Sinterability of magnesium-oxide powder containing spherical agglomerates

The magnesium-oxide (MgO) powders were prepared by calcining basic magnesium carbonate (4MgCO₃·Mg(OH)₂·4H₂O; BMC) powder at a temperature between 600°C and 1200°C for 1 to 5 h. The resulting MgO powders contained spherical agglomerates with diameters of 10–50 m; the external shapes of these BMC agglomerates remained unchanged even after the calcination. With increasing compaction pressure, the densification of MgO powder compacts proceeded by (i) the rearrangement of agglomerates (50 MPa), (ii) the collapse of agglomerates (50–100 MPa), and (iii) the closer packing of primary particles (100 MPa). The MgO compact was fired at 1400 °C for 5 h. The relative density of the sintered MgO compact whose starting powder was prepared by calcining the BMC at 1000°C for 3 h attained 98.0%. The bending strength of this sintered MgO compact attained 214 MPa.     

Preparation of slaking resistant CaO aggregate from lightweight CaCO3 with oxide addition

CaO aggregate was sintered from reagent-grade lightweight CaCO₃ powder by the addition of 0-20% (molar ratio) MgO and ZrO₂, respectively. The results showed that the CaO derived from lightweight CaCO₃ was highly sinterable and compact CaO aggregate with relative density above 96% was obtained after sintering at 1400 °C for 2 h, but further increase of compactness was restrained due to the occurrence of abnormal grain growth. The densification of the aggregate was promoted due to the behavior of oxide addition on restraining the grain growth of CaO. With increasing the amount of oxide addition, the microstructure of CaO aggregate underwent a restructuration process. Homogeneous microstructure, with well growing MgO grains occupying most of the boundary triple points of CaO grain, formed by the addition of 20% MgO. Especially when 20% ZrO₂ was added, CaZrO₃ layer formed around CaO grains. The slaking resistance of the aggregate was appreciably improved due to the promotion of densification, the formation of CaO solid solution (while MgO added) and the modification of microstructure.     

Influence of powder characteristics on gas pressure sintering of Si3N4

The sintering behaviours of four kinds of Si₃N₄ powders were investigated by dilatometry in 10 atm N₂ at 1890, 1930 and 2050° C. The sinterabilities of powders were compared and discussed in relation to the powder characteristics. A large size distribution in the powder accelerated grain and pore growth at <1800° C, which resulted in the inhibition of further densification at >1800° C. The presence of carbon in a powder prevented densification. A powder with a uniform grain size kept the microstructure of the sintered material uniform during sintering at <1800° C and gave a high degree of shrinkage at >1800° C. Densification at >1800° C was accompanied by the dissolution of equi-axial -Si₃N₄ grains and reprecipitation as elongated -Si₃N₄ grains from the oxynitride liquid. The relation between the densification and microstructure is discussed in terms of the relative rates of densification and grain growth.     

Microstructure and permittivity of sintered BaTiO3: influence of particle surface chemistry in an aqueous medium

The influence of changes in the surface chemistry and surface composition of colloidal BaTiO₃, due to its dissolution and adsorption/precipitation of Ba²⁺ in an aqueous medium, on the microstructure and permittivity of sintered powder compacts was investigated. For BaTiO₃ powder with Ba-deficient (Ti-excess) surface prepared at pH 3, grain growth was enhanced at 1350 °C (above the eutectic) and permittivity was reduced (relative to stoichiometric BaTiO₃ prepared at pH 9) with increasing sintering temperature due to the liquid phase formed at grain boundaries. This same sample showed minimal grain growth and moderate enhancement of sinterability at 1300 °C (below the eutectic) attributed to sliding of the Ti-excess surface phase. BaTiO₃ powder treated at pH 3 and subsequently adjusted to pH 10 results in a core-shell structure with a varying near-surface stoichiometry, and produced abnormal grain growth for the compact sintered at 1350 °C. Permittivity of this sample was significantly reduced at 1350 °C due to the formation of the liquid phase, while exhibiting a similar permittivity to that of the stoichiometric sample when sintered at 1300 °C, despite significant microstructural coarsening. We conclude that changes in the surface-phase Ba/Ti ratio of particulate precursors, due to dissolution, adsorption and precipitation reactions in aqueous media, are as significant in determining the mechanical and electronic properties of the sintered material as are variations in the bulk stoichiometry of BaTiO₃.     

An improvement in processing of hydroxyapatite ceramics

Hydroxyapatite ceramics have been fabricated via two different processing routes, a conventional processing route and an emulsion-refined route. The conventional precipitation processing of powder precursors for hydroxyapatite ceramics results in the formation of hard particle agglomerates, which degrade both the compaction and densification behaviour of the resultant powder compacts. An emulsion-refinement step has been shown to be effective in softening particle agglomerates present in the conventionally processed powder precursor. As a result, the emulsion-refined powder compact exhibits both a higher green density and a higher sintered density than the un-refined powder compact, on sintering at temperatures above 800 °C. The effect of powder agglomeration on densification during both the initial and later stage of sintering is discussed. The attainable sintered density of the conventionally processed material was found to be limited by the presence of hard powder agglomerates, which were not effectively eliminated by the application of a pressing pressure of 200 MPa. These hard powder agglomerates, which form highly densified regions in the sintered ceramic body, commenced densification at around 400 °C which is more than 100 °C lower than the densification onset temperature for the emulsion-refined powder compact, when heated at a rate of 5 °C min^–1. The inter-agglomerate voids, manifested by the differential sintering, resulted in the formation of large, crack-like pores, which act as the strength-limiting microstructural defects in the conventionally processed hydroxyapatite. A fracture strength of 170±12.3 MPa was measured for the emulsion-refined material compared to 70±15.4 MPa for the conventionally processed material, when both were sintered at 1100 °C for 2 h.     

Properties of NbC-Co cermets obtained by spark plasma sintering

Sub-micrometer sized NbC-Co powder mixtures with 8, 12, 18 or 24.5 wt.% Co were consolidated by spark plasma sintering (SPS) for 2 min at 1200-1280 °C and 30-60 MPa. The optimum densification conditions were determined by analysing the dimensional change of the NbC-12 wt.% Co powder compact. SPS for 2 min at 1280 °C under a pressure of 60 MPa allowed full densification of the NbC-Co cermets with limited NbC grain growth. The microstructure is characterized as a highly interconnected NbC grain network with an inhomogeneously distributed Co binder. The Vickers hardness increased from 11.70 to 15.40 GPa whereas the fracture toughness decreased from 9.0 to 5.5 MPa m^1 / 2 with decreasing Co content from 24.5 to 8 wt.%.     

Transparent Lu2O3:Eu ceramics by sinter and HIP optimization

Evolution of porosity and microstructure was observed during densification of lutetium oxide ceramics doped with europium (Lu₂O₃:Eu) fabricated via vacuum sintering and hot isostatic pressing (HIP’ing). Nano-scale starting powder was uniaxially pressed and sintered under high vacuum at temperatures between 1575 and 1850 °C to obtain densities ranging between 94% and 99%, respectively. Sintered compacts were then subjected to 200 MPa argon gas at 1850 °C to reach full density. Vacuum sintering above 1650 °C led to rapid grain growth prior to densification, rendering the pores immobile. Sintering between 1600 and 1650 °C resulted in closed porosity yet a fine grain size to allow the pores to remain mobile during the subsequent HIP’ing step, resulting in a fully-dense highly transparent ceramic without the need for subsequent air anneal. Light yield performance was measured and Lu₂O₃:Eu showed ∼4 times higher light yield than commercially used scintillating glass indicating that this material has the potential to improve the performance of high energy radiography devices.     

Reaction sintering of diamond using a binary solvent-catalyst of the Fe-Ti system

Reaction sintering of diamond was investigated using a starting mixed powder of purified natural graphite and a binary solvent-catalyst of the Fe-Ti system under high pressure (7G Pa) and temperature (1700°C) conditions for e treatment time of 1 to 15 min. Diamond sintered compact of about 100% conversion ratio from graphite todiamond was obtained with the binary solvent-catalyst content: 11.4 to 17.0 vol% (30 to 40wt%) Fe and 6.6 to 7.7vol% (10wt%) Ti. The sintered compact having the bulk density of 4.1 to 5.5g cm^–3, consisted of diamond phase and metal carbide (Fe₃C and TiCx) phase. The Vickers microhardnesm (under 1000 g load) mfthesintered diamond phase was >8OO0, while that ofthe metal carbide phase was 1000 to 2000. The transformation from graphite to diamond proceeded in a short time (< 1 min), which was followed by a particle joining between the formed diamond grains, when the densification would be attained at the reaction time of 15 min by pooling out the melt of carbon and solvent-catalyst.     

Preparation of TiB2 sintered compacts by hot pressing

A sintered compact of titanium diboride (TiB₂) was prepared by hot pressing of the synthesized TiB₂ powder, which was obtained by a solid-state reaction between TiN and amorphous boron. Densification of the sintered compact occurred at 20 MPa and 1800° C for 5 to 60 min with the aid of a reaction sintering, including the TiB₂ formation reaction between excess 20 at % amorphous boron in the as-synthesized powder (TiB₂ + 0.2B) and intentionally added 10 at % titanium metal. A homogeneous sintered compact of a single phase of TiB₂, which was prepared by hot pressing for 30 min from the starting powder composition [(TiB₂ + 0.2B) + 0.1 Ti], had a fine-grained microstructure composed of TiB₂ grains with diameters of 2 to 3 m. The bulk density was 4.47 g cm^–3, i.e. 98% of the theoretical density. The microhardness, transverse rupture strength and fracture toughness of the TiB₂ sintered compact were 2850 kg mm^–2, 48 kg mm^–2 and 2.4 MN m^–3/2, respectively. The thermal expansion coefficient increased with increasing temperature up to 400° C and had a constant value of 8.8 x 10^–6 deg^–1 above 500° C.     

Effect of sintering temperature on the microstructure and high-frequency magnetic properties of Ni0.467Zn0.07Co0.015Fe0.511O4 ferrite

In this work, an attempt is made to study the effects of sintering temperature on the microstructure and high-frequency (HF) magnetic properties of a nickel zinc ferrite compound of very low ZnO content of Ni_0.467Zn_0.07Co_0.015Fe_0.511O₄ composition. Samples were prepared by a conventional ceramic route and sintered for 2 h at 1150, 1200, 1250, and 1300 °C. It was shown that the higher the sintering temperature the higher the saturation magnetization and the measured initial permeabilities, and the lower was the H _c of the samples. This was related to the increased sintered densities and grain sizes. The magnitudes of the electrical resistivity of the samples sintered at 1300 °C compared to those of the samples sintered at 1150 °C and 1200 °C showed four orders decrease. This is thought to be due to the grain-size increase and possibly the formation of higher Fe²⁺/Fe³⁺ concentration. The lowest measured quality factor (Q-factor) obtained in the range of 60–210 MHz, corresponds to the samples sintered at 1300 °C. The highest Q-value in the frequency range of 125–210 MHz was obtained for the samples sintered at 1150 °C, which has also shown the highest electrical resistivity.     

Microstructure of cBN-diamond sintered compact prepared by reaction sintering

cBN-diamond composite sintered compacts (diamond content 15–70 wt %) were prepared by reaction sintering at 7–7.5 GPa and 1400–1700 °C for 10–30 min from the starting powder of the hBN-diamond system in the presence of 1 wt % NH₄NO₃ as a volatile catalyst. A fully dense sintered compact with 99% conversion from hBN to cBN was obtained at 7 GPa and 1700 °C after 30 min. An induced transformation from hBN to cBN seemed to occur on the surface of the added diamond seed crystals. Diamond seed crystals (about 30 wt %, grain size 0.2–1.5 m) were found to be well-dispersed in the reaction-bonded cBN matrix. The Vickers microhardness of the sintered compact was 5100 kg mm^–2. The contacts between diamond grains were observed in the sintered compacts containing diamond seed grains of more than 70 wt %. The toughness of the sintered compact tended to increase with decreasing diamond content and the grain size of seed crystals.     

Effects of added diamond powder on the reaction sintering behaviour of diamond in the graphite-nickel system

Polycrystalline diamond sintered compact was prepared under high pressure and temperature conditions (7 GPa, 1700°C, 10 to 30min) from the starting material with the composition of the system 50 wt% carbon (diamond + graphitized pitch coke (GPC))-50 wt% Ni. The effects of added diamond powder on the microstructure of the sintered compact and reaction sintering behaviour were investigated. The grain size of diamond in the sintered compact decreased remarkably from 20 to 40m to 2 to 3m on addition of 10 to 20 wt% diamond powder (grain size: 1m) to the GPC-Ni system. The grain size can be controlled by that of the added diamond powder. A sufficient supply of carbon from GPC plays an important role in the formation of a covalently bonded compact of diamond. The grain growth of the formed diamond is depressed by the coexistence of diamond powder, which controls the solubility of carbon in the metal-carbon system and also the grain growth process by solution-reprecipitation.     

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.     

Yttria-zirconia: Effect of microstructure on conductivity

Complex impedance measurements and detailed analysis of the grain-boundary microstructure have been made on fully stabilized yttria-zirconia sintered bodies as a function of grain size. The prereacted yttria-zirconia powder used in this study was obtained from a commercial source. The powder has very high reactivity and starts sintering around 1200° C. The densification process is complete around 1350° C but the grain growth continues almost linearly with sintering temperature. The grain size variation obtained was between 1 and 30 m. The grain-boundary resistivity when plotted against grain size showed an inflection in the vicinity of 1500° C sintering temperature. These results have been explained in terms of the grain-boundary microstructure changing with the sintering temperature. The thickness of the grain-boundary layer determined from impedance data and transmission electron micrographs are in reasonably good agreement. The activation energy for the grain-boundary resistivity was only slightly higher than that for the lattice resistivity.     

PZT Material by Spray-Dried Precursors and Solid-State Reaction: Cold Consolidation and Sintering

The Nb-doped Pb(Zr_0.52 Ti_0.48)O₃ (PZT) powder, synthesized by spray drying of the solution followed by calcination, was cold consolidated and sintered under different process conditions. The microstructure and final properties were compared with the material produced by the conventional solid-state reaction of the oxides. The spray-dried powder undergoes a complete reaction in the perovskitic phase at 550°C, while the mixed oxides are converted at temperatures not lower than 800–850°C. Because of the hollow spherical structure of the spray-dried powder, low green densities are obtained. Consequently, a grinding process, as well as high-pressure cold isostatic compaction, were applied. The high reactivity of the powder results in the reduction of the densification temperature of 100°C. The final microstructure differs substantially from that developed with mixed-oxide processing and a different sintering mechanism is proposed.     

Densification and microstructure development during the sintering of submicrometre magnesium oxide particles prepared by a vapour-phase oxidation process

The densification behaviour and microstructure development of MgO compacts fired from room temperature up to 1700°C at a heating rate of 10°C min^–1 were examined. Starting materials were seven kinds of MgO powder with primary particle sizes ranging from 11–261 nm; these powders were produced by a vapour-phase oxidation process. The original powders contained agglomerates, due to the spontaneous coagulation of primary particles, which ranged in size from 100–500 nm. The MgO compacts densified during firing by three types of sintering: sintering within agglomerates; sintering between agglomerates and grains; and rearrangement of agglomerates and grains. The MgO compact with the lowest primary particle size (11 nm) densified by the first and second types of sintering, but the effects of these two types of sintering decreased when the primary particle size became 44 nm; here the rearrangement of agglomerates and grains primarily contributed to densification of the compact. All three types of densification became less complete with further increases in primary particle size up to 261 nm. The relative densities of the MgO compacts with smaller primary particle sizes (11–44 nm) became 96–98% when the compacts were fired up to 1700°C.     

Preparation and sintering behaviour of submicronic Bi4Ti3O12 powders

Submicronic powders of Bi₄Ti₃O₁₂ with different morphologies were prepared by both the oxalate coprecipitation and the conventional mixing oxides methods. Compacts of the two calcined powders were sintered at 850–1100 °C in air, and the densification process was studied by non-isothermal and dilatometric experiments. A rapid densification (> 97% theoretical density) below 875 °C took place in the Bi₄Ti₃O₁₂ oxalate powder which was attributed to an extremely uniform pore-size distribution in the green compact. The possible formation of a transient liquid which promotes densification also was taken into account. The development of plate-like morphology in the conventional Bi₄Ti₃O₁₂ powder, broad pore-size distribution, and the plate-like colony formation, hindered rapid densification of the green compacts at low temperature. Microstructural development was studied; preliminary dielectric and electrical results are also reported.     

Slip casting and sintering of monodispersed TiO2 particles

Monodispersed TiO₂ particles were used to prepare a uniformly packed green compact with a high relative density by slip casting. A suspension consisting of monodispersed TiO₂ particles, solvent and binder was cast in the mould. The sintering behaviour of the green compact was investigated. The green compact could be sintered to a relative density of > 99% by treatment at 1050 °C for 120 min. The average grain size of the sintered body was 1.26 m without abnormal grain growth. The green sheet cast on a glass board could be densified with no grain growth. The experimentally obtained relation between densification rate and grain size indicated a volume diffusion mechanism according to Coble's equation.     

Fabrication and microstructure-mechanical property relationships in Ce-TZPs

Ceria-stabilized tetragonal zirconia polycrystals(Ce-TZPs) have been fabricated via conventional sintering of commercially available electrofused and electrorefined CeO₂-doped ZrO₂ powder at 1550°C for various periods from 0.5–30 h. The resultant grain sizes of the sintered materials were in the range 2–15 m. The sintering of such electrorefined powder appears to occur by a liquid state sintering process, evinced in terms of the grain-size dependence on sintering time at 1550°C and by direct TEM observation. The mechanical properties of the sintered materials have been characterized, including single-edge notch bend fracture toughness and three-point bend fracture strength. The grain-size dependence of these properties in the CeO₂-stabilized tetragonal polycrystals is very much different from that in Y₂O₃ stabilized tetragonal zirconia polycrystals (Y-TZPs). The transformation plasticity, which is represented by the yield stress behaviour and the total strain to fracture, plays an important role in the microstructure-property interrelationship in the Ce-TZPs.     

Physical and mechanical properties of YBa2Cu3O7-δ superconductors

The physical and mechanical properties of YBa₂Cu₃O_7–° superconductors are examined. These properties are related to powder preparation method, powder characteristics, sintering behaviour and sintered microstructure. The sintering atmosphere and sintering schedules affect the final microstructure very strongly and determine, in conjunction with starting powder characteristics, the sintered density. The mechanical properties such as Young's modulus, bend strength and critical stress intensity factor (fracture toughness) are measured and related to microstructure as determined by electron microscopy. Control of microstructure by careful powder selection and sintering schedule is seen as key to optimizing the physical and mechanical properties of the material. Finally attention is drawn to fabrication techniques and how these must be optimized in order to realize the mechanical properties which are necessary if these are to be useful as engineering materials. Comparisons between fabrication techniques show that uniaxial powder pressing suffers from limitations in terms of specimen complexity and densification whereas the favoured route, termed viscous processing, gives a more homogeneous microstructure, higher strength and allows near theoretical density to be achieved.