Effect of sintering on TiO2-impregnated alumina foams

The effect of sintering on the bulk properties, morphology and phase composition of ultralight Al₂O₃ foams impregnated with TiO₂ was investigated in comparison with pure alumina foam in the temperature range of 900–1600°C in air. Impregnation was carried out by immersion of pre-sintered alumina foam in a sol of titanium isopropoxide-acetylacetone complex. The changes of the foam linear shrinkage, effective density and porosity were studied along with morphological evolution and relationship between these properties was demonstrated. Titania impregnation increased the linear shrinkage (LS) during sintering by a maximum of 5% relative to pure alumina foams. The change of LS and weight loss of TiO₂/Al₂O₃ foams lead to a final density of 0.19 g/cm³ and porosity of 95%. The initial coating was found to develop a mosaic structure due to early shrinkage of the coating. After sintering at 1600°C the coating reacted with the underlying Al₂O₃ surface and became uniformly distributed. Finally, it was shown that the reacting TiO₂ layer formed the tialite (Al₂TiO₅) phase below 1400°C. This Tialite coating remained intact under 1200°C without stabilizers.     

Phase evolution and microstructure characteristics of Mo-based Tb2O3-Dy2O3 composites synthesized by ball milling and sintering

Mo-based Tb₂O₃-Dy₂O₃ composites used as neutron absorbers in nuclear power reactor were synthesized by powder metallurgy. The comparative studies of Mo-based Tb₂O₃ and Mo-based Dy₂O₃ composites were carried out to deeply understand the phase evolution and microstructure characteristics of Mo-based Tb₂O₃-Dy₂O₃ composites. Ball milling induced terbium oxide and dysprosium oxide in the powder mixtures to be first fined, nano-crystallized, amorphized and finally dissolved into Mo matrix to form the supersaturated nanocrystalline solid solution that was driven by mechanical work, not by negative heat of mixing. Mo lattice parameter increased with increasing ball-milling time, opposite for Mo grain size. A phase transformation of Dy₂O₃ crystal from cubic to monoclinic and then to amorphous was observed during ball milling. The microhardness of sintered bulks was first increased and then decreased with increasing sintering time. The maximum value was obtained at the bulks sintered for 8?h. The microhardness and bulk density were increased with increasing sintering temperature before 1600?°C. The mechanism of ball milling and sintering was also discussed.     

Effects of SiO2 and Cr2O3 on the formation process of ZnO varistors

The effects of SiO₂ and Cr₂O₃ on the formation process of ZnO varistors were investigated. Prior to formation of the Zn_2.33Sb_0.67O₄ spinel phase (Sp-phase), a spinel-like phase forms. However, this phase does not control the varistor microstructure. The Sp-phase and the Bi₂O₃-phase which were formed by the decomposition of the Bi₂(Zn_4/3Sb_2/3)O₆ pyrochlore phase played important parts in the control of the varistor microstructure. That is, the Bi₂O₃ phase produced in the reaction promotes the initial sintering of the varistor and the Sp-phase inhibits the ZnO grain growth. In this reaction, SiO₂ and Cr₂O₃ play a role in decreasing the decomposition temperature of the pyrochlore phase. Decreasing the decomposition temperature below 900° C (where ZnO grain growth begins) leads to the inhibition of ZnO grain growth.     

Effects of nondensifying inclusions on the densification and microstructure of zinc oxide matrix composites

The densification and microstructure development of ZnO containing Zn₇Sb₂O₁₂, ZrO₂, and aggregated ZnO were investigated to elucidate the effect of nondensifying inclusions on the sintering of ceramic/ceramic composites. The inclusion retarded the densification, and the degree of retardation was found to depend on the chemical species of inclusion; Zn₇Sb₂O₁₂ had the largest effect, followed by ZrO₂ and then aggregated ZnO last. The experimental results for aggregated ZnO was explained by the theory which predicts the generation of backstresses. The backstresses give a less significant effect on the densification. For Zn₇Sb₂O₁₂ and ZrO₂, the microstructure of the matrix varied with distance from an inclusion particle; much porosity was observed in the region surrounding the inclusion. Circumferential voids, which are responsible for the suppression of densification, form during the initial stage of sintering. Inclusion particles generate an anchoring effect which retards the densification of the matrix immediately surrounding the inclusion particle during the intermediate stage.Supported by the Inamori Foundation.     

On the phases formation in the calcination process of ZnO varistor

The degree of phase formation was investigated by Sb₂O₃ behaviour in the calcination process, and the change of microstructure and breakdown properties due to the phases formation in ZnO varistor. The samples were calcined at several temperatures for 2 h and were sintered at 1150 to 1300 ° C, respectively, for 1 h. After that,V-I characteristics were investigated. Phase analysis by X-ray diffraction and microstructures observation by SEM were done. As a result, spinel phase was not formed and low nonlinear resistance was shown in the samples without Sb₂O₃. In the samples containing Sb₂O₃, it was shown that the pyrochlore and spinel phase are formed at the conventional calcination temperature or even below that temperature. This primary spinel phase and the spinel phase transformed from pyrochlore phase in sintering process inhibit ZnO grain growth, and so nonlinear resistances should be changed. Hence ZnO grain growth in ZnO-based varistor system is strongly dependent on the Sb₂O₃ behaviour in the calcination process.     

Microstructure and boundary phases of Lu–Al-doped silicon nitride by pressureless sintering

Various silicon nitride materials were fabricated by pressureless sintering using lutetia and alumina as sintering additives. Densification behavior, microstructure, strength and formation of secondary crystalline phases were investigated. The combination of Lu₂O₃/Al₂O₃ sintering aids can promote the densification and evolution of a fine grain microstructure of Lu–Al-doped silicon nitride because of the low viscosity of the liquid. The J′ phase given by Lu₄Si_2−xAl_xO_7+xN_2−x was considered to be secondary crystalline phase at the grain pockets. The composition with a Lu₂O₃/Al₂O₃ weight ratio 10/10 had highest strength of 690 ± 50 MPa.     

Microstructure development in Sb2O3-doped ZnO

The microstructure development in Sb₂O₃-doped ZnO was studied at doping levels up to 2.0 mol%. Dopant Sb₂O₃ reacted with ZnO to form inclusion particles,α-Zn₇Sb₂O₁₂, and inhibited the grain growth of ZnO. With increasing doping level of Sb₂O₃, the growth rate of ZnO decreased whereas that of inclusion particles increased. Some inclusion particles were trapped in ZnO grains at low doping levels of Sb₂O₃, but the volume fraction of trapped inclusion particles decreased with increasing doping level. Stereological analysis of the size and number ratios of ZnO grains and inclusion particles indicated that a compatible assumption is needed to evaluate Zener effect in two-phase sintering.     

Isotropic Mg<Subscript>3</Subscript>Sb<Subscript>2</Subscript> compound prepared by solid-state reaction and ball milling combined with spark plasma sintering

Mg₃Sb₂ compounds were synthesized via low-temperature solid-state reaction (SSR) and ball milling (BM), respectively, followed by spark plasma sintering (SPS) process. The effects of possible sintering pressure-induced orientation in the SPS process have been investigated in terms of the microstructure and thermoelectric transport properties. The results indicate that BM technique causes more severe Mg loss than pure SSR method, leading to distinct Sb phase existing in the product after SPS consolidation process. On the contrary, a single phase of Mg₃Sb₂ is easily obtained with the combination of SSR and SPS techniques. Besides, these BM–SPS and SSR–SPS samples exhibit the similar microstructure as well as the same electrical and thermal transport properties parallel or perpendicular to the direction of sintering pressure. The study suggests that SSR method embodies the advantages of both the composition control and the orientation elimination in Mg₃Sb₂ compound as compared to BM method with the specific parameters in the current work. This investigation is quite favorable for this material fabrication and the future application of thermoelectric modules and devices.     

Thermal evolution of ZnO-Bi2O3-Sb2O3 system in the region of interest for varistors

In facing the design of new processing strategies for ZnO based ceramic varistors, a precise control of its microstructural development during sintering is demanded. Addition of dopants to zinc oxide results in the formation of secondary phases that to a large extent determine the macroscopic electrical properties of the ceramic. In a varistor system based on ZnO with small additions of Bi₂O₃ and Sb₂O₃ these three oxides govern the reactions at high temperature that give place to the secondary phases. These reactions become then the head point from which the functional microstructure is configured. In this way the present work deals with the thermal evolution of the ZnO-Bi₂O₃-Sb₂O₃ system in the region of interest for varistors, revealing the existence of two simultaneous reactions paths during sintering these ceramics.     

Evolution of the microstructure and phase composition of materials based on the fluorohydroxyapatite–zirconia–alumina system during sintering

We have studied the influence of the sintering temperature, Al₂O₃ additions, and liquid-forming sintering aids on the phase composition and microstructure of fluorohydroxyapatite-based composite ceramic materials containing 20 and 60% zirconia. The addition of alumina has been shown to prevent secondary recrystallization processes during sintering and contribute to stabilization of tetragonal zirconia. The addition of the sintering aid has made it possible to lower the sintering temperature to 1200°C.     

Effects of sintering aids on the microstructure and mechanical properties of laminated Si3N4/BN ceramics

Laminated Si₃N₄/BN ceramics with two types of sintering aids, MgO–Y₂O₃–Al₂O₃ (MYA) and La₂O₃–Y₂O₃–Al₂O₃ (LYA), were fabricated through roll compaction and hot-pressing. Sintering aids influence evidently the microstructure and mechanical properties of laminated Si₃N₄/BN ceramics. In comparison with La₂O₃–Y₂O₃–Al₂O₃, MgO–Y₂O₃–Al₂O₃ sintering aid is easier to form a glassy phase with lower viscosity and lower eutectic temperature, which is much easier to migrate into BN interlayers. This results in the denser interlayer microstructure and good bending strength of laminated Si₃N₄/BN ceramics at room temperature, but poor work of fracture (WOF) at room temperature, low strength and work of fracture at elevated temperature. In addition, the LYA sintering aid is good for forming elongated and interlocked β-Si₃N₄ grains and beneficial to the mechanical properties of the laminated Si₃N₄/BN ceramics.     

Zirconia electrolyte cells

The effects of time, temperature and doping additions on the sintered density of CaO and Y₂O₃ stabilized ZrO₂ have been studied, and comparisons of the strength and microstructure of the various materials made. Two mol % Al₂O₃ or 5 mol % TiO₂ additions were the most effective sintering aids, causing densification by a liquid phase mechanism.     

Optimization of the composition and sintering conditions of high-voltage ZnO varistor ceramics

This paper presents a study aimed at optimizing the composition and sintering conditions of highvoltage ZnO varistor ceramics. We demonstrate that, with allowance for the cost of starting materials, the optimal composition of high-voltage ZnO varistor ceramics is as follows (wt %): ZnO, 90; Bi₂O₃, 2.76; Sb₂O₃, 1.92; Al₂O₃, 3.32; and Co₂O₃, 2. The optimal sintering conditions are isothermal holding at a temperature of 975°C for 2 h. The ceramics thus prepared have V _b = 4.5 kV/mm, α = 50, I _l = 1.1 μA/cm², density ρ = 5.67 g/cm³ (relative density of 96.1%).     

Microstructure and properties of sintered mullite developed from Indian bauxite

Dense mullite aggregates with 72% Al₂O₃ have been synthesized by reaction sintering of two varieties of Indian bauxite and silica sol. The bauxites used are of inferior grade with different levels of accessory impurities such as Fe₂O₃, TiO₂, CaO. The phase and microstructure development of sintered samples were investigated by XRD and SEM. It was found that morphology of the sintered grain is very much dependent on the impurity level. Mullite formed from bauxite-1 with low impurity is mostly equiaxed, whereas mullite developed from bauxite 2 with higher impurity particularly CaO is needle shaped. Presence of CaO in bauxite was found to be more detrimental than TiO₂ and Fe₂O₃.     

Effect of ball size on milling efficiency of zinc oxide dispersions

Zinc oxide (ZnO) was wet milled using inert Al₂O₃-ceramic balls having different diameter at different milling intervals and the milling efficiency of the resultant dispersion was followed through particle size analysis and zeta potential measurements. The results indicated that small-sized balls improved the milling efficiency. The highest share (%) of lower-size particles was obtained after 24?h of ball milling.     

Microstructure and properties of low-voltage ZnO varistor ceramics

The influences of pre-firing process and sintering temperature on the microstructure and electrical properties of ZnO-Bi₂O₃-TiO₂-Sb₂O₃-MnO₂-CoO low-voltage varistor ceramics were studied. Especially, twin boundaries and exaggerated grain growth (EGG) were observed here, and the mechanism of EGG was discussed. It seems that the formation of twin boundaries has a correlation with Sb₂O₃ content and sintering temperature, and that twin boundaries have a great influence in grain growth of ZnO.     

Preparation and properties of mullite-bonded porous SiC ceramics using porous alumina as oxide

Mullite-bonded porous silicon carbide ceramics were prepared by an in situ reaction bonding technique and sintering in air with SiC, porous Al₂O₃, and graphite as starting materials. The pores in the ceramics were formed by burning graphite and by stacking particles of SiC and Al₂O₃. The surface of SiC was oxidized to SiO₂ at high temperature. With a further increase in temperature, SiO₂ reacted with Al₂O₃ to form mullite. The reaction-bonding characteristics, phase composition, open porosity, mechanical strength as well as the microstructure of porous SiC ceramics were investigated.     

Effect of powder bed on the microstructure and electrical properties of Bi2O3- and Sb2O3-doped ZnO

The effects of powder bed on the microstructure and electrical properties of Bi₂O₃- and Sb₂O₃-doped ZnO specimens are investigated in the present study. By using a sufficient amount of powder bed, the weight loss of Bi₂O₃ reduces from >95 to <20%. The reduction of weight loss enhances significantly the densification and grain growth of ZnO. Furthermore, the use of powder bed can also reduce the size distribution of ZnO grains. Nevertheless, the presence of Bi₂O₃ residue results in the formation of a pyrochlore phase (Zn₂Bi₃Sb₃O₁₄) during the cooling down stage; its presence is detrimental to the nonlinear characteristics of ZnO–Bi₂O₃–Sb₂O₃ system.     

Mechanical alloying behavior of Ti6Al4V residual scraps with addition of Al2O3 to produce nanostructured powder

The present investigation has been based on production and subsequent comparison of different physical, mechanical and thermal properties of nanostructured Ti6Al4V and Ti6Al4V/Al₂O₃ powders by means of high energy ball milling. In this regard, the structural and morphological changes of powders were investigated by X-ray diffraction, scanning electron microscopy and microhardness measurements. The results revealed that ball milling process reduced the grain size of Ti6Al4V and Ti6Al4V + 10 wt% Al₂O₃ to approximately 20 and 15 nm, respectively. For both compositions also a remarkable change in morphology and particle size occurred during ball milling of powders with different compositions. Moreover, phase evolution during milling and heat treatment was taken into consideration. The as-milled Ti6Al4V + 10 wt% Al₂O₃ powder exhibited higher microhardness (∼900 Hv) comparing to as-milled Ti6Al4V (∼536 Hv) and as-received samples (∼400 Hv).     

Preparation of Al2O3–TiB2 nanocomposite powder by mechanochemical reaction between Al,B2O3 and Ti

Mechanochemical processing is a novel technique for the synthesis of nano-sized materials. This research is based on the production of Al₂O₃–TiB₂ nanocomposite powder using mechanochemical processing. For this purpose, a mixture of aluminum, titanium and boron oxide powders was subjected to high energy ball milling. The structural evaluation of powder particles after different milling times was conducted by X-ray diffractometry (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results showed that during ball milling the Al/B₂O₃/Ti reacted with a combustion mode producing Al₂O₃–TiB₂ nanocomposite. In the final stage of milling, the crystallite sizes of Al₂O₃ and TiB₂ were estimated to be less than 50 nm.