Development of an instrumented and automated single mode cavity for ceramic microwave sintering: Application to an alpha pure alumina powder

This paper describes the development of an instrumented and automated single mode microwave cavity for sintering ceramic powders. This setup includes an optical dilatometer and a motorized plunger to control heating cycles in a wide range of heating rates (from 5 °C ∙ min^− 1 to 200 °C ∙ min^− 1) up to 1600 °C and to allow reliable comparison with conventional sintering. The cavity and the sintering cells for both hybrid and direct microwave sintering were designed using finite element simulation. For accurate temperature measurement, an optical pyrometer calibrated with a specific protocol has been used. Microwave sintering of fine grained (< 100 nm) alpha alumina compacts was thus investigated and compared to conventional sintering. This pure alumina powder has been sintered by direct microwave heating, without any susceptor nor doping element to initiate heating as often achieved in the literature. The comparison of the densification kinetics along an identical thermal cycle evidenced a significant enhancement of sintering under microwaves during the first and intermediate stages.     

Preparation and characterization of precursor powders for yttria-doped tetragonal zirconia polycrystals (Y-TZP) and Y-TZP-Al2O3 composites

Precursor powders for the preparation of tetragonal 2.5 mol% Y₂O₃-ZrO₂ containing 0 to 30 wt% Al₂O₃ were made by coprecipitation. The behaviour of this powder during calcination from room temperature to 1200° C was studied using differential thermal analysis. X-ray diffraction and transmission electron microscopy methods, and measurements of surface area. The uncalcined powder was essentially amorphous. On heating alumina-free powder, zirconia crystallized at 485° C: for increasing alumina content, zirconia crystallized from an amorphous aluminous matrix at increasing temperatures (850° C for 20 wt% Al₂O₃), while the crystallite size decreased and the surface area of the powder increased. The zirconia first crystallized as cubic, but transformed to the tetragonal form near 1100° C. The alumina crystallized as corundum at 1200° C. No monoclinic zirconia could be detected when calcined aluminous material was cooled to room temperature. The sintering behaviour of the calcined powder is discussed.     

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.     

Mechanical properties of hydroxyapatite–zirconia compacts sintered by two different sintering methods

Microwave sintering is traditionally employed to reduce the sintering temperature required to densify powder compacts. The effect of microwave heating on hydroxyapatite (HA)–zirconia (ZrO₂) green bodies has been investigated in order to understand how microwave energy may affect the physical and mechanical properties of the resultant densified composites. Laboratory synthesised nano-sized HA and a commercial nano-sized ZrO₂ powder have been ball milled to create mixtures containing 0–5 wt% ZrO₂ loadings. Compacts were microwave sintered at either 700, 1000 or 1200°C with a 1 h hold time. Comparative firings were also performed in a resistive element furnace using the same heating profile in order to assess the differences between conventional and microwave heating on the physical, mechanical and microstructural properties of the composites. Samples sintered at 700°C show little sign of densification with open porosities of approximately 50%. Composites conventionally sintered at 1000°C were between 65 and 75% dense, whereas the samples microwave sintered at this temperature were between 55 and 65% dense. Samples sintered at 1200°C showed the greatest degree of densification (>80%) with a corresponding reduction in open porosities. TCP generation occurred as a consequence of sintering at 1200°C, even with 0 wt% ZrO₂, and increased degradation of the HA phase to form significant amounts of TCP occurred with increasing additions of ZrO₂, along with increasing open porosity. Nanosized ZrO₂ prevents the densification of the HA matrix by effectively pinning grain boundaries and this effect is more pronounced in the MS materials. Similar strengths are achieved between the microwave and conventionally sintered samples. Greater amount of open porosity and pore interconnectivity are seen in the MS samples, which are considered to be useful for biomedical applications as they can promote osteo-integration.     

Fabrication and characterization of nanocrystalline oxides by crystallization of amorphous precursors

Amorphous zirconia and alumina powders were produced by the electrochemical deposition. Hot-pressing of the zirconia powder at 2.5 GPa and 360°C for 30 min caused simultaneous consolidation of the powder to a dense body and crystallization of a nanocrystalline tetragonal phase. Cold-pressing of the same powder at 2.5 GPa and sintering at 600°C or 800°C for 1 hr resulted in large tetragonal crystallites within the highly porous compacts. Cold-pressing of alumina powder at 2.5 GPa and sintering at 500°C, or 900°C for 2 hrs resulted in partial crystallization of the amorphous phase to a mixture of various polymorphs of alumina. The microstructure was inhomogeneous and composed of both nanometer and submicrometer grains. Microhardness of the hot-pressed partially crystallized nanocrystalline tetragonal zirconia was comparable to that of single crystal monoclinic phase, and decreased after annealing at 450°C for 30 min. Microhardbess of the nanocrystalline alumina was lower by on order of magnitude than that of conventional polycrystalline alumina. The decrease in the microhardness of the sintered specimens was related both to the grain size and the porosity.     

Effects of amorphous silica coatings on the sintering behaviours of SiC whisker-reinforced Al2O3 composites

In this study, pressureless sintering of silicon carbide whisker (SiC_w)-reinforced alumina composites was investigated. SiC whiskers or Al₂O₃ powders were coated with amorphous silica, and sintering behaviour was analysed according to the powder characteristics of the composite. It was found that amorphous silica coatings improved densification as compared with uncoated powders, because the viscous flow allows the release of any tensile stress due to differential shrinkage between the matrix and the silicon carbide whiskers. Mullite occurred when amorphous silica coatings reacted with alumina at 1500 °C, which resisted the viscous sintering of the amorphous silica coatings.     

Effect of heating mode and temperature on sintering of YAG dispersed 434L ferritic stainless steel

This study examines the effect of heating mode, sintering temperature, and varying yttria alumina garnet (YAG) addition (5 and 10 wt%) on the densification and properties of ferritic (434L) stainless steel. The straight 434L stainless steel and 434L–YAG composites were sintered in a conventional and a 2.45 GHz microwave furnace. The composites were sintered to solid-state as well as supersolidus sintering temperature at 1200 and 1400 °C, respectively. Both 434L and 434L–YAG compacts coupled with microwaves and underwent rapid heating (∼45 °C/min). This resulted in about 85% reduction in the processing time. For all compositions microwave sintering results in greater densification. As compared to conventional sintering, microwave sintered compacts exhibit a more refined microstructure, thereby, resulting in higher bulk hardness. The mechanical properties and sliding wear resistance of 434L stainless steel is shown to be sensitive both to the sintering condition as well as YAG addition and has been correlated to the effect of heating mode on the pore morphology.     

Preparation of Fe3AI based iron aluminides from elemental and prealloyed povvders

Abstract

Iron aluminides were prepared by a powder metallurgy process from elemental powders, mixtures of prealloyed and elemental powders, and prealloyed powder. The sintering behaviour of various powders was studied using scanning electron microscopy, optical microscopy, and density measurement. It was found that sintering of elemental powder involved two distinct processes, i.e. alloying and densification, but sintering of prealloyed powder involved densification alone. The addition of prealloyed powder to elemental powders was helpful in restraining the swelling of sintered samples, the degree of swelling of sintered samples being reduced as the amount of prealloyed powder increased. For samples made from Fe-25 at.-%Al prealloyed powder, remarkable shrinkage was measured after sintering at 1250°C for 1 h. Within the correct range, their density increased with sintering temperature and time, but prolonged sintering at high temperature resulted in the loss of aluminium and a two phase microstructure. The difference in sintering behaviour between the various powders was discussed on the basis of thermodynamics.     

Assessment of Microwave Heating for Sintering of Al/SiC and for in‐situ Synthesis of TiC

This work describes sintering of SiC‐reinforced Al‐matrix composites and in‐situ synthesis of TiC in a powder mixture of Ti and C. In the first case, microwave energy is absorbed by SiC grains, heating the metal matrix to sintering and even melting temperature. The composite is processed at <1 kW microwave power. Microwave absorption and the heating rate increase with decreasing SiC particle size. Composites with high SiC content (70 vol.‐%) are processed at 650 °C/1 h in the microwave furnace, whereas conventional resistive heating at the same temperature did not allow sintering of the sample. In the second case, radiative energy allowed the heating of Ti/C samples up to 950 °C, and microwave assistance enhanced the reaction sintering of Ti/C powder mixtures forming TiC at the border of the Ti particles. The results are compared with conventional processing. Optical images and XRD patterns confirmed the formation of TiC for both techniques.     

Fabrication of hydrothermal ageing resistant of 3 mol % yttria-stabilised tetragonal zirconia polycrystalline via microwave sintering

The influence of microwave sintering on the densification, mechanical performances, microstructure evolution and hydrothermal ageing behaviour of pure 3 mol % yttria-stabilised tetragonal zirconia polycrystalline (3Y-TZP) ceramics was compared with conventional sintered samples. Green bodies were sintered via conventional pressure-less and microwave sintering method between 1200 °C to 1400 °C with dwelling time and firing rate at 120 min, 10 °C/min and 1 min, 20 °C/min. Result showed that reduced processing temperature and holding time is possible with microwave sintering technique for fabricating good resistant zirconia sample with bulk density, Young's modulus, and Vicker's hardness that are comparable to samples sintered with conventional method. However, the microwave sintered samples suffered from hydrothermal ageing where their average grain size is above critical size. The enhancement of hydrothermal ageing resistance of the sintered samples is associated with the decreasing grain size of the sintered samples instead of sintering method.     

Effect of sintering process on the microstructures of Bi2O3-doped yttria stabilized zirconia

Variations of microstructures in Bi₂O₃-doped yttria stabilized zirconia (YSZ) with conventional furnace and microwave sintering were investigated in this work. The results demonstrated that a small amount of addition of Bi₂O₃ was effective in reducing the sintering temperature of YSZ from 1500 °C to 1200 °C and promoting the densification rate of the ceramics. It is interesting that microwave sintering is found to suppress the evaporation rate of Bi₂O₃ and formation of the monoclinic-ZrO₂ or other amorphous phases. Compared to conventional furnace sintering, significant improvement in density of Bi₂O₃-doped YSZ at lower sintering temperatures with microwave sintering was observed. Rapid heating rate and short sintering time for restricting serious segregation at grain boundary were observed as well. Employing microwave sintering at the same sintered condition, the density of a specimen was evidently increased by 4.59% in comparison to the specimen sintered with a conventional furnace sintering.     

Microwave hybrid synthesis of silicon carbide nanopowders

Nanosized silicon carbide powders were synthesised from a mixture of silica gel and carbon through both the conventional and microwave heating methods. Reaction kinetics of SiC formation were found to exhibit notable differences for the samples heated in microwave field and furnace. In the conventional method SiC nanopowders can be synthesised after 105 min heating at 1500 °C in a coke-bed using an electrical tube furnace. Electron microscopy studies of these powders showed the existence of equiaxed SiC nanopowders with an average particle size of 8.2 nm. In the microwave heating process, SiC powders formed after 60 min; the powder consisted of a mixture of SiC nanopowders (with two average particle sizes of 13.6 and 58.2 nm) and particles in the shape of long strands (with an average diameter of 330 nm).     

A comparative study of ZnNb2O6 nanoceramics synthesized by high energy ball milling and subsequent conventional and microwave annealing

Ball-milling and subsequent conventional and microwave assisted heating processes have been applied to synthesize ZnNb₂O₆ nanoceramic. X-ray diffraction, simultaneous thermal analysis, scanning electron microscope (SEM), transmission electron microscope (TEM) and BET techniques were utilized to characterize the as-milled and annealed samples. Characterization of synthesized powders revealed that in spite of the very short heating time in the microwave process without soaking time, the powder heated at 550 °C had all physical properties similar to powders synthesized in conventional heating at the 650 °C temperature with a heating rate of 10 °C/min and a soaking time of 1 h. In addition, SEM, TEM and BET observations of synthesized powders showed that the particle size of powders lies in the nano meter range.     

Sintering and grain growth of 3 mol% yttria zirconia in a microwave field

A comparative study of the sintering and grain growth of 3 mol% yttria zirconia using conventional and microwave heating was performed. Extensive measurements of grain size were performed at various stages of densification, and following isothermal ageing at 1500 °C for 1, 5, 10 and 15 h. Microwave heating was found to enhance densification processes during constant rate heating. The grain size/density relationship for the microwave-sintered samples was shifted in the direction of increased density for density values less than 96% of the theoretical value when compared to conventionally heated samples. This suggests that there may be a difference in the predominant diffusion mechanisms operating during the initial and intermediate stages of sintering. Results of the ageing experiments showed that once densification was near completion, grain growth was accelerated in the microwave field, and exaggerated grain growth occurred.     

Spark plasma sintering synthesis of intermetallic T2 in the Mo–Si–B system

Mo₅SiB₂ (T2) was synthesized by sparking plasma sintering (SPS) under different heating rates and sintering temperatures. The powder mixture with a T2 composition (Mo–12.5Si–25B (at.%)) failed to produce the T2 single-phase alloy due to the volatilization of Si during SPS process. Extra 0.5 at.% Si added to this mixture offset the volatilization loss. It has been found that heating of the mixture at low heating rates favored the formation of binary phases in the solid state at medium temperatures. In this work, the T2 alloy with a fine-grained microstructure was obtained via a liquid–solid reaction when the mixture was heated fleetly to the temperatures above the silicon melting point at the rapid heating rate of 200 °C/min. The sintering temperature at 1500 °C for T2 synthesis is beneficial to enhance further densification, as well as to avoid abnormal grain growth at higher temperatures.     

Properties of nanostructured magnesium metatitanate prepared by the sol-gel technique

Nanometric powders of high purity were produced using alkoxides of magnesium and titanium as precursors for the sol-gel method. Formation of geikielite from the sol-gel powders was carried out by stepwise heating of the sol-gel precipitate, and the progress of the process was followed by XRPD, TGA and DTA. Low temperature treatment of the sol-gel products removed the solvent and led to an amorphous precursor powder. At 370 °C crystallization of periclase-spinel (MgO) and inception of geikielite formation took place. At 595 °C a single phase of geikielite was identified. These temperatures of formation are lower than previously reported values. This product shows a lower sintering temperature and improved dielectric properties after sintering, as compared to magnesium metatitanate produced by calcination of ordinary magnesia and titania powders. Approximately a ten-fold improvement for the quality factor Q and a more uniform value of the dielectric constant ε were attained when using nanostructured powders for sintering magnesium metatitanate bodies.     

The fabrication of O′-sialon ceramics by pressureless sintering

Low-porosity ceramics with O′-sialon as the major crystalline phase have been successfully fabricated by pressureless sintering. Densification at 1400 to 1800° C is interpreted using Kingery's liquid-phase sintering model. Particle rearrangement accounts for a significant proportion of the overall densification and is closely related to the amount and viscosity of the liquid present. The oxidation resistance of fabricated samples is good but, at temperatures above 1300° C, decreases with increasing alumina content in the starting mix. Modulus of rupture values of about 420 MPa at room temperature are high enough to encourage further work.     

Consolidation of nanoscale iron powders

The consolidation behavior of two types of nanoscale iron powders-vacuum condensed (nanograins in nanoparticles) and ball-milled (nanograins in microparticles), was studied. The consolidation of two microscale powders, atomized and ground, was also characterized for comparison. Consolidation techniques investigated were cold closed die-compaction, cold isostatic pressing (CIPing), and after CIPing, sintering or hot isostatic pressing (HIPing). The mechanical properties, density, and microstructure of the resulting compacts were found to depend on the original powder type and its consolidation history. Significant differences were found between the microscale and nanoscale powders. An additional reason, besides the dissimilarity in grain size, for the differences observed relates to the fact that the nanograin powders contained significant amounts of oxygen, which ultimately resulted in a distinctly two-phase bulk microstructure. The vacuum condensed powder achieved satisfactory green strength on CIPing, and high hardness (440 Hv) on low temperature sintering. While unnecessary for complete consolidation, HIPing at 500 °C was found to be beneficial and compacts of this powder thus treated were found to have a hardness of 520 Hv and high compressive yield strength (1800 MPa). For ball-milled powders, HIPing was found to be essential for achieving effective consolidation: ball-milled material, which remained friable after CIPing and sintering at 580 °C, achieved exceptionally high hardness (820 Hv) when HIPed at 580 °C and 175 MPa. The ductility was greatly improved when HIPed at temperatures between 700 °C and 850 °C, while preserving its relatively high strength. The behavior of these nanoscale powders can be understood by invoking the usual densification, particle bonding, and grain growth mechanisms. Optimization of these processes may result in unique mechanical properties of ball milled powders.     

Microstructural evolution during the supersolidus liquid phase sintering of nickel-based prealloyed powder mixtures

A novel concept for full-density sintering is described. Two prealloyed powders with slight compositional differences are tailored to separate the solidus temperatures into high-melt and low-melt compositions. A mixture of these two powder compositions allows full-density sintering at a temperature between the two solidus temperatures. For these experiments, the two powders were nickel-based alloys, where the low-melt powder contained boron. The mixed powders were sintered at temperatures above the solidus of the low-melt powder to form a transient liquid that promoted rapid densification of the mixture. Microstructure evolution during sintering was assisted using quenching experiments. Variables in this study included the heating rate, peak temperature, hold time, and powder ratio. Interdiffusion between the two powders controls microstructure evolution, with a dominant role associated with boron diffusion and reaction. The transient liquid phase responsible for densification is linked to boron diffusion and subsequent compound precipitation.     

Sintering of fibrous barium titanate powder compacts with preferred orientation

The sintering of fibrous BaTiO₃ powder particles was investigated. Special emphasis was given to the role of particle orientation in the compact on densification and microstructure development. Compacts were made by dry-pressing. During the initial stage of sintering, the fibrous particles rearranged and bundles of particles were formed. The volume of pores between bundles of particles decreased on further heating. Grain growth started when the sintered density reached ca. 56% of the theoretical density. Higher temperatures of sintering increased the degree of the crystal axis orientation. Thus, highly orientated sintered bodies with high densities were prepared by heating at 1500 °C.