Transient liquid-phase sintering of AlN ceramics with CaF2 additive

Transient liquid-phase (TLP) sintering of CaF₂ additive on the densification behaviors and microstructural development of AlN ceramics are investigated. It is found that 1 wt% CaF₂ can effectively promote densification process. Increasing content of CaF₂ results in finer grain size and slower densification during intermediate sintering stage. XRD results show that grain-boundary phase of CaAl₄O₇ is formed at 1150 °C from reactions of AlN–CaF₂–Al₂O₃. With further temperature increasing, the grain-boundary phases of CA2 and CaAl₁₂O₁₈, which were formed from the reaction between CaF₂ and oxide layers, experienced transformations firstly into CaAl₄O₇ above 1600 °C and into CaAl₂O₄ at higher temperature. SEM and TEM results show that formed grain-boundary phases can evaporate from sintering bodies during further soaking, leaving clean grain boundaries. The efficiency of TLP sintering mechanism is further manifested by the preparation of transparent AlN ceramics with good combination properties.     

Onset coarsening/coalescence of cobalt oxides in the form of nanoplates versus equi-axed micron particles

The change of specific surface area and pore size distribution coupled with N₂ adsorption–desorption hysteresis isotherm, in particular that typical to cylindrical pores, were used to determine the onset coarsening/coalescence in the temperature range of 500–800 °C for Co(OH)₂ derived Co₃O₄ nanoplates and 700–1000 °C for CoO-derived Co₃O₄ powders (backtransformed to CoO above 900 °C) which are equi-axed in shape and microns in size. The vigorous onset coarsening/coalescence of the nanoplates and equi-axed micron particles was found to occur within minutes having apparent activation energy of 37 ± 7 kJ/mol (based on t_0.7, i.e. time for 70% surface area reduction) and 113 ± 8 kJ/mol (based on t_0.3), respectively. The surface area reduction process of the nanoplates was found to be controlled by (1 1 1)-specific coalescence besides a coarsening–repacking process more common to the equi-axed particles. The present static experimental results of coarsening–coalescence of the Co₃O₄ (below 900 °C) or CoO particles (above 900 °C) supports our previous supposition that CoO and Co₃O₄ nanocondensates could readily assemble as nanochain aggregates and further coalesce into a close packed manner below 1000 °C by the radiant heating effect in a dynamic laser ablation process.     

Thermal properties of La2O3-doped ZrB2- and HfB2-based ultra-high temperature ceramics

Thermal properties of La₂O₃-doped ZrB₂- and HfB₂-based ultra high temperature ceramics (UHTCs) have been measured at temperatures from room temperature to 2000 °C and compared with SiC-doped ZrB₂- and HfB₂-based UHTCs and monolithic ZrB₂ and HfB₂. Thermal conductivities of La₂O₃-doped UHTCs remain constant around 55–60 W/mK from 1500 °C to 1900 °C while SiC-doped UHTCs showed a trend to decreasing values over this range.     

Hot corrosion behavior of V2O5-coated Gd2Zr2O7 ceramic in air at 700–850 °C

Gd₂Zr₂O₇ ceramic was prepared by solid state reaction at 1650 °C for 10 h in air, and exhibited a defect fluorite-type structure. Reaction between molten V₂O₅ and Gd₂Zr₂O₇ ceramic was investigated at temperatures ranging from 700 to 850 °C using an X-ray diffractometer (XRD) and scanning electron microscopy (SEM). Molten V₂O₅ reacted with Gd₂Zr₂O₇ to form ZrV₂O₇ and GdVO₄ at 700 °C; however, in a temperature range of 750–850 °C, molten V₂O₅ reacted with Gd₂Zr₂O₇ to form GdVO₄ and m-ZrO₂. Two different reactions observed at 700 °C and 750–850 °C could be explained based on the thermal instability of ZrV₂O₇.     

Effect of calcination temperature on the fabrication of transparent lutetium titanate by spark plasma sintering

Transparent lutetium titanate (Lu₂Ti₂O₇) bodies were fabricated by spark plasma sintering using Lu₂O₃ and TiO₂ powders calcined from 700 °C to 1200 °C. No solid-state reaction was identified after calcination at 700 °C, whereas single-phase Lu₂Ti₂O₇ powder was prepared at 1100 and 1200 °C. The calcination at 700 °C promoted densification at the early stages of sintering, whereas residual pores at grain boundaries resulted in Lu₂Ti₂O₇ bodies with low transparency. Low-density and opaque Lu₂Ti₂O₇ bodies formed owing to the coarsening of the powder calcined at 1200 °C. The Lu₂Ti₂O₇ body sintered using the powder calcined at the moderate temperature of 1100 °C had a density of 99.5% with the highest transmittances of 41% and 74% at wavelengths of 550 nm and 2000 nm, respectively.     

Effect of spinel content on hot strength of alumina-spinel castables in the temperature range 1000–1500°C

Hot modulus of rupture of Al₂O₃-spinel castables containing 5–15 wt% alumina-rich magnesia alumina spinel and 1·7 wt% CaO generally increases with increase in spinel content and temperature from 1000 to 1500°C. The magnitudes of hot modulus of rupture of castables containing 15 wt% spinel and 1·7 wt% CaO are 14·3 MPa at 1400°C and 15·6 MPa at 1500°C, while those of castables containing 20 wt% spinel and 1·7 wt% CaO are 12·5 MPa at 1400°C and 14·7 MPa at 1500°C. The former castables contained 15 wt% spinel of −75 μm size, while the latter contained 10 wt% spinel of +75 μm size and another 10 wt% spinel of −75 μm size. The bond linkage between the CA₆ and spinel grains in the matrix is believed to cause both the spinel content and temperature dependence of hot strength of Al₂O₃-spinel castables, as well as fine grain spinel even in amount less than coarser grain spinel to be more effective for enhancing hot strength. The trend of the magnitude of thermal expansion under load (0·2 MPa) above 1500°C of the castables is not necessarily indicative of the magnitude of hot modulus of rupture at 1400 or 1500°C. ©     

Formation of porous aggregations composed of Al2O3 platelets using potassium sulfate flux

Porous aggregations, with about 10 μm diameter, composed of Al₂O₃ platelet crystals were formed by heating a powder mixture consisting of Al₂(SO₄)₃+2K₂SO₄ (mol ratio) in an alumina crucible at temperatures 1000–1300°C for 3 h and removing the flux component with hot hydrochloric acid after heating. The specific surface area of the aggregations obtained by heating at 1000°C for 3 h was maximum and its value was 5·2 m² g⁻¹. Since the size of Al₂O₃ platelets increased and the number of Al₂O₃ platelets decreased, the specific surface area decreased to 0·7 m² g⁻¹ at 1100°C. When heated at 1300°C, the size of the Al₂O₃ platelets increased with increasing amount of K₂SO₄ in the starting powder mixture. ©     

Characterization of Li–Zn–Fe crystalline phases in low temperature ceramic glaze

Ceramic glaze containing Li₂O and ZnO was prepared at a low firing temperature of 1100 °C. Addition of 0–30 wt.% iron oxide content developed brown color with a metallic sparkling effect from crystallization after soaking at 980–1080 °C. Using XRD, SEM/EDS and Raman microscopy the crystalline phases were determined as lithium zinc ferrite (Li_xZn_1?2xFe_2+xO₄ where x = 0.05–0.20), hematite (α-Fe₂O₃) and anorthite (CaAl₂Si₂O₈). The most preferable metallic sparkling effect was caused by the lithium zinc ferrite phase obtained from the glaze containing 10 wt.% of iron oxide. Thermal analysis by STA after heat treatment indicated that crystallization temperature of lithium zinc ferrite and the effective soaking temperature depended on the iron oxide content in the glaze. The influence of excessive iron oxide content on the crystallization behavior of lithium zinc ferrite, anorthite and hematite phases is discussed.     

Linear thermal expansion coefficients of relaxor-ferroelectric 0.57Pb(Sc1/2Nb1/2)O3–0.43PbTiO3 ceramics in a wide temperature range

The objective of this work was to examine linear thermal expansion of virgin and poled 0.57Pb(Sc_1/2Nb_1/2)O₃–0.43PbTiO₃ ceramics between 30 °C and 600 °C by contact dilatometry. The thermal expansion dL/Lo of the virgin ceramic increases with increasing temperature until approximately 260 °C. The physical and technical thermal expansion coefficients were determined. At 260 °C the physical thermal coefficient is 2.08 × 10^?6 K^?1. Between 260.0 °C and 280.0 °C an anomaly in the thermal expansion vs. temperature and an endothermic peak in the differential scanning calorimetry curves correspond to the phase transition region from tetragonal to cubic phase. At temperatures from 280 °C to 600 °C the thermal expansion dL/Lo increases again.In the derivative of the dL/Lo heating curves of the poled ceramics, additionally to the anomaly at 270 °C, also the anomaly at 160 °C is observed, which is associated with the depolarization of the material during heating.     

Novel nonaqueous precipitation synthesis of alumina powders

Alumina powders were prepared via a novel nonaqueous precipitation method with aluminum powders as aluminum source and anhydrous acetic acid as precipitant. The thermal decomposition and phase transformation of crystal precipitate and the influence of precipitate aging were investigated via TG-DTA-MS, XRD, TEM, BET, FE-SEM and performance tests of sintered bodies. The results show crystal precipitate C₄H₇AlO₇ transforms to amorphous Al₂O₃ at 300 °C, and then to γ-Al₂O₃ at 950 °C, and finally to α-Al₂O₃ at 1050 °C. The particle size of α-Al₂O₃ prepared at 1100 °C is 50–100 nm with BET surface area of 25.98 m²∙g⁻¹. FE-SEM morphology of sintered sample at 1400 °C shows excellent sinterability of the α-Al₂O₃ powders. Aging eliminates aggregation, and leads to highly homogenized and densified particles. It also affects the densification behaviour during sintering and further influences density, thermal expansion coefficient, flexural strength, volume resistivity and electric breakdown strength of sintered bodies     

Poly(N-isopropylacrylamide) assisted microwave synthesis of magnesium aluminate spinel oxide for production of highly transparent films by hot compression

Magnesium aluminate spinel oxides have been prepared via poly(N-isopropylacrylamide) assisted microwave technique. The prepared MgAl₂O₄ powders showed a crystalline cubic structure with spinel phase after calcination at 600 °C only. The poly(N-isopropylacrylamide) amount showed a high effect on the crystallite size and the densification behavior of MgAl₂O₄. The increase of the amount of poly(N-isopropylacrylamide) reduced the sintering temperature of MgAl₂O₄ from 1400 °C to 1050 °C. The hot-pressed of MgAl₂O₄ powders in the presence of 3 wt% of poly(N-isopropylacrylamide) exhibited a full density at sintering temperature 1100 °C in 15 min only. The sintered films showed high transparency (81 ± 2%) in the wavelength range 500–1000 nm.     

Effect of B2O3 and CuO additives on the sintering temperature and microwave dielectric properties of Ba(Zn1/3Nb2/3)O3 ceramics

The B₂O₃ added Ba(Zn_1/3Nb_2/3)O₃ (BBZN) ceramic was sintered at 900 °C. BaB₄O₇, BaB₂O₄, and BaNb₂O₆ second phases were found in the BBZN ceramic. Since BaB₄O₇ and BaB₂O₄ second phases have an eutectic temperature around 900 °C, they might exist as the liquid phase during sintering at 900 °C and assist the densification of the BZN ceramics. Microwave dielectric properties of dielectric constant (ɛ_r) = 32, Q × f = 3500 GHz, and temperature coefficient of resonance frequency (τ_f) = 20 ppm/°C were obtained for the BZN with 5.0 mol% B₂O₃ sintered at 900 °C for 2 h. The BBZN ceramics were not sintered below 900 °C and the microwave dielectric properties of the BBZN ceramics sintered at 900 °C were very low. However, when CuO was added, BBZN ceramic was well sintered even at 875 °C. The liquid phase related to the BaCu(B₂O₅) second phase could be responsible for the decrease of sintering temperature. Good microwave dielectric properties of ɛ_r = 36, Q × f = 19,000 GHz and τ_f = 21 ppm/°C can be obtained for CuO doped BBZN ceramics sintered at 875 °C for 2 h.     

Tribological properties of NiCr–ZrO2(Y2O3)–SrSO4 composites at elevated temperatures

The effect of SrSO₄ content on the tribological properties of NiCr–30wt%ZrO₂(Y₂O₃) (NC30Z) cermet was evaluated over a wide temperature range from room temperature to 1000 °C. The results indicated that the inclusion of SrSO₄ effectively improved the friction coefficients and wear rates of NC30Z cermet above 400 °C. NC30Z–5SrSO₄ composite against alumina ball exhibited satisfactory tribological performance, which was attributed to synergistic lubrication of pseudocubic-SrZrO₃ and NiCr₂O₄ between 400 °C and 800 °C and cubic-SrZrO₃, NiCr₂O₄, NiO and Cr₂O₃ at 1000 °C.     

Degradation of plasma-sprayed yttria-stabilized zirconia coatings via ingress of vanadium oxide

V₂O₅ reaction and melt infiltration in plasma-sprayed 7 wt% Y₂O₃–ZrO₂ (YSZ) coatings were investigated at temperatures ranging from 750 °C to 1200 °C using SEM and TEM combined with EDS. The interlamellar pores and intralamellar cracks, common in plasma-sprayed materials, provide pathway for the molten species. The microstructure of the contaminated coatings is therefore the result of the interplay between the dissolution/reaction rates of the V₂O₅ with YSZ coating and the infiltration rates of the molten species. Near the coating surface, the reaction front proceeds in a planar fashion, via dissolution of the lamella and precipitation of fine-grained reaction products composed of ZrV₂O₇ (for reactions at 750 °C and below), m-ZrO₂ and YVO₄. The thickness of this planar reaction zone or PRZ was found to increase as reaction time and temperature increased. The melted V₂O₅ was observed to infiltrate along the characteristic microstructure of plasma-sprayed coatings, i.e. the interconnected pores and cracks, and react with the YSZ. The thickness of this melt infiltrated reaction zone or MIRZ ranged from 5 μm for reactions at 750 °C for 30 min to 130 μm for reactions at 1000 °C for 90 min. At 1200 °C, only a PRZ was observed (i.e. the thickness of the MIRZ was nominally zero), suggesting that the dissolution reaction within the pores/cracks and subsequent formation of reaction products may limit infiltration. Fifty-hour heat-treatments at 1000 °C and 1200 °C prior to reaction with the V₂O₅ at 800 °C for 90 min were used to change the microstructural features of the coating, such as crack connectivity and pore size. The heat-treatment at 1000 °C was found most deleterious to the coating due to large cracks created via a desintering process that afforded deep penetration of the molten V₂O₅.     

Chemical oxidation of residual carbon from ZnAl2O4 powders prepared by combustion synthesis

The removal of carbon residue from ZnAl₂O₄ nanopowders by annealing at 500–800 °C leads to a decrease of specific surface area from 228.1 m²/g to 47.6 m²/g. At the same time, the average crystallite size increased from 5.1 nm to 14.9 nm. In order to overcome these drawbacks, a new solution for removing the carbon residue has been suggested: chemical oxidation using hydrogen peroxide. In terms of carbon removal, a H₂O₂ treatment for 8 h at 107 °C proved to be equivalent to a heat treatment of 1 h at 600 °C. The benefits of chemical oxidation over thermal oxidation were obvious. The specific surface area was much larger (188.1 m²/g) in the case of the powder treated with H₂O₂. The average crystallite size (5.8 nm) of ZnAl₂O₄ powder treated with H₂O₂ was smaller than the crystallite size (8.2 nm) of the ZnAl₂O₄ powder annealed at 600 °C.     

Calcium-magnesium-alumina-silicate (CMAS) resistance characteristics of LnPO4 (Ln = Nd,Sm, Gd) thermal barrier oxides

Calcium-magnesium-alumina-silicate (CMAS) attack has been considered as a significant failure mechanism for thermal barrier coatings (TBCs). As a promising series of TBC candidates, rare-earth phosphates have attracted increasing attention. This work evaluated the resistance characteristics of LnPO₄ (Ln = Nd, Sm, Gd) compounds to CMAS attack at 1250 °C. Due to the chemical reaction between molten CMAS and LnPO₄, a dense, crack-free reaction layer, mainly composed of Ca₃Ln₇(PO₄)(SiO₄)₅O₂ apatite, CaAl₂Si₂O₈ and MgAl₂O₄, was formed on the surface of compounds, which had positive effect on suppressing CMAS infiltration. The depth of CMAS penetration in LnPO₄ (Ln = Nd, Sm, Gd) decreased in the sequence of NdPO₄, SmPO₄ and GdPO₄. GdPO₄ had the best resistance characteristics to CMAS attack among the three compounds. The related mechanism was discussed based on the formation ability of apatite phase caused by the reaction between molten CMAS and LnPO₄.     

Electrospinning,optical properties and white LED applications of one-dimensional CaAl12O19:Mn4+ nanofiber phosphors

Non-rare-earth, red-emitting CaAl₁₂O₁₉:Mn⁴⁺ nanofiber phosphors have been successfully prepared by an electrospinning technique followed by an annealing process. The as-prepared precursor fibers have smooth surfaces with an average diameter of 5 µm. After annealing at high temperature, the diameter of the fibers gradually reduces due to the decomposition of the organic polymers. The photoluminescence and crystalline properties of the fibers were investigated as a function of Mn⁴⁺ concentration and the annealing temperature. Under ultraviolet and blue light excitation, CaAl₁₂O₁₉:Mn⁴⁺ exhibits a characteristic red emission at 655 nm with three satellite peaks due to the ²E→⁴A₂ transition of Mn⁴⁺. The highest PL intensity is achieved at a 0.5% Mn⁴⁺ concentration and a firing temperature of 1400 °C. In comparison to CaAl₁₂O₁₉:Mn⁴⁺ prepared by a usual solid-state reaction, the luminescence of the as-prepared nanofiber phosphors in the present work has been strongly enhanced by optimizing the morphology and improving the crystallinity and phase purity. The absorption band in the blue region and a bright emission in the red region make the CaAl₁₂O₁₉:Mn⁴⁺ nanofiber phosphor a candidate for achieving high color rendering in YAG:Ce-based WLEDs. A warm WLED with a high CRI of 88.5 at a CCT of 4553 K has been successfully achieved by coating YAG:Ce with CaAl₁₂O₁₉:Mn⁴⁺ nanofiber phosphors on blue InGaN chips.     

High-temperature hot corrosion behavior of gadolinium zirconate by vanadium pentoxide and sodium sulfate in air

Gadolinium zirconate (Gd₂Zr₂O₇) prepared by solid state reaction exhibited a defect fluorite-type structure. Reactions between Gd₂Zr₂O₇ ceramic and vanadium pentoxide (V₂O₅), sodium sulfate (Na₂SO₄), and V₂O₅ + Na₂SO₄ mixture were investigated from 700 to 1000 °C in air using an X-ray diffractometer (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). V₂O₅ reacts with Gd₂Zr₂O₇ to form gadolinium vanadate (GdVO₄) and monoclinic zirconia (m-ZrO₂) at 900 and 1000 °C in air. However, no chemical reaction product between Na₂SO₄ and Gd₂Zr₂O₇ is found at 900 and 1000 °C in air. V₂O₅ reacts with equal molar Na₂SO₄ to form sodium vanadate (NaVO₃) at 610 °C. In the temperature range of 700–1000 °C, Na₂SO₄ + V₂O₅ mixture reacts with Gd₂Zr₂O₇ in air to form the final reaction products of GdVO₄ and m-ZrO₂.     

Charge–discharge curves and discharge capacities of LiNi1−yCoyO2 synthesized from lithium carbonate and nickel and cobalt oxides

LiNi_1?yCo_yO₂ (y=0.1, 0.3, and 0.5) were synthesized by a solid-state reaction method at 800 °C and 850 °C using Li₂CO₃, NiO, and Co₃O₄ as the starting materials. The electrochemical properties of the synthesized LiNi_1?yCo_yO₂ were then investigated. For samples with the same composition, the particles synthesized at 850 °C were larger than those synthesized at 800 °C. The particles of all the samples synthesized at 850 °C were larger than those synthesized at 800 °C. LiNi_0.5Co_0.5O₂ synthesized at 850 °C had the largest first discharge capacity (159 mA h/g), followed in order by LiNi_0.7Co_0.3O₂ synthesized at 800 °C (158 mA h/g) and LiNi_0.9Co_0.1O₂ synthesized at 850 °C (151 mA h/g). LiNi_0.9Co_0.1O₂ synthesized at 850 °C had the best cycling performance with discharge capacities of 151 mA h/g at n=1 and 156 mA h/g at n=5.     

The preparation and properties of La3.5Ru4O13 and La2RuO5

The stability of the La_3.5Ru₄O₁₃ and La₂RuO₅ compounds in the La–Ru–O system in various atmospheres and various temperature ranges was investigated by thermal analysis, X-ray diffraction analysis and electron microscopy. The La_3.5Ru₄O₁₃ compound is stable in oxidizing and neutral atmospheres (N₂ with 10 ppm O₂), while La₂RuO₅ is partially reduced in a neutral atmosphere to form La₂RuO_4.6. In a reducing atmosphere both compounds decompose into metallic Ru and La₂O₃. The thermal expansion coefficients of La₂RuO₅ and La_3.5Ru₄O₁₃ at 800 °C are 11.2 × 10⁻⁶ K⁻¹ and 9.3 × 10⁻⁶ K⁻¹, respectively. The specific electrical resistivity for La_3.5Ru₄O₁₃ is relatively independent of temperature and is 2 × 10⁻² Ω cm at 800 °C, while for La₂RuO₅ it decreases with increasing temperature and is 1 Ω cm at 800 °C.