3Zr2(PO4)3-based composites

Journal of Alloys and Compounds 250 (1997) 515–519

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New low thermal expansion ceramics: Sintering and thermal behavior of Ln 1 / 3 Zr 2 (PO 4 ) 3 -based composites a a b b b a, J.M. Heintz , L. Rabardel , M. Al Qaraoui , M. Alami Talbi , R. Brochu , G. Le Flem * a

b

ICMCB, CNRS, Universite´ Bordeaux I, Av Dr Schweitzer, 33608 Pessac Cedex, France ´ , Faculte´ des Sciences, Universite´ Mohammed V, Av. Ibn Batouta, Rabat, Morocco Laboratoire de chimie du solide appliquee

Abstract Materials with the general formula Mx Zr 2 (PO 4 ) 3 are known to possess low coefficients of thermal expansion (CTE). The present work investigates the thermal properties of new composite materials issued from the decomposition at high temperature of Ln 1 / 3 Zr 2 (PO 4 ) 3 (Ln5La, Gd). The decomposition process was studied and showed that the resulting powder was a LnPO 4 , Zr 2 P2 O 9 and ZrO 2 mixture. Composite materials made of that mixture were sintered and characterized. The effect of sintering aids such as ZnO was considered. Final densities of the composites were about 90% of theoretical density and these materials presented low CTE in the 10 26 8C 21 range. Keywords: Phosphate; Low CTE; Sintering; Thermal behavior; Composite

1. Introduction Low thermal expansion ceramics have been extensively studied in the past years due to their large field of applications ranging from dinnerware, electronic substrates to space shuttle insulation [1]. Their thermal shock resistance and their ability to support thermal cycling may be also used in composite systems [2]. A broad structural family, designated as CTP or NZP type, referring to Ca 1 / 2 Ti 2 (PO 4 ) 3 or NaZr 2 (PO 4 ) 3 compounds presented very interesting thermal properties [3,4]. Extensive substitutions on the cation sites were possible giving rise to a large number of compounds. Therefore, it appeared possible to tailor CTE of these compounds by variations of their chemical compositions. New materials including lanthanum cations and belonging to that structural family were discovered and synthesized in 1991 [5]. Their chemical formula was expressed as Ln 1 / 3 Zr 2 (PO 4 ) 3 where Ln5La, Gd or Eu (called here LnZP). Preliminary results concerning X-ray and dilatometric experiments actually showed promising thermal properties [5]. However, their thermal stability range is limited since their decomposition begins slightly above 900 8C which prevents sintering at high temperature. Densification below 900 8C was tentatively investigated using ZnO as a sinter*Corresponding author. Fax: (33) [email protected]

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0925-8388 / 97 / $17.00  1997 Elsevier Science S.A. All rights reserved PII S0925-8388( 96 )02631-X

ing aid. A small increase in densification was noted but the final densities were not higher than 70% of the theoretical ones. Nevertheless the multiphasic composites resulting from the decompositions of these Rare Earth phosphates still posses low CTE. Therefore the aim of this work was an investigation of both densification and thermal properties of these types of composites. The decomposition process was studied using X-ray diffraction (XRD) and thermal gravimetry analysis (TGA). Microanalysis measurements allowed the composition of the resulting mixtures to be determined. Finally, sintering experiments and CTE measurements in relation to phase decomposition are described.

2. Experimental procedure Single phase LnZP powders were synthesized by a sol-gel process as described previously [5]. Composite powders were obtained from the decomposition of Ln 1 / 3 Zr 2 (PO 4 ) 3 at 1400 8C during 24 h. Phase composition was determined by means of XRD and their relative ratio confirmed by density measurements. Powders were hand-ground before being uniaxially compacted in a lubricated stainless steel die. The forming pressure was 125 MPa and the as-obtained pellets were 6 mm in diameter and 3 mm in thickness. Densities of the pellets were determined from geometric measurements. The green density of the samples was about 50% of

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theoretical densities (d th (LaZP)53.34, d th (GdZP)53.40). Sintering experiments were performed at different temperatures between 900 to 1450 8C at a heating rate of 5 8C min 21 under air. The phase composition of the sintered specimen was checked using XRD. Microstructural observations were obtained by scanning electron microscopy (SEM Jeol JSM 840). Local chemical analyses were performed on sintered pellets which were previously polished and thermally etched using X-ray microanalyser (Tracor EDS). The bulk coefficient of thermal expansion of sintered specimen was determined from ambient temperature to 1000 8C at 5 8C min 21 by means of a dilatometer carefully calibrated on reference samples (Netzsch 402 EP).

d Ln 1 / 3 Zr 2 (PO 4 ) 3 → 1 / 3LnPO 4 1 2 / 3Zr 3 (PO 4 ) 4 and 2 / 3Zr 3 (PO 4 ) 4 → 2 / 3ZrP2 O 7 1 2 / 3Zr 2 P2 O 9 d ZrP2 O 7 → 1 / 2Zr 2 P2 O 9 1 1 / 2P2 O 5 d Zr 2 P2 O 9 → 2ZrO 2 1 P2 O 5 Finally the overall decomposition process can be summarized by the relation: Ln 1 / 3 Zr 2 (PO 4 ) 3 → 1 / 3LnPO 4 1 (1 2 x)Zr 2 P2 O 9 1 2xZrO 2 1 (x 1 1 / 3)P2 O 5

3. Study of the decomposition of Ln 1 / 3 Zr 2 (PO 4 ) 3 phases Decomposition processes are identical for La or GdZP phases but kinetics are slightly different. The decomposition started from 900 8C, revealed by the appearance of the diffraction pattern of ZrP2 O 7 . Above 1000 8C, the presence of b-Zr 2 P2 O 9 and LnPO 4 could be noted. LnZP phases had almost disappeared at 1300 8C and a 5 h (La) or 10 h (Gd) heat treatment at 1400 8C only gave a mixture of bZr 2 P2 O 9 , LnPO 4 and ZrO 2 . According to the Zr 2 P2 O 9 – LnPO 4 –ZrO 2 phase diagram (Fig. 1), Ln 1 / 3 Zr 2 (PO 4 ) 3 belongs to the binary line LnPO 4 –Zr 3 (PO 4 ) 4 . Therefore, the first step of the decomposition process leads to the formation of these two compounds. Zr 3 (PO 4 ) 4 which can also be written as Zr 1 / 4 Zr 2 (PO 4 ) 3 (NZP type structure) also decomposes slowly at 900 8C into a mixture of ZrP2 O 7 and a-Zr 2 P2 O 9 [6]. Then, XRD patterns showed that the as-formed ZrP2 O 7 decomposes itself from 1100 8C to form b-Zr 2 P2 O 9 . The latter starts slowly to decompose at 1400 8C. As the temperature increases, the following chemical reactions can be proposed:

The value of x depends on the nature of the lanthanide cation and on kinetic factors. A TGA study performed on the LnZP powders at 1400 8C (5 8C min 21 heating rate) during 24 h indicated a total weight loss of 14.2% and 11.2% for LaZP and GdZP samples respectively. This mass loss was attributed to P2 O 5 departure. Therefore, the molar coefficients associated to the different phases in the final mixture were determined for this 24 h experiment as: 1 / 3LaPO 4 1 5 / 6Zr 2 P2 O 9 1 1 / 3ZrO 2 (x 5 1 / 6) 1 / 3GdPO 4 1 9 / 10Zr 2 P2 O 9 1 1 / 5ZrO 2 (x 5 1 / 10) These results were confirmed by density measurements carried out on the decomposed powders. Experimental values were very close to the theoretical ones calculated from the previous molar coefficients (Table 1). In the Table 1 Sintering conditions, composition Ln 1 / 3 Zr 2 (PO 4 ) 3 based materials

and La

LnZP powders5Ln 1 / 3 Zr 2 (PO 4 ) 3 powders CTE (10 26 8C 21 ) 1.4

thermal

properties

of

Gd 21.8

LnZP-C powders5LnZP powders heat treated 24 h at 1450 8C LnZP-C composition 0.22 LaPO 4 0.23 GdPO 4 0.56 Zr 2 P2 O 9 0.63 Zr 2 P2 O 9 0.22 ZrO 2 0.14 ZrO 2 LnZP-C experimental density 4.13 4.10 LnZP-C theoretical density 4.11 4.15 LnZP-C pellets sintered at 1450 8C Relative density (%) CTE (10 26 8C 21 )

Fig. 1. Ternary phase diagram 1 / 2 Ln 2 O 3 –1 / 2 P2 O 5 –ZrO 2 .

10 h 84 0.6

5h 91 0.3

(LnZP-C 13 w % ZnO) pellets sintered at 1450 8C during t (h) CTE (10 26 8C 21 ) t50.5 h 2.4 t51 h 2.7 t52 h 2.5 t55 h 1.9 t510 h 1.0

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Fig. 2. Evolution of the relative density of LnZP-C pellets as a function of the sintering temperature and time.

following text, these powders will be labelled as LnZP-C according to their composite nature.

4. Sintering and characterization of the LnZP-C (composite) materials Fig. 2 illustrates the densification kinetics of LnZP-C powders. Although treatment temperatures were rather high, these mixtures of powder were still difficult to densify completely. According to their composition, Zr 2 P2 O 9 was the main constituent and the densification of this compound is known to be very difficult without additives [7,8]. However, final densities around 85–90% of theoretical densities could be reached after several hours of sintering at 1450 8C. The microstructure of a typical LaZP-C specimen sintered for 15 h at 1425 8C is presented in Fig. 3. The core of the pellet was rather dense whereas the outer part, ¯60 mm, appeared very porous. Chemical analyses were per-

Fig. 3. SEM micrograph of the edge of a LaZP-C pellet sintered for 15 h at 1425 8C.

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formed on the same sample and the results were reported on a ternary phase diagram (Fig. 1). All experimental measurements lay along the Zr / La56 line. It confirmed that the main chemical evolution of the LnZP system was related to P2 O 5 elimination. The average composition of the denser part of the pellet was located near the predicted composition of LaZP-C powder. Other measurements performed at the border between the dense and the porous parts showed various compositions containing smaller and smaller quantities of Zr 2 P2 O 9 . The composition of the porous outer part corresponded to the LaPO 4 16ZrO 2 point. It suggests that all Zr 2 P2 O 9 phase was decomposed in that area. This study demonstrates the validity of the decomposition process proposed in the previous section. The total decomposition of Zr 2 P2 O 9 at the outside of the pellet also suggests that this process is activated at solid– gas interfaces. The influence on the composite densification of an oxide such as ZnO was also considered. The main constituents of our material mixture were Zr 2 P2 O 9 and LnPO 4 phosphates. Thermal evolution could lead to the presence of P2 O 5 and induce a liquid phase during sintering. Several eutectics exist between ZnO and compounds of the ZnO–P2 O 5 system in the 860–1300 8C temperature range [9]. An optimum concentration of 3 wt. % led to the best densification when ZnO was used during a sintering treatment of LaZP-C at 1450 8C for 1 h. This result agreed quite well with Zr 2 P2 O 9 sintering studies [7]. In that case (3 wt. % of ZnO addition) densification kinetics was improved since 85–87% of theoretical densities were reached for LaZP-C and 88–90 for GdZP-C after only a 30 min sintering time. Densification of GdZP-C pellets seemed slightly more efficient than LaZP-C pellets. This may be related to the larger relative amount of Zr 2 P2 O 9 within the initial composite mixture: 63% for GdZP-C compared to 56% for LaZP-C. Typical microstructures of ZnO doped LaZP-C sintered pellets are presented in Fig. 4. A backscattered electron image allowed the different phases to be identified. Larger grey grains correspond to Zr 2 P2 O 9 while the medium white grains correspond to LaPO 4 and the smaller ones to ZrO 2 . A closer observation clearly showed the presence of a Zn rich intergranular film (Fig. 5). Chemical analysis performed on these samples showed a slight dispersion of the compositions around the Zr / La56 line and the average values lay close the theoretical LaZP-C composition. But almost no change in phosphorous content versus sintering time could be noted. As previously mentioned, the decomposition of Zr 2 P2 O 9 in ZrO 2 and P2 O 5 within the ceramic is slow and mainly controlled by solid gas interfaces. The thermal expansion behavior of the LnZP-C type ceramics were tested. All of them presented low CTE between 20 to 1000 8C, from 0.3 to 2.7 10 26 8C 21 depending on the processing conditions of the ceramics (Table 1). Fig. 6a illustrates the thermal behavior of a LaZP-C

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Fig. 4. Backscattered electron image of a ZnO doped LaZP-C pellet sintered for 5 h at 1450 8C. Large grey grains5Zr 2 P2 O 9 , medium white grains5LaPO 4 , small white grains5ZrO 2 .

ceramic sintered without additives. A low value of CTE is obtained (0.6 10 26 8C 21 ) during heating, associated to an hysteresis shrinkage phenomenon during cooling. This is related to the development of microcracks within the material [10]. The main constituent of the composite pellet is Zr 2 P2 O 9 which is known to present a large thermal expansion anisotropy (Da 511.4 10 26 8C 21 ). When the grain size of such a material become higher than a critical size, microcracks can develop first upon cooling from the sintering temperature. During reheating of the ceramic, some of the crystallographic thermal expansion is taken up by closure and healing of the microcracks resulting in an apparent lower bulk thermal expansion. When the ceramic is cooled again internal stresses reinitiate microcrack formation leading to the observed hysteresis loop. The duration of the sintering treatment for that LaZP-C material is most likely sufficient for the grain size of Zr 2 P2 O 9 to reach its critical size. On the other hand, pure GdZP-C

Fig. 5. SEM micrograph of an Zn rich intergranular film in the previous LaZP-C pellet sintered for 5 h at 1450 8C.

Fig. 6. Thermal expansion behavior of (a) LaZP-C pellet; (b) GdZP-C pellet.

presented a low CTE value associated to a less pronounced hysteresis phenomenon (Fig. 6b). Decomposition of the initial Gd 1 / 3 Zr 2 (PO 4 ) 3 phase was initiated at a higher temperature than La 1 / 3 Zr 2 (PO 4 ) 3 , which resulted in an lower initial average particle size for Zr 2 P2 O 9 . Evolution of CTE values of ZnO doped LaZP-C composites confirmed that analysis (Table 1). A short sintering time gave CTEs close and slightly higher than that of the Zr 2 P2 O 9 value (1.7 10 26 8C 21 ) and as the sintering time increased, CTEs decreased. While densities did not change, grain growth took place resulting in the microcracking phenomena previously mentioned. Measurements of the size of LaPO 4 and Zr 2 P2 O 9 grains were realized for these specimen using the intercept method. The estimated critical size for Zr 2 P2 O 9 grains around 7 mm [7] was reached after a 10 h sintering treatment. The corresponding ceramic actually presented a bulk CTE less than 10 26 8C 21 . However, the expansion curves did not show the characteristic hysteresis loop. But as the composite was sintered for longer and longer, a more and more marked shrinkage phenomenon took place at high temperature. Moreover the starting temperature of that shrinkage decreased as the sintering time increased. The extension of the sintering time led to partial Zr 2 P2 O 9 decomposition and P2 O 5 release. Therefore the Zn-rich liquid phase became richer

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in P2 O 5 lowering its melting temperature according to the phase diagram [9].

5. Discussion and conclusion Thermal behavior of the LnZP-C composite materials is not simply the rule mixture of the thermal behavior of the individual phases. Thermal expansion of each of the constituent was determined. LaPO 4 and GdPO 4 presented CTE close to 7 10 26 8C 21 , the CTE of ZrO 2 was 7.6 10 26 8C 21 while the CTE of Zr 2 P2 O 9 was 1.7 10 26 8C 21 . In fact, a composite effect is obtained, related to the microstructure and to the manner in which the different constituents interacted. As a matter of fact, densification and thermal properties of the decomposed Ln 1 / 3 Zr 2 (PO 4 ) 3 powder (LnZP-C) seemed very dependant on the characteristics of the main constituent Zr 2 P2 O 9 and especially its grain size. The thinner the microstructure of such a material, the better the thermal and mechanical properties. But, other constituents such as LnPO 4 or ZrO 2 do play a key role in the definition of the grain sizes and the microstructure. Control of grain size in a ceramic can be achieved either by the control of grain size in the initial powder or by limiting coarsening during the sintering treatment. Duplex microstructures where a second phase allows the inhibition of grain growth have been successfully developed [11]. The decomposition process of the LnZP powders allowed a well dispersed and fine grain mixture of the 3 phases to be obtained. Then, sintering of that powder led to a LnZP-C composite material which actually presented an interpenetrating microstructure type. It resulted in a slow Zr 2 P2 O 9 grain growth, slower than the one observed during pure Zr 2 P2 O 9 sintering and comparable to the case where a second phase was added [7,12]. The LnZP-C composite materials were difficult to densify because of the sintering behavior of Zr 2 P2 O 9 . But additions of ZnO were effective to improve their densification kinetics. Densification was promoted by the occurrence of a low melting point liquid phase from the ZnO– P2 O 5 system. However, when sintering was prolonged at

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1450 8C, slow decomposition of Zr 2 P2 O 9 took place, resulting in a P2 O 5 enrichment of the Zn-rich liquid phase and a decrease of its melting temperature. The as-obtained intergranular films were susceptible to influence the composite thermal behavior around 1000 8C during a further heating. So, it has been shown that the choice of the sintering conditions (temperature, time and additives) allowed the microstructure and the thermal expansion of the LnZP-C (Ln5La or Gd) materials to be controlled in order to get dense and low CTE ceramics.

Acknowledgments This study was supported by the French Minister of the ´ ´ Franco-Marocaine Foreign Office under ‘‘Action integree n891 566’’.

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