Lightweight geopolymer made of highly porous siliceous materials with various Na2O/Al2O3 and SiO2/Al2O3 ratios

The syntheses of lightweight geopolymeric materials from highly porous siliceous materials viz. diatomaceous earth (DE) and rice husk ash (RHA) with high starting SiO₂/Al₂O₃ ratios of 13.0-33.5 and Na₂O/Al₂O₃ ratios of 0.66-3.0 were studied. The effects of fineness and calcination temperature of DE, concentrations of NaOH and KOH, DE to RHA ratio; curing temperature and time on the mechanical properties and microstructures of the geopolymer pastes were investigated. The results indicated that the optimum calcination temperature of DE was 800 °C. Increasing fineness of DE and starting Na₂O/Al₂O₃ ratio resulted in an increase in compressive strength of geopolymer paste. Geopolymer pastes activated with NaOH gave higher compressive strengths than those with KOH. The optimum curing temperature and time were 75 °C and 5 days. The lightweight geopolymer material with mean bulk density of 0.88 g/cm³ and compressive strength of 15 kg/cm² was obtained. Incorporation of 40% RHA to increase starting SiO₂/Al₂O₃ and Na₂O/Al₂O₃ ratios to 22.5 and 1.7 and enhanced the compressive strength of geopolymer paste to 24 kg/cm² with only a marginal increase of bulk density to 1.01 g/cm³. However, the geopolymer materials with high Na₂O/Al₂O₃ (>1.5) were not stable in water submersion.     

Grazing incidence X-ray study of Ge-nanoparticle formation in (Ge:SiO2)/SiO2 multilayers

We present the study of formation of Ge-nanoparticles (Ge-NP) in germanosilicate (Ge:SiO₂) multilayer (ML) films under thermal treatment. In anticipation of controllable formation of Ge-NP, ML films were prepared by magnetron deposition at room temperature as 20 bi-layer stacks, each bi-layer comprised of a 7 nm thick layer of (Ge + SiO₂) (molar ratio: 60:40) succeeded by a 7 nm thick layer of pure SiO₂, and then annealed for 1 h, up to T_a = 900 °C. Formation and morphology of Ge-NP were analyzed by combining the information obtained from the grazing incidence small angle X-ray scattering and X-ray diffraction. It was found that precipitation of Ge-NP starts at T_a = 600 °C, while high degree of in-plane confinement and lateral ordering of rather uniform precipitated particles is achieved at T_a =  700-800 °C range. At still higher annealing temperature T_a > 800 °C, volume fraction of precipitated Ge-NP in SiO₂ matrix diminishes due to the out-diffusion of Ge atoms from the film, while Ge-NP are no more well confined to (Ge + SiO₂) layers.     

Piezoelectric and ferroelectric properties of K0.5Na0.5NbO3 ceramics synthesized by spray drying method

Potassium-sodium niobate was synthesized at 800 °C for 1 h using dried precursors in a powder form obtained by the spray drying method. Different samples were sintered from 1060 to 1120 °C for 2 h reaching a relative density as high as 96% of the theoretical value. Piezoelectric and ferroelectric properties were studied for these samples and some of the most prominent results are: k_p, d₃₁, 2P_r, and 2E_C of 0.36, 39 pC/N, 29 μC/cm² and 16.5 kV/cm, respectively, for the sample sintered at 1080 °C. The methodology presented in this study can be used to synthesize submicrometer powders.     

Preparation of LiFePO4/C composite powders by ultrasonic spray pyrolysis followed by heat treatment and their electrochemical properties

LiFePO₄ powders could be successfully prepared from a precursor solution, which was composed of Li(HCOO)·H₂O, FeCl₂·4H₂O and H₃PO₄ stoichiometrically dissolved in distilled water, by ultrasonic spray pyrolysis at 500 °C followed by heat treatment at sintering temperatures ranging from 500 to 800 °C in N₂ + 3% H₂ gas atmosphere. Raman spectroscopy revealed that α-Fe₂O₃ thin layers were formed on the surface of as-prepared LiFePO₄ powders during spray pyrolysis, and they disappeared after sintering above 600 °C. The LiFePO₄ powders prepared at 500 °C and then sintered at 600 °C exhibited a first discharge capacity of 100 mAh g⁻¹ at a 0.1 C charge-discharge rate. To improve the electrochemical properties of the LiFePO₄ powders, LiFePO₄/C composite powders with various amounts of citric acid added were prepared by the present method. The LiFePO₄/C (1.87 wt.%) composite powders prepared at 500 °C and then sintered at 800 °C exhibited first-discharge capacities of 140 mAh g⁻¹ at 0.1 C and 84 mAh g⁻¹ at 5 C with excellent cycle performance. In this study, the optimum amount of carbon for the LiFePO₄/C composite powders was 1.87 wt.%. From the cyclic voltammetry (CV) and AC impedance spectroscopy measurements, the effects of carbon addition on the electrochemical properties of LiFePO₄ powders were also discussed.     

Preparation and characterization of Y2O3–Sm2O3 co-doped ceria electrolyte for IT-SOFCs

Y₂O₃–Sm₂O₃ co-doped ceria (YSDC) powder was synthesized by a gel-casting method using Ce(NO₃)₃·6H₂O, Sm₂O₃ and Y₂O₃ as raw materials. Phase structure of the synthesized powders was characterized by X-Ray diffraction analysis. Sinterability of the powders was investigated by testing the relative density and observing the microstructure of the sintered YSDC samples. Electrical conductivity of the sintered YSDC samples was measured using impedance spectra method. Single solid oxide fuel cells based on the YSDC electrolyte were also assembled and tested. The results showed that YSDC powders with single-phase fluorite structure can be obtained by calcining the dried gelcasts at temperature above 800 °C. Average particle size of the YSDC powder is 50–100 nm. Relative density of more than 95% of the theoretical can be achieved by sintering the YSDC compacts at temperature above 1400 °C. The sintered YSDC sample has an ionic conductivity of 4.74 × 10⁻² S cm⁻¹ at 800 °C in air. Single fuel cells based on the YSDC electrolyte with 50 μm in thickness were tested using humidified hydrogen as fuel and air as oxidant, and maximum power densities of about 190 and 112 mW cm⁻² were achieved at 700 and 600 °C, respectively.     

The influence of oxygen ambient annealing conditions on the quality of Al/SiO2/n-type 4H-SiC MOS structure

The effect of reoxidation process in O₂ on the electrical properties of metal-oxide-semiconductor (MOS) capacitors fabricated on n-type 4H-SiC (0 0 0 1) is investigated. All samples were oxidized in wet oxygen at temperature of 1175 °C. Reoxidation process was carried out on four of the five samples. Samples were annealed at temperature of 700 °C and 800 °C for different process times. The reoxidation process in oxygen improves the quality of the dielectric layer and the interface of Al/SiO₂/n-type 4H-SiC MOS structure. The best quality of the SiO₂/SiC interface can be achieved for the MOS structure annealed in O₂ at higher temperature (800 °C) for longer time. However, higher and more uniformly distributed values of breakdown voltage were obtained for MOS structures reoxidized at temperature of 700 °C.     

Microstructure and mechanical properties of silicon nitride-titanium nitride composites prepared by spark plasma sintering

Si₃N₄-TiN composites were prepared by spark plasma sintering (conventional sintering (SPS1) and in situ reaction sintering (SPS2)). Homogeneous distribution of equiaxed TiN grains in Si₃N₄ matrix results in the highest microhardness (21.7 GPa) and bending strength (621 MPa) of sample SPS1 sintered at 1550 °C. Dispersion of elongated TiN grains in Si₃N₄ matrix results in the highest fracture toughness (8.39 MPa m^1/2) of sample SPS2 sintered at 1300 °C.     

Low permittivity SrCuSi4O10-LMZBS glass composite for LTCC applications

The tetragonal gillespite type SrCuSi₄O₁₀ (SCS) was prepared by the conventional solid-state ceramic route. The SCS sintered at 1100 °C/6 h showed ε_r = 4.0 and tan δ = 1.1 × 10⁻³ at 5 GHz. The SCS has poor sinterability and the addition of lithium magnesium zinc borosilicate glass (20: Li₂O, 20: MgO, 20: ZnO, 20: B₂O₃, 20: SiO₂) lowered the sintering temperature and improved densification. The SCS ceramic with 5 wt.% LMZBS glass sintered at 900 °C has ε_r = 5.0 and tan δ = 1.9 × 10⁻³ at 5 GHz. The composite is chemically compatible with the common electrode material silver.     

Microstructure and electrical conductivity of LaNi0.6Fe0.4O3 prepared by combustion synthesis routes

The continuing development of new materials suitable for solid oxide fuel cells operating at about 650-800 °C is of great interest in recent days. The present investigation deals with the development of a perovskite composition-LaNi_0.6Fe_0.4O₃ (LNF)-prepared following two combustion synthesis routes: citrate-gel (LNC) and urea (LNU). The powders were sintered over a wide temperature range (900-1400 °C) and sintering behavior for LNC and LNU was compared. The thermal expansion coefficient (TEC), electrical and microstructural characteristics of LNF was thoroughly investigated. Electrical conductivities were found to be one and a half times higher than that of most commonly used cathode material, La(Sr)MnO₃. Moreover, the TEC value of LNF was found to be ≈11.4×10⁻⁶ K⁻¹ at 800 °C. The study opens up a possibility of using LNF as a promising cell component for SOFC.     

Effect of the sintering temperature on the properties of Ce0.85La0.10Ca0.05O2−δ electrolyte material

Rare earth and alkaline earth co-doped Ce_0.85La_0.10Ca_0.05O_2−δ electrolyte material with the powder obtained by solid-state reaction method was sintered at 1300, 1400, 1500 and 1600 °C respectively. The results showed that the ionic conductivity of the sample sintered at 1400 °C was slightly lower compared to that sintered at 1500 °C in the temperature range of 300-550 °C, while the sample sintered at 1400 °C showed the highest ionic conductivity in all the samples above 550 °C. The ionic conductivity of ∼0.021 S/cm at 600 °C and the relative density of 98.2% were observed for the sample sintered at 1400 °C. In addition, the highest flexural strength with 145 MPa was also obtained for the sample sintered at 1400 °C. It suggested that the sintering temperature for Ce_0.85La_0.10Ca_0.05O_2−δ electrolyte may be reduced to as low as 1400 °C with desired properties.     

Structure, hardness and thermal stability of TiAlN and nanolayered TiAlN/CrN multilayer films

TiAlN films were deposited on silicon (1 1 1) substrates from a TiAl target using a reactive DC magnetron sputtering process in Ar+N₂ plasma. Films were prepared at various nitrogen flow rates and TiAl target compositions. Similarly, CrN films were prepared from the reactive sputtering of Cr target. Subsequently, nanolayered TiAlN/CrN multilayer films were deposited at various modulation wavelengths (Λ). X-ray diffraction (XRD), energy dispersive X-ray analysis, nanoindentation and atomic force microscopy were used to characterize the films. The XRD confirmed the formation of superlattice structure at low modulation wavelengths. The maximum hardness of TiAlN/CrN multilayers was 3900 kg/mm², whereas TiAlN and CrN films exhibited maximum hardnesses of 3850 and 1000 kg/mm², respectively. Thermal stability of TiAlN and TiAlN/CrN multilayer films was studied by heating the films in air in the temperature range (T_A) of 500-900 °C for 30 min. The XRD spectra revealed that TiAlN/CrN multilayers were stable up to 800 °C and got oxidized substantially at 900 °C. On the other hand, the TiAlN films were stable up to 700 °C and got completely oxidized at 800 °C. Nanoindentation measurements performed on the films after heat treatment showed that TiAlN retained a hardness of 2200 kg/mm² at T_A=700 °C and TiAlN/CrN multilayers retained hardness as high as 2600 kg/mm² upon annealing at 800° C.     

Thermal shock behavior of as-sintered and pre-oxidation Zr2Al4C5–20SiC composites

A water-quenching technique was adopted to evaluate the thermal shock behavior of as-sintered and pre-oxidation Zr₂Al₄C₅–20 vol.%SiC composites in an air atmosphere. The strength retention of the two kinds of composites was measured after varying temperature (ΔTc) up to 800 °C. As-sintered Zr₂Al₄C₅–20 vol.%SiC showed rapidly drop in flexural strength above ΔT of 400 °C. However, the pre-oxidation Zr₂Al₄C₅–20 vol.%SiC composite showed a higher retained strength up to 800 °C, and the critical thermal shock temperature difference (ΔTc) is as high as 660 °C. The main reasons for a great improvement of thermal shock resistance (TSR) were also discussed and analyzed.     

Synthesis and characterization of nanocrystalline La2Mo2O9 fast oxide-ion conductor by an in-situ polymerization method

A nanocrystalline La₂Mo₂O₉ powder was synthesized via the pyrolysis of polyacrylate salt precursor prepared by an in situ polymerization of the metal salts and acrylic acid. The pyrolysis behavior of the polymeric precursor was studied by thermal (TG/DTA) analysis. The obtained product was characterized by X-ray diffraction (XRD) and transmission electron microscope (TEM) analysis. The results revealed that the average particle size is ∼25 nm for La₂Mo₂O₉ with good crystallinity. The synthesized nanocrystalline La₂Mo₂O₉ powder showed good sinterability and reached ∼99% of theoretical density when sintered at 800 °C for 4 h. The La₂Mo₂O₉ sample sintered at 800 °C, yield good microstructure with improved conductivity value of about 0.12 S/cm at 800 °C.     

Preparation and properties of SiO2 matrix composites doped with AlN particles

SiO₂ matrix composites doped with AlN particles were prepared by hot-pressing process. Mechanical properties of SiO₂ matrix composites can be greatly improved by doping with AlN particles. Flexural strength and fracture toughness of 30 vol%AlN-SiO₂ composite sintered at 1400°C reached 200 MPa and 2.96 MPa·m^1/2. XRD analysis indicated that, up to 1400°C, no chemical reaction occurred between SiO₂ matrix and AlN particles suggesting an excellent chemical compatibility of SiO₂ matrix with AlN particles. The influences of hot-pressing temperature and the content of AlN particles on dielectric properties of SiO₂-AlN composites were studied. The temperature and frequency dependency of dielectric properties of SiO₂-AlN composites were also studied. Residual flexural strength of SiO₂-AlN composites decreased with increasing temperature difference. The critical temperature difference was estimated about 600°C.     

Preparation and properties of negative thermal expansion Zr2WP2O12 ceramics

Using solid-state reaction method, Zr₂WP₂O₁₂ powder was synthesized for this study. The optimum heating condition was 1200 °C for 4 h. The obtained powder was compacted and sintered. The relative density of the Zr₂WP₂O₁₂ ceramics with no sintering additive was 60%. That of samples sintered with more than 0.5 mass% MgO was about 97%. The average grain size (D₅₀), as estimated from the polished surface of a sample sintered at 1200 °C for 4 h was about 1 μm. The obtained ceramics showed a negative thermal expansion coefficient of about −3.4 × 10⁻⁶ °C⁻¹. Young's modulus, Poisson's ratio, three-point bending strength, Vickers microhardness, and fracture toughness of the obtained ceramics were, respectively, 74 GPa, 0.25, 113 ± 13 MPa, 4.4 GPa and 2.3 MPa m^1/2.     

The effect of manganese oxide on the sinterability of hydroxyapatite

The sinterability of manganese oxide (MnO₂) doped hydroxyapatite (HA) ranging from 0.05 to 1 wt% was investigated. Green samples were prepared and sintered in air at temperatures ranging from 1000 to 1400 °C. Sintered bodies were characterized to determine the phase stability, grain size, bulk density, hardness, fracture toughness and Young's modulus. XRD analysis revealed that the HA phase stability was not disrupted throughout the sintering regime employed. In general, samples containing less than 0.5 wt% MnO₂ and when sintered at lower temperatures exhibited higher mechanical properties than the undoped HA. The study revealed that all the MnO₂-doped HA achieved >99% relative density when sintered at 1100-1250 °C as compared to the undoped HA which could only attained highest value of 98.9% at 1150 °C. The addition of 0.05 wt% MnO₂ was found to be most beneficial as the samples exhibited the highest hardness of 7.58 GPa and fracture toughness of 1.65 MPam^1/2 as compared to 5.72 GPa and 1.22 MPam^1/2 for the undoped HA when sintered at 1000 °C. Additionally, it was found that the MnO₂-doped samples attained E values above 110 GPa when sintered at temperature as low as 1000 °C if compared to 1050 °C for the undoped HA.     

Microstructure–mechanical properties relations in SiC–TiB2 composite

Densification and mechanical properties (fracture toughness, flexural strength and hardness) of SiC–TiB₂ composite were studied. Pressureless sintering experiments were carried out on samples containing 0–50 vol% of TiB₂ created by in situ reaction between TiO₂, B₄C and carbon. Al₂O₃ and Y₂O₃ were used as sintering additives to create liquid phase and promote densification at sintering temperature of 1940 °C. The sintered samples were subsequently heat treated at 1970 °C. It was found that the presence of TiB₂ serves as an effective obstacle to SiC grain growth as well as crack propagation thus increasing both strength and fracture toughness of sintered SiC–TiB₂ composite. The subsequent heat treatment of sintered samples promoted the elongation of SiC matrix and further improved mechanical properties of the composite. The best mechanical properties were measured in heat-treated samples containing 12–24 vol% TiB₂. The maximum flexural strength of ∼600 MPa was obtained in samples with 12 vol% TiB₂ whereas the maximum fracture toughness of 6.6 MPa m^1/2 was obtained in samples with 24 vol% TiB₂. Typical microstructures of samples with the mentioned volume fractions of TiB₂ consist of TiB₂ particles (<5 μm) uniformly dispersed in a matrix of elongated SiC plates.     

Carbonate coprecipitation synthesis of Sr- and Mg-doped LaGaO3

A carbonate coprecipitation synthesis, simple and easily scalable to industrial level, has been successfully applied to produce Sr- and Mg-doped LaGaO₃. The thermal evolution shows that the main perovskite phase crystallizes already at 1000 °C. Samples sintered at 1400 °C for 10 h have a relative density of 93% and an electrical conductivity of 7.7 ? 10^− 2 S cm^− 1 at 800 °C, similar to the one reported for solid state synthesis. The presence of small amounts of secondary phases (about 17%) such as LaSrGaO₄ and LaSrGa₃O₇ does not seem to deeply affect the electrical properties of the final material.     

Synthesis of BaTi4O9 ceramics by reaction-sintering process

Synthesis of BaTi₄O₉ ceramics by a reaction-sintering process was investigated. The mixture of raw materials for stoichiometric BaTi₄O₉ were pressed and sintered into ceramics without any calcination stage involved. Pure BaTi₄O₉ phases were obtained at 1150-1280 °C. High-sintered density, 98.2-99.5% of theoretical value (4.533 g/cm³), can be obtained for pellets sintered at 1200-1280 °C for 2-6 h. Some rod-shaped grains 3-7 μm in the longitudinal axis appear in pellets sintered at 1230 °C. Both the size and the amount of these rod-shaped grains increase at higher sintering temperature.     

Effect of liquid-forming additives on the sintering and mechanical properties of Al2O3/3Y-TZP (30 vol.%) composite

Al₂O₃/3Y-TZP (30 vol.%) composite was pressurelessly sintered with addition of TiO₂MnO₂ and/or CaOAl₂O₃SiO₂ glass. It was found that TiO₂MnO₂ addition greatly enhanced the densification of the composite by the formation of a low-viscosity liquid at sintering temperature. In contrast, the high-viscosity liquid formed by CaOAl₂O₃SiO₂ glass improved mechanical properties because of its repressing effect on grain growth. The composite could be obtained at a temperature as low as 1400°C by co-doping with TiO₂MnO₂ and CAS glass. Bending strength of 552±64 MPa and fracture toughness of 6.03±0.22 MPa m^1/2 were obtained with a doping level of 2 wt.% TiO₂MnO₂ and 2 wt.% CAS glass.