Effect of powder reactivity on microwave sintering of alumina

Effect of the reactivity of starting alumina powder of varying crystallinity on the sintering behavior in microwave process was studied. From X-ray amorphous to highly crystalline alumina, powders were obtained by conventional heating of compacts made of the precursor amorphous powder by heating it at different temperatures from 800 to 1500 °C. These samples were then sintered in a multimode microwave field of 2.45 GHz for 10 min at 1500 °C. The microwave effect on densification of the various alumina powders was evaluated by comparing the microwave and conventional sintering data. The results show significant microwave enhancement in the densification of the samples without any pretreatment. This enhancement became less significant as the temperature of the pretreatment increased and finally diminished. Since the pretreatment at elevated temperatures made the powder more stable thermodynamically, this study indicates that the sintering enhancement of a ceramic material in microwave is a metastability-related phenomenon.     

Flow behaviour and microstructural evolution in cold worked 70∶30 α-brass

Abstract

Deformation behaviour and microstructures at failure were investigated in a mill cold worked 70∶30 α-brass over the test temperature range of 298–973 K and strain rate range of 10^?5–5×10^?3 s^?1. Tensile properties as a function of temperature revealed three distinct regions, with their temperature sensitivity being maximum at intermediate temperatures (553–673 K) and much less towards the lower and higher temperature ranges. Two values of activation energy for high temperature deformation Q were obtained to be 117·5 kJ mol^?1 below 623 K and 196·4 kJ mol^?1 above this critical temperature. In the respective temperature range the values of stress exponent n were 5·6 and 3·8. Based on the values of Q and n, the deformation mechanism was suggested to be dislocation climb creep with a probable contribution from dislocation pipe diffusion on lowering the temperature. Both grain size and cavity size were found to increase with increasing test temperature, suggesting them to be interrelated and act as an alternative steps for accommodating grain boundary sliding. Static grain growth study, over the temperature range of 773 to 1073 K, led to activation energy for grain growth to be 71 kJ mol^?1, with the time exponent of 0·37.     

Lithium metazirconate for nuclear application: physical and mechanical properties

Lithium metazirconate is a candidate in fusion reactor designs as a tritium-breeding material. An amorphous powder was synthesized by gelling zirconium propylate and lithium acetate: after heat treatment at 900 °C, pure Li₂ZrO₃-l was obtained. Preliminary tests made on sintered pellets, obtained from this powder, demonstrated an appropriate tritium release at low temperature, when the sample porosity was about 20% and the mean pore radius 3 μm. The subject of this paper was to realize a good compromise between mechanical properties and the spread porosity, necessary for tritium diffusion. Dilatometric investigations were performed on compacts of powders treated at 800, 900 and 1000 °C to identify the best temperature and time of sintering. The calcined powders were also uniaxially pressed at 200, 300 and 500 MPa: the green bodies were sintered at 1200, 1250 and 1300 °C for 2 or 4 h, in order to produce ceramics having different porosity. On the sintered bodies, the pore size distribution, the mean pore radius and mechanical properties were evaluated.     

A simple sol–gel technique for preparing hydroxyapatite nanopowders

HA powder was prepared using a sol–gel method with phosphoric pentoxide (P₂O₅) and calcium nitrate tetrahydrate (Ca(NO₃)₂·4H₂O). The effect of sintering temperatures on crystalline degree and composition of the HA phase, and also the effect of aging times on crystal size of the HA powder were studied using XRD and TEM. It was found that at sintering temperatures ranging from 600 to 900 °C, the dominant phase in the powders was HA with small amounts of calcium oxide and β-tricalcium phosphate (β-TCP) at 800 and 900 °C, and only HA phase was observed at 600 and 700 °C. 10–15 nm HA powders were obtained using this technique. This technique has an advantage over other sol–gel methods in more simple and shorter time because of no requiring pH value control and long hydrolysis time.     

Influence of Particle Size and Heating Rate on Decomposition of BC Dry Chemical Fire Extinguishing Powders

Pyrolysis of BC dry chemical fire extinguishing powders which are useful for Class “B” and Class “C” fires was conducted on a thermogravimetric analyzer with sample loading of 10–25 mg under dynamic air atmosphere. The effect of particle sizes (medium value 48.99, 27.24, 4.93 µm) and heating rates (10, 15, and 20°C min^?1) were examined. The pyrolysis kinetics of the samples was analyzed using a distribution activation energy model. It was found that the decomposition temperature decreased and the pyrolysis rate increased after the samples were milled. The agglomeration of particles during production did not have an appreciable influence on the pyrolysis process of the samples in our experimental conditions. The activation energy value was 77.13?219.78 kJ · mol^?1, 58.18?288.67 kJ · mol^?1, and 44.59?209.17 kJ · mol^?1 for the powder of particle size 48.99, 27.24, 4.93 µm. We should use micro powder in fire extinguishing.     

Ionic conductivity in dehydrated zeolites

The ionic conductivity of pressed pellets of dehydrated synthetic offretite, cancrinite and zeolite A, with various alkali metal ions, was determined by low-frequency impedance spectroscopy. Experiments were carried out in the frequency range 10 Hz–10 MHz at temperatures from 100–600°C. The conduction activation energies range between 55 kJ mol^?1 (Na-zeolite A) and 108 kJ mol^?1 (Li-cancrinite). The best conductivity value obtained was that of Na-zeolite A with 2.9×10^?3Ω^?1cm^?1 at 600°C.     

Effect of characteristics of (Sm,Ce)O2 powder on the fabrication and performance of anode-supported solid oxide fuel cells

Effect of characteristics of Sm_0.2Ce_0.8O_1.9 (SDC) powder as a function of calcination temperature on the fabrication of dense and flat anode-supported SDC thin electrolyte cells has been studied. The results show that the calcination temperature has a significant effect on the particle size, degree of agglomeration, and sintering profiles of the SDC powder. The characteristics of SDC powders have a significant effect on the structure integrity and flatness of the SDC electrolyte film/anode substrate bilayer cells. The SDC electrolyte layer delaminates from the anode substrate for the SDC powder calcined at 600 °C and the bilayer cell concaves towards the SDC electrolyte layer for the SDC powder calcined at 800 °C. When the calcinations temperature increased to 1000 °C, strongly bonded SDC electrolyte film/anode substrate bilayer structures were achieved. An open-circuit voltage (OCV) of 0.82–0.84 V and maximum power density of ～1 W cm^?2 were obtained at 600 °C using hydrogen as fuel and stationary air as the oxidant. The results indicate that the matching of the onset sintering temperature and maximum sintering rate temperature is most critical for the development of a dense and flat Ni/SDC supported SDC thin electrolyte cells for intermediate temperature solid oxide fuel cells.     

Sintering studies on submicrometre-sized Y-Ba-Cu-oxide powder

The isothermal sintering behaviour of submicrometre-sized (<50 nm) powders of single-phase YBa₂Cu₃O _x (123) and unreacted stoichiometric mixture of submicrometre-sized (<50 nm) powders of BaCO₃, Y₂O₃ and CuO (which on calcination at 1173 K gives YBa₂Cu₃O _x ) was investigated through dilatometry under different sintering atmospheres. The sintering rate of the powder compacts was impeded by the presence of oxygen. The activation energies,Q, of sintering were determined to be 1218 kJ mol^–1 in argon, 1593 kJ mor^–1 in air and 2142 kJ mol^–1 in oxygen. A decrease in the apparent sintered density with increasing oxygen partial pressure was also observed. X-ray diffraction and thermal analyses (thermogravimetry and differential thermal analysis) showed no reaction during sintering of the single-phase product. Pellets fabricated from uncalcined powder exhibit two stages of sintering, one between 1073 and 1173 K having an activation energyQ=627kJ mol^–1, and a second one above 1173 K withQ=383.7 kJ mol^–1. A.c. susceptibility, resistivity and critical current density were determined as a function of the temperature of the sintered samples.     

Thermal stability of MgO nanoparticles

Thermal stability of nanocrystalline MgO particles with average diameter of 11 nm was investigated by annealing of the cold isostatically pressed compacts between 600 °C and 900 °C for various durations. Sintering time versus grain radius at 800 °C demonstrated a linear line with the slope of ∼ 4 similar to that expected for surface diffusion. High resolution scanning electron microscope images from different specimens showed a porous microstructure of interconnected particles typical for initial sintering. Arrhenius plot of the grain size data revealed the activation energy of 161 ± 11 kJ mol^− 1 for the growth process in agreement with those reported for grain boundary grooving experiments. It was found that MgO particles undergo coarsening already at temperatures as low as 0.31 of the MgO melting point (3125 K). Increase in the particle diameter and decrease in the surface area were associated with surface diffusion mechanism that leads to initial sintering between the particles.     

Novel synthesis of high surface area MgAl2O4 spinel as catalyst support

A modified sol–gel route, by combining gelation and coprecipitation processes, was developed for the synthesis of high surface area MgAl₂O₄ spinel precursors. The obtained precursors were then calcined in flowing air at temperatures ranging from 500 to 900 °C. The formation of new phases upon calcinations was investigated using X-ray diffraction, thermal gravimetric analysis, and Fourier transform infrared spectroscopy (FTIR). Single-phase spinel powder with uniform pore size distribution was formed at temperatures as low as 600 °C. It was found that the thermal stability of the as-synthesized spinels is higher than that reported by other preparation methods. After calcinations at 800 and 950 °C for 8 h, the specific surface area reaches a level of 182 and 136 m²·g⁻¹, respectively. And the degree of crystallinity is higher than other preparation methods as illustrated by samples calcined at 800 °C. The amount of PVA added significantly affects the surface area of the samples. With increasing the ratio of M/PVA, the surface area of the resulting spinels increased accordingly.     

Effect of powder characteristics on the sinterability of hydroxyapatite powders

The effect of different sintering conditions on the sintered density and microstructure of two different hydroxyapatite (HA) powders was examined. The powder characteristics of a laboratory synthesized HA powder (Lab HA) were low crystallinity, a bimodal particle size distribution, a median particle size of 22 m and a high specific surface area (SSA) of 63 m²/g. By contrast, a commercial calcined HA (commercial HA) was crystalline and had a median particle size of 5 m and a low SSA of 16 m²/g. The different powder characteristics affected the compactability and the sinterability of the two HA powders. Lab HA did not compact as efficiently as commercial HA, resulting in a lower green density, but the onset of sintering of powder compacts of the former was approximately 150 °C lower than the later. The effect of compaction pressure, sintering temperature, time and heating rate on the sintered densities of the two materials was studied. Varying all these sintering conditions significantly affected the sintered density of commercial HA, whereas the sintered density of Lab HA was only affected significantly by increasing the sintering temperature. The Vickers hardness, H_v, of Lab HA was greater than commercial HA for low sintering temperatures, below 1200 °C, whereas for higher sintering temperatures the commercial HA produced ceramics with greater values of hardness. These trends can be related to the sinterability of the two materials.     

Structure of polycrystals produced by sintering nanocrystalline powders of cubic and wurtzitic boron nitrides

The structure and some properties of polycrystals produced by sintering nanocrystalline powders of the dense modifications of shock—wave-synthesized BN have been studied. The sintering was conducted at a static pressure of 7.7 GPa and temperatures from 1100 to 1800° C. The highest density (3 g/cm³) and microhardness (up to 20 GPa) have been exhibited by polycrystals produced by sintering the powder containing wurtzitic and cubic modifications in amounts that are approximately equal. In the temperature range from 1100 to 1300° C the wurtzitic phase transformed into the cubic one. In this temperature range the average size of cBN grains changed from 20 to 50 nm. The structure of compacts is characterized by the presence of grain (grain-boundary) interlayers 2–5 nm in thickness.     

An investigation on microwave sintering of Fe, Fe–Cu and Fe–Cu–C alloys

The powder characteristics of metallic powders play a key role during sintering. Densification and mechanical properties were also influenced by it. The current study examines the effect of heating mode on densification, microstructure, phase compositions and properties of Fe, Fe–2Cu and Fe–2Cu–0·8C systems. The compacts were heated in 2·45 GHz microwave sintering furnaces under forming gas (95%N₂–5%H₂) at 1120 °C for 60 min. Results of densification, mechanical properties and microstructural development of the microwave-sintered samples were reported and critically analysed in terms of various powder processing steps.     

Stainless steel foams made through powder metallurgy route using NH4HCO3 as space holder

Stainless steel (316) foams of varying porosities have been made through powder metallurgy route using NH₄HCO₃ as a space holder. Green compacts of stainless steel powder with NH₄HCO₃ were sintered at two different temperatures: 1100 °C and 1200 °C. At higher sintering temperatures, neighboring stainless steel powders fused together to form polycrystalline grain structure with iron–chromium intermetallic phases segregated along the grain boundaries. Whereas, the fusion of neighboring stainless steel powders was limited around the particle–particle contact only when the green compacts were sintered at 1100 °C, which resulted in a larger amount of microporosities in the cell wall. These foams exhibited strain hardening behavior in the plateau region under compressive loading. The yield stress and the flow stress (at lower strain levels) of foams, sintered at 1100 °C were higher. But, the reverse is true for the flow stress at higher strain levels. The exponents and the coefficients of the power law relationships varied with sintering temperature and strain levels.     

Influence of calcination temperature on the properties of spray dried alumina-zirconia composite powders

Alumina-zirconia composite powders containing 10, 12.5, 15 or 20 wt% zirconia were prepared by spray-drying the hydroxide gels. These powders were calcined at 650 and 950 °C. The spray-dried as well as the calcined powders were characterized by means of Coulter counter, Sorptometer, infrared spectroscopy (i.r.), scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and X-ray diffraction (XRD). Initially the spray-dried powders are amorphous and spherical in shape with a diameter of 6 m and crystallize after calcination treatment at 950 °C. Sintered density of the 950 °C calcined powder compacts was higher than 650 °C calcined powder compacts. Compacts made from 650 °C treated powders retained 100% tetragonal phase after sintering irrespective of composition. Some amount of tetragonal phase is transformed into monoclinic phase in the composites containing higher amount of zirconia in the sintered compacts made from 950 °C calcined powders.     

Microcalorimetric studies of the acidity of the small-pore zeolite H-RHO

Microcalorimetry and infrared studies of ammonia adsorption are used to investigate the acidic properties of the small-pore zeolite H-RHO. Shallow-bed calcination of NH₄-RHO at 400°C produces a nearly pure H-RHO with framework hydroxyl Brönsted acid sites having a heat of adsorption of ammonia of ∼170 kJ mol⁻¹. Shallow-bed calcination at 600° or 700°C destroys ∼25% and ∼50% of the framework hydroxyls, respectively, producing ‘super acid’ sites and a new type of hydroxyl characterized by an infrared hydroxyl band at 3640 cm⁻¹. This new hydroxyl site is only weakly acidic; evidence suggests it may have a heat of adsorption of ammonia of ∼60 kJ mol⁻¹. Shallow-bed calcination also produces a small number of strongly acidic Lewis sites. The high acidity of these strong Lewis sites raises the average energy of ammonia adsorption by up to 15 kJ mol⁻¹ for H-RHO calcined at 600° or 700°C. Immobile adsorption of ammonia and preferential location of the most energetic sites in the internal pores of the zeolite structure results in a ‘bell-shaped’ curve for the initial heat of adsorption for H-RHO calcined at 600°C.     

Wear behavior of light-weight and high strength Fe-Mn-Ni-Al matrix self-lubricating steels

The good combination of mechanical and tribological properties for self-lubricating materials is crucial. In this work, novel self-lubricating Fe-16.4 Mn-4.8 Ni-9.9 Al-xC(wt%) steels containing graphite phase were fabricated using mechanical alloying and spark plasma sintering. The compositions of the steels were designed by using thermodynamic calculation, and the effect of carbon addition on the microstructure was further investigated. The steel possesses high hardness of 621 HV, high yield strength of 1437 MPa and good fracture toughness at room temperature. The yield strengths are still above 600 MPa at 600?C.The tribological behavior and mechanical properties from room temperature to 800?C were studied, and the wear mechanisms at elevated temperatures were discussed. The steel has a stable friction coefficient of 0.4 and wear rate in a magnitude of 10~(-6) mm~3/N·m below 600?C. The good tribological properties of the steels were mainly attributed to the high hardness, lubrication of graphite and stable surface oxide layer.     

Preparation and characterization of the bismuth sodium titanate (Na0.5Bi0.5TiO3) ceramic doped with ZnO

This study investigates effects of the zinc oxide (ZnO) addition and the sintering temperature on the microstructure and the electrical properties (such as dielectric constant and loss tangent) of the lead-free piezoelectric ceramic of bismuth sodium titanate (Na_0.5Bi_0.5TiO₃), NBT, which was prepared using the mixed oxide method. Three kinds of starting powders (such as Bi₂O₃, Na₂CO₃ and TiO₂) were mixed and calcined. This calcined NBT powder and a certain weight percentage of ZnO were mixed and compressed into a green compact of NBT–ZnO. Then, this green compact of NBT–ZnO was sintered to be a disk doped with ZnO, and its characteristics were measured. In this study, the calcining temperature was 800 °C, the sintering temperatures ranged from 1000 to 1150 °C, and the weight percentages of ZnO doping included 0.0, 0.5, 1.0, and 2.0 wt%. At a fixed wt% ZnO, the grain size increases with increase in the sintering temperature. The largest relative density of the NBT disk obtained in this study is 98.3% at the calcining temperature of 800 °C, the sintering temperature of 1050 °C, and 0.5 wt% ZnO addition. Its corresponding dielectric constant and loss tangent are 216.55 and 0.133, respectively.     

Processing and sintering of yttrium-doped tungsten oxide nanopowders to tungsten-based composites

Innovative chemical methods are capable of fabricating nanoscale tungsten oxide compounds doped with various rare-earth elements with high purity and homogeneity, which can be processed under hydrogen into nanostructured oxide-dispersed tungsten composite powders having several potential applications. However, hydrogen reduction of doped tungsten oxide compounds is rather complex, affecting the morphology and composition of the final powder. In this study, we have investigated the reduction of tungstic acid in the presence of Y and we provide the experimental evidence that Y₂O₃ can be separated from Y-doped tungstic acid via hydrogen reduction to produce Y₂O₃-W powders. The processed powders were further consolidated by spark plasma sintering at different temperatures and holding times at 75 MPa pressure and characterized. The optimized SPS conditions suggest sintering at 1400 °C for 3 min holding time to achieve higher density composites with an optimum finer grain size (3 µm) and a hardness value up to 420 H _V. Major grain growth takes place at temperatures above 1300 °C during sintering. From the density values obtained, it is recommend to apply higher pressure before 900 °C to obtain maximum density. Oxides inclusions present in the matrix were identified as Y₂O₃·3WO₃ and Y₂O₃·WO₃ during high resolution microscopic investigations.     

Synthesis of spodumene and spodumene — zirconia composite powders using aqueous sol-gel method

Pure spodumene and spodumene-zirconia (5, 10, 15 mol%) composite powders were prepared using aqueous sol-gel method employing lithium formate, aluminium formate, zirconium formate and tetraethoxy silane (TEOS) as starting materials in aqueous medium. The gels prepared by this method were dried at 100°C for 24h and then calcined for 2h at different temperatures ranging from 500°C to 800°C. X-ray powder diffraction (XRD), differential thermal analysis (DTA), thermogravimetric analysis (TGA) and infra-red spectroscopy analysis (IR) were utilized to characterize the gel powders and calcined powders. Transmission electron microscope (TEM) was used to measure the average particle size of the calcined powders.