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 and mechanical properties of dense polycrystalline hydroxyapatite through freeze-drying

High purity hydroxyapatite (HAp) powders were synthesized through the wet-chemical method and subsequent freeze-drying or heat-drying, respectively. Dense polycrystalline HAp ceramics were obtained by sintering the powder compacts in air in the temperature range 1100–1350°C. Results show that the freeze-drying method can produce highly homogeneous, fine-grained HAp powders. The relative density, Vickers hardness and flexural strength of the sintered freeze-dried HAp ceramics increased with the sintering temperature, reaching a maximum at around 1350°C, and the highest values obtained were 99.0%, 820 and 110 MPa, respectively. All these mechanical properties of the freeze-dried HAp ceramics are much higher than those of the conventionally heat-dried HAp ceramics. The drying method can significantly influence the properties of the HAp powders as well as the HAp ceramics.     

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.     

Sintering behaviour and mechanical properties of hydroxyapatite and dicalcium phosphate

The sintering behaviour of powders of two calcium phosphates, namely hydroxyapatite (HA) and dicalcium phosphate (DCP), were studied at various temperatures and in various environments. The density, flexural strength and Knoop hardness of HA sintered in air for 4 h initially increased with the sintering temperature, reaching maxima at around 1150°C, and then decreased due to decomposition of HA into tri- (TCP) and tetracalcium phosphates. Sintering in vacuum caused decomposition of HA at lower temperatures, and consequently the mechanical properties were poorer than those of HA sintered in air. The densification and mechanical properties of DCP sintered in air and vacuum showed similar behaviour to those of HA. In air DCP underwent phase transformation from - to - and to -phases. In vacuum DCP started to decompose into tricalcium phosphate at 1000°C. To reduce dehydroxylation, HA powder was sintered in moisture at various temperatures up to 1350°C and X-ray diffraction study did not indicate any decomposition at the highest sintering temperature. The density, flexural strength and hardness of HA sintered in moisture increased with the sintering temperature and eventually reached plateaux at about 1300°C, but below 1200°C they were lower than those of HA sintered in air at corresponding temperatures. Thus, it is seen that dehydroxylation did not hinder sintering of HA. On the other hand, decomposition obstructed sintering of both HA and DCP.     

Influence of slip-casting and dry-pressing on structure evolution of alumina compacts

A commercial alumina powder was processed to remove aggregates and agglomerates for the study of processing influence on sintering behaviour and structure evolution. Slip-casting a uniformly dispersed slurry formed green compacts with homogeneous particle packing, which subsequently resulted in a nearly defect-free microstructure upon sintering. Dry-pressing the granules that possessed a similar homogeneous microstructure led to compacts with larger voids and particle-packing inhomogeneity. The inhomogeneity persisted through sintering and evolved voids with a size of about 6 m. However, under the same sintering condition, the sintered density of the dry-pressed compact approached that of the slip-cast compact. The grain sizes of both sintered compacts were similar, implying that differences in consolidation effectiveness do not necessarily incur variation in grain growth provided the compacts are prepared from the same starting powder and sintered under the same condition.     

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.     

Influence of temperature and aging time on HA synthesized by the hydrothermal method

The influence of temperature and aging time on the morphology and mechanical properties of nano-sized hydroxyapatite (HA) synthesized by a hydrothermal method is reported here. The pre-mixed reactants were poured into a stirred autoclave and reacted at temperatures between 25–250°C for 2–10 h. HA powders thus obtained were examined using X-ray diffraction (XRD), high-resolution field emission scanning electron microscopy (FESEM) and a particle size analyzer. It was found that the aspect ratio of the particles increased with the reaction temperature. The length of the HA particles increased with the reaction temperature below 170°C, but it decreased when the temperature was raised above 170°C. The agglomerates of HA particles were formed during synthesis, and their sizes were strongly dependent on reaction temperatures. As the reaction temperature increased, the agglomerate size decreased (p = 0.008). The density of the discs pressed from these samples reached 85–90% of the theoretical density after sintering at 1200°C for 1 h. No decomposition to other calcium phosphates was detected at this sintering temperature. A correlation existed (p = 0.05) between the agglomerate sizes of HA particles synthesized at various conditions and their sintered densities. With the increase of the agglomerate size, the sintered density of the HA compact decreased. It was found that both the sintered density and flexural strength increased with increasing aging time and reaction temperature. A maximum flexural strength of 78 MPa was observed for the samples synthesized at 170°C for 5 h with the predicted average at these conditions being 65 MPa. These samples attained an average sintered density of 88%.     

Spark plasma sintering of BiFeO3

Dense BiFeO₃ ceramics were prepared by a novel spark plasma sintering (SPS) technique. The sintering was conducted at temperatures ranging from 675 to 750 °C under 70 MPa pressure. A bulk density value up to 96% of theoretical density was achieved in the process. This contrast to around 90% of the theoretical density achieved by conventional sintering at around 830 °C. It was found that the tendency to form unwanted Bi₂Fe₄O₉ phase is higher at a high sintering temperature for SPS. The dielectric and ferroelectric properties also improved (with respect to conventionally sintered sample) for spark plasma-sintered samples.     

Effects of copper addition on the sintering behavior and mechanical properties of powder processed Al/SiC<Subscript><Emphasis Type="Bold">p</Emphasis></Subscript> composites

The effect of copper addition on powder processed Al-10 vol% SiC composites was studied in regards to their sintering responses. Copper was mixed with aluminum powder either as elemental powders or as the coated layer on SiC particles. After sintering at 600°C for 1 h, Al-SiC composites with no copper addition showed little densification. It also demonstrated very low bend strengths of 49 and 60 MPa, indicating poor bonding between the powders in the sintered composite. The addition of 8% copper to the Al/SiC system effectively improved the sintering response, producing over 95% theoretical density, a bend strength of 231 MPa with the copper coated SiC, and a 90% density with over 200 MPa bend strength with the admixed copper.The as-sintered microstructures of the Al–SiC composites clearly revealed particle boundaries and sharp pores, indicating that only a limited neck growth occurred during sintering. In the case of Al–Cu–SiC composites, however, a liquid phase was formed and spread through particle boundaries filling the interfaces or voids between SiC particles and the matrix powders. The coated copper on SiC particles produced a somewhat better filling of the interface or voids, resulting in a little more densification and better sintered strength. Since the solubility of copper in aluminum is less than 2% at the sintering temperature, the alloying of copper in the aluminum matrix was limited. Most of the copper added was dissolved in the liquid phase during the sintering and precipitated as CuAl₂ phase upon cooling.     

Influence of raw powder preparation routes on properties of hydroxyapatite fabricated by 3D printing technique

A comparison between two routes of raw powder preparation, namely spray drying and grinding, for 3D printing of hydroxyapatite was carried out. Hydroxyapatite particles prepared by the spray drying technique were spherical in shape whereas the grinding route gave irregular-shaped agglomerates. Spray-dried powders had higher tap density than milled powders, however milled powders yielded 3DP specimens with greater green density and strength. After sintering at 1300 °C for 1 and 5 h, samples fabricated from milled powders showed a 32% higher in sintered density, a 20% lower in porosity and approximately two times higher flexural modulus and strength than samples fabricated from spray-dried powders. This difference was related to the better packing characteristics of milled powders which promoted improved inter- and intra-particle densification during high temperature sintering compared to the spray-dried powders which yielded only high intra-particle densification, but lower inter-particle densification.     

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.     

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.     

In-Situ Characterization of SiC–AlN multiphase ceramics

AlN and SiC can react and form a solid solution at temperatures above 1800 °C, a result that may be beneficial for sintering silicon carbide ceramics. The pressureless sintered AlN–SiC multiphase ceramics have reached high density at a temperature of 2100 °C for 1 hr in Ar. Analytical scanning transmission electron microscopy was then used to determine the grain boundary, fracture surface, and the local compositions. Because AlN has a higher solid vaporization pressure than SiC, the vaporization rate of the AlN solid would far exceed that of SiC at a sintering temperature. The vaporizing AlN was deposited on the surface of SiC powder; SiC grains then elongated in a random arrangement. The form of elongated rod crystals of 4H SiC is 5 to 8 m in length and 1 m in width. It resulted in the sample fracture section producing pulling-out and a strong tearing-open effect. The bending strength and the fracture toughness of the material obtained are 420 MPa and 4.40 MPa × m^1/2, respectively.     

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.     

Densification and strengthening of silver-reinforced hydroxyapatite-matrix composite prepared by sintering

Ductile-phase reinforcement of hydroxyapatite (HA) was achieved by addition of silver particulates (5–30 vol %) in HA powder and subsequent sintering of HA–Ag powder compacts. A composite made by sintering 10 vol % Ag and the balance HA at 1200 °C for 1 h in air had flexural strength of 75±7 MPa, which was almost double that of pure HA sintered under an identical condition. The density of HA-10 vol % Ag composite was 90±2% of the theoretical density (as calculated from the rule of mixture) and was lower than that (98.7±0.4%) of pure HA sintered at a similar condition. The X-ray diffraction pattern of the composite did not indicate any decomposition of HA or any reaction between HA and Ag. Ag in the composite melted during sintering, but, due to poor wetting, did not spread in between HA particles. The addition of Ag reduced densification and grain growth during sintering of HA–Ag composites. Indentation cracks in the composites went around Ag inclusions and often stopped at Ag inclusions. The increase in the flexural strength of the composites was thought to be due to crack-bridging and crack-arrest by silver particles.     

The affect of densification and dehydroxylation on the mechanical properties of stoichiometric hydroxyapatite bioceramics

This paper reports the effects of processing densification on the mechanical properties of hydroxyapatite bioceramics. Densification of synthetic hydroxyapatite is conducted in the range 1000-1300 °C. X-ray diffraction and SEM microscopy are used to check the microstructure transformations. Vickers hardness, toughness and Young's modulus are analyzed versus the density and grain size. The sintering temperature and the particle size influence strongly the densification and the resulting mechanical properties. In addition, the critical sintering temperature appears around 1200 °C and the declined strength at the temperature up to 1200 °C is found sensitive to the dehydroxylation process of hydroxyapatite.     

Processing and properties of Pb(Mg1/3Nb2/3)O3-PbZrO3-PbTiO3 ceramic relaxors

The shrinkage phenomenon during the reaction-sintering of PMN-PZT from low-temperature pre-reacted 3PbO + MgNb₂O₆ + PZT powder mixtures has been studied. It was assumed that the pre-reaction treatment leads to the formation of a pyrochlore phase containing very active MgO small particles, and that the strong shrinkage occurring up to 800 °C took place by the diffusion of Mg²⁺ cations into the pyrochlore phase particles, thus controlling the reaction-sintering shrinkage phenomenon. Above that temperature the densification was enhanced by a liquid-phase sintering process. The ceramics sintered at 1050 °C for 2 h showed 96% of the theoretical density, and the dielectric constant of such a sintered ceramic showed a maximum value of 17 000 at 1 kHz. It was also found that the dielectric constant decreased with increasing grain size. Although the role of PZT in enhancing the dielectric constant of otherwise low-purity PMN ceramics is not clear, the increase in K is assumed to be a solid-solution effect. The presence of impurities and the PbO stoichiometry could be influencing the not too high dielectric constant value of PMN-PZT ceramics.     

The influence of preceramic binders on the microstructural development of silicon nitride

A number of polymeric precursors to silicon nitride were prepared and evaluated as binders in cold pressing/pressureless sintering operations. These polymers exhibited ceramic yields in excess of 75% by weight, and powder compacts made using them as binders displayed improved green handling properties. Compacts pyrolysed at 800 °C exhibited unusual microstructures, including the development of whiskers in situ. Based on microstructural observation, compacts sintered under pressureless conditions appeared to show enhanced densification relative to those processed without preceramic binders. Preceramic binders appeared to enhance the formation of -Si₃N₄ and may enhance densification of compacts sintered under pressureless conditions.     

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.     

Solid state reaction, sintering and Pb removal activity of hydroxyapatite/zirconia layered composite

In order to form a layered hydroxyapatite/zirconia ceramic, the solid state reaction and sintering were examined by the three processes of powder mixture, dry-pressing compaction and tape cast. The solid state reaction between hydroxyapatite and zirconia occurred in the thin width of 10–50 m at interface in a layered composite body. In both sintered layer composites from dry compaction and tape cast, the significant deformation of composite bodies was observed, depending on sintering temperatures. By selecting a sintering temperature of 1200°C, we fabricated a layer ceramic composite of hydroxyapatite/zirconia exhibiting the flat film shape. The tape cast process was useful to form a porous sintered composite of hydroxyapatite and zirconia. The porous composite showed the removal performance of aqueous lead from wastewater.