ISSN 10876596, Glass Physics and Chemistry, 2010, Vol. 36, No. 4, pp. 470–477. © Pleiades Publishing, Ltd., 2010. Original Russian Text © T.I. Panova, M.Yu. Arsent’ev, L.V. Morozova, I.A. Drozdova, 2010, published in Fizika i Khimiya Stekla.
Synthesis and Investigation of the Structure of Ceramic Nanopowders in the ZrO2–CeO2–Al2O3 System T. I. Panova, M. Yu. Arsent’ev, L. V. Morozova*, and I. A. Drozdova Grebenshchikov Institute of Silicate Chemistry, Russian Academy of Sciences, nab. Admirala Makarova 2, St. Petersburg, 199034 Russia *email:
[email protected] Received November 30, 2009
Abstract—A new technological scheme has been developed for synthesizing nanocrystalline powders in the ZrO2–CeO2–Al2O3 system. The powders have been used for preparing nanoceramic materials with an open porosity of ~1%. The physicochemical properties of these ceramic materials have been investigated. Key words: coprecipitation, ultrasound treatment of precipitates, nanocrystalline powders, nanoceramics, electrical conductivity DOI: 10.1134/S1087659610040115
INTRODUCTION At present, the preparation of nanomaterials and investigation of their physicochemical properties have become the main direction of the development of promising technologies. These materials, which con sist of nanosized particles, have been extensively stud ied in respect with the possibility of revealing their new properties [1, 2]. A special place among ceramic materials is occu pied by ceramics based on zirconia, which exhibits a high melting temperature, corrosion resistance, wear resistance, low thermal conductivity, high strength, fracture toughness, and thermal shock resistance. Solid electrolytes based on zirconia stabilized by rareearth elements have been widely known and used in different fields of engineering [3]. The development of technologies for synthesizing oxide nanopowders, including those based on stabi lized zirconia, is an important direction in the design of materials of a new generation. The main problem in the development of techno logical schemes for synthesizing nanoceramic materi als consists in retaining nanosizes of precursor parti cles. The high surface energy and thermodynamic instability of nanoparticles are the driving force of the process of spontaneous aggregation of these particles, which results in the loss of their unique properties [2, 4]. Therefore, it is necessary to find ways of sup pressing the agglomeration of nanoparticles and retaining their sizes. Chemical synthesis methods (coprecipitation [5, 6], sol–gel process [7–10], mechanochemical syn thesis [11, 12]) are most effective for preparing nanop owders of zirconia and its solid solutions.
One of the most universally employed synthesis methods is the coprecipitation, which provides a high degree of homogeneity of mixed components. This favors an intensification of the chemical processes of the synthesis of specified compositions and allows one to considerably decrease the temperature of the prep aration of nanoceramic materials. However, apart from the advantages, this method has a substantial dis advantage: the powders produced in this manner are characterized by a high degree of agglomeration and a large scatter in sizes of primary particles and agglom erates. In other words, this method in its classical vari ant does not make it possible to prepare unagglomer ated powders with nanosized particles. The reason lies in the high degree of interparticle interaction, which is characteristic of hydrogels, is associated with the phe nomenon referred to as the syneresis, and leads to contraction of individual regions with the formation of large agglomerates over the entire volume of the pre cipitate [13]. In this respect, the prevention or mini mization of the agglomeration process is one of the most important elements in the chemical technology for producing oxide nanopowders. SAMPLES AND EXPERIMENTAL TECHNIQUE Synthesis of Nanocrystalline Ceramic Materials in the ZrO2–CeO2–Al2O3 System The investigations performed in our work are aimed at developing a new technology for synthesizing nanocrys talline ceramic materials in the ZrO2–CeO2–Al2O3 sys tem and studying their properties. The previously investigated system, which contained 87 mol % ZrO2 and 13 mol % CeO2 [12, 14, 15], was chosen as the model object. This ratio of the components according
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to the phase diagram provides the stabilization of the tetragonal zirconia modification at low temperatures. One of the approaches that allow one to facilitate the preparation of ceramic materials with a nanosized structure is the introduction of modifying additives [16]. The presence of the second phase in the ceramic material affects the recrystallization process during the synthesis and, possibly, will limit the agglomeration of crystalline particles. In this work, we synthesized ceramic samples of the following compositions: (ZrO2)0.87(CeO2)0.13 (no. 1), (ZrO2)0.87(CeO2)0.13 + 5 mol % Al2O3 (no. 2), and (ZrO2)0.87(CeO2)0.13 + 10 mol % Al2O3 (no. 3). At the first stage of the synthesis, we used the chemical precipitation method. The compounds ZrO(NO3)2 ⋅ nH2O, Ce(NO3)5 ⋅ 5H2O, and Al(NO3)3 ⋅ 9H2O served as the initial reactants. Preliminarily, we investigated the precipitation of the zirconium, cerium, and aluminum hydroxides. In order to deter mine the pH value of complete precipitation of the synthesis products, the initial solutions of the nitrate salts (~0.1 M) were titrated by an NH4OH aqueous solution (~1 M). It follows from the analysis of the pH curves of the precipitation (Fig. 1) that zirconium hydroxide precipitates at pH ~2.30, cerium hydroxide precipitates at pH ~7.00, and aluminum hydroxide precipitates at pH ~9.25. It should be noted that the pH values of the precipitation of aluminum hydroxide lie in a very narrow range and a small increase in the above value leads to the dissolution of aluminum hydroxide, which results in the disturbance of the specified oxide ratio. Therefore, it is necessary to con stantly maintain the value pH 9.25 when precipitating hydroxides in the ZrO2–CeO2–Al2O3 system. Similar results were obtained by Kadoshnikova [17]. The following optimum conditions for the coprecipi tation of the components in the ZrO2–CeO2–Al2O3 sys tem were chosen experimentally: (i) it is necessary to use dilute solutions of salts and a precipitant (~0.1 M), (ii) the NH4Cl compound should be added to the precipitant solution in order to maintain the required level of the pH value (~9.25), (iii) the precipitation should be performed with stirring at the rate Vprec = 0.02 cm3/s in the tempera ture range from –6 to 0°C, and (iv) the residence time of the precipitates in the mother solution after the completion of the precipita tion should be as short as possible in order to deceler ate the growth of agglomerates. With the aim of reducing the degree of agglomera tion of particles of the coprecipitated product, at the second stage of synthesis, the gellike precipitates were subjected to ultrasound treatment (in a Sapfir ultra sound bath). The propagation of ultrasonic waves through the coprecipitated gel is accompanied by the generation of cavitation bubbles, which with an increase in the pressure implode with the release of a
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Fig. 1. Particle distribution over sizes d for the coprecipi tated powder (composition no. 2) in the ZrO2–CeO2– Al2O3 system. The particle distribution for composition no. 3 is similar.
large amount of the energy. This energy is spent for destroying agglomerate chains in the gellike precipi tate [18], which leads to an increase in the dispersity of the coprecipitation product. The destruction of agglomerates is also favored by local heating during the ultrasound treatment. Then, the disagglomerated gels were frozen at a temperature of –25°C for 20 h, which allowed us to perform a deep dehydration of the gels and to retain unstable nanostates of coprecipitated powders [19]. The freezing of the gel leads not only to the removal of adsorption and crystallization water but also to its solidification with a maximum rate; as a result, a high chemical homogeneity inherent in the prepared coprecipitation product is retained in the solid phase. The gels thus prepared were defrozen and dried at a temperature of 100°C. The coprecipitated powders of the specified com positions (nos. 1–3) were investigated by different methods and used to prepare ceramic materials. Methods of Investigation The ranges of pH values of the precipitation of the hydroxides were determined using pH monitoring (150 M pH meter). The size of agglomerates of the coprecipitated hydroxides was determined using sedimentation anal ysis on a HORIBA LB550 laser analyzer. The specific surface area of the powders was mea sured by the lowtemperature nitrogen adsorption method (Autosorb1 instrument). 2010
method at a frequency of 1000 Hz in the temperature range 200–800 K. The electrical conductivity of the ceramic materials was calculated according to the for mula σ = (1/R) (l/S) (S cm–1), where S is the surface area of the sample (cm2) and l is the distance between electrodes (cm). The electronic and ionic components of the elec trical conductivity were separated by the polarization method [22]. The microstructure of the sintered samples was examined on an EM125 electron microscope.
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Fig. 2. Xray diffraction patterns of the coprecipitated powders in the ZrO2–CeO2–Al2O3 system after calcina tion at 350°C (1) without freezing of the coprecipitated gel and (2) after freezing of the coprecipitated gel at a temper ature of –20°.
The thermal processes occurring in coprecipitated powders during heating were investigated using differ ential thermal analysis (DTA) on a MOM Q1000 derivatograph in the temperature range 20–800°C. The Xray powder diffraction analysis was performed on a DRON3 diffractometer. The measurements were carried out in the angle range 2θ = 15°–65° at room temperature in air. The Xray diffraction patterns were identified with invoking the database of the ASTM internal standards. The isothermal treatment of the powders was car ried out in a Nabertern furnace with the program con trol in the temperature range 400–1200°C and in a Globarheater furnace at temperatures above 1200°C. The time of isothermal holding was equal to 1.0 h for calcination of the powders and 1.5 h for sintering of the ceramic materials. The heating rate was varied from 450 to 600°C/h depending on the calcination temperature. The open porosity of the ceramic samples was determined according to the technique described in GOST (State Standard) 473.481 [20]. The linear thermal expansion coefficients were cal culated using the method described in [21]. The ultimate strengths of the ceramic materials under threepoint bending were determined according to the technique described in GOST (State Standard) 473.881 [20]. The electrical resistance of the ceramic samples (R, Ω) was measured by the ac twopoint probe
The coprecipitated gels after their ultrasound treat ment and freezing at –20°C were studied using sedi mentation analysis. It was revealed that the dominant size of agglomerates in the prepared powders of com position nos. 2 and 3 lies in a rather narrow range from 30 to 50 nm (Fig. 1). The specific surface area of the powders is equal to 75 m2/g for composition no. 1, 84 m2/g for composition no. 2, and 90 m2/g for com position no. 3. According to the Xray powder diffraction data, the onset of the crystallization of the Xray amorphous powders formed after freezing of the coprecipitated gels (composition nos. 2, 3) is observed at a tempera ture of 350°C (Fig. 2). In the Xray diffraction pat terns, there appear lowintensity reflections corre sponding to the metastable form of the cubic solid solution based on zirconia. This is an indirect proof that the interaction of the coprecipitated hydroxides in the system begins already at the stage of the formation of the Xray amorphous phase. The coprecipitated powders that were not frozen are Xray amorphous at 350°C. Figures 3a and 3b show the DTA and thermogravi metric (TG) curves of the coprecipitated gel (compo sition no. 2) before and after its treatment at a temper ature of –20°C. The data for composition no. 3 are similar. The thermogram in Fig. 3a does not contain thermal effects that accompany the crystallization processes for individual oxides from the formed hydrated compounds after their precipitation. This fact indicates that the zirconium, cerium, and alumi num hydroxides begin to interact already at the copre cipitation stage. It also follows from Fig. 3 that the freezing of the coprecipitated gels leads to a consider able decrease in the H2O content in amorphous hydroxide crystal hydrates as compared to the unfro zen gel (see the TG curves). The shift in the extremum of the endothermic effect characterizing the dehydra tion process toward the lowtemperature range (175°C 155°C) is explained by the weakening of the intermolecular interaction forces between parti cles and the increase in the dispersity of the coprecip itated powder, which, in turn, significantly facilitates the dehydration of the coprecipitated gel. The exo
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Fig. 3. Results of thermal analysis (DTA and TG curves) for the coprecipitated powders in the ZrO2–CeO2–Al2O3 system (a) without freezing of the coprecipitated gel and (b) after freezing of the coprecipitated gel at a temperature of –20°.
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thermic effects in the DTA curves correspond to the crystallization of the solid solution based on zirconia. According to the Xray powder diffraction data, the formed phase has a pseudocubic structure (c'ZrO2) (Fig. 4). The DTA results also allow us to argue that the freezing of the coprecipitated gel to –20°C leads to a decrease in the crystallization temperature of the c'ZrO2 phase (450°C 435°C). The sizes of crystal grains of the c'ZrO2 phase in the compositions under investigation according to the calculation from the Scherrer formula [7] are approximately equal to 3 nm.
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Fig. 4. Sequence of the formation of the tetragonal solid solution in the ZrO2–CeO2–Al2O3 system: (1) 450, (2) 800, and (3) 1300°C.
In the temperature range 500–600°C, the solid solutions with the pseudocubic structure (c'ZrO2) transform into the tetragonal modification (tZrO2), which is evidenced by the splitting of the Xray diffrac tion maximum in the angle range 2θ = 34°–36° (Fig. 5). An increase in the temperature to 1400°C leads to a perfection of the tetragonal structure of the solid solution based on zirconia (Fig. 4). In the Xray diffraction patterns of the powders of composition nos. 2 and 3, the αAl2O3 phase does not manifest itself, which is most likely associated with the partial dissolution of αAl2O3 in the (ZrO2)0.87(CeO2)0.13 solid solution. This is favored by the small radius of Al3+ ions (0.57 Å) as compared to the radii of Zr4+ (0.82 Å), Ce3+ (1.02 Å), and Ce4+ (0.88 Å) ions.
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Fig. 5. Fragments of the Xray diffraction patterns of the coprecipitated powders in the ZrO2–CeO2–Al2O3 system after calcination at temperatures of (1) 500, (2) 550, and (3) 600°C. 2010
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Table 1. Changes in the average size d of crystals of the tZrO2 phase in the nanoceramic materials based on the ZrO2–CeO2–Al2O3 system (composition nos. 1–3) in the temperature range 400–1400°C T, °C
Crystal size d (nm) for composition no.
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22 12 7
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58 28 19
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solution based on zirconia in the ZrO2–CeO2–Al2O3 system are listed in Table 1. The data presented in the table indicate that, in the materials of composition nos. 2 and 3 containing Al2O3, the grains grow slowly and their size at 1200°C does not exceed 22 nm. In the temperature range 1200–1400°C, the growth of grains accelerates and their size increases to 40 nm. Compared to the (ZrO2)0.87(CeO2)0.13 solid solution, the average size of grains of the tZrO2 phase in composition nos. 2 and 3 is considerably smaller. Most likely, this is associated with the fact that the incorporation of the Al2O3 oxide into the crystal lattice of the solid solution based on zirconia favors a decrease in the average size of grains and retards their growth. The nanocrystalline powders formed in the ZrO2– CeO2–Al2O3 system (composition nos. 1–3 after the heat treatment at 600°C were pressed at a pressure of 100 MPa and sintered at a temperature of 1400°C (for 1.5 h). The average grain size in the sintered samples (composition nos. 1–3) was somewhat larger than that in the powdered samples. Most likely, this is associated with the increase in the time of isothermal holding to 1.5 h, which favors a more active occurrence of recrys tallization processes. The shrinkage of the sintered samples was approximately equal to 20%, and their density amounted to 90–92% of the theoretical den sity. The sintering of the nanoceramic materials under the aforementioned temperature conditions made it possible to prepare the samples with a minimum porosity. The properties of the nanocrystalline ceramic materials in the ZrO2–CeO2–Al2O3 system are sum marized in Table 2 (where d is the average grain size, P is the open porosity, α is the thermal expansion coef Table 2. Properties of the ceramic materials in the ZrO2–CeO2–Al2O3 system (sintering at 1400°C for 1.5 h) Ceramic composition no.
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