Synthesis of MoSi2–Al2O3 nanocomposite by mechanical alloying

MoSi₂–Al₂O₃ nanocomposite was synthesized by mechanical alloying (MA) of MoO₃, SiO₂ and Al powder mixture. The structural evolution of the powders was studied by X-ray diffraction (XRD). Both β-MoSi₂ and -MoSi₂ were obtained after 3 h of milling. The spontaneous formation of β-MoSi₂ during milling proceeded by a mechanically induced self propagating reaction (MSR), analogous to that of the self propagating high temperature synthesis (SHS). After 70 h of milling the β-phase transformed to -phase. The crystallite size of -MoSi₂ and Al₂O₃ after milling for 100 h was 12 and 17 nm, respectively. Residual Mo and Si in the 3 and 70 h milled samples formed β-MoSi₂ and Mo₅Si₃ during heating at 1000 °C, respectively.     

Microstructures, densification and mechanical properties of TiC–Al2O3–Al composite by field-activated combustion synthesis

Dense TiC–Al₂O₃–Al composite was prepared with Al, C and TiO₂ powders by means of electric field-activated combustion synthesis and infiltration of the molten metal (here Al) into the synthesized TiC–Al₂O₃ ceramic. An external electric field can effectively improve the adiabatic combustion temperature of the reactive system and overcome the thermodynamic limitation of reaction with x < 10 mol. Thereby, it can induce a self-sustaining combustion synthesis process. During the formation of Al₂O₃–TiC–Al composite, Al is molten first, and reacted with TiO₂ to form Al₂O₃, followed by the formation of TiC through the reaction between the displaced Ti and C. Highly dense TiC–Al₂O₃–Al with relative density of up to 92.5% was directly fabricated with the application of a 14 mol excess Al content and a 25 V cm⁻¹ field strength, in which TiC and Al₂O₃ particles possess fine-structured sizes of 0.2–1.0 μm, with uniform distribution in metal Al. The hardness, bending strength and fracture toughness of the synthesized TiC–Al₂O₃–Al composite are 56.5 GPa, 531 MPa and 10.96 MPa m^1/2, respectively.     

Polyol method for the preparation of nanosized Gd2O3, boehmite and other oxides

A modified microwave-assisted polyol method was applied to prepare nanoparticulate ceramic powders of different oxides, i.e. Gd₂O₃, AlO(OH) (boehmite) and TiO₂. Due to the good dielectric properties of the utilised solvents, such as ethylene glycol, diethylene glycol and 1,4 butanediol, a significant decrease in reaction time was achieved under microwave heating. In the case of AlO(OH) and Gd₂O₃, <5 nm primary particle size were obtained. Boehmite was found to be intercalated with the solvent. The general applicability of the process is shown and the advantages in terms of properties and processibility are described. The powders thus prepared were investigated using X-ray diffractometry, electron microscopy and physisorption.     

Interface phenomena in the Y2O3/(Al–Cu) system

Wetting behavior and the interface reaction in the Y₂O₃/(Cu–Al) system were investigated at 1423 K. A contact angle of about 130° was measured in the Y₂O₃/Cu system. Aluminum addition to copper improves wetting and the transition from non-wetting to wetting (θ ≤ 90°) was observed for the alloy with 50 at.% Al. The microstructure examination of the interface indicates that Al reacts with yttria, yttrium dissolves in the melt and a crater of AlYO₃ is formed at the substrate. The interface interaction in the Y₂O₃/(Cu–Al) system is in a good agreement with the results of a thermodynamic analysis in the Y–Al–Cu–O system. The crater depth and the macroscopic final contact angles are correlated with the Y and Al activities in the melt.     

Synthesis and characterization of Zn/Al2O3 nanocomposite by mechanical alloying

In this study, fabrication and characterization of zinc-based metal matrix nanocomposite reinforced by Al₂O₃ particles was investigated. Aluminum and zinc oxide powder mixture was milled in a planetary ball mill in order to produce Zn/Al₂O₃ nanocomposite. The structural evaluation milled and annealed powders studied by X-ray diffraction, SEM observation and hardness measurement. The zinc crystallite size estimated with broadening of XRD peaks by Williamson-Hall formula. The zinc oxide was found to react with Al through a rapid self-sustaining combustion reaction process. As a result a zinc matrix composite reinforced by Al₂O₃ particulate was formed. The microhardness value of produced nanocomposite powder was about 350 HV which was 10–15 times higher than the microhardness of pure zinc (20–30 HV).     

Templated grain growth of SrBi2Ta2O9 ceramics: Mechanism of texture development

Textured SrBi₂Ta₂O₉ (SBT) ceramics were fabricated via templated grain growth (TGG) technique using platelet-like SBT single crystal templates. The templates (5 wt%) were embedded in a fine-grain SBT powder matrix containing 3 wt% of Bi₂O₃ excess that were subjected to uniaxial pressing and sintering at 1000–1250 °C for up to 24 h. Microstructural characterization by SEM was performed to establish the effect of sintering parameters on the grain growth and texture development. It was found that the ceramics developed a bimodal microstructure with notable concentration of large (longer than 90 μm) aligned grains with c-axis oriented parallel to the pressing direction. The mechanism controlling the texture development and grain growth in SBT ceramics is discussed.     

Effect of temperature on the catalytic oxidation of CO over nano-sized iron oxide

Nanocrystallite iron oxide powders were prepared by co-precipitation method using highly purified FeCl₃ and NH₄OH solution. The prepared powders were tested for the catalytic oxidation of CO to CO₂. The effect of oxidation temperature on the catalytic reaction was isothermally investigated using advanced quadrpole mass gas analyzer system. The mechanism of the catalytic oxidation reaction was estimated from the experimental data. Fe₂O₃ nanocrystallite of 78 nm shows a good response towards catalytic CO oxidation at the temperature range 200–500 °C. The catalytic oxidation efficiency reached 98% at 400–500 °C. The reaction was found to be first order with respect to CO. The average activation energy of the reaction was found to be 26.3 kJ/mol which is smaller than the values reported in the literatures. The mechanism of CO catalytic oxidation was investigated by comparing the CO catalytic oxidation data in the absence and presence of oxygen. It was found that the catalytic oxidation of CO over Fe₂O₃ nanocrystallite proceeded by adsorption mechanism. Based on the experimental results, Fe₂O₃ nanocrystallite powders can be recommended as a promising catalyst for CO oxidation.     

Synthesis, structure refinement and characterization of tetrahydrated acid gadolinium diphosphate HGdP2O7·4H2O

Synthesis and single crystal structure are reported for a new gadolinium acid diphosphate tetrahydrate HGdP₂O₇·4H₂O. This salt crystallizes in the monoclinic system, space group P2₁/n, with the following unit-cell parameters: a = 6.6137(2) Å, b = 11.4954(4) Å, c = 11.377(4) Å, β = 87.53(2)° and Z = 4. Its crystal structure was refined to R = 0.0333 using 1783 reflections. The corresponding atomic arrangement can be described as an alternation of corrugated layers of monohydrogendiphosphate groups and GdO₈ polyhedra parallel to the () plane. The cohesion between the different diphosphoric groups is provided by strong hydrogen bonding involving the W4 water molecule.

IR and Raman spectra of HGdP₂O₇·4H₂O confirm the existence of the characteristic bands of diphosphate group in 980–700 cm⁻¹ area. The IR spectrum reveals also the characteristic bands of water molecules vibration (3600–3230 cm⁻¹) and acidic hydrogen ones (2340 cm⁻¹). TG and DTA investigations show that the dehydration of this salt occurs between 79 and 900 °C. It decomposes after dehydration into an amorphous phase. Gadolinium diphosphate Gd₄(P₂O₇)₃ was obtained by heating HGdP₂O₇·4H₂O in a static air furnace at 850 °C for 48 h.     

Role of glycine-to-nitrate ratio in influencing the powder characteristics of La(Ca)CrO3

Ultrafine La(Ca)CrO₃ (LCC) powders were prepared through glycine-nitrate gel combustion process. The effect of glycine-to-nitrate ratio on batch size, particle size, nature of agglomeration and densification was studied. As-prepared powders when calcined at 700 °C resulted in LCC along with a small amount of CaCrO₄. The primary particle size obtained in case of stoichiometric and fuel-rich precursor was found to be 25–60 and 60–180 nm, respectively. It was found that the final powder had softer agglomerates with increasing glycine-to-nitrate ratio, which in turn improved the sintered density. The powder obtained through fuel-rich precursor could be sintered to ≈98% of its theoretical density at 1300 °C without applying any ball milling operation.     

Synthesis, phase transformation and dielectric properties of sol–gel derived Bi2Ti2O7 ceramics

Sol–gel derived Bi₂Ti₂O₇ ceramic powders have been prepared from methoxyethoxides of bismuth and titanium (molar ratio of Ti/Bi = 1.23 and water/alkoxides = 1.31). The Bi₂Ti₂O₇ phase was stable at a low temperature (700 °C), but it then transformed into mixed phases of Bi₄Ti₃O₁₂ and Bi₂Ti₄O₁₁ at 850–1150 °C. The single phase of Bi₂Ti₂O₇ reoccurred at 1200 °C. Dielectric properties and ferroelectric behavior of samples sintered at 1150 and 1200 °C were examined. Under frequency of 1 MHz, samples sintered at 1150 and 1200 °C had a dielectric constant of 101.3 and 104.2, and a loss tangent of 0.0193 and 0.0145, respectively. Only the sample sintered at 1150 °C showed ferroelectric behavior, where remanent polarization is 3.77 μC cm⁻² and coercive field is 24 kV cm⁻¹. Thus, the Bi₂Ti₂O₇ did not exhibit ferroelectricity, but the mixed phase of Bi₄Ti₃O₁₂ and Bi₂Ti₄O₁₁ did.     

Effect of CeO2 addition on structure and electrical properties of (Na0.5Bi0.5)0.93Ba0.07TiO3 ceramics prepared by citric method

(Na_0.5Bi_0.5)_0.93Ba_0.07TiO₃ ceramics added with 0–0.8 wt.% CeO₂ were prepared by a citrate method, and the influence of the CeO₂ addition on the structure and electrical properties was investigated. The specimens containing various amounts of CeO₂ show the coexistence of rhombohedral and tetragonal phases, with the relative content of the tetragonal phases gradually enhancing with increasing amount of CeO₂. Compared with (Na_0.5Bi_0.5)_0.93Ba_0.07TiO₃, the specimen added with a small amount of CeO₂ (≤0.2 wt.%) display a slightly improved electromechanical coupling factor (k_p) and piezoelectric constant (d₃₃) in conjunction with a reduced dielectric loss (tg δ) and an enhanced mechanical quality factor (Q_m), while higher CeO₂ amounts led to a rapid deterioration of the piezoelectric and ferroelectric properties. The variation of the electrical properties with the CeO₂ addition was tentatively interpreted with respect to doping effect, crystal-structural evolution and stability of ferroelectric domains.     

Electrical properties of Ta2O5 thin films deposited on Cu

The electrical and dielectric properties of reactively sputtered Ta₂O₅ thin films with Cu as the top and bottom electrodes forming a simple metal insulator metal (MIM) structure, Cu/Ta₂O₅/Cu/n-Si, were studied. Ta₂O₅ films subjected to rapid thermal annealing (RTA) at 800°C for 30 s in N₂ ambient crystallized the film, decreased the leakage current density and resulted in reliable time-dependent dielectric breakdown characteristics. The conduction mechanism at low electric fields (<100 kV/cm) is due to Ohmic conduction; however, the Schottky mechanism becomes predominant at high fields (>100 kV/cm). Present studies demonstrate the use of Cu as a potential electrode material to replace the conventional precious metal electrodes for Ta₂O₅ storage capacitors.     

Eu-activated heavy scintillating glasses

Eu³⁺-activated scintillating glasses with molar compositions of 35SiO₂–15B₂O₃–30Ln₂O₃–20AlF₃ (Ln = Y, La, Gd, Lu) have been prepared. The effects of Ln³⁺ ions on the density, transmission, photoluminescence and radioluminescence have been studied. The glasses have high density, ranging from 4.0 to 6.1 g/cm³ in the order of Y < La < Gd < Lu. Gd-containing glass exhibits a much higher light yield than the other glasses. The effect of complete substitution of fluorine by oxygen on the scintillation properties is also investigated.     

Influence of Mn doping on microstructure and DC-accelerated aging behaviors of ZnO–V2O5-based varistors

The microstructure, electrical properties, dielectric characteristics, and DC-accelerated aging behavior of the ZnO–V₂O₅–MnO₂ system sintered were investigated for MnO₂ content of 0.0–2.0 mol% by sintering at 900 °C. For all samples, the microstructure of the ZnO–V₂O₅–MnO₂ system consisted of mainly ZnO grain and secondary phase Zn₃(VO₄)₂. The incorporation of MnO₂ to the ZnO–V₂O₅ system was found to restrict the abnormal grain growth of ZnO. The nonlinear properties and stability against DC-accelerated aging stress improved with the increase of MnO₂ content. The ZnO–V₂O₅–MnO₂ system added with MnO₂ content of 2.0 mol% exhibited not only a high nonlinearity, in which the nonlinear coefficient is 27.2 and the leakage current density is 0.17 mA/cm², but also a good stability, in which %ΔE_1 mA = −0.6%, %Δ = −26.1%, and %Δtan δ = +22% for DC-accelerated aging stress of 0.85E_1 mA/85 °C/24 h.     

Mechanical characterization of Ni1−xZnxFe2O4 prepared by non-conventional methods

The mechanical properties like hardness, H_v and compressive strength, σ of Ni_1−xZn_xFe₂O₄ (x = 0.2, 0.3, 0.4 and 0.5) prepared by the non-conventional flash combustion and citrate-gel decomposition techniques are studied and reported. It is observed that there is an increase in hardness with zinc content as well as sintering temperature. The hardness in the order of 2.0–3.63 GPa and compressive strength in the order of 150–240 MPa are obtained for Ni–Zn ferrites prepared by these non-conventional techniques. The influence of density, porosity and microstructure on hardness and compressive strength of Ni–Zn ferrites with respect to sintering temperature was studied.     

Formation of Mg–Al–Ti/MgO composite via reduction of TiO2

This paper investigates the possibility of reduction of TiO₂ through the mechanical alloying of Mg and TiO₂ powders. Elemental Mg and Al powders were first mixed with TiO₂ according to the nominal composition of Mg5wt.%–Al17.15wt.%–TiO₂. The powder mixture was mechanically milled in a planetary ball mill for different durations. Two types of DSC curves have been observed. The unmilled powders showed no reaction when heated to about 500 °C, while an exothermic reaction in the mechanically milled powders was observed at about 390 °C. Further investigation of the heat treatment of the mechanically milled powders showed the complete disappearance of TiO₂ and the formation of MgAl₂O₄ after annealing at 400 °C, implying that reduction of TiO₂ has occurred via the mechanical activation and annealing processes.     

RuO2–TiO2 mixed oxides prepared from the hydrolysis of the metal alkoxides

The hydrolysis of ruthenium alkoxide/titanium tetraethoxide mixtures to gels and powders containing 30–40 mol% Ru was investigated. Basic or neutral conditions led to powders consisting of 2–10 nm diameter crystalline RuO₂ nanoparticles embedded in a matrix of crystalline (anatase) and amorphous TiO₂. Acid hydrolysis conditions gave gels containing smaller, amorphous RuO₂ nanoparticles (1–3 nm). In all samples the RuO₂ nanoparticles tended to clump into aggregates up to 0.5 μm across. Acid or neutral hydrolysis of ruthenium ethoxide gave samples which displayed lower surface Ru:Ti ratios as measured by XPS compared to the bulk (XRF), and also contained more low-valent Ru (as measured by XRF), probably due to incomplete hydrolysis of the precursors. These samples also contained more Ru metal after calcination (XRD). Calcination (450 °C) was accompanied by Ru-promoted combustion of organic material and led to crystalline (anatase) TiO₂ and Ti_xRu_1−xO₂ solid solution (rutile phase).     

Improvements in mechanical properties of TiB2 ceramics tool materials by the dispersion of Al2O3 particles

TiB₂–Al₂O₃ composites with Ni–Mo as sintering aid have been fabricated by a hot-press technique at a lower temperature of 1530 °C for 1 h, and the mechanical properties and microstructure were investigated. The microstructure consists of dispersed Al₂O₃ particles in a fine-grained TiB₂ matrix. The addition of Al₂O₃ increases the fracture toughness up to 6.02 MPa m^1/2 at an amount of 40 vol.% Al₂O₃ and the flexural strength up to 913.86 MPa at an amount of 10 vol.% Al₂O₃. The improved flexural strength of the composites is a result of higher density than that of monolithic TiB₂. The increase of fracture toughness is a result of crack bridging by the metal grains on the boundaries, and crack deflection by weak grain boundaries due to the bad wetting characters between Ni–Mo and Al₂O₃.     

Consolidation behavior of L12 phase (Al + 12.5 at.% Cu)3Zr powder with nanocrystalline structure during CIP and subsequent sintering

The mechanically alloyed (Al + 12.5 at.% Cu)₃Zr powders were consolidated by cold isostatic pressing (CIP) and subsequent sintering. Effects of CIP pressure and sintering temperature on the stability of metastable L1₂ phase and nanocrystalline structure were investigated. Before sintering, the powders were CIPed at 138, 207, 276, and 414 MPa. The relative densities of the CIP compacts were not greatly affected by the CIP pressure. However, the L1₂ phase of the specimen CIPed at pressures greater than 276 MPa was partially transformed into D0₂₃. The optimum consolidation conditions for maintaining L1₂ phase and nanocrystalline microstructure were determined to be CIP at 207 MPa and sintering at 800 °C for 1 h for which the grain size was 34.2 nm and the relative density was 93.8%. Full density specimens could be prepared by sintering above 900 °C, however, these specimens consisted of L1₂ and D0₂₃ phases. The grain sizes of all the specimens were confirmed by TEM and XRD, and were found to be less than 40 nm. This is one of the smallest grain sizes ever reported in trialuminide intermetallic compounds.     

Characterization and microstructural evolution of SiC(OAl) fibers to SiC(Al) fibers derived from aluminum-containing polycarbosilane

The SiC(OAl) fibers and the SiC(Al) fibers were fabricated by the use of aluminum-containing polycarbosilane (Al–PCS) precursor. The two types of fibers have been characterized. Chemical element analysis, AES, SEM, XRD, RMS and NMR have been employed. The chemical formula of SiC(OAl) fibers is SiC_1.31O_0.25Al_0.018 with C and O rich on the surface. The microstructure of SiC(OAl) fibers is a mixture of β-SiC nanocrystals, free carbon, and an amorphous silicon oxycarbide (Si–C–O phase), which have been confirmed by an amount of SiC₂O₂, SiCO₃, SiO₄ and SiC₃O units in the ²⁹Si MAS NMR spectrum. A small quantity of aluminum is embedded uniformly in the Si–C–O amorphous continuous phase. For SiC(Al) fibers, nearly stoichiometric composition was confirmed as chemical composition of SiC_1.03O_0.013Al_0.024. The fiber is composed of a large number of β-SiC crystallites, a small amount of -SiC crystalline and SiC amorphous phase. The aluminum in the SiC(Al) fibers mainly exists in two manners: Al–C bonds connected with the surfaces of the β-SiC grains and Al–O bonds, or Al₂O₃, to the amorphous phase.