Microstructural and optical properties of the ZnS ceramics sintered by vacuum hot-pressing using hydrothermally synthesized ZnS powders

ZnS nanopowders annealed at low temperatures (≤550?°C) have a pure cubic structure, while a small amount of hexagonal phase formed in specimens annealed at temperatures ≥700?°C. The particle sizes of the ZnS nanopowders increased with the annealing temperature. ZnS ceramics that were sintered using ZnS nanopowders annealed at low temperatures (≤550?°C) exhibited low transmittance, because of their porous microstructure. ZnS ceramics that were synthesized using ZnS powders annealed at high temperatures (≥800?°C) containing large agglomerated particles, also exhibited low transmittance, due to the presence of a liquid phase. A carbonate absorption band was found from the ZnS ceramics with small grains, because carbon ions diffused from the graphite mold into the ZnS ceramics during sintering, probably through the grain boundaries, and formed carbonates. A ZnS ceramic that was sintered at 1020?°C using the nanopowders annealed at 750?°C exhibited dense microstructure, with a large transmittance, 68%, in the wavelength range 6.0–12?μm.     

Effect of annealing process on IR transmission and mechanical properties of spark plasma sintered Yttria

In this study, fully dense Yttria ceramics were successfully fabricated by spark plasma sintering (SPS) at temperatures of 1300 and 1350 $℃$. The effects of post-annealing on IR transmission were investigated by Fourier transform infrared spectroscopy (FTIR) at various temperatures ranging from 1050 to 1250 $℃$. It was found that the optimum annealing temperature depends strongly on the sintering temperature. Annealed samples showed white opaqueness mainly due to the increase and coalescence of pores after annealing and showed an absorption band around 6.6 $\mu m$ which limits usage of yttria in IR applications. Sintering at 1350 $℃$ and annealing at 1250 $℃$ led to the maximum IR transmittance above 80% at wavelength of 5 $\mu m$ for a 3.5-mm-thick sample. The hardness and the fracture toughness of the samples were analyzed in detail and hardness of 9.2 GPa and fracture toughness of 1.65 MPa m^1/2 were obtained for the above sample.     

Effect of heating rate on properties of transparent aluminum oxynitride sintered by spark plasma sintering

High heating rates ranging from 50 to 250°C/min are selected to rapidly sinter transparent aluminum oxynitride (AlON) ceramics by spark plasma sintering (SPS) at 1600°C under 60 MPa using a bimodal AlON powder synthesized by the carbothermal reduction and nitridation method. With 1 minute holding time before cooling, all the specimens show high density and high transparence. The maximum transmittance is up to 74.5%-80.6%, where the maximum transmittance is positively correlated with the heating rate. Further analysis reveals that faster heating rates enable the decrease in the amount of the AlON phase decomposed into the α-Al₂O₃ and AlN phases during heating. These α-Al₂O₃ and AlN phases have to be converted back to AlON at the final stage of sintering, which indicates that a decrease in the amount of the α-Al₂O₃ and AlN phases via the boosted heating leads to the higher transmittance of the AlON ceramics. The high heating rates and short holding duration of the SPS utilized in this study result in the fine grain size of the obtained ceramics (1-6 μm) compared to that of the AlON ceramics fabricated by the conventional sintering method. This effect of high heating rates is confirmed by the coupled densification-grain growth modeling. In turn, the obtained AlON specimens exhibit a Vickers hardness of 15.87-16.62 GPa.     

Microstructure development and optical properties of Fe:ZnSe transparent ceramics sintered by spark plasma sintering

Fe:ZnSe transparent ceramics were prepared by spark plasma sintering. Fe:ZnSe powders synthesized via co-precipitation yielded well-dispersed particles with an average particle size of 550 nm. These powders were in the cubic phase Fe:ZnSe, indicating the successful substitution of Fe²⁺ for Zn²⁺. The highest relative density, 99.4%, was obtained by increasing the pressure and sintering time. The effects of sintering temperature, pressure, and time on the microstructure of SPS prepared ceramics were presented by micrographs. With increasing sintering temperature, from 600°C to 900°C, the average grain size increased from < 1 to 10 μm. The intergranular fracture indicated no neck formation in the sintering process. High pressure was essential for the densification process. The average grain size deceased from approximately 10 to 5 μm when the pressure was increased. Increasing the sintering time from 10 to 120 minutes lead to a change in the microstructure, from inter- to transgranular fracture, and eliminated the micropores. The as-prepared Fe:ZnSe ceramics were composed of single-phased cubic ZnSe. The sample sintered at 900°C under a pressure of 90 MPa for 120 minutes yielded a transmittance of approximately 60% at 1.4 μm and 68% at 7.5 μm and had residual micropores as its main scattering source. There was a strong characteristic absorption peak of Fe²⁺ ions at around 3 μm, which was red-shifted compared to Fe:ZnS transparent ceramics. Fe:ZnSe transparent ceramics have a reddish-brown color and it could be a promising mid-infrared laser material.     

Fabrication of translucent AlN ceramics with MgF2 additive by spark plasma sintering

Translucent AlN ceramics with 0‐2 wt.% MgF₂ additive were prepared by spark plasma sintering. AlN powder was heated temporarily up to 2000°C, before holding at 1850°C for 20 minutes in N₂ gas. The sintered ceramics consisted of a single phase of hexagonal AlN, and showed a transgranular fracture mode. The total transmittance was improved remarkably by the additive, to reach 74% at a wavelength of 800 nm for 1 wt.% MgF₂. For 2 wt.% MgF₂, the transmittance was slightly lower than that for 1 wt.% MgF₂, and an absorption band was observed apparently at around 400 nm. The addition of MgF₂ along with the temporary heating at higher temperatures than the sintering temperature contributed to improve the transmittance remarkably.     

Fabrication of sub-micrometer MgO transparent ceramics by spark plasma sintering

Transparent MgO ceramics were fabricated by spark plasma sintering (SPS) of the commercial MgO powder using LiF as the sintering additive. Effects of the additive amount and the SPS conditions (i.e., sintering temperature and heating rate) on the optical transparency and microstructure of the obtained MgO ceramics were investigated. The results showed that LiF facilitated rapid densification and grain growth. Thus, the MgO ceramics could be easily densified at a moderate temperature and under a low pressure. In addition, the transparency and microstructure of the MgO ceramics were found to be strongly dependent on the temperature and heating rate. For the MgO ceramics sintered at 900 °C for 5 min with the heating rate of 100 °C/min and the pressure of 30 MPa from the powders with 1 wt% LiF, the average in-line transmittance reached 85% in the range of 3 – 5 μm, and the average grain size is ∼0.7 μm.     

Synthesis routes for enhanced piezoelectric properties in spark plasma sintered Ta-doped KNN ceramics

Ta substitution in K_0.5Na_0.5NbO₃ lead free piezoelectrics helps to prevent grain growth and has been shown to improve the piezoelectric properties but always leads to hetereogeneous microstructures. Two synthesis routes have been studied to prepare K_0.5Na_0.5Nb_0.8Ta_0.2O₃ (KNNTa) substituted powders. Then highly densified KNNTa ceramics have been obtained by spark plasma sintering (SPS). The use of a synthesized oxide precursor Nb_1.6Ta_0.4O₅ during the ceramic elaboration process clearly shows through accurate Rietveld study a successful Ta substitution with 92% of Amm2 K_0.485(8)Na_0.515(8)Nb_0.819(6)Ta_0.181(6)O₃ phase, confirmed by SEM-EDS analysis and a more homogeneous chemical composition. This leads to enhanced electromechanical coupling coefficients with an improvement of 50% of k_t, 15% for k_p and low electrical losses, compared to the conventional synthesis method with a simple mixing of commercial precursors.     

Ultraviolet emission transparent Gd:YAG ceramics processed by solid-state reaction spark plasma sintering

Transparent 1% Gd-doped YAG and YAG ceramics were synthesized via solid-state reaction spark plasma sintering using commercially available powder and TESO as a sintering additive. The highest in-line transmission values achieved were 77.1% at 550 nm and 80.6% at 800 nm in the 1% (at.%) Gd-doped YAG transparent ceramic with 99.90% relative density. Ultraviolet emission at 312.5 nm was observed in 1% Gd-doped YAG ceramic via photoluminescence excitation, making it a promising material for applications in solid-state UV devices.     

Electron spin resonance of transparent alumina ceramics fabricated by spark plasma sintering

Transparent α‐alumina ceramics are fabricated using spark plasma sintering. Paramagnetic defects related to the optical properties of the ceramics have been investigated using electron spin resonance (ESR) analyses. An isotropic ESR signal at g = 2.003 (S = 1/2) with a linewidth of 0.5 mT is formed during sintering. The g = 2.003 signal intensity has a weak correlation with the absorbance in the visible region but does not correlate with the real in‐line transmission (RIT) at 650 nm. An ESR signal with a fine structure attributed to Fe³⁺ was detected in both the α‐Al₂O₃ starting powder and the sintered ceramic samples. The degree of c‐axis orientation of the grains has been determined using the Fe³⁺ signal intensity, which depends on the angle between the directions of the c‐axis and the applied magnetic field. The ESR analysis indicated that the c‐axis tends to be oriented in the direction of the sintering pressure. The degree of c‐axis orientation was found to correlate with the RIT in highly densified ceramics.     

High-strength TiB2-B4C composite ceramics sintered by spark plasma sintering

Spark plasma sintering (SPS) is an advanced sintering technique because of its fast sintering speed and short dwelling time. In this study, TiB₂, Y₂O₃, Al₂O₃, and different contents of B₄C were used as the raw materials to synthesize TiB₂-B₄C composites ceramics at 1850°C under a uniaxial loading of 48 MPa for 10 min via SPS in vacuum. The influence of different B₄C content on the microstructure and mechanical properties of TiB₂-B₄C composites ceramics are explored. The experimental results show that TiB₂-B₄C composite ceramic achieves relatively good comprehensive properties and exceptionally excellent flexural strength when the addition amount of B₄C reaches 10 wt.%. Its relative density, Vickers hardness, fracture toughness, and flexural strength reach to 99.20%, 24.65 ± .66 GPa, 3.16 MPa·m^1/2, 730.65 ± 74.11 MPa, respectively.     

Effects of SiC on phase transformation and microstructure of spark plasma sintered α/β-SiAlON composite ceramics

α/β-SiAlON/SiC composite ceramic tool materials were prepared via spark plasma sintering. The effects of content and size of SiC particles and sintering temperature on phase composition, mechanical properties, and microstructure were investigated. The results indicated that SiC restrained the transformation of β-SiAlON to α-SiAlON, but higher SiC content (≥10 wt.%) resulted in a higher Vickers hardness of the composite. The large size of SiC particles raised the densification temperature of α/β-SiAlON composites, and small SiC particles benefited to improve microstructure. There were more equiaxed α-SiAlON grains and β-SiAlON with a larger aspect ratio (${\overline{\alpha }}_{95}$ = 5.1) in the α/β-SiAlON composite containing 100 nm SiC. The sample containing 10 wt.% 100 nm SiC particles sintered at 1700°C had the optimal properties with a Vickers hardness and fracture toughness of 18.5 ± .2 GPa, 6.4 ± .2 MPa m^1/2, respectively.     

Transparent alumina ceramics: Effect of standard and plasma generated stabilizing approaches in colloidal processing

The study is focused on an optimization of the slip-casting process used for the fabrication of the transparent/translucent alumina ceramics; more precisely, on specifying the most appropriate way to stabilize the cast alumina suspensions. An innovative method of the particles’ stabilization by plasma treatment was compared with the classical electrostatic and the most frequently used electrosteric approach. Properties of green bodies (pore size distribution, density) and sintered samples (density, mean grain size, real in-line transmittance) were measured in term to evaluate the impact of the individual stabilization mechanism on the final properties of the transparent/translucent ceramics. The results showed that all tested approaches enable the preparation of the transparent/translucent alumina ceramics by Hot Isostatic Pressing. Ceramics prepared from the plasma treated as well as the electrostatically stabilized powders exhibited narrower pore size distribution, higher density, and lower mean grain size in comparison to ceramics fabricated from only electrosterically stabilized powders. Despite these promising properties the plasma-treated samples resulted in an unexpectedly low RIT of 36% caused by the presence of thin cracks. However, the electrostatically stabilized samples achieved the highest RIT value of 57%.     

Enhanced bipolar fatigue resistance in PMN-PZT ceramics prepared by spark plasma sintering

The bipolar fatigue behaviors of lead magnesium niobate-lead zirconate titanate (PMN-PZT) ceramics sintered by conventional sintering (CS) and spark plasma sintering (SPS) were systematically investigated. Significantly enhanced bipolar fatigue resistance was observed for SPS samples by a comparative analysis of the evolution of both large signal (polarization, strain) and small signal (piezoelectric coefficient, permittivity) hysteresis curves. The enhanced fatigue resistance is not only attributed to the suppressed development of microcracks, which is due to the decrease in grain size and porosity, but is also related to the reduction of domain wall pinning effect induced by migratory point defects, especially oxygen vacancies. Besides, the different phase structure and its evolution upon poling and fatigue are also responsible for the enhanced fatigue resistance.     

Transparent ultrafine Yb3+:Y2O3 laser ceramics fabricated by spark plasma sintering

Conventional ceramic processing techniques do not produce ultrafine‐grained materials. However, since the mechanical and optical properties are highly dependent on the grain size, advanced processing techniques are needed to obtain ceramics with a grain size smaller than the wavelength of visible light for new laser sources. As an empirical study for lasing from an ultrafine‐grained ceramics, transparent Yb³⁺:Y₂O₃ ceramics with several doping concentrations were fabricated by spark plasma sintering (SPS) and their microstructures were analyzed, along with optical and spectroscopic properties. Laser oscillation was verified for 10 at.% Yb³⁺:Y₂O₃ ceramics. The laser ceramics in our study were sintered without sintering additives, and the SPS produced an ultrafine microstructure with an average grain size of 261 nm, which is about one order of magnitude smaller than that of ceramics sintered by conventional techniques. A load was applied during heating to enhance densification, and an in‐line transmittance near the theoretical value was obtained. An analysis of the crystal structure confirmed that the Yb³⁺:Y₂O₃ ceramics were in a solid solution. To the best of our knowledge, this study is the first report verifying the lasing properties of not only ultrafine‐grained but also Yb‐doped ceramics obtained by SPS.     

Thermoelectric properties of In- and Ga-doped spark plasma sintered ZnO ceramics

In this study, indium (In)- and gallium (Ga)-doped zinc oxide (ZnO) ceramics, [Zn_(1?x?y)Ga_xIn_y]O (x = 0, 0.02; y = 0, 0.005, 0.01, 0.02), were fabricated via spark plasma sintering (SPS) at 1423 K. Crystal structure and microstructural analyses were conducted to confirm the solubility of the dopants and understand the correlations between the crystallographic phases and the various compositions. It was confirmed that the solubility of Ga (x = 0.02; y = 0.005) was promoted by doping with In and Ga, and the highest power factor of 0.99 mW K^?2 m^?1 was acquired at 1046 K. Furthermore, the thermal conductivity at 340–530 K was reduced by doping with In and Ga.     

Residual porosity and optical properties of spark plasma sintered transparent polycrystalline cerium-doped YAG

Transparent cerium-doped yttrium aluminum garnet (Ce:YAG) phosphors are promising candidates for high-power white light emitting diode applications. In the present study, Ce:YAG powder was synthesized by a co-precipitation method and highly transparent ceramics were fabricated by spark plasma sintering. The effects of temperature and pressure, as well as post-sintering treatments (annealing or hot isostatic pressing), on residual porosity were studied by electron and confocal laser microscopy. Correlation between residual porosity characteristics (pore size and volume fraction) and optical properties (in-line transmittance and photoluminescence intensity) of the luminescent transparent ceramics was established.     

Effects of A-site ionic size on the phase transition behavior of lead-free niobate ceramics

Understanding the phase structure evolution is important for developing high performance lead-free piezoelectric materials. In this work, the effects of A-site ionic size of monovalent ions on the phase transition behaviors for the lead-free niobate ceramics ANbO₃ (A = Li, Na, Ag, and K) are investigated using XRD analysis and dielectric measurement. The iso-valent ionic doping restrains the relaxation behavior that usually appears in the hetero-valent ionic-doped niobate ceramics. The A-site average ionic size of R_A and its ionic radius differences of ΔR_A are found to be crucial influence factors on the phase transition behaviors of the ANbO₃ ceramics. Small Li⁺ doping stabilize tetragonal phase of the ANbO₃ ceramics with R_A > 1.47 Å, but stabilize rhombohedral phase of the ones with R_A < 1.47 Å. On the other hand, The ANbO₃ ceramics without Li⁺ doping prefer to orthorhombic phase due to indistinctive ionic size differences (ΔR_A < 0.25 Å). Our results suggest that a certain phase and phase transition boundary could be designed by appropriate ionic doping for developing the niobate-based lead free piezoelectric ceramics.     

Microstructure and mechanical properties of the spark plasma sintered Ta2C ceramics

Single phase hexagonal α-Ta₂C ceramics were synthesized by spark plasma sintering and using TaC and Ta as the starting powders. Effects of sintering temperatures and holding times on the densification process, phase formation, microstructure development, and mechanical properties of the α-Ta₂C ceramics were investigated. Densification occurred in the temperature range of 1520–1675 °C in less than 2.5 min. But completion of the Ta₂C formation took about 40 min at 1500 °C, and 5 min at 1900 °C. The materials sintered at 1500 °C consisted of fine equiaxed grains. The Ta₂C grains grew anisotropic to form an elongated self-toughening microstructure at 1700 °C. At 1900 °C, the neighboring Ta₂C individual crystals coalesced to form large Ta₂C blocks to entrap the residual pores. Although higher flexural strength and fracture toughness were reached at 1700 °C, the unstable microstructures of the Ta₂C materials indicated limited applications at high temperatures.     

Thermal conductivity of spark plasma sintered SiC ceramics with Alumina and Yttria

In this study, dense SiC ceramics were fabricated at 1650?1750 °C for 10?60 min by spark plasma sintering (SPS) using 3?10 wt.% Al₂O₃-Y₂O₃ as sintering additives. Effects of sintering temperature, sintering additive content and holding time on microstructure as well as correlations between microstructure and thermal conductivity were investigated. An increase in the sintering temperature promotes grain growth. Extending holding time has little influence on grain size but results in formation of continuous network of sintering additive, which increases interfacial thermal resistance and thus decreases thermal conductivity. For SiC ceramics composed of continuous SiC matrix and discrete secondary phase (yttrium aluminum garnet, YAG), an increase in the sintering additive content results in smaller grain size and lower thermal conductivity. The lower thermal conductivity of the SiC ceramic with higher sintering additive content is mainly due to the smaller grain size rather than the low intrinsic thermal conductivity of YAG.