A model for initial-stage sintering thermodynamics of an alumina powder

A model was proposed to calculate several thermodynamic parameters for the initial-stage sintering of an alumina powder obtained after calcinations at 900 °C for 2 h of a precursor. The precursor was synthesized by an alumina sulphate-excess urea reaction in boiling aqueous solution. The cylindrical compacts of the powder with a diameter of 14 mm were prepared under 32 MPa by uniaxial pressing using oleic acid (12% by mass) as binder. The compacts were fired at various temperatures between 900 and 1400 °C for 2 h. The diameter (D) of the compacts before and after firing was measured by a micrometer. The D value after firing was taken as a sintering equilibrium parameter. An arbitrary sintering equilibrium constant (K_a) was calculated for each firing temperature by assuming K_a = (D_i − D) / (D − D_f), where D_i is the largest value before sintering and D_f is the smallest value after firing at 1400 °C. Also, an arbitrary change in Gibbs energy (ΔG _a°) was calculated for each temperature using the K_a value. The graphs of ln K_avs. 1 / T and ΔG _a° vs. T were plotted, and the real change in enthalpy (ΔH°) and the real change in entropy (ΔS°) were calculated from the slopes of the obtained straight lines, respectively. Inversely, real ΔG° and K values were calculated using the real ΔH° and ΔS° values in the ΔG° = − RT ln K = ΔH° − TΔS° relation. The best fitting ΔH° and ΔS° values satisfying this relation were found to be 157,301 J mol^− 1 and 107.6 J K^− 1 mol^− 1, respectively.     

CO2 gas-activated sintering of carbonated hydroxyapatites

Sintering compacts of carbonated hydroxyapatite (CHA) nanoparticles (3.4 wt% CO₃²⁻) in a CO₂ flow (4 mL/min) proceeded at a temperature which was more than 200 °C lower than that for hydroxyapatite in air (1150 °C). During heating from RT to 1200 °C (5 K/min) the rate of shrinkage of the CHA compacts showed a maximum thrice as high as that in air at about 929 °C. The shrinkage correlates with a mass loss caused by the release of CO₂ due to the thermal decomposition of CO₃²⁻ ions that substitute PO₄³⁻ ions in the CHA lattice. Firing the compacts in the CO₂ flow at 800 and 900 °C for 2 h resulted in an additional carbonatation on the B-sites and a further decrease in the sintering temperature to 890 °C. The compacts fired in the 900-1000 °C range became almost complete ceramics with high densities and mechanical properties close to those of medical implants. Firing at temperatures above 1000 °C resulted in an additional carbonatation on the A-sites. However, this led to a material with low densities and poor mechanical properties. A supposition has been proposed that the effect of CO₂ gas-activated sintering is a result of the intensification of the diffusion in the nanoparticles caused by CO₂ molecules entering the bulk from the CO₂ atmosphere and (or) releasing from the bulk due to the decomposition of carbonates on the B-sites in the lattice.     

Superplastic deformation of hydroxyapatite ceramics with B2O3 or Na2O addition fabricated by pulse current pressure sintering

High-density and fine-grained transparent hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂: HAp) ceramics with B₂O₃ and Na₂O addition were fabricated using pressureless sintering and pulse-current pressure sintering between 1000 and 1100 °C; the superplastic deformation of these HAp specimens was evaluated. The relative density of pure HAp compacts pulse-current pressure sintered at 1000 °C for 10 min under a pressure of 50 MPa attained 99.9% and exhibited translucency. The tensile elongation of the pure HAp specimen, which was measured at 1000 °C under a strain rate of 1.48 × 10⁻⁴ s⁻¹, was as high as 364%. The relative density of HAp compacts with 3.0 mol.% B₂O₃ addition pulse-current pressure sintered under the same conditions as those of pure HAp compacts was 98.9%, whereas the grain size was as low as 0.24 μm. The elongation of HAp specimens, measured at a test temperature of 1000 °C under a strain rate of 1.48 × 10⁻⁴ s⁻¹, was as high as 578%.     

Influence of magnesia on sintering stress of alumina

The sintering stress and the densification of MgO-doped Al₂O₃ were measured with a self-loading apparatus and a thermomechanical analyzer, respectively. The densification started at 950 °C and finished at about 1450 °C. The measured surface tensions were 0.7–0.8 N m⁻¹ in the intermediate sintering stage but drastically decreased to 0.2 N m⁻¹ in the final stage of sintering.     

Thermal behavior and characteristics of fired geopolymers produced from local Cameroonian metakaolin

The thermal behavior along with certain characteristics of geopolymers produced from local Cameroonian metakaolin and heated up to 1000 °C were examined. Geopolymers fired up to 900 °C had the same physical aspect as initial ones and those fired at 1000 °C warped, were glazed and blistered. The TG showed elimination of water according to two stages. The dilatometric curves of preheated samples showed shrinkage between 90 and 250 °C followed by expansion and sintering. The samples heated up to 700 °C were amorphous and new crystalline phases appeared around 900 °C. The microstructure of geopolymers heated between 300 and 900 °C showed progressive disruption and the linear shrinkage increased. The water absorption of the samples fired up to 700 °C increased slightly and tremendously around 900 °C. A drastic decrease of compressive strength was observed with the samples fired between 300 and 900 °C. Hence, the characteristics of geopolymers lessened with elimination of the water which forms hydration spheres around the compensating cations (Na⁺) opposed to tetrahedral groups AlO₄⁻ along with transformation of amorphous phase.     

Impedance analysis of Sr-substituted CePO4 with mixed protonic and p-type electronic conduction

Members of the solid-solution series Ce_1−xSr_xPO_4−δ (x = 0, 0.01, 0.02) with mixed protonic and electronic transport have been synthesized by a nitrate-decomposition method followed by sintering at 1450 °C. Impedance spectroscopy is employed to estimate the bulk electrical conductivity in wet (∼0.03 atm) and dry atmospheres of O₂ and 10%H₂:90%N₂. Conductivity increases with dopant concentration (x), oxygen partial pressure (pO₂) and water vapour partial pressure (pH₂O) reaching ∼3.5 × 10⁻³ S cm⁻¹ at 600 °C for x = 0.02 in wet O₂. Activation energies (E_a) for the bulk conductivity of Ce_0.98Sr_0.02PO_4−δ below 650 °C are 0.44 and 0.78 eV for wet oxidising and wet reducing conditions, respectively. A moderate but positive pO₂⁺ⁿ power-law dependence (n < 1/10) of conductivity is exhibited in the pO₂ range 10^−2.5 to 10⁻¹ atm, consistent with mixed ionic and p-type electronic transport. Thermogravimetric analysis indicates that the Sr-doped materials are stable in a CO₂ atmosphere in the temperature range 25–1200 °C.     

Crystallization behavior and properties of K2O–CaO–Al2O3–SiO2 glass-ceramics

In order to obtain high-strength anorthite glass-ceramics, K₂O–CaO–Al₂O₃–SiO₂ quaternary glass and relevant glass-ceramics were prepared and investigated. The results show that anorthite along with kalsilite or leucite was precipitated from the parent glass. Kalsilite crystals were formed firstly and then converted into leucite through reacting with SiO₂ in the glass phase. The morphology of the crystals was dependent on the heat-treatment temperature. Column crystals were transformed into fine granular grains when the sintering temperature changed from 900 °C to 1100 °C. The activation energy (E_α) and avrami constant (n) were also calculated as 463.81 KJ/mol and 3.74 respectively, indicating that bulk nucleation and three-dimensional crystal growth were the dominating mechanisms in the temperature range 1000–1100 °C. The maximum value of the flexural strength for the glass-ceramics containing leucite was 248 MPa and the coefficient of thermal expansion (CTE) was in the range 5.69~11.94×10⁻⁶ K⁻¹. The leucite is the main reason for the high CTEs and high flexural strength of glass-ceramics.     

Mg-doped poly(?-caprolactone)/siloxane biohybrids

Ionically conducting materials based on a poly(?-caprolactone) (PCL)/siloxane organic/inorganic host framework doped with magnesium triflate (Mg(CF₃SO₃)₂) were synthesized by the sol-gel process. In this matrix short PCL chains are covalently bonded to the siliceous network via urethane linkages. In this study the salt content of samples was identified using the conventional notation n, where n indicates the number of (C(O)(CH₂)₅O) PCL repeat units per Mg²⁺ ion. Xerogels with compositions ranging from n = ∞ to 1 were prepared. The only composition studied that was not entirely amorphous was that prepared with n = 1. Xerogels with n ≥ 7 are thermally stable up to at least 200 °C. The composition with the highest conductivity of the series is that with n = 34 (5.9 × 10⁻⁹ and 9.8 × 10⁻⁷ S cm⁻¹ at 24 and 104 °C, respectively).     

Electrocatalyst materials for fuel cells based on the polyoxometalates [PMo(12 − n)VnO40] (n = 0-3)

We report on the use of the polyoxometalate acids of the series [PMo_(12 − n)V_nO₄₀]^(3 + n)− (n = 0-3) as electrocatalysts in both the anode and the cathode of polymer-electrolyte membrane (PEM) fuel cells. The heteropolyacids were incorporated as catalysts in a commercial gas diffusion electrode based on Vulcan XC-72 carbon which strongly adsorbed a low loading of the catalyst, ca. 0.1 mg/cm². The moderate activity observed was independent of the number of vanadium atoms in the polyoxometalate. In the anode the electrochemistry is dominated by the V^3+/4+ couple. With a platinum reference wire in contact with the anode, polarization curves are obtained withV_OC of 650 mV and current densities of 10 mA cm⁻² at 100 mV at 80 °C. These catalysts showed an order of magnitude more activity on the cathode after moderate heat treatment than on the anode,V_OC = 750 mV, current densities of 140 mA cm⁻² at 100 mV. The temperature dependence of the catalysts was also investigated and showed increasing current densities could be achieved on the anode up to 139 °C and the cathode to 100 °C showing the potential for these materials to work at elevated temperatures.     

Characterisation of activated nanoporous carbon for supercapacitor electrode materials

Commercial nanoporous carbon RP-20 was activated with water vapor in the temperature range from 950 °C to 1150 °C. The XRD analysis was carried out on nanoporous carbon powder samples to investigate the structural changes (graphitisation) in modified carbon that occurred at activation temperatures T ? 1150 °C. The first-order Raman spectra showed the absorption peak at 1582 cm⁻¹ and the disorder (D) peak at 1350 cm⁻¹. The low-temperature N₂ adsorption experiments were performed at −196 °C and a specific surface area up to 2240 m²g⁻¹ for carbon activated at T = 1050 °C was measured. The cell capacitance for two electrode activated nanoporous carbon system advanced up to 60 F g⁻¹ giving the specific capacitance ∼240 F g⁻¹ to one electrode nanoporous carbon ∣1.2 M (C₂H₅)₃CH₃NBF₄ + acetonitrile solution interface. A very wide region of ideal polarisability for two electrode system (∼3.2 V) was achieved. The low frequency limiting specific capacitance very weakly increases with the rise of specific area explained by the mass transfer limitations in the nanoporous carbon electrodes. The electrochemical characteristics obtained show that some of these materials under discussion can be used for compilation of high energy density and power density non-aqueous electrolyte supercapacitors with higher power densities than aqueous supercapacitors.     

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.     

Characteristics of nanosized Bi-based glass powders prepared by flame spray pyrolysis as transparent dielectric layer material

Bi-based glass powders with particle size of 34 nm were prepared by high-temperature flame spray pyrolysis. The glass transition temperature (T_g) of the powders was 442 °C. Dielectric layers fired at temperatures of 480 and 500 °C contained voids, while those fired at temperatures above 540 °C had clean surfaces and no voids. The dielectric layers sintered at temperatures of 560 and 580 °C had transmittances of 70% in the visible range. Further, it was observed that the dielectric layers formed from the nanosized glass powders obtained from spray solutions containing excess boron had higher transmittances (80% in the visible range at a sintering temperature of 580 °C) than the layers formed from spray solutions containing stoichiometric amounts of boron.     

Electrical properties of samaria-doped ceria electrolytes from highly active powders

The correlations of the microstructures and the electrical properties of high reactive Ce_0.8Sm_0.2O_1.9 (SDC) powders, synthesized via an optimal carbonate coprecipitation method, were investigated. Microstructure of the SDC ceramics sintered at 900-1400 °C showed uniform grain and small grain size, compared with those prepared with various methods under similar sintering conditions. These features may be related to high conductivity (σ_600 °C = 0.022 S cm⁻¹) and low activation energy for conduction (0.66 eV). AC impedance spectra were involved to resolve grain interior and grain boundary resistance. Grain boundary contribution to the total resistance showed the values below 1/2 at 200-450 °C, suggesting low grain boundary effect. The motion enthalpy for the grain interior conduction decreased while the association enthalpy increased with sintering temperature up to 1300 °C, which might be possibly originated from the increase in lattice parameters with the sintering temperature.     

Synthesis, characterization and optical properties of fluorinated poly(aryl ether)s containing phthalazinone moieties

A series of novel fluorinated poly(aryl ether)s containing phthalazinone moieties (FPPEs) have been prepared by a modified synthetic procedure for optical waveguide applications. The obtained random copolymers exhibited excellent solubility in polar organic solvents, high glass transition temperatures (T_gs: 185-269 °C), good thermal stabilities (the temperatures of 1% weight loss: 487-510 °C) and good optical properties. By adjusting the feed ratio of the reactants, the refractive indices of TE and TM modes (at 1550 nm) could be well controlled in the range of 1.575-1.498 and 1.552-1.484, respectively. The optical losses of the FPPEs exhibited relatively low values (less than 0.27 dB/cm at 1310 nm). Additionally, the thermo-optic coefficient (dn/dT) values of the FPPEs at 1310 nm and 1550 nm (TE mode) ranged from −0.97 × 10⁻⁴ °C to −1.33 × 10⁻⁴ °C and from −0.96 × 10⁻⁴ °C to −1.29 × 10⁻⁴ °C, respectively.     

Synthesis, characterization and optical properties of cross-linkable poly(phthalazinone ether ketone sulfone)

Cross-linkable poly(phthalazinone ether ketone sulfone) bearing tetrafluorostyrene groups (PPEKS-FSt) has been prepared by copolycondensation reaction for optical waveguide applications. The resulting amorphous polymer exhibits good solubility in some common polar organic solvents (e.g., N,N′-dimethylacetamide, N-methyl-2-pyrrolidinone, chloroform) at room temperature, and can be easily spin-coated into thin films with good optical quality. The glass transition temperature (T_g) and the temperature of 1% weight loss (1% T_d) are 261 °C and 494 °C, respectively, which could be further increased by 31 °C and 14 °C upon thermal cross-linking. The cross-linked polymer thin films exhibit high refractive index (∼1.65, TE mode), high thermo-optic coefficient value (dn/dT) (−1.455 × 10⁻⁴/°C, TE mode), low optical loss (less than 0.24 dB/cm at 1310 nm) and relatively low birefringence (∼0.007).     

Biogas reforming over La-NiMgAl catalysts derived from hydrotalcite-like structure: Influence of calcination temperature

Hydrotalcite-like compound with general formula [M(II)_1 − xM(III)_x(OH)₂]^x+(Aⁿ⁻_x/n) ^· mH₂O, where Aⁿ⁻ is the compensation anion, has been used as precursor of active catalysts for biogas reforming. This precursor was calcined at six different temperatures between 250 and 750 °C and the resulting catalysts were tested in order to evaluate the influence of the calcination temperature on the catalytic activity and stability. XRD characterization showed that from 250 °C the hydrotalcite structure is no longer detected, leading to Mg(Ni,Al)O solid solutions, where no peaks related to lanthanum appear. An increase on the calcination temperature increased the grain size and cell parameter value. 50 h-catalytic tests were carried out at 700 °C, CH₄:CO₂ molar ratio of 1:1 and a mass/feed alimentation ratio (W/F) of 0.4 mg min cm^− 3. Used catalysts were characterized by temperature programmed oxidation (TPO), scanning electron microscopy (SEM) and Raman spectroscopy in order to obtain information about coke deposition. Catalytic tests highlighted the great influence of calcination temperature over catalytic activity and stability, having found that, as a general trend, calcination temperatures below 750 °C decrease both the stability and catalytic activity, with the exception of the catalyst calcined at 550 °C, where a higher activity was achieved but with a comparatively low stability.     

Sintering,microstructure and conductivity of La2NiO4+δ ceramic

Microstructure and electrical conducting properties of La₂NiO_4+δ ceramic were investigated in the sintering temperature range 1200–1400 °C. The results demonstrate that the microstructure and electrical conducting properties of La₂NiO_4+δ ceramic are sensitive to sintering temperature. Compared with a progressive densification development with sintering temperature from 1200 to 1300 °C along with an insignificant change in grain size, there is an exaggerated grain growth in the specimens sintered at higher temperatures. Increasing sintering temperature from 1200 to 1300 °C resulted in an enhancement of electrical conducting properties. Further increase of sintering temperature exceeding 1300 °C reduced the electrical conducting properties. A close relation between the microstructure and electrical conducting properties was suggested for La₂NiO_4+δ ceramic. With respect to the electrical conducting properties, the preferred sintering temperature of La₂NiO_4+δ ceramic was ascertained to be 1300 °C. The specimen sintered at 1300 °C exhibits a generally uniform microstructure together with electrical conductivities of 76–95 S cm⁻¹ at 600–800 °C.     

Wear resistance glass-ceramics with a gahnite phase obtained in CaO-MgO-ZnO-Al2O3-B2O3-SiO2 system

The thermal conditions for obtaining the glass-ceramic material of Al_0.107B_0.374Mg_0.043Zn_0.282Ca_0.100Si_0.927O₃ with a crystalline phase in the form of gahnite (ZnAl₂O₄) were specified. The activation energy E_a and the Avrami parameter n for the crystallisation process were determined with the non-isothermal DTA procedure. The maximum temperatures of crystallisation of phases, depending on the rate of heating, ranged between 800-840 °C for willemite and 870-915 °C for gahnite. The homogeneous crystalline spinel phase was obtained by heat treatment above 1000 °C. Precipitation solely of a ghanite phase from glass-ceramic causes a relative increase in its fracture toughness and wear resistance compared to the two-phase materials, i.e., K_IC = 2.12-1.65 MPam^1/2 and w_s = 0.21 × 10⁻⁴ mm³/Nm to w_s = 1.43 × 10⁻⁴ mm³/Nm.     

Preparation and microwave dielectric properties of 3ZnO·B2O3 ceramics with low sintering temperature

The preparation and dielectric properties of 3ZnO·B₂O₃ ceramics were investigated. Dense 3ZnO·B₂O₃ ceramics were obtained as sintered in the temperature range from 950 to 1000 °C for 3 h. The X-ray diffraction showed that the obtained ceramics were of a monoclinic 3ZnO·B₂O₃ structure. The ceramic specimens fired at 955 °C for 1 h exhibited excellent microwave dielectric properties: ?_r ∼ 6.9, Q × f ∼ 20,647 GHz (@6.35 GHz), and τ_f ∼ −80 ppm/°C. The dependences of relative density, ?_r, and Q × f of ceramics sintered at 955 °C on sintering soaking time showed that they all reached their plateaus as the soaking time was up to 60 min. Meanwhile, 3ZnO·B₂O₃ ceramics had no reaction with silver during cofiring, indicating it is a potential candidate for low-temperature cofired ceramic (LTCC) substrate.     

Transparent polycrystalline MgAl2O4 ceramic fabricated by spark plasma sintering: Microwave dielectric and optical properties

The optical properties and microwave dielectric properties of transparent polycrystalline MgAl₂O₄ ceramics sintered by spark plasma sintering (SPS) through homemade nanosized MgAl₂O₄ powders at temperatures between 1250 °C and 1375 °C are discussed. The results indicate that, with increasing sintering temperatures, grain growth and densification occurred up to 1275 °C, and above 1350 °C, rapid grain and pore growth occurred. The in-line light transmission increases with the densification and decreases with the grain/pore growth, which can be as high as 70% at the wavelength of 550 nm and 82% at the wavelength of 2000 nm, respectively. As the sintering temperature increases, Q×f and dielectric constant ε_r values increase to maximum and then decrease respectively, while τ_f value is almost independent of the sintering temperatures and remains between −77 and −71 ppm/°C. The optimal microwave dielectric properties (ε_r=8.38, Q×f=54,000 GHz and τ_f=−74 ppm/°C) are achieved for transparent MgAl₂O₄ ceramics produced by spark plasma sintering at 1325 °C for 20 min.