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Influence of sintering temperature on structural and optical properties of Y2O3–MgO composite SPS ceramics
Journal Pre-proof Influence of sintering temperature on structural and optical properties of Y2O3–MgO composite SPS ceramics N.A. Safronova, O.S. Kryzhanovska, M.V. Dobrotvorska, A.E. Balabanov, А.V. Tolmachev, R.P. Yavetskiy, S.V. Parkhomenko, R. Brodskii, V.N. Baumer, D. Yu Kosyanov, O.O. Shichalin, E.K. Papynov, Jiang Li PII:
S0272-8842(19)33327-9
DOI:
https://doi.org/10.1016/j.ceramint.2019.11.137
Reference:
CERI 23506
To appear in:
Ceramics International
Received Date: 21 August 2019 Revised Date:
21 October 2019
Accepted Date: 16 November 2019
Please cite this article as: N.A. Safronova, O.S. Kryzhanovska, M.V. Dobrotvorska, A.E. Balabanov, А.V. Tolmachev, R.P. Yavetskiy, S.V. Parkhomenko, R. Brodskii, V.N. Baumer, D.Y. Kosyanov, O.O. Shichalin, E.K. Papynov, J. Li, Influence of sintering temperature on structural and optical properties of Y2O3–MgO composite SPS ceramics, Ceramics International (2019), doi: https://doi.org/10.1016/ j.ceramint.2019.11.137. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Influence of sintering temperature on structural and optical properties of Y2O3–MgO composite SPS ceramics N.A. Safronova1,*, O.S. Kryzhanovska1, M.V. Dobrotvorska1, A.E. Balabanov1, А.V. Tolmachev1, R.P. Yavetskiy1, S.V. Parkhomenko1, R. Brodskii1, V.N. Baumer2, D.Yu. Kosyanov3, O.O. Shichalin3,4, E.K. Papynov3,4, Jiang Li5 1
Institute for Single Crystals, NAS of Ukraine, 60 Nauky Ave., Kharkiv 61072, Ukraine
2
SSI “Institute for Single Crystals”, NAS of Ukraine, 60 Nauky Ave., Kharkiv 61072, Ukraine
3
Far Eastern Federal University, 8 Sukhanova Str., Vladivostok 690950, Russian Federation
4
Institute of Chemistry, Far-Eastern Branch, Russian Academy of Sciences, 159 100-let Vladivostoku Ave., Vladivostok 690022, Russian Federation
5
Key Laboratory of Transparent Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201899, China
Abstract The paper studies infrared-transparent 50:50 vol.% Y2O3–MgO finegrained composite ceramics produced via glycine-nitrate process and spark plasma sintering (SPS). Effect of SPS temperature (T=1100, 1200, 1250 and 1300°С) on consolidation processes of Y2O3–MgO nanopowders and on ceramics properties has been investigated. Morphology, structural-phase state, density, Vickers microhardness and infrared transmittance of Y2O3–MgO nanocomposites have been studied. Transmittance has been found to increase with the SPS temperature reaching 71%@6000 nm wavelength at T=1300°C. Maximal Vickers hardness of 10.9 GPa has been achieved at T=1200°C. The correlation between processing conditions of ceramics and indicated characteristics was investigated. 1
Keywords: A: Sintering; B: Composites; C: Hardness, Optical properties; Y2O3– MgO *
Corresponding author. Tel.: +38(057) 3410277; Fax: +38(057) 3409343.
E-mail address: [email protected] (N.A. Safronova) Institute for Single Crystals, NAS of Ukraine 60 Nauky Ave., Kharkiv, 61072, Ukraine
2
1. Introduction Recently, ceramics nanocomposites have become renowned as new promising infrared (IR) transparent materials with high thermal and mechanical stability. The concept of composite nanoceramics involves the fabrication of a nanocomposite constituting two or more non-soluble components in the solid state. Providing transparency to a nanocomposite in a wide range of wavelengths implies using materials with cubic structure and wide transmission windows. One of such materials is the nanocomposite Y2O3–MgO ceramics promising for infrared windows and domes [1-4]. Both MgO and Y2O3 oxides weakly absorb in the range from 0.3 to 6.0 µm and with long wavelength cutoff leading to a wide transmission window [2]. Additionally, there is a stable bi-phase mixture forming in a wide range of concentrations in the Y2O3–MgO system below the eutectics temperature Т=2110°C [5-6]. Y2O3–MgO nanocomposites were reported to possess exceptional optical transmittance, improved mechanical strength, and hardness comparable to single phase Y2O3 and MgO ceramics [1]. The mentioned above allows considering this polycrystalline bi-phase system as one of the most promising IRtransparent material. To minimize optical loss on scattering, such materials are needed to exhibit a number of special properties, particularly, mean grain size of the composite should be considerably smaller than the incident light wavelength. Materials transparent in near-infrared and mid-infrared range are known to have negligible scattering on grain boundaries if grain size is about 0.1 µm [7]. Apart from that, the more the difference in refraction indexes between two phases (n(Y2O3)=1.843 and n(MgO)=1.642 at 4.85 µm), the less the grain size is required, preferably less than λ/20 [1, 7]). It is noteworthy that reducing grain size increases mechanical stability (Hall-Petch effect) and material’s thermal stability [1, 7]. However, grain size <130 nm leads to the inverse Hall-Petch relation caused by increased contribution of intergranular boundaries with their hardness being lower than that of ceramic bulks [1]. Thus, engineering optical and mechanical properties of nanoceramics via 3
microstructure control (density, porosity, and grain size) is a cornerstone in the strategy for fabrication of promising IR-transparent Y2O3–MgO composite ceramics. Ceramics microstructure depends considerably on starting powders, particularly on their granulometric and physico-chemical properties. In general [8], transparent fine-grained ceramics can be obtained with the density close to theoretical using low-agglomerated powders with particle size ~50 nm. Nanopowders for the synthesis of Y2O3-MgO composite ceramics are most often obtained in three ways: (1) the flame pyrolysis technique - the method requires use of special expensive equipment, and the resulting powder is characterized by a wide particle size dispersion [5]; (2) the sol-gel combustion method using citric acid monohydrate and ethylene glycol as raw materials [9, 10, 11], however, several problems are characteristic – in particular the residual porosity of ceramics; (3) the glycine-nitrate process (GNP) – this method allows to obtain fine particles with high purity in a short time using an eco-friendly reaction [7]. Aggregation inhibition in solution due to formation of stable Mg2+ and Y3+ complexes with glycine serving as ligands is one of the advantages of GNP. This fact along with great amount of gases formed at further thermolysis reaction yields a highly dispersed nanopowder. Then, to control microstructural parameters, namely, to stabilize nanosized state during the transfer from nanopowders to ceramics is to fulfil two key principles. The first one consists in suppression of diffusion mass-transfer by applying such consolidation methods as spark plasma sintering (SPS), hot isostatic pressing (HIP), etc. [4, 5, 7, 9, 10]. SPS approach is considered as the most innovative one due to a wide range of opportunities to vary sintering parameters within an electro-physical treatment, namely, temperature, pressure, and holding time, which allow tuning morphology of the composite ceramics in a wide range. The second principle concerns the formation of ceramics with uniform bi-phase structure. Grain growth during sintering can be limited due to the fact that the 4
grains of each phase pin the boundary of the other phase. This avoids the migration of the grain boundary and increasing its size. This zener pinning effect will significantly depend on the uniformity of the phase distribution in the nanocomposite. The effect is remarkably pronounced for the systems with equal volume fractions of the components. In [7] reports about small number of studies of the optical and mechanical properties of transparent ceramics obtained using GNP. In addition, in [7] the HIP was used as a sintering method. As we know, the literature has not previously reported the use of GNP and SPS methods together for Y2O3-MgO composite ceramics, which is promising because of the suppression of diffusion mass transfer due to fast sintering for several seconds compared to 30-90 min in the HIP. The optimum sintering temperature is one of the most actual problem for SPS method. Additionally, present data on Y2O3–MgO composite SPS ceramics lacks information on microhardness, which is known to be one of the main parameters determining stability to thermal shock [1], a crucial property for IR-windows and domes. Thus, the paper investigates the influence of microstructural parameters on microhardness and optical properties of Y2O3–MgO composite ceramics sintered from GNP-produced nanopowders in the context of SPS temperature.
2.Experimental 2.1. Fabrication of nanopowders and composite ceramics 50:50 vol.% Y2O3–MgO (further denoted as Y2O3–MgO) nanopowders were synthesized using the glycine-nitrate process. Initially, high-purity yttrium oxide Y2O3 (99.999%, Alfa Aesar) and magnesium oxide MgO (99.99%, Alfa Aesar) powders were dissolved in nitric acid HNO3 (69%, Fluka Trace Select) to form Y(NO3)3 and Mg(NO3)2 solutions. Glycine C2H5NO2 (99%, Merck) was dissolved in deionized water. Then, Y(NO3)3 and Mg(NO3)2 solutions taken in proportion 50:50 with respect to volumes of the final oxides were added into glycine solution 5
in a molar ratio corresponding to stoichiometry of redox-reaction yielding Y2O3, MgO, H2O, CO2, N2 taking into account the excess of nitric acid used to dissolve the oxides. The valency ratio value of N/G was 0.83. After that, the solution was stirred and heated to boiling. After water evaporation, obtained gel exhibited selfignition forming powder precursor, which calcined in air at 1000°С for 4 hours. Then obtained Y2O3–MgO powders were loaded into a graphite die with an inner diameter of 15 mm, the internal surface of which was covered with graphite sheet. Sintering experiments were conducted using a SPS-515S (Dr. Sinter*LAB, Japan) unit in the temperature range of 1100-1300°C for 8 min at the pressure 50 MPa and average heating rate of 100°C/min. After sintering, the composite ceramics were mirror polished on both surfaces with diamond slurries of different grades.
2.2. Characterization of nanopowders and composite ceramics Morphology of nanopowders and microstructure of ceramics were studied by the means of scanning electron microscopy (SEM, Hitachi S-5500, Hitachi High-Technologies Corp., Japan) and transmission electronic microscopy (TEM, TEM-125, Selmi, Ukraine). The electron microprobe analysis was performed using a Thermo Scientific energy-dispersive microprobe attachment (USA). Average grain size of nanoceramics was determined by using the linear intercept method. At least 300 Y2O3, MgO grains were analyzed for each measurement. The phase identification was performed by X-ray diffraction method (XRD) on a SIEMENS D-500 X-ray diffractometer (CuKα radiation, graphite monochromator). The phases were identified using the JCPDS PDF cards and EVA retrieval system included in the diffractometer software. Rietveld refinement was performed with the FullProf program. Brunauer–Emmett–Teller (BET) method with nitrogen as an adsorptive at the temperature of liquid nitrogen boiling point T=78 K was used for the specific surface area measurements (Quantachrome Quadrasorb SI Automated Surface Area Pore Size Analyzer, USA). Mean particle size was evaluated using the formula: 6
d=
FS ⋅ 109 ρS
(1),
where ρ is the density of the material (g/m3), S is the specific surface area of the material (m2/g), FS is the shape factor of the particles (6 for cubes and spheres) [12]. Mechanical properties of the ceramics were investigated via the microhardness method based on deformation with a concentrated load. Indentation was applied using a PMT-3 device with the standard tetrahedral indenter at the load of 1 N. The obtained indenter imprint was measured by the means of a computer-aided optical microscope Zeiss Axioskop 40A POL. The Vickers microhardness value HV was determined from the relation HV=1854·Р1/a2 (kgf/mm2), where Р1 is the indenter load (g), a is the imprint diagonal (µm), after that the resulting value was converted to GPa. The measurements were performed on 15-20 imprints, the standard deviation in the measured value of mean microhardness was about 2%. The density of the ceramics was studied by hydrostatic weighing method on an AdventurerTM balance (OHAUS Corp., USA). The infrared optical properties of ceramics were studied using a Fourier-Transform Infrared (FTIR) spectrometer (Vertex-80, Bruker Optik GmbH, Germany) in the wavelength range of 1000-10000 nm. Optical transmittance was measured in vacuum using a beam with the aperture of 3 mm.
3. Results and discussion 3.1 Characterization of Y2O3–MgO nanopowders synthesized by glycine-nitrate process SEM and TEM images of Y2O3–MgO nanopowders synthesized by glycine-nitrate process and annealed in air at 1000°С for 4 hours are shown in Fig. 1. As could be seen, the synthesized powders consist of fine nanoparticles with narrow size distribution. Particle size and agglomeration play a crucial role in 7
powder densification and determine microstructure of the final ceramics. Given the nanopowders are prone to aggregation due to high surface energy, to determine the average particle size it is necessary to compare results of several independent methods. The average particle size directly estimated from the TEM data was found to be 60±6 nm (Fig. 1b). According to BET measurements, specific surface of nanopowders is S=16.10 m2/g. Taking into account theoretical density of 50:50 vol.% Y2O3–MgO ρ=4.309 g/cm3 [1], the average particle size obtained from the BET data was 86 nm. Close size values obtained using TEM and BET methods prove small particle size and weak agglomeration of prepared Y2O3·MgO nanopowder. Sintering of such powders may provide formation of fine-grained and highly-dense ceramics due to a short mass transfer distance to diffuse during sintering [13].
(a)
(b)
Fig. 1. SEM (a) and TEM (b) images of the Y2O3–MgO nanopowders annealed at 1000°С for 4 hours.
3.2 Microstructure evolution of Y2O3−MgO composite ceramics during SPS Obtained fine nanoparticles show excellent sinterability. Densification dynamics of Y2O3–MgO nanopowders was studied via punch press displacement monitoring as a function of temperature (powder mass=1,9 g; pressing die diameter =15 mm) in the series of sintering temperatures: 1100, 1200, 1250, and 1300°С (Fig. 2). Nanopowders’ densification in all cases was found to proceed in several 8
steps. Dilatometry curves show that all the samples tend to consolidate the same way at the beginning (in the temperature range of up to ~670°C), which is caused by agglomerates breakdown and particle rearrangement. Intensive sintering begins at 750°C and enters the active densification at 800°C. At this stage, the most effective densification mechanisms become active, namely, the plastic flow and creep processes. When the heating is stopped, the shrinkage becomes slow. Increasing of the sintering temperature from 1100 to 1200°С has a slight effect on densification. Further temperature growth up to 1250 and then to 1300°C leads to a significant sample’s densification.
Fig. 2. Densification dynamics of Y2O3−MgO nanopowders during SPS at 1100 (a), 1200 (b), 1250 (c) and 1300°C (d) for 8 min under 50 MPa.
The microstructure of Y2O3−MgO SPS ceramic samples sintered at 1100, 1200, 1250 and 1300°C were investigated by FESEM (Fig. 3). Microstructure analysis has shown that sintering temperature Т=1100°C is not high enough, since ceramics sintering is not complete and the sample is characterized by a large number of open pores (Fig. 3a). Further increase of SPS temperature to 1200÷1300°C leads to formation of ceramics with a denser structure. The mass contrast clearly seen on the SEM images enables one to estimate phase distribution of the components, taking into account that dark grains are MgO phase and the bright grains belong toY2O3. As could be seen, the Y2O3 and MgO grains are fine and uniformly distributed in the samples. Energy dispersive X-ray spectra (EDXS)
9
collected from specified area of Y2O3−MgO ceramics (Fig. 3e) prove the uniform distribution of the components in the mixture. Several conclusions could be drawn from the analysis of FESEM data. Primarily, SPS of the nitrate-glycine nanopowders leads to formation of homogenous bi-phase nanoceramics. Additionally, uniform phase separation enables one to stabilize ceramics grain size in the most efficient way due to the pinning effect (the grains of each phase pin the boundary of another phase). The immiscible characteristic of the Y2O3 and MgO (system is a stable two-phase mixture below the eutectic point T=2110°C [14]) helps to limit grain coarsening by phase boundary impingement during sintering [3]. Besides, uniform distribution of two phases leads to long-range mass transport between interphase boundaries, which has a substantial limiting effect of the atomic movement [7]. All mentioned above allows preparing fine-grained ceramics and avoiding excessive grain growth.
(a)
(b)
(c)
(d)
10
(e) Fig. 3. FESEM images of Y2O3−MgO composite SPS ceramics sintered at 1100 (a), 1200 (b), 1250 (c) and 1300°C (d) for 8 min under 50 MPa; EDXS of Y2O3−MgO composite ceramics sintered at T=1300°C (e).
To minimize light scattering is not only to provide submicron composite’s mean grain size, but also to control it by altering SPS parameters. The relationship between the sintering temperature and grain size of nanocomposite ceramics is shown in Fig. 4. Mean grain size grows with temperature from 0.16 and 0.20 µm to 0.24 and 0.28 µm for Y2O3 and MgO, respectively. Obtained results is connected with the tendency to combine the same phase grain nearby, because thermally activated processes of mass transfer target at reducing total energy of the system via shrinking grain boundaries [15, 16]. Grain growth is obviously limited by the number of neighboring grains of the same composition. That is why, uniform phase separation of our system enables to avoid uncontrolled grain growth leading to cut-off of the short light wavelength [1].
11
(a)
(b)
(c)
(d)
Fig. 4. Grain size distribution of Y2O3–MgO composite SPS ceramics sintered at 1200 (a), 1250 (b) and 1300°C (c) for 8 min under 50 MPa; the effect of SPS temperature on average sizes of Y2O3 and MgO grains (d). XRD patterns of Y2O3−MgO fine-grained SPS ceramics sintered at T=1100-1300°С exhibit a number of narrow lines of high intensity; the characteristic pattern is presented on Fig. 5a. XRD analysis has shown that samples are a mixture of cubic yttrium oxide (space group Iа3, JCPDS No. 41-1105) and cubic magnesia (space group Fm3m, JCPDS No. 45-0946). Impurity phases were not revealed at the XRD sensitivity limit. This proves that Y2O3 and MgO do not react during SPS. Fractional content of phases are 58.6 wt.% for Y2O3 and 41.4 wt.% for MgO that is in good agreement with theoretical values (Y2O3 – 58.3 wt.%, MgO – 41.75 wt.%) and corresponds to volume ratio 50:50. Let’s consider evolution of structural parameters of Y2O3 and MgO as a function of sintering temperature. Cubic lattice constant of Y2O3 (а) at 1100°C is practically equal to theoretical value of а=10.6041 Å (Fig. 5b). As was mentioned above, ceramics prepared at T=1100°C is characterized by a highly porous microstructure and their grains do not contact tightly, therefore, there are a lot of open pores (Fig. 3a). Absence of tight contact between the grains of different phases and presence of non-contacting (“free”) grains yield rather low crystal 12
lattice deformation of the components with the lattice parameter being close to theoretical. Further temperature increase to T=1200, 1300°C results in considerable reduction of Y2O3 lattice parameter а to 10.5884-10.5879 Å values, respectively. This is caused by microstrains occurring at the grain boundary. The same effect but with the opposite sign is observed for a parameter of MgO cubic lattice (Fig. 5b). Lattice parameter change may evidence deformation of elementary cell due to adjustment of crystallographic planes of two oxides to each other. According to XRD, Y2O3 crystallize size is 45 nm, while for MgO – 55 nm. Taking into account that grain boundaries of polycrystalline materials are formed via twin formation [17, 18], it can be concluded that each grain of Y2O3 and MgO consists of 4-5 crystal blocks.
(a)
(b)
Fig. 5. XRD pattern of Y2O3−MgO composite SPS ceramics sintered at 1300°C for 8 min under 50 MPa (a); a parameter of Y2O3 and MgO crystal lattices as a function of SPS temperature in the range of 1100-1300°C (b).
3.3 Mechanical and optical properties of SPS sintered Y2O3−MgO composite ceramics Fig. 6a shows the relative density of Y2O3-MgO ceramics as a function of the densification conditions. Spark plasma sintering of starting nanopowders at
13
1100°C did not yield highly-dense ceramics. Relative density of nanocomposites was 82% (3.6 g/cm3), which is far below the theoretical value (4.3 g/cm3 [1]). With an raise of the sintering temperature, diffusion mass transfer processes speed up, and the relative density of the samples increases and approaches 100 % for sintering temperature of T=1300°С. Thus, the optimal sintering temperature for formation of almost pore-free Y2O3-MgO ceramics was determined to be 1300°C. The obtained density data are correspond to the data in [7], where ceramics was obtained from smaller GNP-produced nanopowders by the HIP method. Mechanical properties of nanocomposite ceramics are greatly affected by ceramics density and grain size, which in turn depend on sintering temperature. Fig. 6b shows the microhardness as a function of the densification conditions. The samples sintered at 1100°C are characterized by the lowest microhardness ~5 GPa, while the highest values of 10.9 GPa were demonstrated by samples prepared at 1200°C. It is much higher than those of single-phase Y2O3 and MgO ceramics [1, 4, 19, 20]. Microhardness of nanocomposite ceramics obtained at 1250 and 1300°C reduces by 10 and 20%, respectively, despite the relative density increases from 97 % to 100% (from 4.2 to 4.4 g/cm3) with sintering temperature (Fig. 6a). Diffusion mass transfer processes become accelerated at high temperatures leading to removal of residual pores in the material. Along with pore elimination, raising sintering temperature promotes grain growth in ceramics samples (Fig. 3). Hardness decrease at larger grains is caused by reduction of the grain boundaries extension, which block dislocations motion (similar to Hall-Petch relationship). To prove that, Fig. 6с shows linear correlation between microhardness and square root of the average grain size. Grain size has the decisive effect on microhardness values. Microhardness of the composite ceramics with the lowest porosity obtained at sintering temperature T=1300°C is slightly lower than that of ceramics prepared at lower temperatures. The microhardness values of the ceramics obtained in our work using the SPS method are comparable to those obtained by hot pressing in [7]
14
and about 10 % exceed the microhardness values of ceramics obtained by the SPS method [21] (Fig.6с).
(a)
(b)
(c) Fig. 6. Relative density (a) and microhardness (b) of the Y2O3−MgO SPS nanoceramics sintered at 1100-1300°C for 8 min under 50 MPa; Relation of hardness and average grain size of Y2O3−MgO ceramics sintered at T=1100,1200, 1250 and 1300°C (b) vs. data [7, 21]
Fig. 7 shows IR transmission spectra of the Y2O3−MgO specimens sintered at different temperatures. Let’s consider the contribution of the porosity and average grain size of the nanocomposite ceramics to the optical transmittance. The micropores in the nanocomposite will degrade the optical transmittance due to the scattering losses connected with the great difference between refraction indexes of the pores and phases constituting the composite. Ceramics obtained at 1100°C is opaque due to numerous pores, light scattering sources, and immature grained 15
structure (Fig. 3a). As shown on Fig. 7a, IR transmittance of the Y2O3−MgO nanocomposites tends to increase with the sintering temperature. The transmittance values of the 1200, 1250 and 1300°C ceramics reach 50, 60 and 70% at λ=6000 nm, respectively, apparently owing to the growth in density with the sintering temperature. Besides, reduction in transmittance values is connected with the interphase grain boundary scattering due to significant difference in the refractive index between MgO and Y2O3. The highest transmittance at the level of 71%@6000 nm is observed for nanocomposite sintered at Т=1300°С (mean grain size Y2O3 – 0.24 µm, MgO – 0.28 µm, Fig. 4), however, obtained transmittance is lower than theoretical one of 85%, but comparable with the values reported earlier [5, 7, 9, 10]. In addition, the cut-off wavelength of Y2O3−MgO fine-grained ceramics reaches 9500 nm, which is superior to the majority of the used mid-IR transparent materials. Complex absorption band in the range 2800-4000 nm (Fig. 7a) is caused by water molecules adsorbed from air (MgO phase has a strong hydrophilic property) [4, 9], with the stretching vibration of hydroxyl groups (O-H) being observed. An intense broad peak occurs near 7000 nm corresponding to the asymmetrical and symmetrical stretching vibrations of the carboxylate groups (O–C=O), which were formed during the solution combustion synthesis of nanopowders and remained after annealing [7, 10]. Fig. 7b shows transmission calculated with a model that includes only reflection and Mie scattering. Two main differences could be revealed when comparing predicted results with the experimental data (Fig. 7a). Primarily, optical transmittance in the experiment (Fig. 7a) is lower than in theory (Fig. 7b), which is attributed to scattering on nano- and micropores. Additionally, theoretical calculations does not produce bands in the 2800-4000 nm and ~7000 nm wavelength range, which we assigned above to absorption. However, experimental and theoretical spectra coincide well in the short wavelengths range. According to theoretical results, short wavelength transmission cut-off at ~1500 nm corresponds 16
to grain size of 150-200 nm that is in good agreement with experimentally observed for Y2O3−MgO ceramics (Fig. 4). These results indicate that the mechanical properties and IR-transmittance can be improved by engineering microstructure through optimizing sintering conditions of the nanocomposite, such as sintering temperature.
(a)
(b)
(c) Fig. 7. IR-transmission spectra of Y2O3−MgO composite ceramics: experimental (a) and predicted (b) data; typical appearance of Y2O3−MgO sample SPS sintered at 1300°C for 8 min under 50 MPa (c).
4. Conclusions 50:50 vol.% Y2O3–MgO ceramic nanocomposites were successfully prepared by the means of GNP and SPS technique. Fine weakly agglomerated 17
Y2O3–MgO nanopowders annealed in air at T=1000°C (dSEM=60 nm, dBET=86 nm) were sintered in the range T=1100-1300°C for 8 min at 50 MPa under SPS conditions. Raising sintering temperature has been demonstrated to increase mean grain size of both phases due to thermally activated processes of mass transfer (Y2O3: 0.16-0.24 µm, MgO:0.20-0.275 µm). At sintering temperatures ranging in 1200-1300°C, Y2O3 and MgO crystal lattices have been shown to deform due to microstrains arising on the interphase boundaries. Vickers microhardness of the nanocomposite has been found to be greatly affected by the grain size (Hall-Petch strengthening). The maximum Vickers hardness of 10.9 GPa is achieved for ceramics sintered at T=1200°C. Subsequent microhardness reduction (T=12501300°C) is caused by grain growth with the creep limit being decreased. The correlations between the optical properties and the SPS temperature were investigated. IR transmittance of the Y2O3–MgO nanocomposites tends to increase with the SPS temperature that can be attributed to increase of density, grain growth, and decrease of extension of interphase grain boundaries. As a result, Y2O3–MgO ceramics sample sintered at T=1300°С is characterized by the highest IRtransmittance of 71%@6000 nm wavelength.
Acknowledgements The authors are grateful to Dr V.G. Kuryavyi and Dr. A.A. Kuchmizhak for their help in characterization of the ceramic samples. This work was supported by the National Academy of Sciences of Ukraine within the frame of the project “Nanoceramics”. D.Yu.K. appreciate the financial support from the Ministry of Science and Higher Education of the Russian Federation (Grant No. 3.2168.2017/4.6).
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Figure captions
Fig. 1. SEM (a) and TEM (b) images of the Y2O3–MgO nanopowders annealed at 1000°С for 4 hours.
Fig. 2. Densification dynamics of Y2O3−MgO nanopowders during SPS at 1100 (a), 1200 (b), 1250 (c) and 1300°C (d) for 8 min under 50 MPa.
Fig. 3. SEM images of Y2O3−MgO composite SPS ceramics sintered at 1100 (a), 1200 (b), 1250 (c) and 1300°C (d) for 8 min under 50 MPa; EDXS of Y2O3−MgO composite ceramics sintered at T=1300°C (e).
Fig. 4. Grain size distribution of Y2O3–MgO composite SPS ceramics sintered at 1200 (a), 1250 (b) and 1300°C (c) for 8 min under 50 MPa; the effect of SPS temperature on average sizes of Y2O3 and MgO grains (d). Fig. 5. XRD pattern of Y2O3−MgO composite SPS ceramics sintered at 1300°C for 8 min under 50 MPa (a); a parameter of Y2O3 and MgO crystal lattices as a function of SPS temperature in the range of 1100-1300°C (b).
Fig. 6. Density and Vickers hardness of the Y2O3−MgO SPS nanoceramics sintered at 1100-1300°C for 8 min under 50 MPa (a); Relation of hardness and average grain size of Y2O3−MgO ceramics sintered at T=1100,1200, 1250 and 1300°C (b). Fig. 7. IR-transmission spectra of Y2O3−MgO composite ceramics: experimental (a) and predicted (b) data; typical appearance of Y2O3−MgO sample SPS sintered at 1300°C for 8 min under 50 MPa (c).
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Declaration of interests Manuscript - Influence of sintering temperature on structural and optical properties of Y2O3–MgO composite SPS ceramics Authors - N.A. Safronova, O.S. Kryzhanovska, M.V. Dobrotvorska, A.E. Balabanov, А.V. Tolmachev, R.P. Yavetskiy, S.V. Parkhomenko, R. Brodskii, V.N. Baumer, D.Yu. Kosyanov, O.O. Shichalin, E.K. Papynov, Jiang Li The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
S0272-8842(19)33327-9
DOI:
https://doi.org/10.1016/j.ceramint.2019.11.137
Reference:
CERI 23506
To appear in:
Ceramics International
Received Date: 21 August 2019 Revised Date:
21 October 2019
Accepted Date: 16 November 2019
Please cite this article as: N.A. Safronova, O.S. Kryzhanovska, M.V. Dobrotvorska, A.E. Balabanov, А.V. Tolmachev, R.P. Yavetskiy, S.V. Parkhomenko, R. Brodskii, V.N. Baumer, D.Y. Kosyanov, O.O. Shichalin, E.K. Papynov, J. Li, Influence of sintering temperature on structural and optical properties of Y2O3–MgO composite SPS ceramics, Ceramics International (2019), doi: https://doi.org/10.1016/ j.ceramint.2019.11.137. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Influence of sintering temperature on structural and optical properties of Y2O3–MgO composite SPS ceramics N.A. Safronova1,*, O.S. Kryzhanovska1, M.V. Dobrotvorska1, A.E. Balabanov1, А.V. Tolmachev1, R.P. Yavetskiy1, S.V. Parkhomenko1, R. Brodskii1, V.N. Baumer2, D.Yu. Kosyanov3, O.O. Shichalin3,4, E.K. Papynov3,4, Jiang Li5 1
Institute for Single Crystals, NAS of Ukraine, 60 Nauky Ave., Kharkiv 61072, Ukraine
2
SSI “Institute for Single Crystals”, NAS of Ukraine, 60 Nauky Ave., Kharkiv 61072, Ukraine
3
Far Eastern Federal University, 8 Sukhanova Str., Vladivostok 690950, Russian Federation
4
Institute of Chemistry, Far-Eastern Branch, Russian Academy of Sciences, 159 100-let Vladivostoku Ave., Vladivostok 690022, Russian Federation
5
Key Laboratory of Transparent Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201899, China
Abstract The paper studies infrared-transparent 50:50 vol.% Y2O3–MgO finegrained composite ceramics produced via glycine-nitrate process and spark plasma sintering (SPS). Effect of SPS temperature (T=1100, 1200, 1250 and 1300°С) on consolidation processes of Y2O3–MgO nanopowders and on ceramics properties has been investigated. Morphology, structural-phase state, density, Vickers microhardness and infrared transmittance of Y2O3–MgO nanocomposites have been studied. Transmittance has been found to increase with the SPS temperature reaching 71%@6000 nm wavelength at T=1300°C. Maximal Vickers hardness of 10.9 GPa has been achieved at T=1200°C. The correlation between processing conditions of ceramics and indicated characteristics was investigated. 1
Keywords: A: Sintering; B: Composites; C: Hardness, Optical properties; Y2O3– MgO *
Corresponding author. Tel.: +38(057) 3410277; Fax: +38(057) 3409343.
E-mail address: [email protected] (N.A. Safronova) Institute for Single Crystals, NAS of Ukraine 60 Nauky Ave., Kharkiv, 61072, Ukraine
2
1. Introduction Recently, ceramics nanocomposites have become renowned as new promising infrared (IR) transparent materials with high thermal and mechanical stability. The concept of composite nanoceramics involves the fabrication of a nanocomposite constituting two or more non-soluble components in the solid state. Providing transparency to a nanocomposite in a wide range of wavelengths implies using materials with cubic structure and wide transmission windows. One of such materials is the nanocomposite Y2O3–MgO ceramics promising for infrared windows and domes [1-4]. Both MgO and Y2O3 oxides weakly absorb in the range from 0.3 to 6.0 µm and with long wavelength cutoff leading to a wide transmission window [2]. Additionally, there is a stable bi-phase mixture forming in a wide range of concentrations in the Y2O3–MgO system below the eutectics temperature Т=2110°C [5-6]. Y2O3–MgO nanocomposites were reported to possess exceptional optical transmittance, improved mechanical strength, and hardness comparable to single phase Y2O3 and MgO ceramics [1]. The mentioned above allows considering this polycrystalline bi-phase system as one of the most promising IRtransparent material. To minimize optical loss on scattering, such materials are needed to exhibit a number of special properties, particularly, mean grain size of the composite should be considerably smaller than the incident light wavelength. Materials transparent in near-infrared and mid-infrared range are known to have negligible scattering on grain boundaries if grain size is about 0.1 µm [7]. Apart from that, the more the difference in refraction indexes between two phases (n(Y2O3)=1.843 and n(MgO)=1.642 at 4.85 µm), the less the grain size is required, preferably less than λ/20 [1, 7]). It is noteworthy that reducing grain size increases mechanical stability (Hall-Petch effect) and material’s thermal stability [1, 7]. However, grain size <130 nm leads to the inverse Hall-Petch relation caused by increased contribution of intergranular boundaries with their hardness being lower than that of ceramic bulks [1]. Thus, engineering optical and mechanical properties of nanoceramics via 3
microstructure control (density, porosity, and grain size) is a cornerstone in the strategy for fabrication of promising IR-transparent Y2O3–MgO composite ceramics. Ceramics microstructure depends considerably on starting powders, particularly on their granulometric and physico-chemical properties. In general [8], transparent fine-grained ceramics can be obtained with the density close to theoretical using low-agglomerated powders with particle size ~50 nm. Nanopowders for the synthesis of Y2O3-MgO composite ceramics are most often obtained in three ways: (1) the flame pyrolysis technique - the method requires use of special expensive equipment, and the resulting powder is characterized by a wide particle size dispersion [5]; (2) the sol-gel combustion method using citric acid monohydrate and ethylene glycol as raw materials [9, 10, 11], however, several problems are characteristic – in particular the residual porosity of ceramics; (3) the glycine-nitrate process (GNP) – this method allows to obtain fine particles with high purity in a short time using an eco-friendly reaction [7]. Aggregation inhibition in solution due to formation of stable Mg2+ and Y3+ complexes with glycine serving as ligands is one of the advantages of GNP. This fact along with great amount of gases formed at further thermolysis reaction yields a highly dispersed nanopowder. Then, to control microstructural parameters, namely, to stabilize nanosized state during the transfer from nanopowders to ceramics is to fulfil two key principles. The first one consists in suppression of diffusion mass-transfer by applying such consolidation methods as spark plasma sintering (SPS), hot isostatic pressing (HIP), etc. [4, 5, 7, 9, 10]. SPS approach is considered as the most innovative one due to a wide range of opportunities to vary sintering parameters within an electro-physical treatment, namely, temperature, pressure, and holding time, which allow tuning morphology of the composite ceramics in a wide range. The second principle concerns the formation of ceramics with uniform bi-phase structure. Grain growth during sintering can be limited due to the fact that the 4
grains of each phase pin the boundary of the other phase. This avoids the migration of the grain boundary and increasing its size. This zener pinning effect will significantly depend on the uniformity of the phase distribution in the nanocomposite. The effect is remarkably pronounced for the systems with equal volume fractions of the components. In [7] reports about small number of studies of the optical and mechanical properties of transparent ceramics obtained using GNP. In addition, in [7] the HIP was used as a sintering method. As we know, the literature has not previously reported the use of GNP and SPS methods together for Y2O3-MgO composite ceramics, which is promising because of the suppression of diffusion mass transfer due to fast sintering for several seconds compared to 30-90 min in the HIP. The optimum sintering temperature is one of the most actual problem for SPS method. Additionally, present data on Y2O3–MgO composite SPS ceramics lacks information on microhardness, which is known to be one of the main parameters determining stability to thermal shock [1], a crucial property for IR-windows and domes. Thus, the paper investigates the influence of microstructural parameters on microhardness and optical properties of Y2O3–MgO composite ceramics sintered from GNP-produced nanopowders in the context of SPS temperature.
2.Experimental 2.1. Fabrication of nanopowders and composite ceramics 50:50 vol.% Y2O3–MgO (further denoted as Y2O3–MgO) nanopowders were synthesized using the glycine-nitrate process. Initially, high-purity yttrium oxide Y2O3 (99.999%, Alfa Aesar) and magnesium oxide MgO (99.99%, Alfa Aesar) powders were dissolved in nitric acid HNO3 (69%, Fluka Trace Select) to form Y(NO3)3 and Mg(NO3)2 solutions. Glycine C2H5NO2 (99%, Merck) was dissolved in deionized water. Then, Y(NO3)3 and Mg(NO3)2 solutions taken in proportion 50:50 with respect to volumes of the final oxides were added into glycine solution 5
in a molar ratio corresponding to stoichiometry of redox-reaction yielding Y2O3, MgO, H2O, CO2, N2 taking into account the excess of nitric acid used to dissolve the oxides. The valency ratio value of N/G was 0.83. After that, the solution was stirred and heated to boiling. After water evaporation, obtained gel exhibited selfignition forming powder precursor, which calcined in air at 1000°С for 4 hours. Then obtained Y2O3–MgO powders were loaded into a graphite die with an inner diameter of 15 mm, the internal surface of which was covered with graphite sheet. Sintering experiments were conducted using a SPS-515S (Dr. Sinter*LAB, Japan) unit in the temperature range of 1100-1300°C for 8 min at the pressure 50 MPa and average heating rate of 100°C/min. After sintering, the composite ceramics were mirror polished on both surfaces with diamond slurries of different grades.
2.2. Characterization of nanopowders and composite ceramics Morphology of nanopowders and microstructure of ceramics were studied by the means of scanning electron microscopy (SEM, Hitachi S-5500, Hitachi High-Technologies Corp., Japan) and transmission electronic microscopy (TEM, TEM-125, Selmi, Ukraine). The electron microprobe analysis was performed using a Thermo Scientific energy-dispersive microprobe attachment (USA). Average grain size of nanoceramics was determined by using the linear intercept method. At least 300 Y2O3, MgO grains were analyzed for each measurement. The phase identification was performed by X-ray diffraction method (XRD) on a SIEMENS D-500 X-ray diffractometer (CuKα radiation, graphite monochromator). The phases were identified using the JCPDS PDF cards and EVA retrieval system included in the diffractometer software. Rietveld refinement was performed with the FullProf program. Brunauer–Emmett–Teller (BET) method with nitrogen as an adsorptive at the temperature of liquid nitrogen boiling point T=78 K was used for the specific surface area measurements (Quantachrome Quadrasorb SI Automated Surface Area Pore Size Analyzer, USA). Mean particle size was evaluated using the formula: 6
d=
FS ⋅ 109 ρS
(1),
where ρ is the density of the material (g/m3), S is the specific surface area of the material (m2/g), FS is the shape factor of the particles (6 for cubes and spheres) [12]. Mechanical properties of the ceramics were investigated via the microhardness method based on deformation with a concentrated load. Indentation was applied using a PMT-3 device with the standard tetrahedral indenter at the load of 1 N. The obtained indenter imprint was measured by the means of a computer-aided optical microscope Zeiss Axioskop 40A POL. The Vickers microhardness value HV was determined from the relation HV=1854·Р1/a2 (kgf/mm2), where Р1 is the indenter load (g), a is the imprint diagonal (µm), after that the resulting value was converted to GPa. The measurements were performed on 15-20 imprints, the standard deviation in the measured value of mean microhardness was about 2%. The density of the ceramics was studied by hydrostatic weighing method on an AdventurerTM balance (OHAUS Corp., USA). The infrared optical properties of ceramics were studied using a Fourier-Transform Infrared (FTIR) spectrometer (Vertex-80, Bruker Optik GmbH, Germany) in the wavelength range of 1000-10000 nm. Optical transmittance was measured in vacuum using a beam with the aperture of 3 mm.
3. Results and discussion 3.1 Characterization of Y2O3–MgO nanopowders synthesized by glycine-nitrate process SEM and TEM images of Y2O3–MgO nanopowders synthesized by glycine-nitrate process and annealed in air at 1000°С for 4 hours are shown in Fig. 1. As could be seen, the synthesized powders consist of fine nanoparticles with narrow size distribution. Particle size and agglomeration play a crucial role in 7
powder densification and determine microstructure of the final ceramics. Given the nanopowders are prone to aggregation due to high surface energy, to determine the average particle size it is necessary to compare results of several independent methods. The average particle size directly estimated from the TEM data was found to be 60±6 nm (Fig. 1b). According to BET measurements, specific surface of nanopowders is S=16.10 m2/g. Taking into account theoretical density of 50:50 vol.% Y2O3–MgO ρ=4.309 g/cm3 [1], the average particle size obtained from the BET data was 86 nm. Close size values obtained using TEM and BET methods prove small particle size and weak agglomeration of prepared Y2O3·MgO nanopowder. Sintering of such powders may provide formation of fine-grained and highly-dense ceramics due to a short mass transfer distance to diffuse during sintering [13].
(a)
(b)
Fig. 1. SEM (a) and TEM (b) images of the Y2O3–MgO nanopowders annealed at 1000°С for 4 hours.
3.2 Microstructure evolution of Y2O3−MgO composite ceramics during SPS Obtained fine nanoparticles show excellent sinterability. Densification dynamics of Y2O3–MgO nanopowders was studied via punch press displacement monitoring as a function of temperature (powder mass=1,9 g; pressing die diameter =15 mm) in the series of sintering temperatures: 1100, 1200, 1250, and 1300°С (Fig. 2). Nanopowders’ densification in all cases was found to proceed in several 8
steps. Dilatometry curves show that all the samples tend to consolidate the same way at the beginning (in the temperature range of up to ~670°C), which is caused by agglomerates breakdown and particle rearrangement. Intensive sintering begins at 750°C and enters the active densification at 800°C. At this stage, the most effective densification mechanisms become active, namely, the plastic flow and creep processes. When the heating is stopped, the shrinkage becomes slow. Increasing of the sintering temperature from 1100 to 1200°С has a slight effect on densification. Further temperature growth up to 1250 and then to 1300°C leads to a significant sample’s densification.
Fig. 2. Densification dynamics of Y2O3−MgO nanopowders during SPS at 1100 (a), 1200 (b), 1250 (c) and 1300°C (d) for 8 min under 50 MPa.
The microstructure of Y2O3−MgO SPS ceramic samples sintered at 1100, 1200, 1250 and 1300°C were investigated by FESEM (Fig. 3). Microstructure analysis has shown that sintering temperature Т=1100°C is not high enough, since ceramics sintering is not complete and the sample is characterized by a large number of open pores (Fig. 3a). Further increase of SPS temperature to 1200÷1300°C leads to formation of ceramics with a denser structure. The mass contrast clearly seen on the SEM images enables one to estimate phase distribution of the components, taking into account that dark grains are MgO phase and the bright grains belong toY2O3. As could be seen, the Y2O3 and MgO grains are fine and uniformly distributed in the samples. Energy dispersive X-ray spectra (EDXS)
9
collected from specified area of Y2O3−MgO ceramics (Fig. 3e) prove the uniform distribution of the components in the mixture. Several conclusions could be drawn from the analysis of FESEM data. Primarily, SPS of the nitrate-glycine nanopowders leads to formation of homogenous bi-phase nanoceramics. Additionally, uniform phase separation enables one to stabilize ceramics grain size in the most efficient way due to the pinning effect (the grains of each phase pin the boundary of another phase). The immiscible characteristic of the Y2O3 and MgO (system is a stable two-phase mixture below the eutectic point T=2110°C [14]) helps to limit grain coarsening by phase boundary impingement during sintering [3]. Besides, uniform distribution of two phases leads to long-range mass transport between interphase boundaries, which has a substantial limiting effect of the atomic movement [7]. All mentioned above allows preparing fine-grained ceramics and avoiding excessive grain growth.
(a)
(b)
(c)
(d)
10
(e) Fig. 3. FESEM images of Y2O3−MgO composite SPS ceramics sintered at 1100 (a), 1200 (b), 1250 (c) and 1300°C (d) for 8 min under 50 MPa; EDXS of Y2O3−MgO composite ceramics sintered at T=1300°C (e).
To minimize light scattering is not only to provide submicron composite’s mean grain size, but also to control it by altering SPS parameters. The relationship between the sintering temperature and grain size of nanocomposite ceramics is shown in Fig. 4. Mean grain size grows with temperature from 0.16 and 0.20 µm to 0.24 and 0.28 µm for Y2O3 and MgO, respectively. Obtained results is connected with the tendency to combine the same phase grain nearby, because thermally activated processes of mass transfer target at reducing total energy of the system via shrinking grain boundaries [15, 16]. Grain growth is obviously limited by the number of neighboring grains of the same composition. That is why, uniform phase separation of our system enables to avoid uncontrolled grain growth leading to cut-off of the short light wavelength [1].
11
(a)
(b)
(c)
(d)
Fig. 4. Grain size distribution of Y2O3–MgO composite SPS ceramics sintered at 1200 (a), 1250 (b) and 1300°C (c) for 8 min under 50 MPa; the effect of SPS temperature on average sizes of Y2O3 and MgO grains (d). XRD patterns of Y2O3−MgO fine-grained SPS ceramics sintered at T=1100-1300°С exhibit a number of narrow lines of high intensity; the characteristic pattern is presented on Fig. 5a. XRD analysis has shown that samples are a mixture of cubic yttrium oxide (space group Iа3, JCPDS No. 41-1105) and cubic magnesia (space group Fm3m, JCPDS No. 45-0946). Impurity phases were not revealed at the XRD sensitivity limit. This proves that Y2O3 and MgO do not react during SPS. Fractional content of phases are 58.6 wt.% for Y2O3 and 41.4 wt.% for MgO that is in good agreement with theoretical values (Y2O3 – 58.3 wt.%, MgO – 41.75 wt.%) and corresponds to volume ratio 50:50. Let’s consider evolution of structural parameters of Y2O3 and MgO as a function of sintering temperature. Cubic lattice constant of Y2O3 (а) at 1100°C is practically equal to theoretical value of а=10.6041 Å (Fig. 5b). As was mentioned above, ceramics prepared at T=1100°C is characterized by a highly porous microstructure and their grains do not contact tightly, therefore, there are a lot of open pores (Fig. 3a). Absence of tight contact between the grains of different phases and presence of non-contacting (“free”) grains yield rather low crystal 12
lattice deformation of the components with the lattice parameter being close to theoretical. Further temperature increase to T=1200, 1300°C results in considerable reduction of Y2O3 lattice parameter а to 10.5884-10.5879 Å values, respectively. This is caused by microstrains occurring at the grain boundary. The same effect but with the opposite sign is observed for a parameter of MgO cubic lattice (Fig. 5b). Lattice parameter change may evidence deformation of elementary cell due to adjustment of crystallographic planes of two oxides to each other. According to XRD, Y2O3 crystallize size is 45 nm, while for MgO – 55 nm. Taking into account that grain boundaries of polycrystalline materials are formed via twin formation [17, 18], it can be concluded that each grain of Y2O3 and MgO consists of 4-5 crystal blocks.
(a)
(b)
Fig. 5. XRD pattern of Y2O3−MgO composite SPS ceramics sintered at 1300°C for 8 min under 50 MPa (a); a parameter of Y2O3 and MgO crystal lattices as a function of SPS temperature in the range of 1100-1300°C (b).
3.3 Mechanical and optical properties of SPS sintered Y2O3−MgO composite ceramics Fig. 6a shows the relative density of Y2O3-MgO ceramics as a function of the densification conditions. Spark plasma sintering of starting nanopowders at
13
1100°C did not yield highly-dense ceramics. Relative density of nanocomposites was 82% (3.6 g/cm3), which is far below the theoretical value (4.3 g/cm3 [1]). With an raise of the sintering temperature, diffusion mass transfer processes speed up, and the relative density of the samples increases and approaches 100 % for sintering temperature of T=1300°С. Thus, the optimal sintering temperature for formation of almost pore-free Y2O3-MgO ceramics was determined to be 1300°C. The obtained density data are correspond to the data in [7], where ceramics was obtained from smaller GNP-produced nanopowders by the HIP method. Mechanical properties of nanocomposite ceramics are greatly affected by ceramics density and grain size, which in turn depend on sintering temperature. Fig. 6b shows the microhardness as a function of the densification conditions. The samples sintered at 1100°C are characterized by the lowest microhardness ~5 GPa, while the highest values of 10.9 GPa were demonstrated by samples prepared at 1200°C. It is much higher than those of single-phase Y2O3 and MgO ceramics [1, 4, 19, 20]. Microhardness of nanocomposite ceramics obtained at 1250 and 1300°C reduces by 10 and 20%, respectively, despite the relative density increases from 97 % to 100% (from 4.2 to 4.4 g/cm3) with sintering temperature (Fig. 6a). Diffusion mass transfer processes become accelerated at high temperatures leading to removal of residual pores in the material. Along with pore elimination, raising sintering temperature promotes grain growth in ceramics samples (Fig. 3). Hardness decrease at larger grains is caused by reduction of the grain boundaries extension, which block dislocations motion (similar to Hall-Petch relationship). To prove that, Fig. 6с shows linear correlation between microhardness and square root of the average grain size. Grain size has the decisive effect on microhardness values. Microhardness of the composite ceramics with the lowest porosity obtained at sintering temperature T=1300°C is slightly lower than that of ceramics prepared at lower temperatures. The microhardness values of the ceramics obtained in our work using the SPS method are comparable to those obtained by hot pressing in [7]
14
and about 10 % exceed the microhardness values of ceramics obtained by the SPS method [21] (Fig.6с).
(a)
(b)
(c) Fig. 6. Relative density (a) and microhardness (b) of the Y2O3−MgO SPS nanoceramics sintered at 1100-1300°C for 8 min under 50 MPa; Relation of hardness and average grain size of Y2O3−MgO ceramics sintered at T=1100,1200, 1250 and 1300°C (b) vs. data [7, 21]
Fig. 7 shows IR transmission spectra of the Y2O3−MgO specimens sintered at different temperatures. Let’s consider the contribution of the porosity and average grain size of the nanocomposite ceramics to the optical transmittance. The micropores in the nanocomposite will degrade the optical transmittance due to the scattering losses connected with the great difference between refraction indexes of the pores and phases constituting the composite. Ceramics obtained at 1100°C is opaque due to numerous pores, light scattering sources, and immature grained 15
structure (Fig. 3a). As shown on Fig. 7a, IR transmittance of the Y2O3−MgO nanocomposites tends to increase with the sintering temperature. The transmittance values of the 1200, 1250 and 1300°C ceramics reach 50, 60 and 70% at λ=6000 nm, respectively, apparently owing to the growth in density with the sintering temperature. Besides, reduction in transmittance values is connected with the interphase grain boundary scattering due to significant difference in the refractive index between MgO and Y2O3. The highest transmittance at the level of 71%@6000 nm is observed for nanocomposite sintered at Т=1300°С (mean grain size Y2O3 – 0.24 µm, MgO – 0.28 µm, Fig. 4), however, obtained transmittance is lower than theoretical one of 85%, but comparable with the values reported earlier [5, 7, 9, 10]. In addition, the cut-off wavelength of Y2O3−MgO fine-grained ceramics reaches 9500 nm, which is superior to the majority of the used mid-IR transparent materials. Complex absorption band in the range 2800-4000 nm (Fig. 7a) is caused by water molecules adsorbed from air (MgO phase has a strong hydrophilic property) [4, 9], with the stretching vibration of hydroxyl groups (O-H) being observed. An intense broad peak occurs near 7000 nm corresponding to the asymmetrical and symmetrical stretching vibrations of the carboxylate groups (O–C=O), which were formed during the solution combustion synthesis of nanopowders and remained after annealing [7, 10]. Fig. 7b shows transmission calculated with a model that includes only reflection and Mie scattering. Two main differences could be revealed when comparing predicted results with the experimental data (Fig. 7a). Primarily, optical transmittance in the experiment (Fig. 7a) is lower than in theory (Fig. 7b), which is attributed to scattering on nano- and micropores. Additionally, theoretical calculations does not produce bands in the 2800-4000 nm and ~7000 nm wavelength range, which we assigned above to absorption. However, experimental and theoretical spectra coincide well in the short wavelengths range. According to theoretical results, short wavelength transmission cut-off at ~1500 nm corresponds 16
to grain size of 150-200 nm that is in good agreement with experimentally observed for Y2O3−MgO ceramics (Fig. 4). These results indicate that the mechanical properties and IR-transmittance can be improved by engineering microstructure through optimizing sintering conditions of the nanocomposite, such as sintering temperature.
(a)
(b)
(c) Fig. 7. IR-transmission spectra of Y2O3−MgO composite ceramics: experimental (a) and predicted (b) data; typical appearance of Y2O3−MgO sample SPS sintered at 1300°C for 8 min under 50 MPa (c).
4. Conclusions 50:50 vol.% Y2O3–MgO ceramic nanocomposites were successfully prepared by the means of GNP and SPS technique. Fine weakly agglomerated 17
Y2O3–MgO nanopowders annealed in air at T=1000°C (dSEM=60 nm, dBET=86 nm) were sintered in the range T=1100-1300°C for 8 min at 50 MPa under SPS conditions. Raising sintering temperature has been demonstrated to increase mean grain size of both phases due to thermally activated processes of mass transfer (Y2O3: 0.16-0.24 µm, MgO:0.20-0.275 µm). At sintering temperatures ranging in 1200-1300°C, Y2O3 and MgO crystal lattices have been shown to deform due to microstrains arising on the interphase boundaries. Vickers microhardness of the nanocomposite has been found to be greatly affected by the grain size (Hall-Petch strengthening). The maximum Vickers hardness of 10.9 GPa is achieved for ceramics sintered at T=1200°C. Subsequent microhardness reduction (T=12501300°C) is caused by grain growth with the creep limit being decreased. The correlations between the optical properties and the SPS temperature were investigated. IR transmittance of the Y2O3–MgO nanocomposites tends to increase with the SPS temperature that can be attributed to increase of density, grain growth, and decrease of extension of interphase grain boundaries. As a result, Y2O3–MgO ceramics sample sintered at T=1300°С is characterized by the highest IRtransmittance of 71%@6000 nm wavelength.
Acknowledgements The authors are grateful to Dr V.G. Kuryavyi and Dr. A.A. Kuchmizhak for their help in characterization of the ceramic samples. This work was supported by the National Academy of Sciences of Ukraine within the frame of the project “Nanoceramics”. D.Yu.K. appreciate the financial support from the Ministry of Science and Higher Education of the Russian Federation (Grant No. 3.2168.2017/4.6).
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Figure captions
Fig. 1. SEM (a) and TEM (b) images of the Y2O3–MgO nanopowders annealed at 1000°С for 4 hours.
Fig. 2. Densification dynamics of Y2O3−MgO nanopowders during SPS at 1100 (a), 1200 (b), 1250 (c) and 1300°C (d) for 8 min under 50 MPa.
Fig. 3. SEM images of Y2O3−MgO composite SPS ceramics sintered at 1100 (a), 1200 (b), 1250 (c) and 1300°C (d) for 8 min under 50 MPa; EDXS of Y2O3−MgO composite ceramics sintered at T=1300°C (e).
Fig. 4. Grain size distribution of Y2O3–MgO composite SPS ceramics sintered at 1200 (a), 1250 (b) and 1300°C (c) for 8 min under 50 MPa; the effect of SPS temperature on average sizes of Y2O3 and MgO grains (d). Fig. 5. XRD pattern of Y2O3−MgO composite SPS ceramics sintered at 1300°C for 8 min under 50 MPa (a); a parameter of Y2O3 and MgO crystal lattices as a function of SPS temperature in the range of 1100-1300°C (b).
Fig. 6. Density and Vickers hardness of the Y2O3−MgO SPS nanoceramics sintered at 1100-1300°C for 8 min under 50 MPa (a); Relation of hardness and average grain size of Y2O3−MgO ceramics sintered at T=1100,1200, 1250 and 1300°C (b). Fig. 7. IR-transmission spectra of Y2O3−MgO composite ceramics: experimental (a) and predicted (b) data; typical appearance of Y2O3−MgO sample SPS sintered at 1300°C for 8 min under 50 MPa (c).
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Declaration of interests Manuscript - Influence of sintering temperature on structural and optical properties of Y2O3–MgO composite SPS ceramics Authors - N.A. Safronova, O.S. Kryzhanovska, M.V. Dobrotvorska, A.E. Balabanov, А.V. Tolmachev, R.P. Yavetskiy, S.V. Parkhomenko, R. Brodskii, V.N. Baumer, D.Yu. Kosyanov, O.O. Shichalin, E.K. Papynov, Jiang Li The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.