Regularities of sintering of zirconium diboride-molybenum alloys

Summary In the process of sintering of mixtures of zirconium diboride with 5, 10, and 15% Mo, specimen growth resulting from heterodiffusion is observed at the instant of formation of solid solution of Mo in ZrB₂ during slow heating to high temperatures or during the initial period of isothermal holding in the case of very rapid heating. At temperatures of up to 1700–1750°C, growth predominates over shrinkage, and specimen dimensions increase with increasing holding time; at temperatures above 1800°C, positive shrinkage takes place, but is very slight in the case of rapid heating to the isothermal holding temperature.During isothermal holding in the temperature range 1800–2200°C, very intensive shrinkage is observed during the initial period (20–30 min). Subsequently, this shrinkage slows down, and may be described as viscous flow caused by diffusional processes. The energy of activation of the densification process, calculated from the shear viscosity values obtained, was found to be 367±48, 352±28, and 379±46 kJ/mole for alloys of ZrB₂ with 5, 10, and 15% Mo, respectively, i.e., less than the energy of activation of densification of zirconium diboride (678±55 kJ/mole).Thus, the presence of molybdenum activates diffusion processes during sintering.     

Influence of ammonium sulfate on YAG nanopowders and Yb:YAG ceramics synthesized by a novel homogeneous co-precipitation method

Homogeneous and dispersed Y_3 Al_5 O_(12)(yttrium aluminum garnet,YAG) nanopowders were synthesized via a homogeneous co-precipitation method from the mixed solutions of yttrium nitrate,aluminum nitrate and a small amount of ammonium sulfate using hot urea as the precipitant.The method has the superiorities that co-precipitation of cations is ensured and continuous decomposition of the hot urea is achieved to obtain the narrow size distribution particles.The addition of small amount of ammonium sulfate surfactant,although has no influence on YAG garnet phase formation,has significant effect on dispersion,particles distribution and sinterability of the resultant YAG and Yb:YAG powders.Compared with the undoped sample,the green body of Yb:YAG doped with ammonium sulfate has higher total shrinkage,linear shrinkage rate and relative density through sintering at 1600 ℃.The resultant Yb:YAG powders can be sintered into transparent ceramics at 1700 ℃ through vacuum sintering.The influence of the sulfate ions on characteristics of the resultant powders was thoroughly studied.     

Synthesis of γ-TiAl by Reactive Spark Plasma Sintering of Cryomilled Ti and Al Powder Blend: Part II: Effects of Electric Field and Microstructure on Sintering Kinetics

The current study shows the dramatic effect of an electric field (EF) and use of nanosized cryomilled grains on accelerating sintering kinetics during spark plasma sintering of blended elemental powder compacts of Ti₅₃Al₄₇ targeted to produce γ-TiAl intermetallic compounds. The EF had the dominating effect since it reduced the activation barrier for diffusion through Al₃Ti leading to faster growth of Al₃Ti; the precursor to γ-TiAl. The Avrami exponent (n) determined for the micrograin compact lies between 1.0 and 1.5, which indicates that reaction sintering is controlled by bulk diffusion in these compacts, while for cryomilled compacts this is between 0.7 and 1.0 suggesting the important role of dislocations and grain boundaries on the transformation during reaction sintering. The activation energies were found to be in increasing order as: cryomilled compacts with EF (182 kJ/mol); micrograin compacts with EF (290 kJ/mol); cryomilled compacts without EF (331 kJ/mol); and micrograin compacts without EF (379 kJ/mol). The cryomilled microstructure also enhanced the sintering kinetics because of the availability of faster diffusing paths in Al and Ti including larger grain boundary area and dislocation density.     

Liquidlike sintering behavior of nanometric Fe and Cu powders: Experimental approach

Nanometric Fe and Cu powders were sintered in vacuum, He, and H₂ atmospheres after uniaxial cold pressing. The shrinkage behavior of samples was studied using three different dilatometric techniques: constant heating rate, isothermal annealing, and the Dorn method. Density greater than 90 pct was obtained at sintering temperatures of 900 °C. In nanometric powders, densification and grain coarsening occurred in a narrow temperature interval. Despite the low oxide content in the starting powders (1.5 to 4 wt pct), the reducing atmosphere plays a relevant role in the sintering process. The self-diffusion activation energies obtained for nanometric Fe were 116 and 60 kJ/mole in vacuum and H₂, and those obtained for nanometric Cu were 70 and 43 kJ/mole in He and H₂. According to the present results, the activation energies obtained from both nanometric powders in H₂ could be associated with those for self-diffusion in liquid Fe (65 kJ/mole) and Cu (41 kJ/mole).     

Enhanced low-temperature sintering of tungsten

The effects of various transition metal additions on the sintering of a well-characterized, fine tungsten powder were analyzed using both isothermal and constant heating rate experiments in the temperature range 900 to 1400°C. Approximately four atomic mono-layers of palladium on the tungsten powder surface were found to be the optimal enhancer, followed by nickel, cobalt, platinum, and iron. The addition of Cu to the tungsten had no appreciable effect on the sintering kinetics. Sintering enhancement by these transition metals is related to their periodic chart position (i.e., electron structure). An overall non-Arrhenius shrinkage temperature dependence is attributed to grain growth in the activator-treated specimens. The activation energy for tungsten densification was determined to be 430 to 450 kJ/mol, which is in general agreement with a grain boundary diffusion process.     

The interaction of hydrogen with the interface of AI2O3 particles in iron

A mathematical model to calculate the trap binding energy and trap density is suggested considering the theories of hydrogen trapping and hydrogen retrapping. When iron containing 2.0 wt pct Al₂O₃ is heated with a uniform heating rate of 3 K-min^-1, a hydrogen peak is observed at 853 K in the evolution ratevs temperature plot. This is due to hydrogen evolution from the Al₂O₃/lattice interface. The trap activation energy and trap binding energy of hydrogen at the Al₂O₃/lattice interface are estimated as 79 kJ ⋅ mol^-1 and 71.4 kJ ⋅ mol^-1, respectively, fitting experimental data to the model. This indicates that the Al₂O₃/lattice interface acts as an irreversible trapping site for hydrogen. By combining the trap binding energy and trap activation energy, the energy level of hydrogen around the Al₂O₃/lattice interface is suggested. The saddle point energy of hydrogen at Al₂O₃/lattice interface, 7.56 kJ ⋅ mol^-1 is nearly equivalent to the activation energy for hydrogen diffusion through a normal lattice, 6.9 kJ ⋅ mol^-1. Formerly Graduate Student, Korea Advanced Institute of Science and Technology.     

Enhanced low-temperature sintering of tungsten

The effects of various transition metal additions on the sintering of a well-characterized, fine tungsten powder were analyzed using both isothermal and constant heating rate experiments in the temperature range 900 to 1400‡ C. Approximately four atomic monolayers of palladium on the tungsten powder surface were found to be the optimal enhancer, followed by nickel, cobalt, platinum, and iron. The addition of Cu to the tungsten had no appreciable effect on the sintering kinetics. Sintering enhancement by these transition metals is related to their periodic chart position(i.e., electron structure). An overall non-Arrhenius shrinkage temperature dependence is attributed to grain growth in the activator-treated specimens. The activation energy for tungsten densification was determined to be 430 to 450 kJ/mol, which is in general agreement with a grain boundary diffusion process.     

Modification of the sintering kinetics of palladium by a surface oxide

The presence of a surface oxide layer on palladium powders gives rise to a densification-free sintering process controlled by a surface diffusion mechanism with an activation energy of 59.2 kJ/mol₋₁. At temperatures above the oxide dissociation temperature in air, 1062 K, palladium powders sinter with an activation energy of 120.4 kJ/mol"1, indicative of a grain boundary diffusion. AtT < 1061 K and under a 10₋₈ atm vacuum, sintered powders showed densification but with an activation energy too high to be grain boundary diffusion-related. These results are interpreted in terms of the role of the surface oxide and are compared with previously published results on the sintering of Pd.     

A two-stage approach of manufacturing FeAl40 iron aluminides by self-propagating synthesis and pressureless sintering

A two-stage sintering process was successfully used to sinter FeAl to densification levels of just above 95% at a temperature of 1300°C. In the first stage, mixed iron and aluminium powders were synthesised at 750°C via self-propagating high-temperature synthesis (SHS) to form brittle and porous Fe₂Al₅. Then the pellets were crushed and milled to various sizes and mixed with iron powders in the nominal composition of FeAl40 and pressurelessly sintered at a higher temperature to obtain a higher densification by taking advantage of the less violent exothermic reaction of Fe₂Al₅ and Fe. The intermediate and end products in SHS and sintering were characterised by SEM/EDX and XRD. The porosity level of the final FeAl40 product was controlled by the heating rate and powder size, which was also strongly influenced by the temperature, holding time and the ratio of the two powders.     

Morphological and calorimetric studies on the

Amorphous Al₅₀Zr₅₀ alloy powders have been prepared by rod-milling technique using mechanical alloying (MA) method. The amorphization and crystallization processes of the alloyed powders were followed by optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), differential thermal analysis (DTA), and differential scanning calorimetry (DSC). The results have shown that the formation of amorphous Al₅₀Zr₅₀ alloy powders occurs through three stages, agglomeration, disintegration, and homogenization. At the disintegration stage, the alloyed powders contain many fine layers of Al and Zr. An amorphous phase has been formed at about 880 K as a result of heating these layered particles in a thermal analyzer. The crystalline-to-amorphous transformation at this stage of milling is attributed to a thermally assisted solid-state amorphizing reaction. The present study corroborates the similarity of the amorphization process through the MA with the solid-state interdiffusion reaction in multilayered thin films. The amorphization temperature, T_a, and the activation energy of amorphization, E_a, are 675 and 156 kJ/mol, respectively. In addition, the enthalpy change of amorphization, ΔH_a, was evaluated to be -3.5 kJ/mol. On the other hand, the crystallization temperature, T_x, and enthalpy change of crystallization, ΔH_x, were 1000 K and −68 kJ/mol, respectively.     

Sintering of a highly heat-resistant ceramics by using combination SHF-heating and equipment for its realization

This article examines the main problems in sintering a highly heat-resistant ceramics by SHF heating. It is shown that it is expedient to use combination heating from an SHF source and additional heaters made of resistive composites to speed up sintering and equalize the temperature distribution in the ceramics. The additional heaters heat the product externally by convection and radiation. The synthesis of mullite from powders obtained by the sol-gel method and gradient powders of Al₂O₃/SiO₂ is described to illustrate the use of combinated SHF heating.     

Effects of heating rate on densification and grain growth during field-assisted sintering of α-Al2O3 and MoSi2 powders

Two types of powders, electrically conductive MoSi₂ and insulating α-Al₂O₃, were sintered by a field-assisted sintering technique (FAST) using heating rates from 50 °C to 700 °C/min. The Al₂O₃ powders were sintered to 99 pct density at 1100 °C for 2 minutes under 45 Mpa pressure. For Al₂O₃, no exaggerated grain growth was observed and the final grain size inversely scaled with the heating rate. Such a grain growth behavior fits the literature models based on multiple transport mechanisms for constant-heating-rate sintering. The density reached by MoSi₂ under similar sintering conditions was 91 pct. The grain size was independent of the heating-rate value. Specific electrical field and pressure effects are shown to contribute to enhanced densification and minimal coarsening in each material.     

Influence of MgO,Al2O3 and CaO/SiO2 on the viscosity of blast furnace type slag with high Al2O3 and 5 wt-% TiO2

The effect of MgO, Al₂O₃ and CaO/SiO₂ on the viscosity of CaO–SiO₂–Al₂O₃–MgO–5 wt-% TiO₂ slag was studied in the temperature range of 1673–1773?K. At a fixed CaO/SiO₂ ratio of 1·17 and 12 wt-% Al₂O₃, the viscosity of the slag decreased with increasing MgO content because of depolymerisation of the silicate structures. At a fixed CaO/SiO₂ ratio of 1·17 and 8 wt-% MgO, the viscosity of the slag increased with increasing Al₂O₃ content. At 8 wt-% MgO and 12 wt-% Al₂O₃ wt-%, increasing the CaO/SiO₂ ratio from 1·07 to 1·50 resulted in lower slag viscosity. The temperature dependencies of the viscosity on MgO addition, Al₂O₃ addition, and CaO/SiO₂ ratio were analyzed, and the apparent activation energies of each system were found to be between 178 and 232?kJ/mol, 273 and 360?kJ/mol, and 204 and 233?kJ/mol, respectively. Five different viscosity models were employed to predict slag viscosity, and the Riboud model was found to be the best for predicting this parameter.     

Sintering kinetics and alumina yield in lime-soda sinter process for alumina from coal wastes

An investigation of the application of the lime-soda sinter process to alumina extraction from coal wastes has been carried out. In the sintering stage, the optimal operating conditions have been obtained for the highest yield of alumina. The kinetics of sodium aluminate formation have also been studied in the sintering stage. The sinter mixes have been fired isothermally in air in the temperature range 1100 to 1350 °C. Alumina recovery of about 80 pct has been obtained by sintering coal-waste mixes having molar ratios of Na₂O/Al₂O₃ = 1.3 and CaO/SiO₂ = 1.8 at 1200 to 1250 °C for 20 to 30 minutes. Insoluble alumina compounds are responsible for the incomplete recovery. The major sinter components are identified as sodium aluminate and β-dicalcium silicate. The nucleation and growth kinetics equation is used to correlate the experimental data of sodium aluminate formation obtained under atmospheric pressure in the temperature range 1000 to 1200 °C. An activation energy of 286 kJ/mol has been calculated for fine coal-waste powder mixtures. Formerly Graduate Student in Metallurgy.     

Effect of LiF as Sintering Agent on the Densification and Phase Formation in Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-4 Wt Pct Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> Ceramic Compound

Different amounts of LiF were added to an Al₂O₃-4 pct Nb₂O₅ basic ceramic, as sintering agent. Improved new ceramics were obtained with LiF concentrations varying from 0.25 to 1.50 wt pct and three sintering temperatures of 1573 K, 1623 K, and 1673 K (1300 °C, 1350 °C, and 1400 °C). The addition of 0.5 wt pct LiF yielded the highest densification, 94 pct of the theoretical density, in association with a sintering temperature of 1673 K (1400 °C). Based on X-ray diffraction (XRD), this improvement was due not only to the presence of transformed phases, more precisely Nb₃O₇F, but also to the absence of LiAl₅O₈. The preferential interaction of LiF with Nb₂O₅, instead of Al₂O₃, contributed to increase the alumina sintering ability by liquid phase formation. Scanning electron microscopy (SEM) results revealed well-connected grains and isolated pores, whereas the chemical composition analysis by energy dispersive energy (EDX) indicated a preferential interaction of fluorine with niobium, in agreement with the results of XRD. It was also observed from thermal analysis that the polyethylene glycol binder burnout temperature increased for all LiF concentrations. This may be related to the formation of hydrogen bridge bonds.     

Non-Isothermal Kinetics of Reduction Reaction of Oxidized Pellet Under Microwave Irradiation

 The microwave heating characteristics of the mixture with oxidized pellet and coal was studied, and the non-isothermal reduction dynamics is discussed. The results show that, the slow-heating stage of the temperature rising process can be segmented into two heating temperature curves approximately that have good linear relationship. They can be seen as temperature programming. In the first stage, between 827 and 1073 K, the reaction mechanism obeys diffusion controlled model. In the second stage, between 1093 and 1323 K, the reaction mechanism also obeys diffusion controlled model. The apparent activation energies are found to be 7513 kJ/mol for the first stage and 5317 kJ/mol for the second stage. That is lower than the apparent activation energy under conventional heating. The microstructure of the reduced pellets shows that microwave can improve the kinetics of the reduction. Microwave has anxo-action to the reaction obviously.     

The sintering of 304L stainless steel powder

An examination of the shrinkage kinetics for a 304L stainless steel powder showed that initial densification is controlled by a strain assisted volume diffusion mechanism. At temperatures above 1330 K, grain growth reduces the shrinkage rate; however, at lower sintering temperatures, the shrinkage rate is temporarily increased by the proximity of the moving grain boundaries to the interparticle necks. The activation energies of volume diffusion (240±20 kJ/mol) and grain growth (285±35 kJ/mol) were in good agreement with prior results.     

The sintering of 304L stainless steel powder

An examination of the shrinkage kinetics for a 304L stainless steel powder showed that initial densification is controlled by a strain assisted volume diffusion mechanism. At temperatures above 1330 K, grain growth reduces the shrinkage rate; however, at lower sintering temperatures, the shrinkage rate is temporarily increased by the proximity of the moving grain boundaries to the interparticle necks. The activation energies of volume diffusion (240 ± 20 kJ/mol) and grain growth (285 ± 35 kJ/mol) were in good agreement with prior results.     

Low-Temperature Densification Sintering and Properties of Monoclinic-SrAl2Si2O8 Ceramics

The dense monoclinic-SrAl₂Si₂O₈ ceramics have been prepared by a two-step sintering process at a sintering temperature of 1173 K (900 °C). Firstly, the pre-sintered monoclinic-SrAl₂Si₂O₈ powders containing small SiO₂·Al₂O₃ crystal phases were obtained by continuously sintering a powder mixture of SrCO₃ and kaolin at 1223 K (950 °C) for 6 hours and 1673 K (1400 °C) for 4 hours, respectively. Subsequently, by the combination of the pre-sintered ceramic powders with the composite flux agents, which are composed of a SrO·3B₂O₃ flux agent and α-Al₂O₃, the low-temperature densification sintering of the monoclinic-SrAl₂Si₂O₈ ceramics was accomplished at 1173 K (900 °C). The low-temperature sintering behavior and microstructure evolvement of the monoclinic-SrAl₂Si₂O₈ ceramics have been investigated in terms of Al₂O₃ in addition to the composite flux agents. It shows that due to the low-meting characteristics, the SrO·3B₂O₃ flux agent can urge the dense microstructure formation of the monoclinic-SrAl₂Si₂O₈ ceramics and the re-crystallization of the grains via a liquid-phase sintering. The introduction of α-Al₂O₃ to the SrO·3B₂O₃ flux agent can apparently lead to more dense microstructures for the monoclinic-SrAl₂Si₂O₈ ceramics but also cause the re-precipitation of SiO₂·Al₂O₃ compounds because of an excessive Al₂O₃ content in the SrO·3B₂O₃ flux agent.     

The effect of small additions of disperse alumina inclusions on some characteristics of sintered nickel

Summary Materials containing 0.5, 1, 2, 3, 4, and 5 vol. % Al₂O₃ were obtained from mixtures of nickel oxide and alumina powder reduced with hydrogen. The apparent density of the reduced powder and the relative density of pressed compacts vary nonuniformly with increasing alumina content. The material with 1 vol. % Al₂O₃ has the maximum values of these characteristics. The sintering shrinkage and relative density of sintered specimens are minima for this composition. Disperse alumina inclusions appear to exert a dual effect on the shrinkage of contacts during sintering: On the one hand, they decrease shrinkage by disturbing contact between the nickel particles; on the other hand, they facilitate the evacuation of the water vapor forming during sintering. The disperse inclusions break up the nickel submicrograins and affect the microdistortions of the nickel lattice.