Spark plasma sintering of silicon nitride/barium aluminum silicate composite

Si₃N₄ based composites were successfully sintered by spark plasma sintering using low cost BaCO₃, SiO₂ and Al₂O₃ as additives. Powder mixtures were sintered at 1600–1800 °C for 5 and 10 min. Displacement-temperature-time (DTT) diagrams were used to evaluate the sintering behavior. Shrinkage curve revealed that densification was performed between 1100 and 1700 °C. The specimen sintered at 1700 °C showed the maximum relative density (99.8±0.1%), flexural strength (352±16 MPa), Vickers harness (11±0.1 GPa) and toughness (5.6±0.05 MPa m^1/2).     

Effect of crystallization temperature on dielectric and energy-storage properties in SrO-Na2O-Nb2O5-SiO2 glass-ceramics

The SrO-Na₂O-Nb₂O₅-SiO₂ (SNNS) glass-ceramics were prepared through the melt-quenching combined with the controlled crystallization technique. XRD results showed Sr₆Nb₁₀O₃₀, SrNb₂O₆, NaSr₂Nb₅O₁₅ with tungsten bronze structure and NaNbO₃ with the perovskite structure. With the decrease of crystallization temperature, dielectric constant firstly increased and then decreased, while breakdown strength (BDS) was increased. High BDS of the glass-ceramics is attributed to the dense and uniform microstructure at low crystallization temperature. The optimal dielectric constant of 140±7 at 900 °C and BDS of 2182±129 kV/cm at 750 °C were obtained in SNNS glass-ceramics. The theoretical energy-storage density was significantly improved up to the highest value of 15.2±1.0 J/cm³ at 800 °C, which is about 5 times than that at 950 °C. The discharged efficiency increased from 65.8% at 950 °C to 93.6% at 750 °C under the electric field of 500 kV/cm by decreasing crystallization temperature.     

Ni4Nb2O9 ceramics prepared by the reaction-sintering process

Fabrication of Ni₄Nb₂O₉ ceramics via a reaction-sintering process was investigated. A mixture of raw materials was sintered into ceramics by bypassing calcination and subsequent pulverization stages. Ni₄Nb₂O₉ phase appeared at 1300 °C and increased with increasing soak time. Ni₄Nb₂O₉ content was found >96% in 1350 °C/2 h sintering pellets. A density of 5.71 g/cm³ was obtained for pellets sintered at 1350 °C for 2 h. This reaches 96.5% of the theoretical density. As the sintering temperature increased to 1350 °C, an abnormal grain growth occurred and grains >100 μm could be found. ?_r of 15.4–16.9 are found in pellets sintered at 1200–1300 °C. Q × f increased from 9380 GHz in pellets sintered at 1200 °C to 14,650 GHz in pellets sintered at 1250 °C.     

Synthesis of BaCeO3 and BaCe0.9Y0.1O3−δ from mixed oxalate precursors

An oxalate precipitation route is proposed for the synthesis of BaCe_1−xY_xO₃ (x = 0 and 0.1) after calcination at 1100 °C. The precipitation temperature (70 °C) was a determinant parameter for producing a pure perovskite phase after calcination at 1100 °C for 1 h. TG/DTA measurements showed that the co-precipitated (Ba, Ce and Y) oxalate had a different thermal behaviour from single oxalates. Despite a simple grinding procedure, sintered BaCe_0.9Y_0.1O_3−δ pellets (1400 °C, 48 h) presented 90.7% of relative density and preliminary impedance measurements showed an overall conductivity of around 2 × 10⁻⁴ S cm⁻¹ at 320 °C.     

Synthesis and electrical properties of Ce1−xGdxO2−x/2 (x = 0.05–0.3) solid solutions prepared by a citrate–nitrate combustion method

Ce_1?xGd_xO_2?x/2 (GDC) powders with different Gd³⁺ contents (x = 0.05–0.3) were prepared by a simple citrate–nitrate combustion method. The influence of the Gd³⁺ doping content on the crystal structure and the electrical properties of GDC were examined. Many analysis techniques such as thermal analysis, X-ray diffraction, nitrogen adsorption analysis, scanning electron microscopy and AC impedance analysis were employed to characterize the GDC powders. The crystallization of the GDC solid solution occurred below 350 °C. The GDC powders calcined at 800 °C showed a typical cubic fluorite structure. The lattice parameter of GDC exhibited a linear relationship with the Gd³⁺ content. As compared with that sintered at other temperatures, the GDC pellet that sintered at 1300 °C had a high relative density of 97%, and showed finer microstructure. The conductivity of GDC was firstly increased and then decreased with the increase of the Gd³⁺ content. The sintered GDC sample with the Gd³⁺ content of 0.25 exhibited the highest conductivity of 1.27 × 10^?2 S cm^?1 at 600 °C.     

Influence of sintering conditions on piezoelectric properties of KNbO3 ceramics

A homogeneous KNbO₃ (KN) phase was formed in specimens that were sintered at 1020 °C and 1040 °C, without formation of the K₂O-deficient secondary phase, indicating that the amount of evaporation of K₂O during sintering was very small. However, the KN liquid phase was formed during sintering and assisted the densification of the KN ceramics. A dense microstructure was developed in the specimen sintered at 1020 °C for 6 h and abnormal grain growth occurred in this specimen. A similar microstructure was observed in the specimens sintered at 1040 °C for 1.0 h. The dielectric and piezoelectric properties of the KN ceramics were considerably influenced by the relative density. The KN ceramics sintered at 1020 °C for 6 h, which showed a large relative density that was 95% of the theoretical density, exhibited promising electrical properties: ɛ^T₃₃/ɛ_o of 540, d₃₃ of 109 pC/N, k_p of 0.29, and Q_m of 197.     

Densification and ablation behavior of ZrB2 ceramic with SiC and/or Fe additives fabricated at 1600 and 1800 °C

The effects of Fe and SiC additions on the densification, microstructure, and ablation properties of ZrB₂-based ceramics were investigated in this study. The sample powders were conventionally mixed by cemented carbide ball then sintered by spark plasma sintering. The ablation rates and behavior of the ceramics were investigated under an oxyacetylene torch environment at about 3000 °C. A sample with high relative density (96.3%), high flexural strength (415.6 MPa), and low linear ablation rate (−0.4 µm/s) was obtained via SPS at 1600 °C. Adding 4 vol% Fe was more beneficial to the density of ZrB₂ sintered at 1600 °C as compared to ZrB₂ sintered at 1800 °C. The ablation behavior and rates were similar among samples sintered at 1600 °C and 1800 °C.     

Mechanical and optical properties of spark plasma sintered transparent Y2O3 ceramics

Commercial Y₂O₃ nanopowder was used to fabricate transparent Y₂O₃ ceramics by spark plasma sintering under the pressure of 100 MPa for 20 min with the heating rate of 100 °C/min. The microstructures, mechanical and optical properties of the Y₂O₃ ceramics sintered at different temperatures were investigated in detail. Densification occurred up to a sintering temperature of 1500 °C, and above 1500 °C, rapid grain growth and pore growth occurred. The highest relative density of 99.58% and the minimum average grain size of 0.58±0.11 µm were obtained at 1500 °C. The flexural strength, hardness and fracture toughness of the optimal spark plasma sintered Y₂O₃ ceramic were 122 MPa, 7.60 GPa and 2.06 MPa.m^1/2, respectively. The Y₂O₃ ceramic sintered at 1500 °C had the in-line transmission of about 11–54% and 80% in the wavelength range of 400–800 nm and 3–5 µm, respectively.     

Mechanical properties of LaFeO3 ceramics

The fracture strength, fracture toughness and apparent Young’s modulus of LaFeO₃ ceramics in the temperature region 25–800 °C are reported. The fracture strength of the material was observed to increase from 202 ± 18 MPa at room temperature to 235 ± 38 MPa at 800 °C. The room temperature fracture toughness was 2.5 ± 0.1 MPa m^1/2. The fracture toughness decreased to 2.1 ± 0.1 MPa m^1/2 at 600 °C, followed by an increase to 3.1 ± 0.3 MPa m^1/2 at 800 °C. The temperature dependence of the fracture toughness correlates well with the crystallographic strain, |(a−c)|/(a+c), and ferroelastic toughening of LaFeO₃ materials is inferred. Non-elastic stress–strain behaviour of the LaFeO₃ materials due to ferroelasticity was confirmed by cyclic compression experiments, and residual strain was observed in the material after unloading.     

Microstructure,mechanical and thermal properties of in situ toughened boron carbide-based ceramic composites co-doped with tungsten carbide and pyrolytic carbon

Boron carbide (B₄C)-based ceramics were pressureless sintered to a relative density of 96.1% at 2150 °C, with the co-incorporation of tungsten carbide and pyrolytic carbon. The as-batched boron carbide power was 7.89 m² g^?1 in surface area. A level of fracture toughness as high as 5.80 ± 0.12 MPa m^1/2 was achieved in the BW-6C composite. Sintering aids of carbon and tungsten boride were formed by an in situ reaction. The toughness improvement was attributed to the presence of thermal residual stress as well as the W₂B₅ platelets. The thermal conductivity and thermal expansivity of the BW-6C composite as a function of temperature are also reported in this work. Our current study demonstrated that the B₄C–W₂B₅ composites could be potential candidate materials for structural applications.     

Effect of milling type on the microstructural and mechanical properties of W-Ni-ZrC-Y2O3 composites

This study reports the effect of milling type on the microstructural, physical and mechanical properties of the W-Ni-ZrC-Y₂O₃ composites. Powder blends having the composition of W-1 wt% Ni-2 wt% ZrC-1 wt% Y₂O₃ were milled at room temperature for 12 h using a Spex™ 8000D Mixer/Mill or cryomilled in the presence of externally circulated liquid nitrogen for 10 min using a Spex™ 6870 Freezer/Mill or sequentially milled at room temperature and cryogenic condition. Then, powders were compacted in a hydraulic press under a uniaxial pressure of 400 MPa and green bodies were sintered at 1400 °C for 1 h under Ar/H₂ atmosphere. Phase and microstructural characterization of the milled powders and sintered samples were performed using X-ray diffractometer (XRD), TOPAS software, scanning electron microscope/energy dispersive spectrometer (SEM/EDS), X-ray fluorescence (XRF) spectrometer and particle size analyzer (PSA). Archimedes density and Vickers microhardness measurements, and sliding wear tests were also conducted on the sintered samples. The results showed that sequential milling enables the lowest average particle size (214.90 nm) and it is effective in inhibiting W grain coarsening during sintering. The cryomilled and sintered composite yielded a lower hardness value (5.80±0.23 GPa) and higher wear volume loss value (149.42 µm³) than that of the sintered sample after room temperature milling (6.66±0.39 GPa; 102.50 µm³). However, the sequentially milled and sintered sample had the highest relative density and microhardness values of 95.09% and 7.16±0.59 GPa and the lowest wear volume loss value of 66.0 µm³.     

Al2O3–SiC composites prepared by warm pressing and sintering of an organosilicon polymer-coated alumina powder

Al₂O₃/SiC micro/nano composites were prepared by axial pressing of poly(allyl)carbosilane-coated submicrometre alumina powder at elevated temperature (called also warm pressing, or plastic forming) with subsequent pressureless sintering in the temperature interval between 1700 and 1850 °C. Warm pressing at 350 °C and 50 MPa resulted in green bodies with high mechanical strength and with markedly higher density than in green bodies prepared by cold isostatic pressing of the same powder at 1000 MPa. The sintering of warm pressed specimens moreover yielded the composites with higher final density (less than 4% of residual porosity) with the microstructure composed of micrometer-sized alumina grains (D₅₀ < 2 μm) with inter- and intragranular SiC precipitates. High sintering temperatures (>1800 °C) promoted the formation of intergranular platelets identified by TEM as 6H polytype of α-SiC. The maximum hardness (19.4 ± 0.5 GPa) and fracture toughness (4.8 ± 0.1 MPa m^1/2) were achieved in the composites containing 8 vol.% of SiC, and sintered for 3 h at 1850 °C. These values are within the limits reported for nanocomposites Al₂O₃/SiC by other authors and do not represent any significant improvement in comparison to monolithic alumina.     

Diphasic aluminosilicate gels with two stage mullitization in temperature range of 1200–1300 °C

The crystallization of mullite in amorphous diphasic gel aged for 6 months has been studied using non-isothermal differential scanning calorimetry (DSC) and powder X-ray diffraction with Rietveld structure refinement analysis. The diphasic premullite gels undergo structural changes by aging even when they are calcined at 700 °C. These changes imply segregation of the sample to Al₂O₃-rich and SiO₂-rich regions. From the Al₂O₃-rich region crystallizes poorly defined AlSi spinel at 977 °C followed by two-step mullite crystallization in the temperature interval of 1200–1300 °C. Two overlapped exothermic peaks on DSC scan of aged gel were observed; the first at 1233 °C and the second at 1261 °C. The former is attributed to mullite crystallization by transformation of AlSi spinel, by which excess alumina occurs, which in the second step of mullitization reacts with amorphous SiO₂-rich phase. The activation energy for mullite crystallization in the first step was E_a=935±14 kJ mol⁻¹ and the Avrami exponent n=2.5. The values E_a=1119±25 kJ mol⁻¹ and n=1.2 were obtained for mullite formation in the second step. If amorphous SiO₂-rich phase is extracted from the sample, the value E_a=805±26 kJ mol⁻¹ is obtained. Mullite crystallizing from AlSi spinel (when SiO₂-rich phase has been extracted) differentiates compositionally from that formed by both reactions. Smaller unit cell parameters and higher amount of oxygen vacancies are incorporated into tetrahedral positions of mullite structure, as was determined by Rietveld structure refinement method.     

Signature of multiferroicity and pyroelectricity close to room temperature in BaFe12O19 hexaferrite

In this paper, we have reported the signature of multiferroicity and pyroelectricity in BaFe₁₂O₁₉ hexaferrite close to room temperature. The BaFe₁₂O₁₉ hexaferrite samples are synthesized by co-precipitation method at different sintering temperature ranging from 800 to 1200 °C and study their structural, ferroelectric, magnetic, magnetoelectric and pyroelectric properties. X-ray Diffraction patterns show the pure phase formation for all samples. Morphological changes are examined through the scanning electron microscope. The maximum ferroelectric polarization (0.66 μC/cm²) is observed for the sample sintered at 1200 °C, however maximum magnetic polarization 74 emu/g is observed for sample sintered at 1000 °C. Magneto-electric coupling measurements are also performed through dynamic method and average magneto-electric coupling coefficient (~ 7.05 × 10⁻⁷ mV/cm Oe²) is observed at room temperature for the sample sintered at 1200 °C. Furthermore, maximum pyroelectric constant (147 × 10⁻¹³C/cm² °C) is observed at 75 °C for BaFe₁₂O₁₉ samples sintered at 1200 °C. The observation of both multiferroicity and pyroelectricity close to room temperature in BaFe₁₂O₁₉ hexaferrite is interesting and useful for multifunctional devices.     

Effects of MgO addition on the microwave dielectric properties of high thermal-conductive silicon nitride ceramics sintered with ytterbia as sintering additives

1 mol% of MgO was added together with 7 mol% of Yb₂O₃ as sintering additives to silicon nitride powder to fabricate advanced silicon nitride ceramics with both high thermal conductivity and low dielectric loss at 2 GHz. The mixed powder was CIPed at a pressure of 120 MPa and was gas-pressure sintered at 1900 °C to >98% of theoretical density. The sintered Si₃N₄ sample exhibited a high thermal conductivity of ～100 W m^?1 K^?1 and a loss tangent (tan δ) of ～4 × 10^?4, concurrently. The tan δ was further reduced by half after the heat treatment at 1300 °C for 24 h. The improvement in tan δ due to the annealing was explained from the point of crystallization of the intergranular glassy phase.     

Pressureless sintering of titanium carbide doped with boron or boron carbide

Titanium carbide ceramics with different contents of boron or B₄C were pressureless sintered at temperatures from 2100 °C to 2300 °C. Due to the removal of oxide impurities, the onset temperature for TiC grain growth was lowered to 2100 °C and near fully dense (>98%) TiC ceramics were obtained at 2200 °C. TiB₂ platelets and graphite flakes were formed during sintering process. They retard TiC grains from fast growth and reduced the entrapped pores in TiC grains. Therefore, TiC doped with boron or B₄C could achieve higher relative density (>99.5%) than pure TiC (96.67%) at 2300 °C. Mechanical properties including Vickers’ hardness, fracture toughness and flexural strength were investigated. Highest fracture toughness (4.79 ± 0.50 MPa m^1/2) and flexural strength (552.6 ± 23.1 MPa) have been obtained when TiC mixed with B₄C by the mass ratio of 100:5.11. The main toughening mechanisms include crack deflection and pull-out of TiB₂ platelets.     

Devitrification behaviour of plasma-sprayed glass coatings

Glass and glass-ceramic coatings on ceramic tiles have been manufactured by plasma-spraying high-performance CAS (in wt%—SiO₂, 60%; Al₂O₃, 15%; CaO, 23%; others, traces) and CZS (in wt%—SiO₂, 50%; CaO, 31%; ZrO₂, 16.5%; Al₂O₃, 2%; others, traces) glass frits. The CZS system has a surface crystallization at about 1050 °C. Such behaviour would not easily allow to obtain a fully crystalline bulk glass-ceramic, but the defectiveness of the plasma-sprayed coating supplies many nucleation sites. Thus, it becomes completely crystalline and well sintered after a 850 °C for 30 min + 1050 °C for 15 min treatment. The CAS frit, designed not to produce significant crystallization, is well sintered after a 850 °C for 30 min + 950 °C for 30 min thermal treatment, but remains too brittle due to its glassy nature. A 1050 °C treatment allows a few pseudowollastonite crystals to form in a glassy matrix; their formation also hinders sintering. Thus, mechanical properties are inferior to heat-treated plasma-sprayed CZS.     

Effects of packing fraction and bond valence on microwave dielectric properties of A2+B6+O4 (A2+: Ca,Pb, Ba; B6+: Mo,W) ceramics

Microwave dielectric properties of A²⁺B⁶⁺O₄ (A²⁺: Ca, Pb, Ba; B⁶⁺: Mo, W) ceramics were investigated as a function of packing fraction and bond valence. For A²⁺B⁶⁺O₄ specimens sintered at 800–1100 °C for 3 h, a single phase with a tetragonal scheelite structure was detected, and the theoretical density was higher than 93% throughout the composition. Although the ionic polarizability of Ba²⁺ ion was larger than that of Ca²⁺ ion, the dielectric constant (K) of BaB⁶⁺O₄ showed a smaller value than that of CaB⁶⁺O₄. These results could be attributed to changes of the packing fraction due to the effective ionic size. The Q·f value was largely dependent on the packing fraction, as well as the percentages of theoretical density. The temperature coefficients of the resonant frequencies (TCFs) of the specimens were affected by the bond valence of oxygen. The specimens of CaMoO₄ sintered at 1000 °C for 3 h showed the K of 10.8, Q·f of 76,990 GHz and TCF of ?22.8 ppm/°C, respectively.     

Structural,electromagnetic, and dielectric properties of lithium-zinc ferrite ceramics sintered by pulsed electron beam heating

Structural, electromagnetic, and dielectric properties of Li_0.4Fe_2.4Zn_0.2O₄ lithium-zinc ferrite sintered by 2.4 MeV pulsed electron beam heating at 1050 °С for 2 h were investigated. The formation of ferrite with a single-phase cubic spinel structure was confirmed by X-ray diffraction analysis. The average grain size of ferrite ceramic was determined by SEM analysis and its value was 1.7 µm. The radiation-thermal sintered samples are characterized by a saturation magnetization of 67.8 emu/g, the Curie temperature of 508 °C, AC electrical resistivity of 2.4×10⁴ Ω cm (at 25 °C). The frequency dependences of permittivity and the loss tangent were obtained in (20 – 2×10⁶) Hz frequency range. The behavior of ε′ is characterized by high dispersion caused by relaxation polarization in the investigated frequency range. The results were compared to the LiZn ferrite characteristics sintered by traditional thermal heating.     

Densification and mechanical properties of hot-pressed ZrN ceramics doped with Zr or Ti

The densification of hot-pressed ZrN ceramics doped with Zr or Ti have been investigated at 1500–1700 °C. It is shown that either Zr or Ti additive can facilitate the densification process. ZrN with 20 mol% Zr or Ti (named ZNZ and ZNT) sintered at 1700 °C can achieve above 98% relative densities whereas densification temperature up to 2000 °C is necessary for pure ZrN. The densification improvements are attributed to solid solution of Zr or Ti into ZrN to form non-stoichiometric ZrN_1?x or (Zr, Ti)N_1?x. The microstructures and mechanical properties of ZNZ and ZNT samples have been examined. Large grain size and flat fracture surface existed in ZNT sample sintered at 1700 °C, which lead to poor toughness as low as 2.3 MPa m^1/2. On the contrary, the fracture toughness of ZNZ sample sintered at 1700 °C was up to 5.9 MPa m^1/2, attributed to fine and uniform grain size distribution.