass [47]. A high OH content results in a lower Tg a lower viscosity and both the kinetic and thermodynamic barriers to nucleation may be reduced, yielding a higher volume nucleation rate. This has been demonstrated for glasses melted in a steam atmosphere [12,13]. Another method of producing the monoclinic form of celsian rather than hexacelsian is to add fine monoclinic seed crystals to the celsian sol prior to gelation and heat treatment [48,49]. The concept has been used by Kumagai and Messing [50] to control the transformation and sintering of a boehmite solgel by seeding with a-alumina particles. Bulk celsian gel seeded with 10 wt% fine seed crystals could be sintered to > 95% theoretical density at temperatures < 1050°C [49]. Further, the seeds facilitated formation of monoclinic celsian on heat treatment of the densified gel glass, the amount of monoclinic increasing with decreasing seed size. For example, in a gel containing 10 wt% of fine (mean size 3.2 Ixm) seeds heat treated at ll00°C for 1 h, > 92% of the glass crystallised to monoclinic celsian with only 7.5% of residual hexacelsian [49]. TEM of seeded gels after heat treatment (Fig. 7) clearly revealed epitaxial growth of monoclinic celsian from the crystal seeds, while some homogeneous nucleation and growth of hexacelsian also occurred. The easier epitaxial crystallisation in the seeded gels was also demonstrated by the decreased peak crystallisation temperature Tc, which was reduced with increasing seed content. For example a reduction in Tc by as much as 70°C was observed [49]. It is useful to summarise the main features of the work on sol-gel-derived celsian. Densification of the gel-derived glass powder occurred at temperatures significantly lower than for the melt-derived glass powder. Crystallisation of the gel occurred also at lower temperatures than for the melted glass, in bulk or powder form. Volume crystal nucleation occurred in the gel, but apparently only surface crystallisation

9

in the melted glass. There was no clear evidence that nucleation at pore surfaces in the bulk gel was a dominant process; the effect of residual hydroxyl may be more important. The desired phase (in this case monoclinic) can be produced by compositional change (e.g., by adding Li20 or ZnO [51]) or by adding crystal seeds to the sol prior to gelation and heat treatment. Several of these features are expected to apply to other sol-gel-derived compositions and will aid the development of new glass ceramics. For example, sol-gel-derived powder processing routes with hot-pressing may be used to produce fibre- or whisker-reinforced glass ceramic matrix composites. Among other interesting recent developments in sol-gel-derived glass ceramics are the preparation of Li20-AI203-4SiO 2 porous, crack-free glass ceramic monoliths based on [3-eucryptite or [~-spodumene phases, with very low expansion coefficients [52]. Glasses were prepared by an alkoxide sol-gel route and converted to glass ceramics by heat treatment. Small monolithic gels in the ZrO2-AI203-SiO 2 system, also prepared by an alkoxide process, were converted to glass ceramics with high fracture toughness by heat treatment at 900-200°C to precipitate tetragonal ZrO 2 [53]. The high toughness was attributed to the stress-induced transformation of tetragonal to monoclinic ZrO 2.

4. Glass ceramic matrix composites Glass ceramic matrix composites were briefly discussed above in connection with the sol-gel processing of celsian. Here, further discussion is concentrated mainly on materials derived from melted glasses. As pointed out by Prewo [54], glass matrix composites have great commercial potential owing to their ease of densification, low cost and high performance. Fibre composites may be prepared by infiltrating the fibres with a powdered glass slurry to produce a prepreg tape which can be dried and hot-pressed, or alternatively processed by matrix transfer moulding [54]. Ease of compaction is facilitated by viscous flow of the glass. After densification, a glass ceramic matrix with superior toughness and strength may be obtained by controlled heat treatment. A variety of reinforcing fibres have been

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P.F. James./Journal of Non-Crystalline Solids 181 (1995) 1-15

Fig. 8. Different growth morphologies observed in cordierite crystallized from previous flit boundaries: (a) dendritic and (b) cellular; (a) is a secondary electron SEM image of an etched section while (b) is a bright field TEM image [66].

P.F. James./Journal of Non-Crystalline Solids 181 (1995) 1-15

used including boron, carbon, alumina and silicon carbide. Glass ceramic matrix compositions can be tailored to match the thermal expansion coefficient of the fibres and control matrix-fibre interactions, and include those with major phases of 13-spodumene, cordierite, barium osumilite, anorthite, mullite and hexacelsian. The high flexural strengths (> 800 MPa) and fracture toughnesses ( > 24 MN m -3/2) up to about 1000°C achieved with lithium aluminosilicate (LAS) glass ceramic matrices reinforced by silicon carbide fibres (Nicalon) [55] are attributed to low fibre-matrix interracial strength as a result of a carbon-rich interface created during fabrication, which prevents cracks from propagating from matrix to fibre. The interface region is clearly of paramount importance in controlling the mechanical properties in these composites. Good strengths but somewhat lower toughnesses have been achieved with SiCwhisker-reinforced barium osumilite and bariumstuffed cordierite glass ceramic matrices [56]. These materials have potential applications in automotive components and cutting tools owing to their advantageous thermal and mechanical properties. Relatively

11

inexpensive powder processing methods may be used to produce complex-shaped parts [57]. Recent studies on LAS/SiC composites have established the importance of the pull-out of fibres in the crack wake to composite tougness, and how pull-out is influenced by the fibre properties and by the sliding resistance of the interface [58]. Also, detailed microstructural studies and matrix-fibre interface characterisations have been carried out on LAS/Nicalon fibre composites [59-62], on anorthite (calcium aluminosilicate) glass ceramic/ Nicalon fibre composites [63,64], and on barium osumilite glass ceramic/Nicalon fibre composites [65], which again demonstrate the importance of the interface properties. The effects of the fibre surface on crystallisation of the glass matrix during formation of glass ceramic-fibre composites has been recently demonstrated by Glendenning and Lee in the cordierite-SiC fibre system [66]. Melt-derived cordierite glass is known to crystallise by surface nucleation [67]. Composites were made by hot-pressing melt-derived cordierite glass frit with Nicalon SiC fibres. The

Fig. 9. Crystallisation at the previous frit boundary and on the surface of a SiC fibre seen in cross- section in a cordierite-SiC composite heated to 1000°C (SEM image) [66].

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P.F. James./Journal of Non-Crystalline Solids 181 (1995) 1-15

matrix was fully densified and completely amorphous after hot-pressing and was crystallised in a separate heat treatment. Interestingly, the cordierite glass ceramic matrix exhibited a 'memory effect' by which crystallisation occurred at the previous flit boundaries, presumably at impurities and possibly residual porosity. Consequently, the flit particle size is a critical parameter in the processing route used. The crystals nucleated at the boundaries grow either with a dendritic morphology or a cellular morphology (Fig. 8). Further, the fibre surfaces also acted as crystallisation sites (Fig. 9) a fact which must be considered when processing CMCs by this route. In addition to the established use of melted glass flits in processing CMCs by the glass ceramic route, sol-gel-derived powders have considerable potential. Refractory compositions may be produced more readily than by conventional melting and densification can be achieved at lower processing temperatures as demonstrated by the celsian work [42-49]. However, matrix densification and interface development, for example during hot-pressing, should precede controlled crystaUisation and formation of the glass ceramic matrix if successful materials are to be developed.

5. Glass ceramics as substrates, in bonding to metals and as joining media

In recent years silicon microelectronics devices have become increasingly complex with an increasing density of circuit components. The type of packaging to house the silicon chip is critical to device performance, and ceramics and glass ceramic substrates are the basis of such packaging [68]. Alumina substrates have dominated the field because of low cost, good strength and reasonable thermal conductivity to enable heat to be removed during circuit operation. However, glass ceramics have been identified as the best candidates for high-performance packaging in advanced mainframe computer systems [68]. Cofired multilayer cordierite glass ceramics combined with copper are now in use by IBM because of the low dielectric constant of cordierite and the match of its thermal expansion to silicon. The process employs a SiO2-A1203-MgO-P205B203 glass powder compact which is sintered to full

density above 800°C and crystallised by a surface nucleation process to et-cordierite and some clinoenstatite at around 900-950°C, below the melting point of copper. An excess of MgO above the stoichiometric cordierite composition aids sintering and increases the thermal expansion coefficient to match the silicon. B203 lowers the glass viscosity and raises the crystallisation temperature, allowing densification prior to crystallisation, and the P205 encourages formation of et-cordierite [68]. Work on glass ceramic substrates and glass ceramic coated metals for use in hybrid circuitry and microelectronics was recently reviewed by Partridge et al. [7]. Glass ceramics in the systems L i 2 0 Al203-SiO2(LAS) , ZnO-A1EO3-SiOE(XkS) and MgO-A1203-SiO 2 (MAS, including clinoenstatilebased materials) were produced by bulk nucleation and via sintered powder processing techniques involving compaction, screen printing and tape casting. The glass ceramics are compatible with both thickand thin-film circuitry and can be used in a variety of applications. Glass-ceramic-coated metal substrates offer the advantages of good thermal dissipation necessary when the packing density of circuit components is high, in conjunction with ruggedness and large size. Of particular importance [7,69] is the achievement of a strong bond between the glass ceramic and metal or alloy, using essentially vitreous enamelling techniques to achieve good wetting to the metal followed by a crystallisation step to the glass ceramic, which provides the increased refractoriness required to withstand the firing conditions for thickfilm inks. In addition, the glass ceramic composition can be tailored to provide a match in thermal expansion to the metal or alloy. The glass ceramic-metal substrate has a variety of applications. One example is in automotive electronic components where hostile environments are a problem. A regulator circuit has been developed on a glass-ceramic-coated stainless steel substrate known as Keralloy, as a successful alternative to an alumina substrate [10]. The device, which has excellent stability to thermal cycling, can be easily mounted to a vehicle through holes in the metal. The use of such substrates in heating elements for domestic or industrial use has also been described, for example in cooker hotplate and coffee pot elements [10]. Because of their high electrical resistivity and

P.F. James./Journal of Non-CrystaUine Solids 181 (1995) 1-15

ability to bond to metals, glasses have been used for many years in the electronics industry to produce hermetic seals on vacuum tubes and other devices. Glass ceramics can be used in the same way [1] but have the additional advantages of higher strength and better resistance to static fatigue. Also thermal expansion matching is possible over a wider range than can be obtained with glasses. Examples are a ZnOAI203-SiO 2 glass ceramic bonded to molybdenum for use in a high-voltage lead through seal assembly [9] and a lithium zinc silicate glass ceramic used in the manufacture of hermetic seals to nickel-based superalloys [70]. As well as providing a bond to metals in which the glass ceramic provides a bulk component closely matched in thermal expansion [1,6], glass ceramics are very useful as media for joining together ceramic to ceramic and ceramic to metal [9] in cases where the major components are matched in expansion or where mismatch occurs. Glass powder techniques are used to provide the glass bonding phase which is fired on and crystallised to provide a strong refractory join, the interface reactions being extremely important. For example, alumina parts have been joined by MAS and LAS glass ceramics, silicon nitride ceramics by nitrogen-containing glass ceramics, and Y203-stabilised zirconia ceramic to chrome-iron alloy by an LAS glass ceramic [9].

6. Other compositions and applications In this review it is not possible to discuss in detail many important topics, some of which will be briefly covered here. Recently, glass ceramics based on chain silicate structures have been developed [11,71]. These have both high flexural strengths (abraded) of 200-300 MPa and high fracture toughnesses (up to 5.0 MPa m 1/2) and include compositions based on enstatite (MgSiO3), potassium fluor-richterite (KNaCaMg 5Si8022F2) and fluorcanasite (K2Na4Ca5Si12030F4) as the major phases. Fluorcanasite glass ceramics, which are used as substrates in magnetic memory discs [11], exhibit a highly crystalline microstructure of interpenetrating blades. Fracture toughnesses are considerably higher than those of 'ordinary ' glass ceramics. Potassium richterite glass ceramics have

13

commercial applications in high-performance dinnerware and as cups and mugs [11]. Machineable glass ceramics based on cleavable fluormica phases were mentioned earlier. These materials have excellent machineability but not noticeably high strengths. In a recent study by Uno et al. [72], a glass ceramic combining high flexural strength (500 MPa), good fracture toughness (3.2 MPa m 1/2) and good machineability was reported. The microstructure consists of a plate-like calcium mica Ca0.sMg3(Si3AlO10)F 2 phase in which fine tetragonal ZrO 2 particles (20-50 nm size) are imbedded, to which the improved strength is attributed. This material would appear to have many potential applications. A machineable CaO-AI203-Y203-SiO 2 glass ceramic having needle-like crystals of Ca 4Y60(SiO4)6 has also been reported [73]. A novel application of glass ceramics is military armour. Ceramics are known to offer excellent protection against ballistic projectiles because of their mode of fracture and energy-absorbing properties; two ceramics-alumina and boron carbide-have been used extensively. Alumina with a density of 3.8 g cm -3 has a weight disadvantage, boron carbide is lighter (2.4 g cm -3) but expensive. A glass ceramic has been reported with a ballistic performance similar to that of alumina, a density similar to that of boron carbide but a cost approximately half that of alumina [10]. New optical glass ceramics have recently been discussed by Beall [11]. These include integrated lens arrays, luminescent materials, and zero-expansion passive optical materials, the latter recently used in the ring-laser gyroscope. An interesting new range of glass ceramics with very low thermal expansion (as low as ( 6 - 2 3 ) × 10-7oc- 1) has been recently reported in the calcium, strontium and barium aluminoborate systems [74]. These glass ceramics were produced from powdered glass compacts, sintered and crystallised below 1000°C. They have high strength, high fracture toughness and excellent electrical properties. Potential applications in substrates for microelectronic packaging, sealing frits for low-expansion materials, coatings for metals and thermal-shock-resistant ceramics, will, however, depend on achieving improvements in the chemical durabilities of these new glass ceramics.

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P.F. James./Journal of Non-Crystalline Solids 181 (1995) 1-15

Finally, topics which have not been discussed but which deserve mention are the potentially large field of oxynitride glass ceramics, still at a relatively early stage of study and development [75,76], glass ceramics for piezoelectric and pyroelectric devices [8], superconducting glass ceramics [77] and porous glass ceramics for humidity sensors [78]. 7. Conclusions and future prospects

Only some of the new compositions and applications have been highlighted in this review. A few current growth areas have been picked out including phosphate-based compositions; sol-gel-derived materials; matrices in composites; packaging and sealing; and high-strength, high-toughness and machineable materials. It is likely that new compositions with exciting property combinations will continue to be developed in silicate-based systems, also increasingly in the much less studied phosphate and borate systems, and even in non-oxide systems such as the oxynitride glass ceramics. The established method of volume nucleation and growth of bulk samples will continue in importance but the powdered glass route to bulk materials by compaction, sintering and surface crystallisation, as at present used in electronics packing, will be increasingly used. Moreover, sol-gel routes to produce precursor glass powders or bulk samples for subsequent crystallisation will be frequently examined as an option, where compositions difficult to produce by melting are involved and also where specific controlled microstructures and properties are required, for example fine-scale porosity. In all, the domestic and industrial applications of glass ceramics are expected to continue to grow, with extension to new areas such as biomaterials, and the prospects are bright indeed for this well established but relatively new class of materials. Figs. 1 and 2 are reproduced by permission of IOP Publishing Ltd; Figs. 3, 4 and 5 are reproduced by permission of the Journal of Materials Science. References [1] P.W. McMillan, Glass Ceramics, 2nd Ed. (Academic Press, London, 1979).

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