《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_Phase equilibria in the refractory oxide systems of zirconia

Availableonlineatwww.sciencedirect.com Science Direct E噩≈RS ELSEVIER Joumal of the European Ceramic Society 28(2008)2363-2388 www.elsevier.com/locate/jeurceramsoc Phase equilibria in the refractory oxide systems of zirconia, hafnia and yttria with rare-earth oxides E.R. Andrievskaya Institute of Materials Science Problems, National Ukrainian Academy of Sciences, Krzhizhanovsky St. 3. Kiev 03142 Ukraine Available online 5 March 2008 The systematic study of phase equilibria in the ternary systems HfO2(ZrO2) -]O3 has been first carried out. Phase reactions and crystal lization of ceramic alloys in the binary systems ZrO2-Ln2O3, HfO2-Ln2O3, Y2O3-Ln2O3 and phase equilibria in the series of ternary systems HfO2-Y2O3-Ln2O3 and ZrO2-Y2O3-Ln2O3 have been developed at high temperature. The most general regularities of the phase reactions in liquid and solid states inherent in these systems have been considered dependent on lanthanide ion radii. Taking into account literature data and newly developed results in binary and termary systems, the analysis of the main regularities revealed in the constitution of phase diagrams, particularly its dependence on lanthanide ionic radius, was carried out. It was shown that temperature and composition of eutectic reaction, temperature of the pyrochlore phase decomposition, lattice parameters of solid solutions and other parameters of the binary phases linearly depend on ionic radius of lanthanide. For the first time it has been found that the affiliation of lanthanide oxides to cerium or yttrium subgroups predetermines phase relations in the systems and topology of the ternary phase diagrams. The data obtained are the basis for the novel prospective ceramic materials for both structural and functional applications in energetic, medicine, nuclear industry, thermal barrier coatings, solid oxide fuel cells, etc. 2008 Elsevier Ltd. All rights reserved Keywords: Zirconia; Hafnia: Y tria; Rare-earth oxides: Phase equilibria Introduction ceramics allows obtaining the high toughness ceramic material with the fracture toughness of 20-25 MPamcomparable with Stabilized zirconia is a unique material for many extensive one for some steels and metallic alloys. This progress invoked applications: engineering ceramics, thermal barrier coatings, new efforts in phase diagram research, under both equilibrium ceramic implants, electroceramics, high-temp erature magneto- and non-equilibrium conditions. Certainly, the precise definition hydrodyhamic electrodes, fuel-cells, and oxygen sensors, etc. of the eutectic location is an important prerequisite for the direc This variety is grounded on use of combination of mechanical, tionally solidification work. It substantially depends on deviation electrical, thermal and other properties from equilibrium. The directionally solidified eutectic ceramic is one of the This overview is dedicated to the study of the phase most attractive kinds of advanced zirconia-based ceramic mate agrams, phase equilibria in multicomponent ceramic com- rials demonstrating a unique combination of properties like high positions, especially those of them, which are close to the strength in combination with high fracture toughness For oxide eutectic points. To make this work more systematic, the phase systems based on zirconia and alumina, the bending strength of diagrams were studied in the series of the ternary systems 2.3 GPa remaining stable up to 1200C and fracture toughness HfO2(ZrO2-Y2O3-Ln2O3, where symbol Ln means rare-earth of 4.3 MPam(at room temperature)were revealed if the eutec- metals. The overall purpose of the present research is the devel tics were crystallized in the ceramic matrices. 2 Top level of the opment of phase equilibria in the ternary systems based on ZrO mechanical properties at room and high temperatures has been or HfO2 and oxides of the mIB subgroup (Y2O3 and rare-earth demonstrated in the carbide-boride-based systems Paderno oxides(REO)such as La, Sm, Eu, Gd, Er) and properties of has found that the directional solidified eutectic ZrB2-LaB6 the phases existing in these systems. Double-doped zirconia or doped hafnia, the objects of ternary phase diagrams, which in turn were not studied well. Our efforts for the last decade to E-mail address: ragulya@ipms. kiev. fill this space of knowledge resulted in the development of the 0955-2219/S-see front matter o 2008 Elsevier Ltd. All rights reserved. doi: 10.1016/j-jeurceramsoc 2008.01.009

Available online at www.sciencedirect.com Journal of the European Ceramic Society 28 (2008) 2363–2388 Phase equilibria in the refractory oxide systems of zirconia, hafnia and yttria with rare-earth oxides E.R. Andrievskaya Institute of Materials Science Problems, National Ukrainian Academy of Sciences, Krzhizhanovsky St. 3, Kiev 03142, Ukraine Available online 5 March 2008 Abstract The systematic study of phase equilibria in the ternary systems HfO2(ZrO2)–Y2O3–Ln2O3 has been first carried out. Phase reactions and crystal￾lization of ceramic alloys in the binary systems ZrO2–Ln2O3, HfO2–Ln2O3, Y2O3–Ln2O3 and phase equilibria in the series of ternary systems HfO2–Y2O3–Ln2O3 and ZrO2–Y2O3–Ln2O3 have been developed at high temperature. The most general regularities of the phase reactions in liquid and solid states inherent in these systems have been considered dependent on lanthanide ion radii. Taking into account literature data and newly developed results in binary and ternary systems, the analysis of the main regularities revealed in the constitution of phase diagrams, particularly its dependence on lanthanide ionic radius, was carried out. It was shown that temperature and composition of eutectic reaction, temperature of the pyrochlore phase decomposition, lattice parameters of solid solutions and other parameters of the binary phases linearly depend on ionic radius of lanthanide. For the first time it has been found that the affiliation of lanthanide oxides to cerium or yttrium subgroups predetermines phase relations in the systems and topology of the ternary phase diagrams. The data obtained are the basis for the novel prospective ceramic materials for both structural and functional applications in energetic, medicine, nuclear industry, thermal barrier coatings, solid oxide fuel cells, etc. © 2008 Elsevier Ltd. All rights reserved. Keywords: Zirconia; Hafnia; Yttria; Rare-earth oxides; Phase equilibria 1. Introduction Stabilized zirconia is a unique material for many extensive applications: engineering ceramics, thermal barrier coatings, ceramic implants, electroceramics, high-temperature magneto￾hydrodyhamic electrodes, fuel-cells, and oxygen sensors, etc. This variety is grounded on use of combination of mechanical, electrical, thermal and other properties. The directionally solidified eutectic ceramic is one of the most attractive kinds of advanced zirconia-based ceramic mate￾rials demonstrating a unique combination of properties like high strength in combination with high fracture toughness. For oxide systems based on zirconia and alumina, the bending strength of 2.3 GPa remaining stable up to 1200 ◦C and fracture toughness of 4.3 MPa m1/2 (at room temperature) were revealed if the eutec￾tics were crystallized in the ceramic matrices.1,2 Top level of the mechanical properties at room and high temperatures has been demonstrated in the carbide–boride-based systems. Paderno3 has found that the directional solidified eutectic ZrB2–LaB6 E-mail address: ragulya@ipms.kiev.ua. ceramics allows obtaining the high toughness ceramic material with the fracture toughness of 20–25 MPa m1/2 comparable with one for some steels and metallic alloys. This progress invoked new efforts in phase diagram research, under both equilibrium and non-equilibrium conditions. Certainly, the precise definition of the eutectic location is an important prerequisite for the direc￾tionally solidification work. It substantially depends on deviation from equilibrium. This overview is dedicated to the study of the phase diagrams, phase equilibria in multicomponent ceramic com￾positions, especially those of them, which are close to the eutectic points. To make this work more systematic, the phase diagrams were studied in the series of the ternary systems HfO2(ZrO2)–Y2O3–Ln2O3, where symbol Ln means rare-earth metals. The overall purpose of the present research is the devel￾opment of phase equilibria in the ternary systems based on ZrO2, or HfO2 and oxides of the IIIB subgroup (Y2O3 and rare-earth oxides (REO) such as La, Sm, Eu, Gd, Er) and properties of the phases existing in these systems. Double-doped zirconia or doped hafnia, the objects of ternary phase diagrams, which in turn were not studied well. Our efforts for the last decade to fill this space of knowledge resulted in the development of the 0955-2219/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2008.01.009

2364 E.R. Andrievskaya Journal of the European Ceramic Sociery 28(2008)2363-2388 ternary diagram. Thus, the present paper aims to show the main the mentioned systems belong to the limited solubility type of phase relations in the binary and ternary compositions consid- diagrams. The main features inherent in these systems are poly ered prospective for new materials. -82 morphism of REO, zirconia(hafnia) and intermediate phases as and advanced coatings. In many applications, the ternary solid compensate the excess charge on defects and this makes oxides solutions of fluorite-type, C-type of the REO and intermediate sensitive with respect to environment (i.e. phase transformations pyrochlore-type phase are the targeted materials. In some cases, become dependable on oxygen partial pressure) high strength of ceramics is necessary to combine with high ionic The key feature of all solid solutions in these phase diagram onductivity or low thermal conductivity. Such a combination is is the prevalence of a steric factor over energetic. The linear not attainable in a two-component solid solution, but becomes dependences of temperature-concentration coordinates versus realistic in three-component systems. Summary of current and ionic radii of rare-earth elements were revealed for many phase potential applications of ceramics based on the systems with diagram elements. All phase diagrams of the binary systems different lanthanides ZrO2(HfO2-Y2O3-Ln2O3 is presented in HfO2(ZrO2-Ln2O3 are of eutectic type(Figs. 1-4). The min- Table 1 imal liquidus temperature in the ZrO2(HfO2--Ln2O3 systems HfO2-Y2O3-Ln2O3 and ZrO2-k he ternary systems correspond to the reactions of eutectic type F+XLfor lan- The phase relations ies in the wide range of temperature and concentrations using subgroup XRD, DTA in He at temperatures up to 2500C, thermal anal- The coordinates(temperature and concentration) of eutectic ysis in air(including solar furnace up to 3000C), petrography points vary linearly with effective ionic radius of lanthanide. The and electron microscopy. 83-87 temperature of reaction increases and the concentration of lan- REO selected are representatives of lanthanides from the thanide oxide ineutectic point decreases with ion radius decrease beginning, the middle and end of the raw. This allows deducing (Table 2, Fig. 5) the main regularities of constitution in the series of the ternary Note, that under effective ionic radius in solid solution of diagrams HfO2(ZrO2H-Y203-Ln2O3. The comparison of the substitution type Mel-rLnr O2 we understand the ratio of addi phase diagrams based on zirconia and hafnia permits lucida- tivity: Reff=xRLn++(1-xRMe+ where u is a concentration tion the difference between the phase diagram constitution of the in mol%, R is the cation radius. The linear approximation is systems included the components--crystallographic analogs but sufficient, taking into account low accuracy of temperature mea- differed by cation radius. surement above 2000C, as well as the tendency for some oxides The crystallization of the alloys in the ZrO2 to change oxidation degree in vacuum or inert gas medium. The (HfO2-Y203-Ln2O3 systems was investigated using the eutectic reactions in the systems with HfO2 occur at tempe data liquidus and solidus surfaces. The crystallization atures 50-100C higher, than that in systems based on ZrO2 paths for the alloys and the schematics of the reactions were which correspond to difference in melting points for pure hafnia constructed. The equilibrium phase diagrams have been and zirconia(Table 2) deduced In the selected systems, the substitution-type solid solu- Basic interest to this research originates from the diversity tions are known to be formed by components and intermediat of polymorphic modifications inherent to the mentioned oxides, phases. The substantial phenomenon in the solid solutions intermediate phases and solid solutions stable or metastable, as the non-stoichiometry. Thus, the solid solutions based on cubic well as from the effect of electronic structure and ionic radii modifications of HfO,(ZrO,) form so called"defect fluorite of lanthanides on phase stability and boundaries of phase fields. structure.227 The substitution of hafnium or zirconium ions The overview of general regularities in constitution of phase dia- by lanthanide ion leads to increased concentration of oxygen grams of the ternary systems HfO2(ZrO2)-Y2O3-Ln2O3 seems vacancies--the defects compensating the lack of positive ionic to be logically starting from the analysis of the bounded binary charge in the cation sublattice. 28 The formation of solid solu- systems; then discuss several ternary systems and consider the tions also occurs by substitution of lanthanide ion by hafnium character of phase crystallization, and then introduce some or zirconium ion, provided the electron compensation of excess potential system for directional solidification study charge, 62 The model of substitution-type solid solution based on 2. General characteristics and regularities of phase MeO2 (Me=Hf, Zr)can be presented by Kroger-Vink formula: reactions in the systems HfO2-Ln203 and ZrO2-Ln20 Me++1- Ln+102-2-(/2). This formula seems to be correct for all three types of solid solutions based on monoclinic(M), tetrag- Phase interaction of zirconia, hafnia and practically all lan- onal (T) and cubic(fluorite-type, F)polymorphs of MeO2 thanide oxides(REO)(except Pm2O3, CeO2)are thoroughly Additives of the REO markedly decrease temperatures of phase studied in the temperature range from 1500C up to melting transformations T Tt F in hafnia and zirconia In the 2900 C4.57-60,63,65,67,68, 88-22 At temperatures below 1500.C series of phase diagrams, temperature of melting and solid-state these systems are studied a little, because the diffusion mobility transformations directly depends on the ratio between ionic radi is extremely low and the rate of solid solution and intermedi- of cations rHf++/rLn". The solubility in the monoclinic phase ate phase formation is infinitesimal. The phase diagrams of all as a rule, does not exceed 2 mol%, and in the tetragonal it is less

2364 E.R. Andrievskaya / Journal of the European Ceramic Society 28 (2008) 2363–2388 ternary diagram. Thus, the present paper aims to show the main phase relations in the binary and ternary compositions consid￾ered prospective for new materials.4–82 From the practical use viewpoint, these systems are worth studying because of needs in new structural, functional ceramics and advanced coatings. In many applications, the ternary solid solutions of fluorite-type, C-type of the REO and intermediate pyrochlore-type phase are the targeted materials. In some cases, high strength of ceramics is necessary to combine with high ionic conductivity or low thermal conductivity. Such a combination is not attainable in a two-component solid solution, but becomes realistic in three-component systems. Summary of current and potential applications of ceramics based on the systems with different lanthanides ZrO2(HfO2)–Y2O3–Ln2O3 is presented in Table 1. The phase relations in the ternary systems HfO2–Y2O3–Ln2O3 and ZrO2–Y2O3–Ln2O3 were stud￾ies in the wide range of temperature and concentrations using XRD, DTA in He at temperatures up to 2500 ◦C, thermal anal￾ysis in air (including solar furnace up to 3000 ◦C), petrography and electron microscopy.83–87 REO selected are representatives of lanthanides from the beginning, the middle and end of the raw. This allows deducing the main regularities of constitution in the series of the ternary diagrams HfO2(ZrO2)–Y2O3–Ln2O3. The comparison of the phase diagrams based on zirconia and hafnia permits elucida￾tion the difference between the phase diagram constitution of the systems included the components—crystallographic analogs but differed by cation radius. The crystallization of the alloys in the ZrO2 (HfO2)–Y2O3–Ln2O3 systems was investigated using the data on liquidus and solidus surfaces. The crystallization paths for the alloys and the schematics of the reactions were constructed. The equilibrium phase diagrams have been deduced. Basic interest to this research originates from the diversity of polymorphic modifications inherent to the mentioned oxides, intermediate phases and solid solutions stable or metastable, as well as from the effect of electronic structure and ionic radii of lanthanides on phase stability and boundaries of phase fields. The overview of general regularities in constitution of phase dia￾grams of the ternary systems HfO2(ZrO2)–Y2O3–Ln2O3 seems to be logically starting from the analysis of the bounded binary systems; then discuss several ternary systems and consider the character of phase crystallization, and then introduce some potential system for directional solidification study. 2. General characteristics and regularities of phase reactions in the systems HfO2–Ln2O3 and ZrO2–Ln2O3 Phase interaction of zirconia, hafnia and practically all lan￾thanide oxides (REO) (except Pm2O3, CeO2) are thoroughly studied in the temperature range from 1500 ◦C up to melting 2900 ◦C.4,57–60,63,65,67,68,88–221 At temperatures below 1500 ◦C these systems are studied a little, because the diffusion mobility is extremely low and the rate of solid solution and intermedi￾ate phase formation is infinitesimal. The phase diagrams of all the mentioned systems belong to the limited solubility type of diagrams. The main features inherent in these systems are poly￾morphism of REO, zirconia (hafnia) and intermediate phases as well as aleovalent character of ion substitution in the majority of solid solutions.57,58,222–226 Ionic or electronic charge carriers compensate the excess charge on defects and this makes oxides sensitive with respect to environment (i.e. phase transformations become dependable on oxygen partial pressure). The key feature of all solid solutions in these phase diagrams is the prevalence of a steric factor over energetic. The linear dependences of temperature–concentration coordinates versus ionic radii of rare-earth elements were revealed for many phase diagram elements. All phase diagrams of the binary systems HfO2(ZrO2)–Ln2O3 are of eutectic type (Figs. 1–4). The min￾imal liquidus temperature in the ZrO2(HfO2)–Ln2O3 systems correspond to the reactions of eutectic type F + X L for lan￾thanide oxides of cerium subgroup and F + H L—for yttrium subgroup. The coordinates (temperature and concentration) of eutectic points vary linearly with effective ionic radius of lanthanide. The temperature of reaction increases and the concentration of lan￾thanide oxide in eutectic point decreases with ion radius decrease (Table 2, Fig. 5). Note, that under effective ionic radius in solid solution of substitution type Me1−xLnxO2 we understand the ratio of addi￾tivity: Reff = xRLn 3+ + (1 − x)RMe 4+, where ч is a concentration in mol%, R is the cation radius. The linear approximation is sufficient, taking into account low accuracy of temperature mea￾surement above 2000 ◦C, as well as the tendency for some oxides to change oxidation degree in vacuum or inert gas medium. The eutectic reactions in the systems with HfO2 occur at temper￾atures 50–100 ◦C higher, than that in systems based on ZrO2, which correspond to difference in melting points for pure hafnia and zirconia (Table 2). In the selected systems, the substitution-type solid solu￾tions are known to be formed by components and intermediate phases. The substantial phenomenon in the solid solutions is the non-stoichiometry. Thus, the solid solutions based on cubic modifications of HfO2(ZrO2) form so called “defect fluorite” structure.227 The substitution of hafnium or zirconium ions by lanthanide ion leads to increased concentration of oxygen vacancies—the defects compensating the lack of positive ionic charge in the cation sublattice.228 The formation of solid solu￾tions also occurs by substitution of lanthanide ion by hafnium or zirconium ion, provided the electron compensation of excess charge.62 The model of substitution-type solid solution based on MeO2 (Me = Hf, Zr) can be presented by Kroger-Vink formula: ¨ Me4+1−xLn3+xO2−2−(x/2). This formula seems to be correct for all three types of solid solutions based on monoclinic (M), tetrag￾onal (T) and cubic (fluorite-type, F)—polymorphs of MeO2. Additives of the REO markedly decrease temperatures of phase transformations T M and T F in hafnia and zirconia. In the series of phase diagrams, temperature of melting and solid-state transformations directly depends on the ratio between ionic radii of cations rHf4+/rLn 3+. The solubility in the monoclinic phase, as a rule, does not exceed 2 mol%, and in the tetragonal it is less

E.R. Andrievskaya/ Journal of the European Ceramic Sociery 28(2008)2363-2388 Table 1 High performance applications based on zirconia Function Materials Applications ZrO2. Al203 B-Al2O3, Y203-Zr02, Ce02 m battery ), oxygen sensor, fuel cell membrane, gas generator, nical ceramic ZrO,, TiO, CeOz, rare-earth Chemical sensor, catalyst, catalyst support, oxides Structural ceramics ZrO,(TZP), cordierite, Automotive, heat exchangers, metal filters. Al,TiO thermal barrier coatings, diesel port liners ological Doped zirconia Artificial valves for hearts LngZrO7, CaAlzO4 Refractory, TBC Neutrons absorption HfO(ZrO)-Y203 (Eu]O3) Rods for nuclear reactors than 5 mol% Ln2O3. The fluorite-type solid solutions dissolve type solid solution transformed into the ordered pyrochlore-type up to 80 mol% REO. The excess concentration of compensating phases vacancies in the oxygen sublattice and presence of ions larger The solid solutions based on polymorphs of REO are also than zirconium or hafnium ions, both promote transformation of of substitution type, where the electron-ionic compensation of martensitic type in ZrO, and HfO2 at temperatures lower than excessive charge takes place. 62,23>In case of interstitial oxygen 1 170 and 1830Cin pure oxides, respectively. The effect of size ion, the compensation of positive charge occurs and correspond- and charge of added ion on properties of solid solutions based ing to the formula: Ln+2-xMe#*10-3+0.5r on monoclinic modifications of REO and ZrOz were studied by The solubility of hafnia or zirconia in hexagonal(A) and Yoshimuraet al. 79,229-23 In the case of lanthanide oxides, the monoclinic(B)modifications of rEo is small. The widest lattice parameters of monoclinic ZrO solid solutions dissolving homogeneity field belongs to the solid solutions based on C- 2 mol%, Ln2O3, varies linearly with ionic radius of lanthanide type of REO. These solid solutions have cubic structure, the (Fig. 6). The periods a, b, c and volume of elementary cell derivative from the fluorite-type structure. For the lanthanides increase isotropically, and the angle of monoclinity decreases of cerium subgroup, however, this phase was not revealed with ion radius increase in high-temperature fields of the phase diagrams, but low The solid solution based on tetragonal zirconia(T-ZrO2), suf- temperature fields were not studied yet because of extremely fers transformation T M at temperature on 400C lower than slow diffusion activity and abnormally long duration of homog- in pure MeOz. Nonequivalence of sites in low-symmetrical lat- enization tices(such as monoclinic and tetragonal) result in substantial Deformations of lattices and phase transformations stimulated 2.1. Intermediate phases in the systems by doping. In the homogeneity field of T-ZrO2, the period Hf02(ZrO2H-Ln203(Y203) of elementary cell linearly depends on lanthanide ionic radius (Fig.7)19 The analysis of existing data has revealed a considerable The filuorite-type solid solution with cubic lattice is the most similarity between binary systems based on hafnia and zirco- stable in these systems and demonstrates maximal solubility nia. In the systems HfO 2(ZrO2-Ln2O3(Y203), formation of among them. The solubility limit of rEo in F-HfO2 (ZrO2) several intermediate phases were found: phases with crystalline depends on lanthanide ionic radius(Table 3). In the systems structure of the pyrochlore-type(Fd3m, stoichiometry formula ZrO2-Ln2 O3, the lattice parameter d of the fluorite-type solid Ln2Hf207(n2Zr207); a phase with hexagonal structure(R3 solutions of the same concentration changes linearly with ionic formulated as Ln M3012), as well as the phases of perovskite- radius of lanthanide and can be approximated by empirical equa- type cubic structure with rhombic distortions (sD 6-Pbnt formulated as LnMeO3) d=5.120+[0.212(Ra3+-R4+)-0.0023Jm 2.2. The pyrochlore-type phases where m is the mol% of rEO, RLn+ and R2++ are the radii of corresponding ions. The calculated approximation fits the It is well known that the ionic conductivity of solid solu- experimental values of d. tions of fluorite-type passes through the maximum value, which Features of transformation from cubic to tetragonal phases corresponds to the maximum concentration of the mobile were investigated theoretically and experimentally by Kata- defects-anion vacancies, for instance. At high concentration mura et al. 14.215 The kinetics and mechanisms which control of dopant, the vacancies are turned for integration into slug the formation of fluorite solid solutions in the binary and gish clusters such as bi-, tri-vacancies, etc. The defect structure ternary systems were studied by Glushkova and Duran. 66, 232-234 enables to initiate rearrangement of the lattice as a whole, They showed the competition between diffusion controlled ordering, and formation of new phase, as it was found in case mass transport and nucleation of new phase when fluorite- of ordering the pyrochlore-type solid solutions. This ordering

E.R. Andrievskaya / Journal of the European Ceramic Society 28 (2008) 2363–2388 2365 Table 1 High performance applications based on zirconia87 Function Materials Applications Electrical insulation ZrO2, Al2O3 IC circuit substrate Ion-conductivity -Al2O3, Y2O3–ZrO2, CeO2, Gd2O3–CeO2 Solid electrolytes (sodium battery), oxygen sensor, fuel cell membrane, gas generator, membrane reactor Chemical ceramics ZrO2, TiO2, CeO2, rare-earth oxides Chemical sensor, catalyst, catalyst support, desiccant, gas, adsorption/storage Structural ceramics ZrO2(TZP), cordierite, Al2TiO5, Automotive, heat exchangers, metal filters, thermal barrier coatings, diesel port liners Biological ceramics Doped zirconia Artificial valves for hearts Thermal insulation Ln2Zr2O7, CaAl2O4 Refractory, TBC Neutrons absorption HfO2(ZrO2)–Y2O3(Eu2O3) Rods for nuclear reactors than 5 mol% Ln2O3. The fluorite-type solid solutions dissolve up to 80 mol% REO. The excess concentration of compensating vacancies in the oxygen sublattice and presence of ions larger than zirconium or hafnium ions, both promote transformation of martensitic type in ZrO2 and HfO2 at temperatures lower than 1170 and 1830 ◦C in pure oxides, respectively. The effect of size and charge of added ion on properties of solid solutions based on monoclinic modifications of REO and ZrO2 were studied by Yoshimura et al. 179,229–231 In the case of lanthanide oxides, the lattice parameters of monoclinic ZrO2 solid solutions dissolving 2 mol%, Ln2O3, varies linearly with ionic radius of lanthanide (Fig. 6). The periods a, b, c and volume of elementary cell increase isotropically, and the angle of monoclinity decreases with ion radius increase. The solid solution based on tetragonal zirconia (T-ZrO2), suf￾fers transformation T M at temperature on 400 ◦C lower than in pure MeO2. Nonequivalence of sites in low-symmetrical lat￾tices (such as monoclinic and tetragonal) result in substantial deformations of lattices and phase transformations stimulated by doping. In the homogeneity field of T-ZrO2, the period of elementary cell linearly depends on lanthanide ionic radius (Fig. 7).179 The fluorite-type solid solution with cubic lattice is the most stable in these systems and demonstrates maximal solubility among them. The solubility limit of REO in F-HfO2 (ZrO2) depends on lanthanide ionic radius (Table 3). In the systems ZrO2–Ln2O3, the lattice parameter d of the fluorite-type solid solutions of the same concentration changes linearly with ionic radius of lanthanide and can be approximated by empirical equa￾tion: 228,231 d = 5.120 + [0.212(RLn 3+ − RZr 4+) − 0.0023]m where m is the mol% of REO, RLn3+ and Rzr4+ are the radii of corresponding ions. The calculated approximation fits the experimental values of d. Features of transformation from cubic to tetragonal phases were investigated theoretically and experimentally by Kata￾mura et al.214,215 The kinetics and mechanisms which control the formation of fluorite solid solutions in the binary and ternary systems were studied by Glushkova and Duran.66,232–234 They showed the competition between diffusion controlled mass transport and nucleation of new phase when fluorite￾type solid solution transformed into the ordered pyrochlore-type phases. The solid solutions based on polymorphs of REO are also of substitution type, where the electron-ionic compensation of excessive charge takes place.62,235 In case of interstitial oxygen ion, the compensation of positive charge occurs and correspond￾ing to the formula: Ln3+2−xMe4+xO2−3+0.5x. The solubility of hafnia or zirconia in hexagonal (A) and monoclinic (B) modifications of REO is small. The widest homogeneity field belongs to the solid solutions based on C￾type of REO. These solid solutions have cubic structure, the derivative from the fluorite-type structure. For the lanthanides of cerium subgroup, however, this phase was not revealed in high-temperature fields of the phase diagrams, but low￾temperature fields were not studied yet because of extremely slow diffusion activity and abnormally long duration of homog￾enization. 2.1. Intermediate phases in the systems HfO2(ZrO2)–Ln2O3(Y2O3) The analysis of existing data has revealed a considerable similarity between binary systems based on hafnia and zirco￾nia. In the systems HfO2(ZrO2)–Ln2O3(Y2O3), formation of several intermediate phases were found: phases with crystalline structure of the pyrochlore-type (Fd3m, stoichiometry formula Ln2Hf2O7(Ln2Zr2O7); a phase with hexagonal structure (R3, formulated as Ln4M3O12), as well as the phases of perovskite￾type cubic structure with rhombic distortions (sD16 2h −Pbnm, formulated as LnMeO3). 2.2. The pyrochlore-type phases It is well known that the ionic conductivity of solid solu￾tions of fluorite-type passes through the maximum value, which corresponds to the maximum concentration of the mobile defects—anion vacancies, for instance. At high concentration of dopant, the vacancies are turned for integration into slug￾gish clusters such as bi-, tri-vacancies, etc. The defect structure enables to initiate rearrangement of the lattice as a whole, ordering, and formation of new phase, as it was found in case of ordering the pyrochlore-type solid solutions. This ordering

Table 2 The coordinates of invariant points in the systems HfO2-Lng0g and ZrO2-LngO3 Ln? 03 Systems HfO2-Ln2Oa Systems ZrO2-Ln2O3 onic radius, by Ref(nm) mperature of Ref(nm) Ahrrenius(nm) Tectic HtO? peritectic HfO2 peritectic°C Tectic eutectIc°C) 0.1014 0.10068 ~2200 0.102 0.0428 224012 2130 Tb ~2350 0.089 236014 008702 2350 0.087 0848 .15 0.08416

2366 Table 2 E.R. Andrievskaya / Journal of the European Ceramic Society 28 (2008) 2363–2388 The coordinates of invariant points in the systems HfO2–Ln2O3 and ZrO2–Ln2O3 Ln2O3 Systems HfO2–Ln2O3 Systems ZrO2–Ln2O3 58 Ionic radius, by Ahrrenius (nm) Ref (nm) Composition eutectic HfO2 (mol%) Temperature of eutectic (◦C) Composition of peritectic HfO2 (mol%) Temperature of peritectic (◦C) Ref (nm) Composition eutectic Temperature of eutectic (◦C) La 0.114 0.1014 35 2070 – – 0.10068 0.37 1980 77 2330 103 Ce 0.107 0.0983 30 – – – – – – Pr 0.106 0.09816 28 2125 45 ∼2200 0.09704 0.32 2100 75 2420 103 Nd 0.104 0.09698 27 2140 103 65 2450 0.0962 0.30 2120 ∼48 ∼2270 Sm 0.102 0.09428 26 2240 122 67 2550 0.0945 0.25 2180 ∼52 ∼2335 Eu 0.098 0.093 25 2150 139 0.0928 0.26 2130 196 Gd 0.097 0.09282 22 2310 122 – – 0.09244 0.24 2260 Tb 0.093 0.09015 19 2300 122 ∼30 ∼2350 0.08955 0.23 2250 Dy 0.092 0.08934 19 2310 122 ∼32 ∼2390 0.08892 0.22 2280 Ho 0.091 0.08905 15 2340 154 ∼31 ∼2380 0.08866 0.18 2300 Y 0.092 0.08976 16 2400 154 ∼30 2430 0.08976 0.16 2360 Er 0.089 0.08735 15 2360 154 ∼30 ∼2400 0.08702 0.18 2350 Tm 0.087 0.08574 14 2380 154 ∼33 ∼2460 – – – Yb 0.086 0.08496 13 2430 154 ∼35 ∼2500 0.0848 0.15 2410 Lu 0.085 0.08416 – – 12 2510 154 –– –

E.R. Andrievskaya/ Journal of the European Ceramic Sociery 28(2008)2363-2388 T 十+ ++185t++4"+ Py+A mol Py f F+Ay! Ay ++ M r +++++甘 |,a mo Fig. 1. Phase diagrams of the binary systems with hafnia and lanthanide oxides from lanthana to gadolinia:(a)HfO2-La203, 03(b)HfO2-Pr203, 03(c) HfO2-Nd2O3, (d) HfO2-Sm2O3, (e)HfO2-Eu2O3, (f) HfO2-Gd2O3: (O)DTA in helium: (+)annealing and quenching technique. () thermal analysis in air under solar fumace;(O)single-phase regions, (O hase regions by the data of XRD and annealing and quenching occurs by the way of cation rearrangement, distributed in the wide homogeneity fields, but much narrower ones than fluorite disordered solid solution of the fluorite-type and filling cites type solid solutions. The deviation from stoichiometry in the in the pyrochlore lattice, as well as by the oxygen ion shift- Ln2Hf20, phase appears, when x ions of Hf++ are substituted by ing in the anion sublattice, i.e. by changing of the oxygen xions of Ln+ On substitution, the x/2 additional oxygen vacan- ions coordination and abrupt decreasing of oxygen vacan- cies are formed Ln2+rHf2-O7-(/2). When the 'ions of Ln cies concentration.236 All these pyrochlore-type phases possess are substituted by ions of Hf++ the number of oxygen vacan-

E.R. Andrievskaya / Journal of the European Ceramic Society 28 (2008) 2363–2388 2367 Fig. 1. Phase diagrams of the binary systems with hafnia and lanthanide oxides from lanthana to gadolinia: (a) HfO2–La2O3, 103 (b) HfO2–Pr2O3, 103 (c) HfO2–Nd2O3, 103 (d) HfO2–Sm2O3, 122 (e) HfO2–Eu2O3, (f) HfO2–Gd2O3; 122 () DTA in helium; (+) annealing and quenching technique, () thermal analysis in air under solar furnace; () single-phase regions, () two-phase regions by the data of XRD and annealing and quenching. occurs by the way of cation rearrangement, distributed in the disordered solid solution of the fluorite-type and filling cites in the pyrochlore lattice, as well as by the oxygen ion shift￾ing in the anion sublattice, i.e. by changing of the oxygen ion’s coordination and abrupt decreasing of oxygen vacan￾cies concentration.236 All these pyrochlore-type phases possess wide homogeneity fields, but much narrower ones than fluorite￾type solid solutions. The deviation from stoichiometry in the Ln2Hf2O7 phase appears, when x ions of Hf4+ are substituted by x ions of Ln3+. On substitution, the x/2 additional oxygen vacan￾cies are formed Ln2+xHf2−xO7−(x/2). When the ч ions of Ln3+ are substituted by ч ions of Hf4+ the number of oxygen vacan-

2368 E.R. Andrievskaya Journal of the European Ceramic Sociery 28(2008)2363-2388 F 200+Fr + m c L 24002400 22022 2000 2000 770+10 Tc(e) 2800 2800 260 L 2600 -2600 430+25 2200 2000 F+C 2000 2000 T+F mol 03 Yb.o mol 2900±30 00+302600 2200 T+F 780±10 F+M 1351020304050607080905 2° Fig. 2. Phase diagrams of the binary systems with hafnia and lanthanide oxides from terbia to lutetia:(a)HfO2-Tb2O3, 2(b)HfO2-Dy203, 122(c)HfO2-Y203, 54 (d)HfO2-Ho2O3, 154(e)HfO2z-Er203, 54(f) HfO2-Yb203, 54(g)HfO2-Lu2O3 154

2368 E.R. Andrievskaya / Journal of the European Ceramic Society 28 (2008) 2363–2388 Fig. 2. Phase diagrams of the binary systems with hafnia and lanthanide oxides from terbia to lutetia: (a) HfO2–Tb2O3, 122 (b) HfO2–Dy2O3, 122 (c) HfO2–Y2O3, 154 (d) HfO2–Ho2O3, 154 (e) HfO2–Er2O3, 154 (f) HfO2–Yb2O3, 154 (g) HfO2–Lu2O3. 154

E.R. Andrievskaya/ Journal of the European Ceramic Sociery 28(2008)2363-2388 2369 2600 2600 2200 22 80Pr3 2600 2800 2400 2000 20001 mol mol T 2500 T,C 2300 L+H X C+H 2000 C+b 8 mol Fig 3. Phase diagrams of the binary systems with hafnia and lanthanide oxides from lanthana to gadolinia: (a)ZrO2-La203,63(b)ZrO2-Pr203, 63(c)ZrO2-Nd203, 63 (d)ZrO2-Sm203, 63(e)ZrO2-Eu2O3, (f) ZrO2-Gd2O3 63(O)experimental points: ()thermal analysis in air under solar furnace; (O)single-phase regions. (O gions by the data of XRd and annealing and quenching. cies in the anion sublattice of Ln2- Hf2+r07-(/2) decreases in solid solutions of fluorite type. 237 The thermodynamic stabil y/2.The pyrochlore-type phases demonstrate concentration ity of the pyrochlore type compounds can be determined using limits of stability, which are shown in Tables 4 and 5, Fig 8. empirical rule, which identifies the ratio between ionic radii The pyrochlore-type phases and their solid solutions are formed R=(r+Ln/+*HD), which must be higher than 1.2. In accordance y zirconia or hafnia with cerium subgroup of REO (from La with Colong, 235 the average ionic radi for the pyrochlore- to Gd), while the rEo of yttrium subgroup form disorder type phases can be approximately calculated by additive rule

E.R. Andrievskaya / Journal of the European Ceramic Society 28 (2008) 2363–2388 2369 Fig. 3. Phase diagrams of the binary systems with hafnia and lanthanide oxides from lanthana to gadolinia: (a) ZrO2–La2O3, 63 (b) ZrO2–Pr2O3, 63 (c) ZrO2–Nd2O3, 63 (d) ZrO2–Sm2O3, 63 (e) ZrO2–Eu2O3, (f) ZrO2–Gd2O3; 63 () experimental points; () thermal analysis in air under solar furnace; () single-phase regions, () two-phase regions by the data of XRD and annealing and quenching. cies in the anion sublattice of Ln2−xHf2+xO7−(x/2) decreases in ч /2.235 The pyrochlore-type phases demonstrate concentration limits of stability, which are shown in Tables 4 and 5, Fig. 8. The pyrochlore-type phases and their solid solutions are formed by zirconia or hafnia with cerium subgroup of REO (from La to Gd), while the REO of yttrium subgroup form disordered solid solutions of fluorite type.237 The thermodynamic stabil￾ity of the pyrochlore type compounds can be determined using empirical rule, which identifies the ratio between ionic radii R = (r3+ Ln/r4+Hf), which must be higher than 1.2. In accordance with Colong,235 the average ionic radii for the pyrochlore￾type phases can be approximately calculated by additive rule:

E.R. Andrievskaya Journal of the European Ceramic Sociery 28(2008)2363-2388 T, c(a) 2800 2600 2600 220 1800 00 Tb,O, 2800 H州 M+F 80H0O zr0.2040 T, cI(h) 2800 2400 4. Phase diagrams of the binary systems with hafnia and lanthanide oxides from terbia to lutetia: (a)ZrO2-Tb203, 6(b and c)ZrOx-Dy2O3, 63, 194(d)ZOz-Y20 6(e)ZrOx-Ho2O3, 63(f and g)ZrO2-Er203, 219, 221(h)ZrO2-Yb2O363

2370 E.R. Andrievskaya / Journal of the European Ceramic Society 28 (2008) 2363–2388 Fig. 4. Phase diagrams of the binary systems with hafnia and lanthanide oxides from terbia to lutetia: (a) ZrO2–Tb2O3, 63 (b and c) ZrO2–Dy2O3, 63,194 (d) ZrO2–Y2O3, 156 (e) ZrO2–Ho2O3, 63 (f and g) ZrO2–Er2O3, 219,221 (h) ZrO2–Yb2O3. 63

E.R. Andrievskaya/ Journal of the European Ceramic Sociery 28(2008)2363-2388 T.℃ m%,(b) HfO2-Ln2o3 2c4n23 2400 HOz-Ln2Oa 30 2000 1800 0.0840088009200960.1000.104 960.1000.104 Fig. 5. Dependence of melting temperature(a)and composition(b)for eutectics in the systems MeO2-Ln2O3 vs effective ionic radius of lanthanide(Table 2, RLn3+ and RMe are taken by Ahrens scale " (a) V,nm l(b) 05320 0.1415 05315 b 05210 05160 05155 Yb ErY Sm Nd La 05150 0.09 0.10 Yb ErY Sm Nd La Fig. 6. Dependence of lattice parameters(a)and volume of elementary cell(b)for the solid solution based on monoclinic ZrO2 vs ionic radius of dopant. 230 058:。N axls·c 0.518 0514 0514 0.510 (a) mol RO15 mol RO15 Fig. 7. Dependence of lattice parameters(a)and volume of elementary cell(b)for the solid solution based on tetragonal ZrO2 versus ionic radius of dopant. 179

E.R. Andrievskaya / Journal of the European Ceramic Society 28 (2008) 2363–2388 2371 Fig. 5. Dependence of melting temperature (a) and composition (b) for eutectics in the systems MeO2–Ln2O3 vs. effective ionic radius of lanthanide (Table 2, RLn3+ and RMe4+ are taken by Ahrens scale 257). Fig. 6. Dependence of lattice parameters (a) and volume of elementary cell (b) for the solid solution based on monoclinic ZrO2 vs. ionic radius of dopant.230 Fig. 7. Dependence of lattice parameters (a) and volume of elementary cell (b) for the solid solution based on tetragonal ZrO2 versus ionic radius of dopant.179

E.R. Andrievskaya Journal of the European Ceramic Sociery 28(2008)2363-2388 Table 3 The solubility of components in the systems HfO2-Ln2O3 lonic radius, by Ahrens(nm) Limit of solubility(mol%) 0.114 27-402 T5433 F57 0.095 30-38 5555 83/64 5550 11111 0.092 Er 826702 2.5 0.085 Table 4 Properties of the Ln2Hf20, phases a(nm) △Ht( k/mol) Dmes(g/cm) TEC x LagHf, O7 10776 0960 d?Hf,O7 10648 8010 uhF,07 82-4 Gd Hf, 07 Table 5 Properties of the phases Ln2Zr20758 Phase △H°t(kJ/mol) Tm(K) TEC×10-6 Pr2Zr07 Nd, ZrzO7 00000 8839 11.717 106.8 10.733 1.0554 9.347 Gd Zr?O7 10528 11519 R IMe =/Hf2_Ln,= [(2-x)rHf+XrLnI [(2-x)rLn +x'rHeI RI x1)rHf xrLn R=421.2 (for the pure pyrochlore phases) (2-x1rLn+x/Hf RI =1.2 2rHf where R, is the ratio of average radii for ions taking into account their substitution. The boundary value R1=1.2 responds to two values of x and xr. The results of cal The boundary ratio is Fig8. Calculated scheme for detemination of the homogeneity field size for correct for gadolinium hafnate but wrong for terbium haf the phases of pyrochlore type in the systems HfO2-Ln2O3 235

2372 E.R. Andrievskaya / Journal of the European Ceramic Society 28 (2008) 2363–2388 Table 3 The solubility of components in the systems HfO2–Ln2O3 Ln3+ Ionic radius, by Ahrens (nm) Limit of solubility (mol%) Source Py M T F X H A B C La 0.114 27–40 2 5 15 15 12 10 – – 103 Pr 0.107 27–40 2 4 17/65 12 10 8 – – 103 Nd 0.104 27–39 2 3 18/63 11 9 5 – – 103 Sm 0.100 29–39 2 3 27/62 10 8 6 4 – 122 Eu 0.099 30–38 1.5 3 55 10 8 5 4 – 139 Gd 0.095 30–38 1.5 2.5 83/64 10 9 5 3.5 – 122 Tb 0.093 30–38 1.5 2.5 58 8 5 4 3 30 122 Dy 0.092 – 1.5 2.5 62 5 4.5 – 3 32 122 Ho 0.091 – 1 2.0 63 – 4 – 2 30 154 Y 0.092 – 1 2.0 57 – 3 – – 30 154 Er 0.089 – 1 2.0 60 – 3 – – 30 154 Yb 0.086 – 1 2.0 62 – 2.5 – – 35 154 Lu 0.085 – 1 2.0 60 – – – 40 154 Table 4 Properties of the Ln2Hf2O7 phases58 Phase a (nm) −H◦ f (kJ/mol) Tm (K) Dmes (g/cm3) TEC × 10−6/ ◦ La2Hf2O7 1.0776 – 2560 7.84 7.85 Pr2Hf2O7 1.0960 104 2610 7.90 9.13 Nd2Hf2O7 1.0648 85 2730 8.11 9.27 Sm2Hf2O7 1.0568 82 2760 8.20 10.60 Eu2Hf2O7 1.0540 – 2735 8.29 10.82 Gd2Hf2O7 1.0502 41 2790 8.34 – Table 5 Properties of the phases Ln2Zr2O7 58 Phase a (nm) −H◦ f (kJ/mol) Tm (K) Dmes (g/cm3) TEC × 10−6/ ◦ La2Zr2O7 1.0808 126.1 2160 5.89 9.129 Pr2Zr2O7 1.0714 120.3 2220 6.14 5.65 Nd2Zr2O7 1.0668 111.0 2320 6.28 11.717 Sm2Zr2O7 1.0594 106.8 2350 6.48 10.733 Eu2Zr2O7 1.0554 80.0 2350 6.63 9.347 Gd2Zr2O7 1.0528 75.8 2450 6.69 11.519 Fig. 8. Calculated scheme for determination of the homogeneity field size for the phases of pyrochlore type in the systems HfO2–Ln2O3. 235 rMe = rHf2−xLnx = 1 2 [(2 − x)rHf + xrLn] r Me = rLn2−xHfx = 1 2 [(2 − x )rLn + x rHf] Rl = rLn rMe = 2rLn (2 − xl)rHf + xlrLn = 1.2 R = r 3+ Ln r 4+ Hf ≥ 1.2 (for the pure pyrochlore phases) Rl = r Me rHf = (2 − x l )rLn + x l rHf 2rHf = 1.2 where Rl is the ratio of average radii for ions taking into account their substitution. The boundary value Rl = 1.2 responds to two values of xl and x l . The results of cal￾culation are presented in Fig. 8. The boundary ratio is correct for gadolinium hafnate but wrong for terbium haf￾nate.