Effect of compaction pressure on the sinterability of submicron powders of tetragonal zirconium dioxide

Conclusions We studied the effect of compaction pressure on the pore structure of the paniculate compacts obtained using two types of agglomerated submicron powders of tetragonal zirconium dioxide, on the structure evolution during the sintering process, and on the strength of the obtained material. It was established that the characteristics of the agglomerates present in the powders have a significant effect on their behavior during compaction and sintering. At a given compaction pressure, the powders having weaker agglomerates densify up to a higher density and give a more uniform distribution of pores in the preform. The low-density compacts obtained using agglomerated powders having a high specific surface area sinter faster and attain high strength levels at a lower temperature; however, the sintered materials obtained from such compacts contain several structural defects in the form of large pores and have a lower strength. The uniformity of the distribution of pore volume with respect to size (or the specific content of the interagglomerate pores) forms the main criterion of the quality of particle packing in the compacts obtained from agglomerated powders. The compacts having a low content of the interagglomerate pores give a defect-free dense and strong material after sintering. The presence of the anion impurities in the original powders leads to a decrease of density during the sintering process after the attainment of a threshold density at which formation of closed porosity occurs. Pressure sintering (HIP) forms an effective method of suppressing the decrease of density.Translated from Ogneupory, No. 2, pp. 5–11, February, 1993.     

Sintering of Nanosized MnZn Ferrite Powders

The sintering and microstructural evolution of nanosized MnZn ferrite powders prepared by a hydrothermal method were investigated. The microstructure of sintered ferrite compacts depends strongly on the strength of the agglomerates formed during the compacting of nanosized ferrite powders. It was found that at 700°C the theoretical density of sintered compacts can almost be reached, while above 900°C an increase of porosity was identified. The formation of extra porosity at higher sintering temperatures is caused mainly by the oxygen release which accompanies the dissolution of relatively large grains of residual alpha-Fe₂O₃ in the spinel lattice.     

Densification of Sol-Gel-Derived Mullite Ceramics after Cold Isostatic Pressing up to 1 GPa

Molecular-designed ultrafine mullite precursor powders with a stoichiometric composition were prepared by copolymerization of alkoxides. The precursor powders were calcined in the range from 800° to 1200°C and consolidated by ultra-high-pressure cold isostatic pressing up to 1 GPa. Ultrahigh isostatic pressure of 1 GPa led to a closed packing structure in the green compacts. Interaggregate pores in the green compacts were collapsed by the ultrahigh cold isostatic pressure to reduce the pore size below 6 nm. As a result, the maximum density of the green compacts reached 70% of theoretical. These closely packed green compacts of precursor powders with a stoichiometric composition and calcined at relatively low temperatures could be sintered to >95% of theoretical at 1500°C. Relatively low-temperature sintering below the liquid formation temperature resulted in fine microstructure of the resultant mullite ceramic with a grain size below 300 nm.     

Synthesis and Low-Temperature Sintering of Gd-doped CeO2 Ceramic Materials Obtained by a Coprecipitation Process

Well-dispersed ceria–gadolinia oxide powders were obtained from thoroughly isopropanol-washed coprecipitated hydroxides and oxalates, followed by a controlled drying at low temperature and calcining at 550°C. The characteristics of the calcined powders and the microstructure of the green compacts were found to be of great importance in the sintering behavior. Green bodies with high agglomerate sizes need higher sintering temperatures for attaining a final density >99% D _th, while those having soft agglomerates with lower sizes were almost fully densified at a sintering temperature as low as 1250°C. The densification process was studied by isothermal and constant heating rate dilatometry, and microstructural development by scanning electron microscopy. By controlling the processing variables, it was possible to obtain this low-temperature, nearly fully dense (better than 99%) sample with a homogeneous microstructure.     

Hierarchically Porous Ceramics from Diatomite Powders by Pulsed Current Processing

Hierarchically porous ceramic monoliths have been fabricated by pulsed current processing (PCP) of diatomite powders. The partial sintering behavior of the porous diatomite powders during PCP or spark plasma sintering was evaluated at temperatures between 600° and 850°C. Scanning electron microscopy and mercury porosimetry measurements showed that the PCP method was able to bond the diatomite powder together into relatively strong monoliths without significantly destroying the internal pores of the diatomite powder at a temperature range of 700°–750°C. Little fusion at the particle contact points occurred at temperatures below 650°C while the powder showed partial melting and collapse of both the interparticle pores and the internal structure at temperatures above 800°C.     

Submicrometer Transparent Alumina by Sinter Forging Seeded γ-Al2O3 Powders

Seeding boehmite with α-Al₂O₂, followed by calcination at 600°C, results in an agglomerated alumina powder (<53 μm) that can be sinter forged to full density at 1250°C. Compressive strains as high as ɛ_x=−0.9, and radial flow (ɛ_x= 1.0) during sinter forging remove large, interagglomerate pores. The fully dense alumina has a grain size of 0.4 pm and is visually transparent. It is proposed that deformation of dense agglomerates is the primary mecha- nism responsible for large pore elimination and compact densification. The sinter forging of sol-gel-derived alumina powders offers a new technology to prepare highly transparent, optical ceramics at lower temperatures than conventional routes.     

Diffusion Sintering: II, Initial Sintering Kinetics of Alumina

The initial sintering kinetics of alumina have been studied by measuring the isothermal shrinkage of compacts of several alumina powders in air. The shrinkage of these compacts can best be described by a grain-boundary vacancy diffusion model for the temperature range 1200° to 1600°C. The behavior of the compacts is consistent with the model after an initial shrinkage has occurred. The magnitude of this initial shrinkage is constant for identical specimens and is independent of the sintering temperature.     

Measurement of Densification of Zinc Oxide Compacts Using an Optical, Noncontact, Noninvasive Extensometer

An optical noninvasive, noncontact extensometer was used to measure the shrinkage of zinc oxide powder compacts during sintering. Powder compacts were uniaxially and isostatically pressed from micrometer, submicrometer, and nano powders and sintered in a thermal oven at temperatures up to 1100°C. The nanometer-size sample started to densify at ∼400°C, about 200°C below the densification threshold of the micrometer-size sample. The results are in good agreement with those obtained using a contact dilatometer.     

Experimental Analysis of Sintering of MgO Compacts

Experimental sintering studies On undoped and cao-doped Mgo powder compacts in Static air and flowing Water Vapor atmospheres were conducted at 1230° to 1600°C. Corresponding microstructural changes of specimens during sintering were examined by scanning electron microscopy. Kinetic and microstructural data were analyzed to determine sintering mechanisms during the initial and intermediate stages of sintering.     

Preparation of SiO2 Glass from Model Powder Compacts: I, Formation and Characterization of Powders, Suspensions, and Green Compacts

Dense SiO₂ glass was produced at ∼1000°C by using highly ordered compacts of spherical, nearly monosized, amorphous SiO₂ particles. In Part I of this study, the formation and characterization of powders, suspensions, and green bodies are described. Thermogravimetry and DTA revealed that substantial loss of bound water occurs in powders calcined at temperatures as low as 200°C. Surface area and density measurements were used to show that the water loss occurs without micropore formation. FTIR spectroscopy revealed that residual silanol groups persist to the highest temperatures (1050°C) studied. The state of particulate dispersion in suspensions was modified by pH adjustment and monitored by rheological measurements. Flocculated suspensions (low pH) produce inhomogeneous, low-density powder compacts with highly bimodal pore-size distributions. Uniform green bodies (with higher packing densities) were prepared using well-dispersed suspensions (high pH). Two-dimensional, close-packed hexagonal arryas of particles were observed in these compacts. Pore-size distributions were narrower, but still bimodal due to the presence of three-particle and four-particle pore channels. The sintering behavior of these compacts is described in part II.     

Characterization of Sinterable Oxide Powders: I, BeO

The discovery of readily sinterable BeO powder facilitated the production of dense, high-strength BeO ceramics and created interest in the properties and preparation of sinterable BeO. BeO powders prepared by the thermal decomposition of Be(OH)₂ were studied in an effort to relate sinterability with other more basic characteristics of the oxide powder. Different BeO powders made from analyzed hydroxides showed a wide range of sinterability. The temperatures required for sintering the powder compacts to theoretical density ranged from 2300° to above 3200°F. Lattice parameters, thermal decomposition, surface areas, and refractive indices were determined on these powders after calcination in air at 750°, 1470°, 1830°, and 2190°F. Although unexpected variations in these properties were observed, no simple relation with sinterability was found. Occluded and surface impurities appeared to have a critical effect on sintering behavior.     

Densification and Phase Transformation of β-SiAlON–Cubic Boron Nitride Composites Prepared by Spark Plasma Sintering

β-SiAlON–cubic boron nitride (cBN) composites were prepared from β-SiAlON and cBN powders at 1600°–1900°C under a pressure of 100 MPa by spark plasma sintering. The effects of cBN content and sintering temperature on densification and phase transformation of the β-SiAlON–cBN composites were studied. When 10–30 vol% cBN was added to β-SiAlON, the shrinkage rate of the compacts increased. The compacts of β-SiAlON–BN composites originally containing 10–30 vol% cBN ceased to shrink at a temperature lower than that of β-SiAlON and the density of the composites increased. The densification of β-SiAlON–BN composites originally containing >40 vol% cBN was suppressed. The phase transformation of cBN to hexagonal BN in the β-SiAlON–BN composite was inhibited to a greater degree than that in the cBN body.     

Sintering Behavior and Electrical Properties of Nanosized Doped-ZnO Powders Produced by Metallorganic Polymeric Processing

Homogeneous and nanosized (28 nm crystallite size) doped-ZnO ceramic powders were obtained by a metallorganic polymeric method. Calcining and granulating resulted in green compacts with uniform powder packing and a narrow pore-size distribution (pore size 19 nm). Dense ceramic bodies (>99% of theoretical) were fabricated by normal liquid-phase sintering at 850° and 940°C for 1–5 h. Apparently, the low pore-coordination number allowed a uniform filling of the small pores by the liquid formed in the early stages of sintering, and, consequently, high shrinkage and rapid densification occurred in a short temperature interval (825°–850°C). At these sintering temperatures, limited grain growth occurred, and the grain size was maintained at <1 μm. Ceramics so-fabricated showed a nonlinear coefficient, α, of ≥70, and a breakdown voltage, V _b (1 mA/cm²), of ≥1500 V/mm. The high electrical performance of the doped-ZnO dense ceramics was attributed to liquid-phase recession on cooling, which enhanced the ZnO-ZnO direct contacts and the potential barrier effect.     

Conventional and Microwave-Induced Sintering Mechanisms for Reaction-Bonded Aluminum Oxide with Zirconium Oxide Additions

Powder compacts consisting of Al, Al₂O₃, and ZrO₂ were heated by microwave radiation. Tracing the phase evolution during reaction bonding revealed the reaction mechanism. In the case of conventional heating, the compacts expanded slightly at temperatures of <700°C due to Al surface oxidation and expanded sharply at temperatures greater than 700°C as oxidation proceeded from the surface to the interior. Then, the compacts shrank at 1550°C due to sintering. For the case of microwave heating, the compacts expanded at temperatures of <550°C due to the formation of Al₃Zr. This Al₃Zr formation was caused by the preferential heating of ZrO₂ relative to Al and Al₂O₃ by microwave radiation. Then, Al₃Zr was oxidized to form Al₂O₃ and ZrO₂ at temperatures of >1000°C. Finally, the compacts shrank at 1550°C due to sintering, similarly to conventional sintering.     

Sintering Behavior of Doped Lanthanum and Yttrium Manganite

The sintering behavior of doped manganite powders was found to be highly dependent on changes in calcination conditions and A/B cation ratio. Coarsening of combustion synthesized powders by calcination allowed for higher green densities in dry-pressed compacts, which resulted in higher sintered densities for powders calcined in the temperature range 800°-1200°C. Sintered densities decreased for calcination temperatures greater than 1200°C. Preparation of manganites with a deficiency of A-site cations improved the densification behavior substantially. This effect was attributed to an increased concentration of A-site vacancies which enhanced the diffusion of A-site cations during sintering. Modification of doped manganites by alteration of composition and calcination conditions allowed their sintering shrinkage to be "tailored" to more closely match the shrinkage of yttria-stabilized zirconia.     

Microstructural evolution during sintering in MgO powders precipitated from sea water under induced agglomeration conditions

Green compacts pressed by means of uniaxial compaction with Magnesia (MgO) powders precipitated from sea water and calcined at different temperatures were sintered under H₂ atmosphere at 1700 °C. The calcination, carried out between 900 and 1200 °C had a great influence in the final density and the microstructure. The densification of MgO agglomerated powders seems to be predictably related to grain growth and thus coarsening kinetics. At calcination temperatures higher than 900 °C, the volume of large pores was increased notably suggesting that the inhibited grain growth adversely affected the thermodynamics of pore sintering. Relative densities between 74 and 98% of theoretical density were reached in compacts obtained at different compaction pressures. The microstructural differences were examined by Scanning Electron Microscopy (SEM).     

Presintering Heat Treatment, Densification, and Mechanical Properties of Silicon Carbide

Silicon carbides were subjected to extended heat treatments at 1400° or 1600°C prior to a 1-h isochronal sintering at 2000°C in argon, and the resulting microstructures, final densities, and room-temperature four-point bend strengths were compared to those of samples prepared without the pretreatments. The samples heat-treated at 1600°C exhibited higher relative densities and flexure strengths. Microstructural observation revealed that the heat treatment at 1600°C modified the initial powder compact microstructure, decreasing the development of large, dense domains in the early and intermediate sintering stages. This evolution was considered to promote a more uniform subsequent densification. The final size and distribution of the grains as well as of the pores were found to be affected favorably by the pretreatment.     

Enhanced Densification of In2O3 Ceramics by Presintering with Low Pressure (5 MPa)

Sintering of In₂O₃ has been carried out in air to full density. Because of the difference in densification between the agglomerates and the matrix, large interagglomerate pores were observed to form at the initial stage of sintering. Such pore formation could be prevented by applying a small external pressure which resulted in the beneficial rearrangement of agglomerates.     

Spark Plasma Sintered Silicon Nitride Ceramics with High Thermal Conductivity Using MgSiN2 as Additives

Silicon nitride ceramics were prepared by spark plasma sintering (SPS) at temperatures of 1450°–1600°C for 3–12 min, using α-Si₃N₄ powders as raw materials and MgSiN₂ as sintering additives. Almost full density of the sample was achieved after sintering at 1450°C for 6 min, while there was about 80 wt%α-Si₃N₄ phase left in the sintered material. α-Si₃N₄ was completely transformed to β-Si₃N₄ after sintering at 1500°C for 12 min. The thermal conductivity of sintered materials increased with increasing sintering temperature or holding time. Thermal conductivity of 100 W·(m·K)⁻¹ was achieved after sintering at 1600°C for 12 min. The results imply that SPS is an effective and fast method to fabricate β-Si₃N₄ ceramics with high thermal conductivity when appropriate additives are used.     

Fabrication of Low-Porosity Indium Tin Oxide Ceramics in Air from Hydrothermally Prepared Powder

Compositionally homogeneous indium tin oxide (ITO) ceramics with low porosity were obtained successfully by sintering hydrothermally prepared powders. The fabrication technique began with the preparation of microcrystalline, homogeneously tin-doped (5 wt%) indium oxyhydroxide powder, under hydrothermal conditions. Low-temperature (∼500°C) calcination of the hydrothermally derived powder led to the formation of a substitutional-vacancy-type solid solution of In₂Sn_{1− x} O_{5− y} , and further heating of this phase at temperatures of >1000°C resulted in the formation of the tin-doped indium oxide phase, which had the C -type rare-earth-oxide structure. The sintering of uniformly packed, calcined powder compacts at 1450°C for 3 h in air resulted in low-porosity (∼0.7%) ITO ceramics.