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The influence of condition of the monazite structured Lao.9Yo.1PO4 nanocrystals sintering on thermal and mechanical properties of the material
M. O. Enikeeva1'2 *, O. V. Proskurina1'2, E. S. Motaylo2, D. P. Danilovich2, V. V. Gusarov1 1Ioffe Institute, 26, Politekhnicheskaya St., 194021, St. Petersburg, Russia 2Saint Petersburg State Institute of Technology, 26, Moskovsky Ave., 190013, St. Petersburg, Russia
DOI 10.17586/2220-8054-2021-12-6-799-807
A lanthanum orthophosphate- and yttrium-based monazite structured Lao.9 Y0.1PO4 nanocrystalline material was synthesized. The influence of the thermal treatment temperature on its thermal and mechanical properties has been determined. It was shown that the hydrothermal treatment of coprecipitated phosphates at 230 °Cfor 110 minutes yielded monazite structured nanoparticles with the crystallite size of about 16 nm. Nanopowder thermal treatment for 20 min in the annealing-quenching mode at 1000-1400 °C resulted in obtaining compact materials, the porosity of which varied from 52 to 27% with the grain growth from ~20 to ~100 nm. Active sintering occurred at temperatures above 1300 °C. Nanocrystalline materials obtained by heat treatment at 1200 °C with a grain size of ~40 nmhave a porosity of 42%, microhardness of Hv (25 °C) = 4±0.5 GPa, thermal conductivity A(25 °C) = 0.4 W-m-1 •K-1, and can be used as thermal insulation material.
Keywords: hydrothermal treatment, lanthanum and yttrium orthophosphates, thermal diffusivity, specific heat capacity, thermal conductivity.
Received: 25 November 2021 Revised: 6 December 2021
1. Introduction
Many modern functional materials are produced from complex oxide compounds, e.g. ferrites, tungstates, phosphates and others [1-5]. Various branches of modern industry require materials with thermal insulation and barrier properties [6-9]. In this regard, rare earth element (RE) orthophosphates with monazite and zircon type structures are promising [10-14]. Monazites structured RE orthophosphates have a high melting temperature (Tm ^2173-2345 K) [15,16], low water solubility [17], high isomorphic capacity [18-21], resistance to aggressive media [22] and radiation damage [23-25]. The physicochemical properties of RE orthophosphates determine the great interest in the study of materials based on them. The mechanical and thermal properties influence the long-term stability, thermal insulation properties and integrity of ceramic products.
Functional materials are obtained on the basis of individual RE orthophosphates, solid solutions, including nanocrystalline ones [26-35]. RE orthophosphate nanoparticles are of scientific interest to researchers, both from the fundamental and from practical points of view. A thorough study of the mechanisms of formation and stability of RE orthophosphate phases will help broaden their potential application in modern technology. At present, the obtaining of monazite REPO4 (RE= La - Gd) based materials are aimed at producing composites for the radioactive waste form [21,36-39], thermal barrier coatings [11,12], and for the manufacture of luminescent materials [26,40].
Pre-ceramic powders synthesis is an important stage for obtaining functional materials. The hydrothermal treatment synthesis makes it possible to obtain monazite structured single-phase nanopowders without their subsequent thermal treatment [41-43]. Under hydrothermal conditions, the process of structural transformation of the rhabdo-phane structured La(Y)PO4 • nH2O solid solutions into monazite structured La(Y)PO4 solid solutions occurs at lower temperatures (T «180 °C) than during aerobic calcination (T >500 °C) [21,44,45]. Besides, hydrothermal conditions ensure the obtaining of nanocrystalline powders and their greater uniformity in size and composition.
In the literature sources, one can find studies on the obtaining of non-porous macrocrystalline materials based on phases with monazite- or zircon-type structures by high-temperature calcination of rhabdophane structured phases [20, 36, 38]. The obtaining of nanocrystalline powders under hydrothermal conditions and subsequent sintering of the pressed samples in the annealing-quenching mode make it possible to produce porous nanocrystalline materials. Porous ceramics based on monazite structured phases can be considered as a thermal insulating material, since it can potentially have low thermal conductivity.
Despite numerous papers on the synthesis and study of the properties of materials based on monazite structured phosphates [5,16,46-48], there are practically no studies concerning nature-like nanocrystalline single-phase materials with yttrium or other RE additives.
Due to the above reasons, it is urgent to study the effects that the sintering temperature of monazite structured Lao.9Yo.1PO4 nanocrystalline pre-ceramic powders have on the thermal and mechanical properties of nature-like nanomaterials obtained from them.
2. Experimental
Monazite structured La0 9Y01PO4 polycrystalline pre-ceramic nanopowders were synthesized in two stages. At the first stage, a solution of ammonium dihydrogen phosphate NH4H2PO4 was added to a solution of lanthanum nitrate hexahydrate La(NO3)3 6H2O (puriss.) and yttrium nitrate hexahydrate Y(NO3)36H2O (puriss.) with constant stirring. The ratio of the reagent solutions was selected to ensure the stoichiometry specified for the reaction products. The resulting suspension with pH=1 was stirred for 15 minutes and placed in a Teflon liner of a steel autoclave (at 0.7 autoclave capacity). The autoclave was heated to 230 °C inside the furnace at ^10 MPa inside the autoclave. The autoclave was kept in the furnace for 110 minutes at this temperature. After heat treatment, the autoclave was taken out from the furnace and naturally cooled down to room temperature. The solid phase was separated from the solution by decantation and rinsed with distilled water. The powder was dried at 85 °C for 48 hours.
The powder was molded into cylindrical pellets with an average diameter of 15 mm and a thickness of about 1.5 mm by uniaxial dry pressing at 4 MPa.
Sample sintering was carried out in the annealing-quenching mode as follows: the pressed tablets were placed in a heated furnace in a platinum crucible: sample 1 at 1000 °C, sample 2 at 1100 °C, sample 3 at 1200 °C, sample 4 at 1300 °C, and sample 5 at 1400 °C for 20 min for all samples. Then the samples were taken out and cooled down to room temperature (25 °C).
The X-ray phase analysis of the samples employed powder diffractograms recorded on a Rigaku SmartLab 3 powder diffractometer (CuKa emission) in the 20=10-80° angle range with a step of 0.01° and a scanning rate of 0.8°/min. The monazite phase was identified using the PDF 2 database. The average crystallite sizes were determined using the SmartLab Studio II software from Rigaku for all the peaks, using the Halder-Wagner method and taking into account the instrumental broadening of the instrument. The same SmartLab Studio II software was used to plot a lognormal crystallite size distribution for the monazite 020 peak.
The elemental composition and morphological features of powders and sintered samples were determined using a Tescan Vega 3 SBH scanning electron microscope (Tescan, Czech Republic) with an x-act detector Oxford Instruments (Oxford Instruments, United Kingdom) attachment for the X-ray spectral microanalysis. Due to the overlapping of the energy lines of the yttrium La-series (1.922 keV) and the phosphorus Ka-series (2.013 keV), the analysis of the nanopowder composition was carried out in the range up to 20 keV in 4-5 different regions, and the resulting value was obtained by averaging the obtained data.
The samples microhardness was determined by a PMT-3 microhardness tester (LOMO, Russia) by pressing a tetrahedral diamond pyramid in (Vickers method) at a load of 200 g for 10 s. At least five measurements were done for each sample.
The thermal properties of the materials were determined by the laser flash method using the LFA 457 MicroFlash (Netzsch, Germany) in the 25-825 °C range.
The specific heat values of the monazite structured La0 9 Y0.1PO4-based materials (at 25-825 °C) were processed by the least squares method to obtain the coefficients a, b and c according to the Mayer-Kelly equation Cp = a + bT + cT-2 [49].
3. Results and discussion
The elemental analysis of the powder obtained by hydrothermal treatment at 230 °C for 110 minutes showed that the (La+Y):P atomic ratio is 48:52 (±1-2%), and the La:Y atomic ratio is 9:1 (±1-2%), i.e., it can be considered as corresponding to the La0 9Y01PO4 stoichiometry within the error of the method.
The X-ray diffraction data on the initial powder and cylindrical pellets sintered from it (samples 1-5) are shown in Fig. 1. All samples have a monazite structure (PDF 00-012-0283). The monazite phase unit cells in all the samples have practically the same parameters: a=6.8147±0.0025 A, b=7.0416±0.0050 A, c=6.4916±0.0050 A, V=303.32 A3, and Pcaic.=5.0104 g/cm3.
Figure 2 shows the dependence of the average crystallite size of the La0 9Y01PO4 solid solution on the ceramics sintering temperature. The average crystallite size of the La0 9Y01PO4 phase after hydrothermal treatment is about 16 nm. When sintering samples in the 1000-1200 °C range for 20 minutes (samples 1,2, and 3), the average crystallite size gradually increases to 40 nm. An increase in the sintering temperature to 1300-1400 °C (samples 4, 5) leads to a sharp increase in the crystallite size up to 100±10 nm. The observed dependence allows one to conclude that the process of sintering and grain growth activates in the 1200 °C< T <1300 °C range.
Fig. 1. X-ray diffraction patterns of the nanocrystalline powder (Powder) and samples after heat treatment at 1000 (1), 1100 (2), 1200 (3), 1300 (4), and 1400 0C (5)
Fig. 2. Dependence of the average crystallite size on the sintering temperature
Figure 3 shows the crystallites size distribution for the initial powder and sintered samples, determined from the profile of the 020 peak of the monazite phase. It follows from the obtained data that crystallites of the initial powder have a narrow size distribution. The thermal treatments at 1000 0 C and 1100 0 C (samples 1 and 2) lead to an insignificant broadening of the distribution, while the heat treatment at 1200 0C (sample 3) broadens the size distribution of crystallites more noticeably, and the size distribution broadens significantly at 1300 0C and 1400 0C (samples 4 and 5).
Figure 4 demonstrates images of chips and polished sections of the samples sintered at 1000-1400 0 C (specimens 1-5). The images of splinters (Fig. 4 a,c,e) and polished sections (Fig. 4 b,d,f) of samples 1-3 clearly show the fibrous microstructure of the samples formed by anisotropic nanoparticles of La0.9 Yo.iPO4 monazite. The images of splinter from samples 1 and 2 demonstrate a significant fraction of voids between the chaotically stacked monazite nanoparticle agglomerates. The topography of the polished sections from samples 1 and 2 (Fig. 4 b,d) has a similar appearance. Microstructural changes in the material begin at T ^1200 0C, the agglomerated fibers in its polished section (Fig. 4 f) are less pronounced than in the case of samples 1 and 2 (Fig. 4 b,d) obtained at lower temperatures.
The analysis of changes in the microstructure of materials after sintering, revealed by studies on a scanning electron microscope (SEM), and of the data on the crystallites growth during thermal treatment, allows a conclusion that
0 50 100 150 200 250 300
Crystallite size, nm
Fig. 3. Size distribution of crystallites of the initial powder (Powder) and samples after heat treatment at 1000 (1), 1100 (2), 1200 (3), 1300 (4), and 1400 0C (5)
Table 1. Characteristics of the La0.9 Yo.iPO4-based materials
Sample Sintering temperature, 0C Bulk density p, g/cm3 Porosity P, % A (T = 25°C), W-m-1 •K-1
Compacted powder - 2.242 55.3 -
1 1000 2.389 52.3 0.12±0.08
2 1100 2.767 44.8 0.42±0.08
3 1200 2.871 42.7 0.45±0.08
4 1300 3.627 27.6 1.35±0.08
5 1400 3.646 27.2 1.39±0.08
the activation of sintering and recrystallization processes lies in the temperature range of 1200-1300 0C. It should be noted that the obtained temperature range correlates well with the melting point value for the surface (nonautonomous) phase, calculated using the expression proposed in [50,51].
Fig. 4 g,i shows the microstructure of samples 4 and 5 obtained at temperatures >1300 0C, which differs from samples 1, 2, and 3 by the absence of a fibrous structure. Due to a sharp increase in the contact area of grain boundaries and their recrystallization, pore formation occurs in the samples. In the image of the material obtained at 1300 0C (Fig. 4 h), the pore size varies in the range from 50 nm to several ^m, which confirms active sintering of this material. An increase in the sintering temperature to up to 1400 0C leads to an increase in the pore size up to 3-5 ^m (Fig. 4 j).
Table 1 presents the values of the total porosity and apparent density of the samples (powder and samples 1-5). As a result of dry uniaxial pressing of the powder at 4 MPa, the resulting material has a total porosity of P = 55% before the high-temperature treatment. Sintering at temperatures <1200 0 C reduces the total porosity of the samples down to P = 42%. The obtained values of the total porosity are consistent with the qualitative SEM data (Fig. 4 a-j) for samples 1-3, which show a large fraction of voids between the agglomerated La0.9 Y0.1 PO4 monazite nanoparticles. As a result of an increase in the sintering temperature up to 1300 and 1400 0 C (samples 4, 5), the material compactness increases to an apparent density of 3.65 g/cm3, and the total porosity decreases down to P ^27%.
Figure 5 shows the dependence of microhardness (Hv) at 25 0C on total porosity (P) of the samples (Table 1). A decrease in the total porosity of the material from 52% to 27%, which leads to an increase in Hv from 5±0.5 GPa to 17±0.5 GPa, shows that intense sintering and an increase in the hardness of porous materials based on a monazite structured La0.9 Y01PO4 solid solution occurs at temperatures >1300 0C. The obtained porous nanocrystalline materials have lower microhardness values than the known lanthanum orthophosphate-based monazite ceramics [46]. This is due to the difference in the microstructure and functional purpose of these materials. Large porosity and small microhardness values of the synthesized materials are due to the fact that this work was aimed at studying the possibility
Fig. 4. SEM micrographs of samples heat treated at temperatures 1000 (1), 1100 (2), 1200 (3), 1300 (4) and 1400 0C (5): (a,c,e,g,i) splinters of cylindrical samples; (b,d,f,h,j) - polished sections of samples
Fig. 5. Dependence of microhardness Hv on the total porosity P of the Lao.9 Y0.1 PO4-based materials
Fig. 6. Materials thermal diffusivity a after heat treatment at 1000 (1), 1100 (2), 1200 (3), 1300 (4), and 1400 0 C (5)
of obtaining heat-insulating monazite structured RE phosphate-based materials, in contrast to the above works focused on obtaining non-porous ceramic materials for the radioactive waste form.
The dependence of thermal diffusivity (a) of materials (samples 1-5) on temperature (T = 25 — 825 0C) is shown in Fig. 6. For all the samples, a decrease in the a coefficient with the increasing temperature is inherent. The values of thermal diffusivity for samples 1-3 with total porosity of P=52-44% (Table 1) is lower than those for the samples with total porosity of ^27% (samples 4, 5). The materials obtained at 1000-1200 0C displayed an almost linear character of the temperature diffusivity dependence on temperature; the values of a do not exceed 0.3 mm2/s. As a result of material sintering and compaction, an increase in the thermal diffusivity occurs over the entire investigated temperature range. The obtained values depend on the total porosity and the size of the contact boundaries formed during the pressing of the powder. Fig. 6 shows that a(T) values coincide within the error for samples 4 and 5 sintered at T =1300 0C and T =1400 0C, which is confirmed by their almost identical total porosity of -27%. The highest thermal diffusivity for the produced materials is 0.70±0.02 mm2/s at T =25 0C for the samples obtained at 1300-1400 0 C. The values of thermal diffusivity for the LaPO4 monazite-based dense materials decrease monotonically from 1.629 mm2/s (T=20 0C) down to 0.410 mm2/s (T=800 0C) [47]. The minimum difference between the values of a obtained in the present study and those given in the literature is observed at high temperatures: a(800 0C)=0.410 mm2/s (Pca/c. =0%), and a(825 0C)=0.302±0.020 mm2/s (sample 5, P -27%) in the present study.
Fig. 7. Specific heat capacity (Cp) of the monazite structured La0.9 Y0.1 PO4-based materials
Hence, it can be concluded that the thermal diffusivity, most likely, is less dependent on the total porosity of the monazite-based material.
The specific heat values of the sintered samples in Fig. 7 were calculated using the Mayer-Kelly empirical dependence equation (Cp = a + bT + cT-2), the coefficients in which were determined from experimental data in the 25-825 0C range (a = 0.66 ± 0.06, b = (2.7 ± 0.6) • 10-4, c = (—18.6 ± 5.3) • 103). The specific heat values obtained for the monazite structured La0.9Y01PO4-based material are higher than those for individual lanthanum orthophosphate with the same structure [30,47,48]. The character of Cp(T) dependence is in good agreement with the authors' data for the monazite structured lanthanum orthophosphate, differing by 0.1-0.3 J-(g-K)-1, which may be due to the presence of yttrium in the phase.
The values of thermal conductivity coefficient A of samples 1-5 at T =25 0 C are presented in Table 1. The compaction of the La0.9 Y01PO4-based materials by increasing the processing temperature leads to an increase in the thermal conductivity from 0.12±0.08 W-m-1 •K-1 at P = 52.3% up to 1.35 ± 0.08 W-m-1 •K-1 for samples 4 and 5 with a total porosity of ^27%.
4. Conclusion
Pre-ceramic nanocrystalline powders have been obtained on the basis of a monazite structured La0.9Y0.1 PO4 solid solution. The powders are represented by quasi-one-dimensional particles with a crystallite size of about 16 nm. The nanoparticles were synthesized under hydrothermal conditions at 230 0C and a processing time of 110 minutes. The heat treatment of compacted nanocrystalline La0.9Y01PO4 powder by the annealing-quenching method in the 1000-1400 0 C range for 20 minutes resulted in polycrystalline compact materials with a total porosity of 27 to 52%. It is shown that the activation of nanopowder sintering and crystallite growth occurs in the 1200-1300 0C range, which correlates with the melting temperature of the surface (nonautonomous) phase.
The dependence of the thermophysical characteristics and microhardness of the material on the porosity has been determined. The maximum microhardness (Hv) value of 17±0.5 GPa was obtained for the material with the lowest porosity. It was shown that the values of thermal diffusivity (a) of a material with total porosity (P) of 27% decreased monotonically with an increase in temperature from a (T = 25 0C)= 0.70 ± 0.02 mm2/s to a (T = 825 0C)= 0.30 ± 0.02 mm2/s. It was determined that the specific heat capacity of the monazite structured La0.9Y01PO4-based material is higher than that of an individual lanthanum orthophosphate. The thermal conductivity of materials A(T = 25 0C) varies depending on the porosity within a wide range from 0.12 ± 0.08 W-m-1 -K-1 at P ^52% to 1.35 ± 0.08 W-m-1 K-1 at P «27%.
Acknowledgement
The X-ray diffraction studies, determination of thermal diffusivity, examination by scanning electron microscopy, and the elemental analysis of samples were carried out on the instruments of the Engineering Center of the St. Petersburg State Technological Institute (Technical University).
The present work was financially supported by the Russian Foundation for Basic Research (Project No. 18-2912119).
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