Showing papers on "Sintering published in 2004"

Zirconates as New Materials for Thermal Barrier Coatings

Abstract: Zirconates with high melting points were investigated for application as materials for thermal barrier coatings at operating temperatures >1300°C. SrZrO3, BaZrO3, and La2Zr2O7 powders were synthesized and sintered to compacts with various levels of porosity. The sintering results indicated a promising low-sintering activity of the investigated materials. Thermal properties of these dense materials were determined. Thermal expansion coefficients were slightly lower than those of Y2O3-stabilized ZrO2 (YSZ); thermal conductivities of SrZrO3 and BaZrO3 were comparable or slightly higher than those of YSZ. La2Zr2O7 had a lower thermal conductivity. SrZrO3 was not suitable for application as a thermal barrier coating because of a phase transition at temperatures between 700° and 800°C. Mechanical properties (hardness, fracture toughness, and Young’s modulus) of dense BaZrO3 and La2Zr2O7 samples were determined by indentation techniques and showed lower hardness and Young’s modulus compared to YSZ. BaZrO3 and La2Zr2O7 powders were optimized for application as powders for plasma spraying. Plasma-sprayed coatings were produced and characterized. Thermal cycling with a gas burner at 1200°C showed a massive attack of the BaZrO3 coating, with loss of BaO. On the other hand, the La2Zr2O7 coating showed excellent thermal stability and thermalshock behavior.

Room-temperature saturated ferroelectric polarization in BiFeO3 ceramics synthesized by rapid liquid phase sintering

Abstract: Single-phased ferroelectromagnet BiFeO3 ceramics with high resistivity were synthesized by a rapid liquid phase sintering technique. Saturated ferroelectric hysteresis loops were observed at room temperature in the ceramics sintered at 880 °C for 450 s. The spontaneous polarization, remnant polarization, and the coercive field are 8.9 μC/cm2, 4.0 μC/cm2, and 39 kV/cm, respectively, under an applied field of 100 kV/cm. It is proposed that the formation of Fe2+ and an oxygen deficiency leading to the higher leakage can be greatly suppressed by the very high heating rate, short sintering period, and liquid phase sintering technique. The latter was also found effective in increasing the density of the ceramics. The sintering technique developed in this work is expected to be useful in synthesizing other ceramics from multivalent or volatile starting materials.

3D FE simulation for temperature evolution in the selective laser sintering process

Abstract: A thermal model of selective laser sintering (SLS) has been developed. The model allows for the non-linear behavior of thermal conductivity and of specific heat due to temperature changes and phase transformations. The temperature evolution and the formation of the sintered part are simulated by a 3D finite element analysis based on continuous media theory. It is shown that the effect of sintering has a strong influence on thermal evolution through changing the thermal properties of the material. The results of the model were experimentally tested and confirmed by temperature measurements.

Balling processes during selective laser treatment of powders

Abstract: The particularities of the selective laser processing of single‐component metal powder layers were investigated, especially the occurrence of the balling‐processes under different processing conditions. During laser processing, sintered, semi‐sintered/semi‐melted or completely melted cakes can be formed. Size and shape of the laser processed parts can change depending on the energy and time parameters of the laser irradiation and on the properties of initial powder layers.

Bulk glasses and ultrahard nanoceramics based on alumina and rare-earth oxides

Abstract: Although often regarded as a network-former in conventional silicate glasses, Al(2)O(3) alone cannot be obtained as a bulk glass. Until now, glasses comprising continuously linked [AlO(x)] polyhedra have been prepared in only a few systems under very fast cooling conditions, which limits their dimensions to a few millimetres. Yet it is desirable to prepare bulk, or monolithic, alumina-rich glasses, with the prospect of superior mechanical, chemical and optical properties. Here we report a novel process for preparing very-high-alumina glasses and nanoscale glass-ceramics. Fully dense bulk articles in net shape are obtained through viscous sintering of glass microbeads. Additional heat treatment of the consolidated glasses leads to fully crystallized transparent glass-converted nanoceramics with a hardness similar to that of alumina. This method avoids the impracticably high applied pressures (more than 1 GPa) that have been required in most cases to prepare nanocrystalline ceramics by sintering, owing to the concurrent nature of densification and grain growth under pressureless conditions. The reported techniques can be extended to form glasses and nanoceramics in other oxide systems that do not include a conventional glass-forming component.

Low-temperature sintering and microwave dielectric properties of zinc metatitanate-rutile mixtures using boron

Abstract: Mixtures of zinc metatitanate and rutile (ZnTiO3+xTiO2, where x= 0-0.5) have been prepared via the conventional mixed-oxide method. Centrifugal planetary milling with zirconia beads 1 mm in diameter produced very fine powders (mean particle size of 0.2 µm), which allowed the synthesis of ZnTiO3 and sintering at temperatures 94% of the theoretical density have been attained for the specimens that were sintered at 875°C for 4 h with B2O3 additions of <1 wt%. Microwave dielectric properties of the aforementioned compositions were as follows: dielectric constant of 29-31, normalized quality factor of 56000-69000 GHz, and a temperature coefficient of resonance frequency between -10 and +10 ppm/°C. Sintering was enhanced by the formation of a ZnO-B2O3 liquid phase, which affected the microwave properties, because of variation in the phase composition.

Characterisation of porous silicon nitride materials produced with starch

Abstract: Porous silicon nitride is gaining interest for a number of applications including metal–ceramic thermal engineering components, biomaterials and catalyst supports. This paper describes the fabrication of porous silicon nitride ceramic materials using a fugitive additive, corn starch, which allows samples to be produced with different volume fractions of porosity from ∼0 to 0.25. The initial composition consisted of 92 wt.% Si 3 N 4 , 6 wt.% Y 2 O 3 and 2 wt.% Al 2 O 3 . Sintering was carried out at 1800 °C for 2 h under nitrogen. Relative density as a function of the fugitive additive content has been measured. Microstructural analysis reveals a dense matrix of elongated β-Si 3 N 4 grains surrounded by intergranular glass phase and containing large pores and cavities. Pore size, geometry and grain size have been measured for certain compositions. Young's modulus and modulus of rupture have been determined as a function of the volume fraction of porosity. The Young's modulus–porosity relationship has been compared with previous work in the literature and it was found that this dependency is close to that for a model for spherical pores in cubic stacking arrangement.

Amorphous Si(Al)OC ceramic from polysiloxanes: bulk ceramic processing, crystallization behavior and applications

Abstract: Here we report on bulk Si–Al–O–C ceramics produced by pyrolysis of commercial poly(methylsilsesquioxane) precursors. Prior to the pyrolysis the precursors were cross-linked with a catalyst, or modified by the sol-gel-technique with an Al-containing alkoxide compound, namely alumatrane. This particular procedure yields amorphous ceramics with various compositions (Si1.00O1.60C0.80, Si1.00Al0.04O1.70C0.48, Si1.00Al0.07O1.80C0.49, and Si1.00Al0.11O1.90C0.49) after thermal decomposition at 1100 °C in Ar depending on the amount of Al-alkoxide used in the polymer reaction synthesis. The as-produced ceramics are amorphous and remain so up to 1300 °C. Phase separation accompanied by densification (1300–1500 °C) and formation of mullite at T > 1600 °C are the stages during heat-treatment. Bulk SiAlOC ceramics are characterized in terms of microstructure and crystallization in the temperature regime ranging from 1100 to 1700 °C. Aluminum-free SiOC forms SiC along with cracking of the bulk compacts. In contrast, the presence of Al in the SiOC matrix forms SiC and mullite and prevents micro cracking at elevated temperatures due to transient viscous sintering. The nano-crystals formed are embedded in an amorphous Si(Al)OC matrix in both cases. Potential application of polysiloxane derived SiOC ceramic in the field of ceramic micro electro mechanical systems (MEMS) is reported.

Soft- and hard-agglomerate aerosols made at high temperatures.

Abstract: Criteria for aerosol synthesis of soft-agglomerate, hard-agglomerate, or even nonagglomerate particles are developed on the basis of particle sintering and coalescence. Agglomerate (or aggregate) particles are held together by weak, physical van der Waals forces (soft agglomerates) or by stronger chemical or sintering bonds (hard agglomerates). Accounting for simultaneous gas phase chemical reaction, coagulation, and sintering during the formation and growth of silica (SiO2) nanoparticles by silicon tetrachloride (SiCl4) oxidation and neglecting the spread of particle size distribution, the onset of hard-agglomerate formation is identified at the end of full coalescence, while the onset of soft-agglomerate formation is identified at the end of sintering. Process conditions such as the precursor initial volume fraction, maximum temperature, residence time, and cooling rate are explored, identifying regions for the synthesis of particles with a controlled degree of agglomeration (ratio of collision to prima...

Carbothermal reduction synthesis of nanocrystalline zirconium carbide and hafnium carbide powders using solution-derived precursors

Abstract: Zirconium carbide (ZrC) and hafnium carbide (HfC) powders were produced by the carbothermal reduction reaction of carbon and the corresponding metal oxide (ZrO2 and HfO2, respectively). Solution-based processing was used to achieve a fine-scale (i.e., nanometer-level) mixing of the reactants. The reactions were substantially completed at relatively low temperatures (<1500°C) and the resulting products had small average crystallite sizes (∼50–130 nm). However, these products contained some dissolved oxygen in the metal carbide lattice and higher temperatures were required to complete the carbothermal reduction reactions. Dry-pressed compacts prepared using ZrC-based powders with ∼100 nm crystallite size could be pressurelessly sintered to ∼99% relative density at 1950°C.

Synthesis of nanocrystalline yttria powder and fabrication of transparent YAG ceramics

Abstract: Synthesis of nanocrystalline yttria powder from Y(NO 3 ) 3 solution and ammonia water was investigated. It was found that the precursor precipitate is Y 2 (OH) 5 (NO 3 ).H 2 O. The addition of small amount of ammonia sulfate in yttrium nitrate solution can reduce the agglomeration and particle size of the produced yttria powders. Nanocrystalline yttria powder (60 nm in average size) was obtained by calcining the precursor at 1100 °C for 4 h. Using this yttria powder and a commercial ultrafine Al 2 O 3 powder, fully transparent YAG ceramics was fabricated by vacuum sintering at 1700 °C for 4 h through a solid-state reaction method. It was found that the addition of 0.5 wt.% tetraethyl orthosilicate (TEOS) is suitable for the fabrication of transparent YAG ceramics. If the amount of TEOS is less than 0.05 wt.%, abnormal grain growth occurs, and pores are entrapped in the grains. If the amount of TEOS is more than 3 wt.%, large amount of liquid phase is yielded, leaving some residual inclusions at grain boundaries. These are detrimental for the transparency of YAG ceramics.

Low‐Fired (Zn,Mg)TiO3 Microwave Dielectrics

Abstract: A dielectric ceramic comprised of (Zn1-xMgx)TiO3 (x= 0 to x= 0.5) with low sintering temperature and promising microwave properties was prepared by applying a semichemical synthesis route and a microbeads milling technique. X-ray diffractometry and thermal analyses results indicated that the phase stability region of the hexagonal (Zn,Mg)TiO3 extended to higher temperatures as the amount of magnesium increased. The dielectric properties in this system exhibited a significant dependence on the sintering conditions, especially near the phase decomposition temperature. From 950°C, the temperature compensation characteristics occurred as the phase composition changed from hexagonal (Zn,Mg)TiO3 to two phases: (Zn,Mg)2TiO4 and rutile. The magnesium content for zero temperature coefficient (tauf) was ~3 mol% at 950°C; however, tauf increased with the sintering temperatures because of the shift of the decomposition temperature.

Transparent nanocrystalline MgO by rapid and low-temperature spark plasma sintering

Abstract: We investigated superfast densification of nanocrystalline MgO powders by spark plasma sintering (SPS) between 700 °C and 825 °C under applied pressures of 100 and 150 MPa. Fully-dense transparent nanocrystalline MgO with a 52-nm average graun size was fabricated at 800 °C and 150 MPa for 5 min. In-line transmissions of 40% and 60% were measured compared to MgO single crystal, for the yellow and red wavelengths, respectively. Densification occurs by particles sliding over each other; the nanometric graun size and pores lead to the optical transparency. The light brownish color of the nanocrystalline MgO is due to the oxygen vacancy color centers, originating from the reducing atmosphere of the SPS process.

Glass-forming ability, sinterability and thermal properties in the systems RO–BaO–SiO2 (R = Mg, Zn)

Abstract: Glass-ceramics in the systems RO–BaO–SiO 2 (R = Mg, Zn) have good thermal properties for sealing materials on planar-type solid oxide fuel cells (SOFC) at high and intermediate temperatures. In this work, the glass forming regions in the two systems have been determined. Composition lines with different BaO/RO ratios have been defined varying the silica content. The dilatometric properties of the melted glasses (coefficient of thermal expansion, glass transition temperature and softening temperature) have been measured to identify the glasses that best fulfill the cell requirements, such as a thermal expansion coefficient in the range (8.5–12) × 10 −6 K −1 . Since the sealing process is carried out employing paste technology during the heating and working schedule of the cell, it is important to study the interaction between sintering and crystallization to make dense materials with an adequate microstructure. The sintering and crystallization kinetics have been characterized for some selected glass compositions by using hot-stage microscopy (HSM) and differential thermal analysis (DTA). A new parameter S C = T X − T MS which takes into account the onset crystallization temperature ( T X ), temperature of maximum shrinkage ( T MS ) is proposed as an empirical method to evaluate the ability of glass sintering of the studied compositions and indicate good materials for glass-ceramic SOFC products.

Densification and Mechanical Properties of B4C with Al2O3 as a Sintering Aid

Abstract: The densification behavior and mechanical properties of B4C hot-pressed at 2000°C for 1 h with additions of Al2O3 up to 10 vol% were investigated. Sinterability was greatly improved by the addition of a small amount of Al2O3. The improvement was attributed to the enhanced mobility of elements through the Al2O3 near the melting temperature or a reaction product formed at the grain boundaries. As a result of this improvement in the density, mechanical properties, such as hardness, elastic modulus, strength, and fracture toughness, increased remarkably. However, when the amount of Al2O3 exceeded 5 vol%, the level of improvement in the mechanical properties, except for fracture toughness, was reduced presumably because of the high thermal mismatch between B4C and Al2O3.

Development of a novel combustion synthesis method for synthesizing of ceramic oxide powders

Abstract: A novel combustion synthesis method has been developed to prepare electronic ceramic oxide powders—Ni 05 Zn 05 Fe 2 O 4 , ZnO, LiCoO 2 , BaFe 12 O 19 and YBa 2 Cu 3 O 7− x ( x ≦025) Organic compounds (eg, glycine, urea, citric acid, alanine, or carbohydrazide) to be mixed directly with metal nitrates without adding water, is the key technique of this method Metal nitrates acting as oxidants were also used as cation sources, whereas an organic compound was employed as fuel By directly mixing and thoroughly dehydrated, the reactant mixture having appropriate stoichiometric ratio of metal nitrates to organic fuel would transform into a flammable precursor Once ignited in the air at room temperature, the precursor underwent a combustion process and yielded voluminous loose powders Judging from experimental results, the feasibility of the proposed combustion synthesis method was proved to be efficient, quick and simple in becoming one of the suitable methods for mass production of ceramic oxide powder Furthermore, a relatively low sintering temperature of 950 °C for 2 h is sufficient to sinter the as-synthesized Ni–Zn ferrite powders up to about 97% of theoretical density due to their size in nano-scale The as-sintered Ni–Zn ferrite showed good magnetic properties as characterized by using VSM and impedance analyzer

Yttrium Silicate Coatings for Oxidation Protection of Carbon–Silicon Carbide Composites

Abstract: Yttrium silicate (Y 2 SiO 5 ) coatings complement SiC coatings for protecting ceramic multilayer composite materials based on carbon-fiber-reinforced SiC composites (C-SiC). Thick (100 μm), dense Y 2 SiO 5 coatings were prepared by dip coating, using concentrated aqueous slips. The resulting phases were studied by taking into account the simultaneous presence of oxide and non-oxide materials, which affected the chemical stability of the coatings. Thick, mechanically stable coatings were obtained by sintering in carbon crucibles and a SiC bed in an argon-flow furnace. Pure Y 2 SiO 5 coatings completely separated from the SiC substrates. A high percentage of Y 2 Si 2 O 7 was necessary to fit the thermal expansion coefficients and ensure the stability of the coatings. Oxidation resistance of the coated substrates was investigated by isothermal and stepwise oxidation tests.

Numerical simulation of solid state sintering; model and application

Abstract: A comprehensive model for solid state sintering is presented which combines several previous models for partial aspects of sintering and grain coarsening. Some additional aspects are discussed and the model is extended for external loads several times higher than the sintering stress. Model parameters for a SiC powder are presented. For a face seal made of SiC uniaxial die compaction is simulated and different green density distributions after compaction are obtained for two different pressing schedules. Next, the sintering behavior is simulated using these density distributions as initial conditions. The resulting distortions of the face seal differ significantly for the two pressing schedules. For a special element with average green density the predictions of the sintering model are investigated in more detail. Finally, the shape predictions of the model are compared with a simplified model for solid state sintering.

Consolidation and properties of binderless sub-micron tungsten carbide by field-activated sintering

Abstract: The rapid sintering of nano-structured WC hard materials in a short time is introduced with a focus on the manufacturing potential of this spark plasma sintering process. The advantage of this process allows very quick densification to near theoretical density and prohibition of grain growth in nano-structured materials. A dense pure WC hard material with a relative density of up to 97.6% was produced with simultaneous application of 60 MPa pressure and electric current of 2800 A within 2 min. A larger current caused a higher rate of temperature increase and therefore a higher densification rate of the WC powder. The finer the initial WC powder size the higher is the density and the better are the mechanical properties. The fracture toughness and hardness values obtained were 6.6 MPa m1/2 and 2480 kg/mm2, respectively under 60 MPa pressure and 2800 A using 0.4 μm WC powder.

Effect of Silicon Substitution on the Sintering and Microstructure of Hydroxyapatite

Abstract: The substitution of between 0 and 1.6 wt% silicon (Si-HA) in hydroxyapatite (HA) inhibited densification at low temperatures (1000°–1150°C), with these effects being more significant as the level of silicon substitution was increased. For higher sintering temperatures (1200°–1300°C), the sintered densities of HA and Si-HA compositions were comparable. Examination of the ceramic microstructures by scanning electron microscopy (SEM) showed that silicon substitution also inhibited grain growth at higher sintering temperatures (1200°–1300°C). The negative effect of silicon substitution on the sintering of HA at low temperatures (1000°–1150°C) was reflected in the hardness values of the ceramics. However, for higher sintering temperatures, e.g., 1300°C, where sintered densities were comparable, the hardness values of Si-HA compositions were equal to or greater than that of HA, reflecting the smaller grain sizes observed for the former.

Effects of dopants and hydrogen on the electrical conductivity of ZnO

Abstract: The effects of dopants on the electrical conductivity of ZnO were investigated through the ac impedance spectroscopy. The doping of Al increased the electrical conductivity of ZnO greatly, whereas the doping of Li decreased it both in the grain and in the grain boundary. The doping of the 3d transition metals (Co, Mn, and Cu) made the grain boundary more resistive, but the doping effect on the electrical conductivity inside the grain was varied depending on the doping elements. The doping of Co had no significant effects on the electrical conductivity of the grain, and the doping of Mn made the grain a little more resistive. The doping of Cu made the grain much more resistive. In addition, hydrogen was introduced into ZnO by the ion implantation method. The electrical conductivity in the hydrogen-implanted ZnO layer increased by four orders of magnitude. The mechanisms for the doping effects were discussed in this investigation.

Low-Temperature Fabrication of Transparent Yttrium Aluminum Garnet (YAG) Ceramics without Additives

Abstract: A carbonate precursor of yttrium aluminum garnet (YAG) with an approximate composition of NH4AlY0.6(CO3)1.9(OH)2z 0.9H2O was synthesized via a coprecipitation method from a mixed solution of ammonium aluminum sulfate and yttrium nitrate, using ammonium hydrogen carbonate as the precipitant. The precursor precipitate was characterized using chemical analysis, differential thermal analysis/thermogravimetry, X-ray diffractometry, and scanning electron microscopy. The sinterability of the YAG powders was evaluated by sintering at a constant rate of heating in air and vacuum sintering. The results showed that the precursor completely transforms to YAG at ;1000°C via the formation of a yttrium aluminate perovskite (YAP) phase. YAG powders obtained by calcining the precursor at temperatures of <1200°C were highly sinterable and could be densified to transparency under vacuum at 1700°C i 1 h without additives.