Dilatometric Characteristics of Weakly Sintered Ceramics published by Ural Federal University eISSN 2411-1414; chimicatechnoacta.ru ARTICLE 2022, vol. 9(4), No. 20229412 DOI: 10.15826/chimtech.2022.9.4.12 1 of 6 Dilatometric characteristics of weakly sintered ceramics Yury I. Komolikov a, Larisa V. Ermakova b* , Vladimir R. Khrustov c, Victor D. Zhuravlev b a: M.N. Mikheev Institute of Metal Physics, Ural Branch of the Russian Academy of Sciences, Ekaterin- burg 620108, Russia b: Institute of Solid State Chemistry, Ural Branch of the Russian Academy of Sciences, Ekaterinburg 620990, Russia c: Institute of Electrophysics, Ural Branch of the Russian Academy of Sciences, Ekaterinburg 620016, Russia * Corresponding author: larisaer@ihim.uran.ru This paper belongs to a Regular Issue. © 2022, the Authors. This article is published in open access under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). Abstract Thermal expansion of refractory ceramics CaZrO3, MgAl2O4, La2Zr2O7 and YSZ-12 was studied. The samples of the complex oxides were syn- thesized by solution combustion synthesis with glycine; the fuel:oxi- dant ratio was varied depending on the character of redox reaction. The linear thermal expansion coefficient (LTEC) of ceramics was measured on the samples with an initial density 23–52%. The maximal sinterability of 89–92% after 6 h annealing at 1550 °С was demon- strated by La2Zr2O7 and YSZ-12, and the minimal values (78–82%) – by CaZrO3 and MgAl2O4. All materials have close LTEC values, from 9.0 to 9.6·10–6 K–1. Keywords refractory oxide weakly sintered ceramics linear thermal expansion coefficient thermal expansion ceramic density Received: 29.06.22 Revised: 25.08.22 Accepted: 01.09.22 Available online: 13.09.22 1. Introduction Thermal stability of ceramic materials based on zirconium and/or aluminum oxides makes it possible to use them as functional ceramics and thermal barrier coatings at high temperatures [1–5]. However, under the action of high tem- peratures, especially during abrupt heating or cooling, inter- nal stresses appear in the material – usually compression stresses, tensile stresses of cut, and more rarely bending stresses. As a result, cracks are formed in ceramics or coat- ings leading to their destruction. The number and the value of stresses depend on the elastic properties and thermal ex- pansion of anisotropic phases and/or crystals in ceramics [6, 7]. For thermal barrier coatings (TBC) it is particularly important to determine the linear thermal-expansion coeffi- cient (LTEC), since for providing good contact between lay- ers of gradient coating it is necessary to select compositions with close, gradually varying LTEC values. This prevents cracking or exfoliation of coating from the substrate in the process of thermal cycling [8–10]. The most widespread methods of TBC application are: 1) kinds of plasma spraying: Atmospheric Plasma Spraying (APS), Low-pressure Plasma Spray (LPPS), Solution Precur- sor Plasma Sprayed (SPPS); 2) Chemical Vapor Deposition (CVD); 3) Chemical Gas-dynamic Spraying (CGDS) [11–15]. Of much interest is the sol-gel procedure allowing one to apply coatings on complex-shaped articles (as distinct from APS and Electron Beam Vacuum Plasma Deposition (EB- PVD)), as well as multilayer coatings. Besides, its cost is comparable with the cost of production of coatings by the APS method [16, 17]. The principle of this method consists in the formation of a coating from a deposit as a result of hydrolysis (pyrolysis, thermolysis, combustion of polymer-salt composition, xero- gel). The temperature of coating formation from oxide pow- der depends on the temperature at which initial salts and components decompose in this technique. Then the coating is heat treated and sintered at chosen temperatures. The key controlled parameter affecting the characteristics (po- rosity, homogeneity, thickness) and quality of coating is the rate of oxide coating formation from gel (xerogel). However, the initial density of coating in these technol- ogies is much lower than that for materials produced with the use of plasma methods. As a result, such processes as formation of crystal phases, coarsening and sintering of crystallites, inevitable emergence of structural defects, cracking, etc., take place in coatings during the high-tem- perature treatment. For successful development of this coating method it is necessary to study the behavior of low- density and porous ceramics during sintering. Therefore, the aim of this work is to perform dilatometric studies of weakly sintered refractory materials CaZrO3, MgAl2O4, http://chimicatechnoacta.ru/ https://doi.org/10.15826/chimtech.2022.9.4.12 mailto:larisaer@ihim.uran.ru http://creativecommons.org/licenses/by/4.0/ https://orcid.org/0000-0001-7839-1441 http://orcid.org/0000-0001-5933-4310 https://crossmark.crossref.org/dialog/?doi=https://doi.org/10.15826/chimtech.2022.9.4.12&domain=pdf&date_stamp=2022-9-13 Chimica Techno Acta 2022, vol. 9(4), No. 20229412 ARTICLE 2 of 6 La2Zr2O7 and ZrO2-12 produced by Solution Combustion Synthesis (SCS) [18–21]. 2. Experimental 2.1. Starting materials Zirconium and calcium carbonates, magnesium, lantha- num and yttrium oxides, aluminum nitrate nonahydrate, nitric acid, glycine were used as initial reagents. Metal carbonates and oxides were preliminarily dissolved in nitric acid. The resulting solutions were mixed in the re- quired stoichiometric amount, and powders of CaZrO3, MgAl2O4, La2Zr2O7 and ZrO2-12 mol.% Y2O3 were pro- duced by Solution Combustion Synthesis. The powders were prepared in a wide aluminum reactor with a capac- ity of 3 dm3. To reduce heat removal, the reactor had as- bestos sheet insulation. Other conditions for the produc- tion of samples are given below. The sample of CaZrO3 prepared at the Donetsk plant of chemical reagents (DPCR) was used as a reference. 2.2. Preparation of CaZrO3 CaZrO3 was synthesized at  = 1.0 in accordance with equa- tion 1, i.e. stoichiometric ratio of fuel and oxidizer, which ensures the maximum speed and temperature of the SCS process [22]: ZrO(NO3)2 + Ca(NO3)2 + 2.22NH2CH2COOH → CaZrO3 + 3.59N2 + 4.44CO2 + 7.97H2O (1) SCS reaction under stoichiometric conditions ( = 1.0) leads to complete oxidation of organic fuel (glycine), the precursors are white in color (Figure 1а). The produced CaZrO3 powders were mixed, placed in corundum crucibles and annealed in a muffle furnace at a temperature of 1100 оС for 5 h to complete the crystallization. Upon anneal- ing, the product decreased in volume by 5–10%. 2.3. Preparation of La2Zr2O7 Synthesis was performed from a solution of lanthanum and zirconyl nitrates containing 160 g/dm3 of precursors ex- pressed as La2Zr2O7 with glycine ( = 1.0) according to equation 2: 2La(NO3)3 + 2ZrO(NO3)2 + 5.55NH2CH2COOH → La2Zr2O7 + 11.11CO2 + 13.875H2O + 7.775N2 (2) Due to a large amount of gases evolved during the reac- tion, the precursor takes the shape of foam or whipped cream, which decreases the degree of particle sintering. The precursor was mixed and annealed in corundum cruci- bles for 5 h at 900 °C. The annealed product was averaged by way of short-term agitation. 2.4. Preparation of MgAl2O4 Aluminum-magnesium spinel was synthesized by equation 3 in oxidative regime ( = 0.6) [22] from a solution of mag- nesium and aluminum nitrates containing 67 g/dm3 or pre- cursors expressed as MgAl2O4. Figure 1 Appearance of CaZrO3 (а) and La2Zr2O7 (b) powders after SCS ( = 1.0). Mg(NO3)2 + 2Al(NO3)3 + 2.67NH2CH2COOH → MgAl2O4 + 3.2N2 + 3.27NO2 + 6.675H2O (3) This synthesis regime was chosen to reduce the rate of combustion and prevent the removal of the material outside the reactor. Upon completion of combustion, a downy pow- der of light brown color was formed. The precursor was an- nealed at 850 °С for 5 h. The product annealed at 850 оС was ground and additionally annealed at 900 °С for 5 h. 2.5. Preparation of YSZ-12 The powder was synthesized by reaction 4 with a consider- able excess of glycine ( = 1.52) by calcining the solution of yttrium and zirconyl nitrates containing 128.5 g/dm3 of pre- cursors expressed as YSZ-12. 0.88ZrO(NO3)2 + 0.24Y(NO3)3 + 2.07NH2CH2COOH → (ZrO2)0.88(Y2O3)0.12 + 2.275N2 + 10.35H2O + (4.14–x–y)СO2 + xCO + yC (4) Since the SCS reaction occurs in the reduction combus- tion regime, the produced downy and voluminous powder has a gray-brown color due to carbon impurity remaining in the synthesis product as a result of incomplete oxidation of carbon atoms in glycine. The content of unburnt carbon was not estimated; that is why the quantities of the carbon- containing components in equation 4 are given as variables x and y. Upon annealing in air at 900 °С, the powder be- came white. In order to complete the reaction of synthesis, the material was ground and annealed for 5 h at 1100 °С with subsequent grinding. Chimica Techno Acta 2022, vol. 9(4), No. 20229412 ARTICLE 3 of 6 2.6. Sample preparation procedure for measure- ment of LTEC For the preparation of molding material, each produced powder with a preassigned composition was further me- chanically activated with addition of PVC as a binder [23]. The samples for measurements were molded by semidry uniaxial static pressing under a pressure of 160 MPa and preliminarily sintered at 1000 °C. The ceramics for investi- gation was represented by cylinders of d = 4.5±0.2 mm in diameter and l = 10.0±0.2 mm in length. The opposite faces of the cylinders were made plane-parallel, the distance be- tween faces (l0) was fixed before the experiment. 2.7. Research methods The X-ray diffraction studied were performed on a Shi- madzu XRD-7000 X-ray diffractometer in Cu Kα radiation (λ = 1.5456 Å) in the 2θ angle interval from 10 to 70° in stepwise scanning mode with ∆(2θ) = 0.03° and exposition of 3 s. The determination of the phase composition, struc- ture refinement and coherent scattering region (CSR) defi- nition were carried out using the ICDD and ICSD files and WINXPOW and POWDER CELL 2.4 programs. The structural and morphological characteristics were studied on a JEOL JSM 6390 LA scanning electron micro- scope. The specific surface of the powders was determined by the BET method (Tri Star 3000V6.03A) from thermal de- sorption of nitrogen. The density of the sintered ceramics was determined by hydrostatic weighing in alcohol on a Shumadzu AUW–220 D balance equipped with a special at- tachment. The linear thermal expansion of ceramic samples was studied on a Dilatometer DIL 402 C (NETZSCH, Germany). Heating to 1550 °C was carried out with a constant rate of 5 °C/min; simultaneously, air blowing at a rate of 100 cm3/h provided constant atmosphere and a uniform temperature field. The samples were cooled with the fur- nace to room temperature. The results of dilatometric stud- ies were used to calculate the integral (average) LTEC (αav) in the temperature interval (T1–T0) (equation 5): 𝛼𝑎𝑣 = 1 𝐿0 ∙ (𝐿1 − 𝐿0) (𝑇1 − 𝑇0) , (5) where L0, L1 are initial and final length of the sample; T0, T1 are initial and final temperature, respectively. The dimen- sional accuracy (ΔL) was 0.133%; it was determined by the nonlinearity of the dilatometer displacement meter trans- fer function. 3. Results and discussion In combustion reactions, fine-dispersed oxide precursor is formed as a result of decomposition of metalorganic com- plexes [23]. Generous and rapid gas liberation promotes the formation of high-porous, chemically non-equilibrium pow- ders with a small bulk weight and low density. Usually, these are bulk powders composed of aggregates of nano- and microparticles (Figures 2, 3). Aggregates of powders, for example CaZrO3, practically do not sinter at tempera- tures up to 900–1000 °C without preliminary grinding. Af- ter mechanical activation (long-term grinding), the original CaZrO3 aggregates are partially destroyed and form more dense, albeit more fine, in comparison with the powders obtained by the solid-phase method (Figure 2). The morphology of aluminum-magnesium spinel upon annealing at 900 oC differs from other materials and is represented by denser particle aggregates, although less dense than for CaZrO3 produced by the solid-phase method (Figure 4). All samples after SCS are mixed-phase materials, and the morphological features of the powders allow their ac- tive sintering to be carried out at lower temperatures. In the process of high-temperature annealing, the samples be- come single-phase. The crystal-chemical characteristics of the examined materials are listed in Table 1. The densities of the samples after sintering at 1550 °C for 2 h (dilatometer) and 6 h (muffle furnace) are shown in Table 2. In the former case, sintering was performed in a dilatometer with measurement of the sample length var- iation, in the latter case – in a chamber furnace; the heat- ing in all cases was carried out at a rate of 5 °С/min. The obtained results indicate that the density of the samples is practically independent of the holding time of ceramics at 1550 °C. Figure 2 Morphology of CaZrO3 powder produced by SCS method (а) and at DPCR (b). Chimica Techno Acta 2022, vol. 9(4), No. 20229412 ARTICLE 4 of 6 Figure 3 Morphology of La2Zr2O7 (а) and YSZ-12 powders (b). Table 1 Physicochemical characteristics of powders. Sample Structure Phase composition Unit cell parameters, Å CSR, nm CaZrO3 Pbnm Traces of ZrO2 cub. 5.753±0.001 8.011±0.001 5.589±0.001 114±1 CaZrO3 (DPCR) Pbnm CaZrO3 68.9% ZrO2 cub. 5.1% CaCO3 26.0% – – MgAl2O4 Fd–3m 100% 8.083±0.001 18±1 La2Zr2O7 Fd–3m 100% 10.763±0.001 21±1 YSZ-12 Fm–3m 100% 5.145±0.001 90±1 Note that the initial density of MgAl2O4 and La2Zr2O7 samples did not exceed 30% of the theoretical one. Upon 2 h annealing at 1550 °С both CaZrO3 samples exhibited moderately high density values, 76–77%; approximately the same density values were observed for the MgAl2O4 sample (Table 2). The sample of La2Zr2O7 ceramic had the maximal value of density, 98%. Three-fold enhancement of the annealing time, from two to six hours, did not lead to any considerable increase in density (Table 2). This effect is likely to be due to stronger sintering inside powder aggregates of these mate- rials and the absence of diffusion processes between parti- cle aggregates [24]. Dilatometric analysis of thermal shrinkage makes it pos- sible to trace the regularities and specific features of the examined compacts. Figure 5 displays the temperature de- pendences of shrinkage during heating to 1550 °С at a rate of 5 °С/min, and Figure 6 – the time dependences allowing one to estimate the effect of isothermal exposure on shrink- age at 1550 °С. From Figure 5 it is seen that all materials begin to sinter actively at a temperature slightly above 1100 °С. The excep- tion is provided by CaZrO3 samples of both types (curves 1 and 2). They are characterized by a higher initial sintering temperature, about 1200 °С. Probably, in this way the effect of preliminary annealing of powders performed at elevated temperature manifests itself, leading to coarsening and densification of agglomerates. The maximal shrinkage rate is typical of lanthanum zirconate and magnesium alumi- nate. The degree of shrinkage of these materials exceeds the limits of the instrument operating range (50%). Therefore, the above-mentioned curves are extrapolated up to the maximal temperature of the experiment. Figure 4 Morphology of MgAl2O4 powder upon annealing at 900 °С. Figure 5 Dilatometric curves during heating at a constant rate of 5 °С/min with isothermal exposure of 2 h at 1550 °С. 1 – CaZrO3 (SCS); 2 – CaZrO3 (DPCR); 3 – YSZ-12; 4 – MgAl2O4; 5 – La2Zr2O7. Chimica Techno Acta 2022, vol. 9(4), No. 20229412 ARTICLE 5 of 6 Table 2 Theoretical, ρ(theor), initial, ρ(init), densities of samples after pressing and preliminary sintering at 1000 °С, and final, ρ(1550), density of samples after annealing at 1550 °С and LTEC values. Sample Material ρ(theor), g/сm 3 ρ(init), g/сm 3 / (%) ρ(1550), g/сm 3 / (%) LTEC, 10–6 K–1 2 h 6 h 1 CaZrO3 CS 4.78 2.495/(52) 3.626/(76) 3.713/(78) 9.2 2 CaZrO3, DPCR 4.78 2.418/(51) 3.693/(77) 3.781/(81) 9.2 3 YSZ-12 5.90 2.585/(44) 5.378/(91) 5.24/(89) 9.0 4 MgAl2O4 4.10 0.987/(29) 2.785/(80) 2.833/(82) 9.6 5 La2Zr2O7 6.06 1.397/(23) 5.915/(98) 5.924/(98) 9.3 Figure 6 A time dependences of the relative linear shrinkage (L–L0)/L0 in the heating mode at a rate of 5 °C/min to a tempera- ture of 1550 °C, followed by holding for 2 hours: 1 – CaZrO3 (SCS); 2 – CaZrO3 (DPCR); 3 – YSZ-12; 4 – MgAl2O4; 5 – La2Zr2O7. The change in the shrinkage of materials with the time of isothermal holding at 1550 °C can be traced from the curves in Figure 6. The nonzero slope of the dilatometric curves during isothermal exposure in the range of 304– 424 min indicates that the sintering process of the three materials CaZrO3 (CS), CaZrO3 (DPCR), and YSZ-12 is far from completion. An increase in the annealing time did not give a noticeable result: the final density of the three ma- terials was less than 80%, and the density of the YSZ-12 sample even decreased (Table 2). This effect can be asso- ciated with significant agglomeration of powders and their internal sintering, since in this case closed pores are formed that prevent compaction [24]. Based on the fact that very loose aggregates of materials with a low initial density were obtained in combustion re- actions, the reasons for differences in density after sinter- ing can be (in descending order of probability): 1) the for- mation of closed porosity; 2) the strength of the formed ag- gregates of complex oxides; 3) different melting points of the compounds. Possible ways to solve the problem of ob- taining denser ceramics may include: 1) lowering the SCS temperature; 2) lowering the degree of agglomeration of powders after synthesis; 3) annealing at a temperature be- low 1000 °C to avoid reaching chemical equilibrium so as to obtain multiple-phase powders. 4. Conclusions The effect of weak preliminary sintering (to 23–52% den- sity) on the production and subsequent sintering of dense oxide ceramics CaZrO3, YSZ-12, MgAl2O4 and La2Zr2O7 was studied. It was established that weakly sintered ceramics MgAl2O4 and La2Zr2O7 demonstrate continuous shrinkage during annealing in the region of 1100–1550 °С reaching 98% of the theoretical density of ceramics. Preliminary an- nealing at 1100 °С adversely affects the degree of shrinkage of CaZrO3 and YSZ-12 compacts leading to coarsening and densification of particle agglomerates. Supplementary materials No supplementary materials are available. Funding This study was carried out in the framework of the state assignment for the Institute of Solid State Chemistry UB RAS (No. АААА-А19-119031890026-6), Mikheev Institute of Metal Physics UB RAS (No. АААА-А18-118020690196-3) and the Institute of Electrophysics UB RAS (No. 122011200363-9). Acknowledgments None. Author contributions Conceptualization: Yu.I.K., V.D.Zh. Data curation: V.D.Zh. Formal Analysis: Yu.I.K, V.R.Kh., L.V.E. Funding acquisition: V.D.Zh. Investigation: V.R.Kh., L.V.E. Methodology: Yu.I.K., V.R.Kh., L.V.E. Project administration: V.D.Zh. Resources: Yu.I.K, V.R.Kh., L.V.E., V.D.Zh. Software: L.V.E. Supervision: V.D.Zh. Validation: Yu.I.K., V.D.Zh. Visualization: V.R.Kh., Yu.I.K. Writing – original draft: Yu.I.K., V.D.Zh. Writing – review & editing: V.D.Zh., L.V.E. Chimica Techno Acta 2022, vol. 9(4), No. 20229412 ARTICLE 6 of 6 Conflict of interest The authors declare no conflict of interest. Additional information Author IDs: Yury I. 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