Isothermal sintering of barium-zinc-titanate ceramics

Mixtures of BaCO₃-ZnO-TiO₂ were mechanically activated using high-energy planetary ball mill during 20, 40 and 80 min. As the time of mechanical activation increased, the decrease in particle size was observed. The effect of milling on microstructure was investigated by scanning electron microscopy, as well. Sintering process was performed in air at 1100 and 1200 °C for 2 h. Various phases are present within mixtures sintered at 1100 and 1200 °C, and almost pure BaZn₂Ti₄O₁₁ phase was obtained after 80 min of milling and sintering at 1200 °C for 2 h.     

The sintering kinetics of porcelain bodies made from waste glass and fly ash

Porcelain is a material produced from kaoline, quartz and potassium-feldspar. Recently, research of new materials, for example non-hazardous wastes, that are able to replace traditional fluxing agents without changing the process or quality of the final products has been realized. The aim of this work is to study the possibility of the use of glass powder waste and fly ash together for manufacturing porcelain. Instead of quartz, fly ash was used at the selected porcelain composition. The waste glass was added partially and fully in replacement of potassium-feldspar. Samples were fired in an electric furnace with a heating rate of 10 °C/min at 1100, 1150 and 1200 °C for a period of 1, 2, 3 and 5 h. The sintered samples were characterised by XRD (X-ray diffraction) and SEM (scanning electron microscopy). Sintering activation energies were determined based on the bulk density result. At 10, 15, 20 and 25 wt.% glass waste addition, the apparent activation energies were calculated to be 145, 113.5, 70.4 and 53.74 kJ/mol, respectively. It was found that the sintering activation energy decreased with increasing waste glass addition.     

High piezoelectric properties of BaTiO3-xLiF ceramics sintered at low temperatures

BaTiO₃-xLiF ceramics were prepared by a conventional sintering method using BaTiO₃ powder about 100 nm in diameter. The effects of LiF content (x) and sintering temperature on density, crystalline structure and electrical properties were investigated. A phase transition from tetragonal to orthorhombic symmetry appeared as sintering temperatures were raised from 1100 °C to 1200 °C or as LiF was added from 0 mol% to 3 mol%. BaTiO₃-6 mol% LiF ceramic sintered at 1000 °C exhibited a high relative density of 95.5%, which was comparable to that for pure BaTiO₃ sintered at 1250 °C. BaTiO₃-4 mol% LiF ceramic sintered at 1100 °C exhibited excellent properties with a piezoelectric constant d₃₃ = 270 pC/N and a planar electromechanical coupling coefficient k_p = 45%, because it is close to the phase transition point in addition to high density.     

Influence of cooling mode on the electrical properties and microstructure of Ba1.022−xSmxTiO3 ceramics sintered in a reducing atmosphere

We investigated the effects of the Sm-dopant content and the cooling rate on the electrical properties and microstructure of Ba_1.022−xSm_xTiO₃ (BST) ceramics, which were sintered at 1200 °C for 30 min in a reducing atmosphere and then reoxidized at 800 °C for 1 h. The results indicated that the cooling rate affected the electrical properties and the microstructure of the BST samples, whose room-temperature resistivity increased with increasing cooling rate. The semiconducting BST ceramics showed a pronounced positive temperature coefficient of resistivity effect, with a resistance jump greater by 3.16 orders of magnitude, along with a low room-temperature resistivity of 157.4 Ω cm at a cooling rate of 4 °C/min. The room-temperature resistivity of the specimen was lower after sintering for 30 min at 1150 °C during cooling.     

Mechanochemical synthesis and characterisation of BaTa2O6 ceramic powders

Mechanochemical synthesis was used to prepare BaTa₂O₆ powders from BaCO₃ and Ta₂O₅ precursors in a planetary ball mill. Effect of milling time and heat treatment temperature on the formation of BaTa₂O₆ and on the microstructure was investigated. Intensive milling of starting materials resulted in crystallization of BaTa₂O₆ even after 1 h of milling time and single phase BaTa₂O₆ was obtained after 10 h of milling under optimal conditions. The powder derived from 10 h of mechanical activation had crystallite size of 22 nm. But the increase in milling time did not decrease the crystallite size further. High energy milling activated the powders that although 1 h of milling led to formation of single phase BaTa₂O₆ at 1200 °C, this temperature decreased to 900 °C after 5 h of milling. No significant grain growth was observed when the milled powders were heat treated below 900 °C. However, annealing at 1100 and 1200 °C gave an average BaTa₂O₆ grain size of 180 and 650 nm, respectively. An unidentified phase started to form at 1100 °C increasing to high amounts at 1200 °C and they had different shapes and sizes than BaTa₂O₆ grains. These elongated large grains were thought to be due to liquid phase formation caused by iron contamination.     

Effects of CaTiO3 addition on the densification and microwave dielectric properties of BiSbO4 ceramics

In this study, the effects of CaTiO₃ addition on the sintering characteristics and microwave dielectric properties of BiSbO₄ were investigated. Pure BiSbO₄ achieved a sintered density of 8.46 g/cm³ at 1100 °C. The value of sintered density decreased with increasing CaTiO₃, and sintering at a temperature higher than 1100 °C led to a large weight loss (>2 wt%) caused by the volatile nature of the compound. Samples either sintered above 1100 °C or with a CaTiO₃ content exceeding 3 wt% showed poor densification. SEM micrographs revealed microstructures with bimodal grain size distribution. The size of the smaller grains ranged from 0.5 to 1.2 μm and that of the larger grains between 3 and 7 μm. The microwave dielectric properties of the (1−x) BiSbO_4−x CaTiO₃ ceramics are dependent both on the x value and on the sintering temperature. The 99.0 wt% BiSbO₄–1.0 wt% CaTiO₃ ceramic sintered at 1100 °C reported overall microwave dielectric properties that can be summarized as ε_r≈21.8, Q×f≈61,150 GHz, and τ_f≈−40.1 ppm/°C, all superior to those of the BiSbO₄ ceramics sintered with other additives.     

Effects of bismuth oxide on the sinterability of hydroxyapatite

The sinterability of Bi₂O₃-doped hydroxyapatite (HA) has been studied and compared with the undoped HA. Varying amounts of Bi₂O₃ ranging from 0.05 wt% to 1.0 wt% were mixed with the HA. The study revealed that most sintered samples composed of the HA phase except for compacts containing 0.3, 0.5 and 1.0 wt% Bi₂O₃ and when sintered above 1100 °C, 1000 °C and 950 °C, respectively. In general, the addition of 0.5 wt% Bi₂O₃ was identified as the optimum amount to promote densification as well as to improve the mechanical properties of sintered HA at low temperature of 1000 °C. Throughout the sintering regime, the highest value of relative bulk density of 98.7% was obtained for 0.5 wt% Bi₂O₃-doped HA when sintered at 1000 °C. A maximum Young's modulus of 119.2 GPa was measured for 0.1 wt% Bi₂O₃-doped HA when sintered at 1150 °C. Additionally, the ceramic was able to achieve highest hardness of 6.08 GPa and fracture toughness of 1.21 MPa m^1/2 at sintering temperature of 1000 °C.     

Suspension stability and sintering influence on yttria-stabilized zirconia fabricated by colloidal processing

The stability of nano-zirconia 3YSZ powder in suspension was extensively studied by the colloidal method, and the optimum sintering temperature of the green sample fabricated through slip casting was determined. Zirconia suspensions with 10 vol% powder loading were prepared with distilled water, and HNO₃ was used to adjust the pH of the suspension to pH 1–6. All of the suspensions were subjected to sedimentation test, and the results showed that the suspensions adjusted to pH 2 had the lowest sediment volume. This finding indicates that a suspension with pH 2 produces higher packing density. Viscosity test was carried out for the suspensions added with dispersant ranging from 0.3 wt% to 0.7 wt% polyethyleneimine (PEI) with and without pH adjustment. The suspension containing 0.5 wt% PEI with pH 2 adjustment produced the lowest viscosity because of interparticle bond breakage in the aggregates, thus forming colloidally stable suspensions. The zirconia suspension containing 0.5 wt% PEI and whose pH was adjusted to pH 2 was chosen to be slip casted into cylindrical shape. Green samples were sintered at various sintering temperatures that ranged from 1100 °C to 1500 °C through a two-step sintering method. The sample sintered at 1500 °C was found to be porosite-free, and its highest relative density was 99.6% of the theoretical density. Morphological studies detected pores in the microstructure of the samples sintered at low sintering temperatures (1100 and 1200 °C). By contrast, the samples sintered at 1400 and 1500 °C were fully densified. However, the grain size of the sample sintered at 1500 °C was 230 nm, which indicated excessive grain growth. The Vickers hardness of the sample sintered at 1400 °C was found to be highest (12.9 GPa) and comparable to results found in the literature.     

Synthesis of Zn3Nb2O8 ceramics using a simple and effective process

Synthesis of Zn₃Nb₂O₈ ceramics using a simple and effective reaction-sintering process was investigated. The mixture of ZnO and Nb₂O₅ was pressed and sintered directly without any prior calcination. Single-phase Zn₃Nb₂O₈ ceramics could be obtained. Density of these ceramics increased with soaking time and sintering temperature. A maximum density 5.72 g/cm³ (99.7% of the theoretical density) was found for pellets sintered at 1170 °C for 2 h. Pores were not found and grain sizes >20 μm were observed in pellets sintered at 1170 °C. Abnormal grain growth occurred and grains >50 μm could be seen in Zn₃Nb₂O₈ ceramics sintered at 1200 °C for 2 h and 1200 °C for 4 h. Reaction-sintering process is then a simple and effective method to produce Zn₃Nb₂O₈ ceramics for applications in microwave dielectric resonators.     

Phase and microstructural evolution of yttrium-doped nanocrystalline alumina: A contribution of advanced microscopy techniques

Well-dispersed nano-crystalline transition alumina suspensions were mixed with yttrium chloride aqueous solutions, with the aim of producing Al₂O₃-Y₃Al₅O₁₂ (YAG) composite powders. DTA analysis allowed to highlight the role of yttrium on the α-phase crystallization path. Systematic XRD and HRTEM analyses were carried out in parallel on powders calcined in a wide temperature range (600-1300 °C) in order to follow phase and microstructural evolution. A thin, homogeneous yttrium-rich layer was yielded on the alumina particles surface; yttrium diffusion into the alumina matrix was negligible up to 1150 °C whereas, starting from 1200 °C, aggregates of partially sintered alumina particles appeared, stuck together by yttrium-rich thin films. Moreover, in the yttrium-richer zones, such as alumina grain boundaries and triple joints, yttrium-aluminates precipitated at alumina particles surface. Finally, at 1300 °C, alumina-YAG composite powders were produced, in which YAG was homogenously distributed among the alumina grains.     

Effect of YF3 on the phase stability and sinterability of hydroxyapatite—Partially stabilized zirconia composites

Pure hydroxyapatite (HA), HA and partially stabilized zirconia composites (PSZ) with YF₃ and HA–PSZ composite containing 5 wt% PSZ without YF₃ were sintered in air at 900 °C, 1100 °C and 1300 °C for 1 h. The reactions and transformation of the phases in the composites were determined by X-ray diffraction. All the composites with or without YF₃ showed desirable thermal stability below 1300 °C and besides various amounts of CaZrO₃, any amount of tri-calcium phosphate (TCP) was not observed. Above 1100 °C, composites with YF₃ showed higher thermal stability than the composites without YF₃. On the other hand, pure HA started to decompose and TCP was observed at 1300 °C. Composites with YF₃ showed improved thermal stability than the composite containing 5 wt% PSZ without YF₃ and pure HA at lower sintering temperatures such as 900 °C and 1100 °C. However, it was observed that the increasing amount of YF₃ addition caused negative effect on the thermal stability of the composites. 5ZHA composites with YF₃ showed the highest relative density among all of the composites with or without YF₃.     

Characterization of sintered coal fly ashes

Çan, Çatala?z?, Seyitömer and Af?in-Elbistan thermal power plant fly ashes were used to investigate the sintering behavior of fly ashes. For this purpose, coal fly ash samples were sintered to form ceramic materials without the addition of any inorganic additives or organic binders. In sample preparation, 1.5 g of fly ash was mixed in a mortar with water. Fly ash samples were uniaxially pressed at 40 MPa to achieve a reasonable strength. The powder compacts were sintered in air. X-ray diffraction analysis revealed that quartz (SiO₂), mullite (Al₆Si₂O₁₃), anorthite (CaAl₂Si₂O₈), gehlenite (Ca₂Al₂SiO₇) and wollastonite (CaSiO₃) phases occurred in the sintered samples. Scanning electron microscopy investigations were conducted on the sintered coal fly ash samples to investigate the microstructural evolution of the samples. Different crystalline structures were observed in the sintered samples. The sintered samples were obtained having high density, low water adsorption and porosity values. Higher Al₂O₃ + SiO₂ contents caused to better properties in the sintered materials.     

Sintering behaviour of nano-crystalline γ-Al2O3 powder without additives at 2-7 GPa

Al₂O₃ ceramics were fabricated without additives under high pressure (2-7 GPa) at different temperatures (600-1200 °C) using nanocrystalline alumina powder with metastable γ-Al₂O₃ phase as the starting material.It was shown that high pressure increases the nucleation rate while reducing the growth rate of the transformed α phase so that its grain size decreases and nano-scale grains in the sintered structure can be achieved.On the other hand the sintered samples at 7 GPa and high temperature (1000 °C) have shown micron-scale large grain sizes compared to those sintered at lower pressures, for the same temperature and sintering time. This could be attributed to the higher input energy in the system at high pressure and high temperature conditions, thereby reaching the final stage in sintering more quickly.In this work, the best combination of grain size (∼200 nm) and density (98.0% TD) was obtained under the sintering condition of 1000 °C at 7 GPa with a holding time of 1 min.Thus for high pressure/high temperature conditions, the sintering time should be reduced to prevent grain growth.     

Carbonation of fly ash in oxy-fuel CFB combustion

Oxy-fuel combustion of fossil fuel is one of the most promising methods to produce a stream of concentrated CO₂ ready for sequestration. Oxy-fuel FBC (fluidized bed combustion) can use limestone as a sorbent for in situ capture of sulphur dioxide. Limestone will not calcine to CaO under typical oxy-fuel circulating FBC (CFBC) operating temperatures because of the high CO₂ partial pressures. However, for some fuels, such as anthracites and petroleum cokes, the typical combustion temperature is above 900 °C. At CO₂ concentrations of 80-85% (typical of oxy-fuel CFBC conditions with flue gas recycle) limestone still calcines, but when the ash cools to the calcination temperature, carbonation of fly ash deposited on cool surfaces may occur. This phenomenon has the potential to cause fouling of the heat transfer surfaces in the back end of the boiler, and to create serious operational difficulties. In this study, fly ash generated in a utility CFBC boiler was carbonated in a thermogravimetric analyzer (TGA) under conditions expected in an oxy-fuel CFBC. The temperature range investigated was from 250 to 800 °C with CO₂ concentration set at 80% and H₂O concentrations at 0%, 8% and 15%, and the rate and the extent of the carbonation reaction were determined. Both temperature and H₂O concentrations played important roles in determining the reaction rate and extent of carbonation. The results also showed that, in different temperature ranges, the carbonation of fly ash displayed different characteristics: in the range 400 °C < T ? 800 °C, the higher the temperature the higher the CaO-to-carbonate conversion ratio. The presence of H₂O in the gas phase always resulted in higher CaO conversion ratio than that obtainable without H₂O. For T ? 400 °C, no fly ash carbonation occurred without the presence of H₂O in the gas phase. However, on water vapour addition, carbonation was observed, even at 250 °C. For T ? 300 °C, small amounts of Ca(OH)₂ were found in the final product alongside CaCO₃. Here, the carbonation mechanism is discussed and the apparent activation energy for the overall reaction determined.     

Synthesis and electrical properties of nanostructured Ba2SnYO5.5 proton conductor

Ba₂SnYO_5.5 nanopowders were synthesized by a gel polymerization method. In this process, a gel containing Ba, Sn and Y cations has been obtained by the polymerization of acrylic acid using N,N′-methylene bis-acrylamide as a cross-linking reagent and hydrogen peroxide as an igniting reagent. The gel was calcined at 1200 °C, giving rise to the Ba₂SnYO_5.5 single-phase nanopowders with the grain size ranging from 50 nm to 70 nm. Nanopowders were sintered at 1150 °C by spark plasma sintering (SPS) to obtain dense nanostructure materials (> 95%) containing grains whose size ranges between 70 nm and 100 nm. Nanostructured Ba₂SnYO_5.5 shows a good chemical stability in wet atmosphere. However, its protonic conductivity decreases compared with that of microcrystallin Ba₂SnYO_5.5 ceramics.     

Comparison of sintering and compressive strength tendencies of a model coal mineral mixture heat-treated in inert and oxidizing atmospheres

The chemical interactions responsible for sintering in a coal mineral mixture were investigated in air and in N₂. A mineral mixture was made up by mixing kaolin, pyrite, quartz, calcite, hydromagnesite, FeCO₃ and anatase in a fixed ratio. The mineral mixture was pelletized and heat-treated up to 1100 °C in order to evaluate sintering by recording the compressive strength values and visual assessment with scanning electron microscopy (SEM). Chemical interactions responsible for the trends in the compressive strength results were investigated with simultaneous thermogravimetric and differential thermal analysis (TG/DTA), as well as X-ray diffraction. The results indicated that the formation of anhydrite (CaSO₄) was responsible for increased mechanical strength in the mineral mixture pellets heated in air at temperatures higher that 400 °C. CaSO₄ formed from the reaction of the decomposition products of pyrite and calcite (SO_x and CaO). The TG/DTA results also indicated that the reaction with pyrite in air caused the decomposition of calcite in the mixture at a lower temperature than was observed for calcite only. The pellets heated in N₂ did not increase in mechanical strength during heat-treatment due to the lack of CaSO₄ formation in the inert atmosphere. However, SEM analysis indicated that sintering did occur at the higher temperatures in N₂. A decrease was observed in the compressive strength values obtained in air at temperatures from 900 °C to 1100 °C. Reasons for the decreased compressive strengths may include increased porosity, decomposition of CaSO₄, and changes in the characteristics of the aluminosilicate phases.     

Effect of the addition ZrO2–Al2O3 on nanocrystalline hydroxyapatite bending strength and fracture toughness

Nanocrystalline hydroxyapatite powder has been synthesized from a Ca(NO₃)₂·4H₂O and (NH₄)₂HPO₄ solution by the precipitation method. In the next step we prepared ZrO₂–Al₂O₃ powder. After preparation, the powder was dried at 80 °C and calcined at 1200 °C for 1 h. Various amounts (HAP–15 wt% ZA, HAP–30 wt% ZA) of powder were mixed with the hydroxyapatite by ball milling. The powder mixtures were pressed and sintered at 1000 °C, 1100 °C and 1200 °C for 1 h. In order to study the structural evolution, X-ray diffraction (XRD) was used. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used to estimate the particle size of the powder and observe fracture surfaces. Results show that the bending strength of pressed nanocrystalline HAP was improved significantly by the addition 15 wt% of ZrO₂–Al₂O₃ powders at 1200 °C, but the fracture toughness was not changed, however when 30 wt% of ZA powders were added to nanocrystalline HAP, the bending strength and fracture toughness of the specimens decreased at all sintering temperature.     

Effect of carbon nanotubes on sintering behavior of alumina prepared by sol–gel method

Alumina (Al2O3)/carbon nanotube (CNT) (99/1 by weight) composite was prepared by mixing CNT dispersion with AlCl₃-based gel, followed by high temperature sintering at a temperature up to 1150 °C in argon. Composite alumina precursor showed phase transition order from amorphous to γ-Al₂O₃ after sintered at 900 °C for 2 h, partially to θ-Al₂O₃ after sintered at 1000 °C for 2 h, and then partially to α-Al₂O₃ after sintered at 1150 °C for 2 h. By comparison, control alumina precursor directly transformed from amorphous to α-Al₂O₃ after sintered at a relatively low temperature of 600 °C for 2 h. Composite alumina showed porous structure with pore diameter ranging from 100 nm to 2 µm, whereas control alumina was relatively pore-free. The elevated alumina-crystal phase transition temperatures and the formation of porous structure were ascribed to the presence of CNTs in alumina precursor. The composite alumina sintered at 900 °C for 2 h containing only γ-Al₂O₃ had a BET surface area of 138 m²/g, which was significantly higher than that of control alumina sintered at 1150 °C for 2 h containing only α-Al₂O₃, ~15 m²/g.     

Formation of rod-like Si3N4 grains in porous SRBSN bodies using 6Y2O3–2MgO sintering additives

The porous reaction-bonded silicon nitride (RBSN) bodies using (6 wt.% Y₂O₃–2 wt.% MgO) 6Y2M were fabricated by nitridation process at 1350 °C for 8 h. The porous gas pressure sintered (GPSed)-RBSN bodies post-sintered at 1550–1850 °C for 6 h show a microstructure with low aspect ratios having high porosity. The compressive strength of samples sintered at 1650 °C, 1750 °C and 1850 °C were about 146 MPa, 251 MPa and 285 MPa, respectively. The duration time for sintering had a significant effect on the microstructure and grain morphology of the GPSed-RBSN bodies. Even though the GPSed-RBSN was carried out at the comparatively low temperature (1550 °C) for 9 h, high aspect ratio of rod-like Si₃N₄ grains with about 9 was observed. The material properties of samples such as porosity, phase ratio (β/(α + β)) and compressive strength of sample sintered at 9 h were about 43.2%, 99% and 141 MPa, respectively.