Synthesis of rod-like β-Si3N4 seed crystals with tailored morphology

β-Si₃N₄ seed crystals were synthesized by sintering (α+β)-Si₃N₄ powders with Y₂O₃+MgO additives at 1800 °C. Full α- to β-phase transformation was achievable at 1800 °C for 1 h. The pre-existing β-Si₃N₄ particles acted as nuclei during a sintering process. The length and mean aspect ratio of β-Si₃N₄ seed grains could be tailored by careful control of α/β-Si₃N₄ ratio, which resulted in various nuclei and driving force. The sample A95B5 with 5% β-nuclei shows a bimodal size distribution containing large amount of abnormal elongated β-Si₃N₄ grains with remarkable large diameter. With increasing the β-phase content from 5 wt% to 100 wt%, the average diameter and aspect ratio of the β-Si₃N₄ single crystals decreased from 1.43 µm to 0.92 µm and from 4.36 to 2.79, respectively.     

Effect of Al metal precursor on the phase formation and mechanical properties of fine-grained SiAlON ceramics prepared by spark plasma sintering

This paper reports the effects of using nano-precursor powders, including α-Si₃N₄ or amorphous-Si₃N₄, and particularly the partial replacement of AlN by Al metal precursor, on the properties of a fixed Ca-α-SiAlON composition, consolidated by spark plasma sintering at 1450 °C and 1600 °C. The observed changes in mechanical properties are related to the phases present in the microstructures. In addition to allowing the consolidation of dense and fine ceramics at lower temperatures, the partial replacement of AlN by Al metal precursor has shown remarkable improvement in the mechanical properties of samples sintered at 1600 °C containing α-Si₃N₄, a result of fine α-SiAlON grains and nitrogen-rich grain boundary phase. Another key contribution of Al metal precursor is found in lowering the sintering temperature while keeping, or even improving, the mechanical properties of the sintered samples, as observed in samples sintered at 1450 °C.     

High-pressure spark plasma sintering of silicon nitride with LiF additive

High-pressure spark plasma sintering of Si₃N₄ with Y₂O₃, Al₂O₃ and LiF additives was employed to fabricate high quality dense ceramics comprising approximately 92% α-Si₃N₄ phase and 8% β-Si₃N₄ phase. The relatively high pressure applied (up to 650 MPa) had a substantial effect on densification by enhancing particle rearrangement, making it possible to obtain dense Si₃N₄ at a significantly lower sintering temperature (1350 °C). Consequently, virtually no α to β phase transformation transpired during the liquid phase sintering process. The LiF additive had an indispensable influence on the densification process by lowering the viscous glass formation temperature, which also contributed to enhanced particle rearrangement. The nearly fully dense samples (theoretical density ≥99%) obtained displayed a good combination of mechanical properties, namely elastic modulus (304–316 GPa), hardness (1720–1780 HV2) and fracture toughness (6.0 MPa m^1/2).     

Comparison among different sintering routes for preparing alumina-YAG nanocomposites

Al₂O₃-YAG (50 vol.%) nanocomposite powders were prepared by wet-chemical synthesis and characterized by DTA-TG, XRD and TEM analyses. Amorphous powders were pre-heated at different temperatures (namely 600 °C, 800 °C, 900 °C and 1215 °C) and the influence of this thermal treatment on sintering behavior, final microstructure and density was investigated. The best performing sample was that pre-calcined at 900 °C, which yields dense bodies with a micronic/slightly sub-micronic microstructure after sintering at 1600 °C. A pre-treatment step to induce controlled crystallisation of the amorphous powder as well as a fast sintering procedure for green compacts, were also performed as a comparison.Finally, the previously stated thermal pre-treatment of the amorphous product was coupled to an extensive mechanical activation performed by wet planetary/ball milling. This procedure was highly effective in lowering the densification temperature, so that fully dense Al₂O₃-YAG composites, with a mean grain size smaller than 200 nm, were obtained by sintering in the temperature range 1370–1420 °C.     

Effects of the sintering atmosphere on the BaZn1/3Ta2/3O3 based Cu multilayer ceramic capacitors

Recent papers report that BaZn_1/3Ta_2/3O₃ (BZT) ceramic can be sintered at a temperature as low as 1050 °C owing to the use of flux agents like B₂O₃ + LiF combined with a slight non-stoichiometry, whereas its usual sintering temperature is 1400 °C. This low sintering temperature (below the Cu's melting point = 1083 °C) opens the route to fabricate copper based multilayer ceramic capacitors, in condition that a reductive atmosphere is used during the sintering. This paper presents the effect of three various sintering atmospheres (air, H₂ (1%) in N₂ and H₂ (1%) in Ar) on the stability and the dielectric properties of BZT. It is researched a suitable sintering atmosphere to prevent Cu from oxidation and to preserve the dielectric properties of BZT. Using the appropriate atmosphere, copper based multilayer ceramic capacitors, with attractive dielectric properties, have been successfully processed.     

Synthesis and spark plasma sintering of the α-Si3N4 nanopowder

A two-step process (milling and then heat treatment) was used for the preparation of α-Si₃N₄ nanopowder. The influence of the milling time and heat treatment temperature as processing parameters were investigated on the formation of α-Si₃N₄. Silicon nitride ceramic was produced by spark plasma sintering at 1700 °C for 15 min, using MgSiN₂ additive. The optimum sample was produced in a 30 h milling time, heat treatment at 1300 °C, and a 22 °C/min heating rate conditions. X-ray fluorescence analysis showed that the purity of the final product is above 98%. Nanoindentation hardness and Young’s modulus of the SPS-ed sample were measured as 17±2.0 GPa and 290±11.0 GPa, respectively.     

Cu2ZnSnS4 ceramic target: Determination of sintering temperature by TG–DSC

In this work, mixed powders of Cu₂S, ZnS and SnS₂ were used to sinter CZTS ceramic target. Thermal properties of Cu-poor and Zn-rich mixed powders were investigated by thermogravimetry and differential scanning calorimetry (TG–DSC). It has been found that the formation temperature of CZTS was not obvious in DSC curve. Obvious weight loss begun from 800 °C and a strong endothermic peak of melting appeared at around 880 °C depending on the relative tin content in mixed powders. For different compositions in Cu-poor and Zn-rich region, all TG and DSC curves showed similar profile as a function of temperature. To reduce volatilization and low melting point compound of SnS and Cu–Sn–S when sintering, a sintering temperature of 750 °C was adopted. CZTS targets (360 mm×80 mm) have been successfully fabricated with Cu-poor and Zn-rich composition, kesterite phase and high compactness of above 93%.     

Spark plasma sintering of TaC–HfC UHTC via disilicides sintering aids

Ta_0.8Hf_0.2C ceramic has the highest melting point among the known materials (4000 °C). Spark plasma sintering is a new route for consolidation of materials, specially ultra high temperature ceramics (UHTCs), which are difficult to be sintered at temperatures lower than 2000 °C.The purpose of this study is to consolidate Ta_0.8Hf_0.2C by spark plasma sintering at low temperature using MoSi₂ and TaSi₂ as sintering aid. In this regard, effect of different amounts of sintering aids and carbides ratio on densification behavior and mechanical properties of Ta_1?xHf_xC were investigated.Fully consolidation of Ta_0.8Hf_0.2C was achieved in presence of 12 vol.% sintering aid after sintering at 1650 °C for 5 min under 30 MPa. The first stage of sintering was due to plastic deformation of sintering aids particles and consequent rearrangement. The second stage was occurred via Ta_1?xHf_xC solid solution and liquid phase formation.     

A synergistic effect of Li-B-Si glass and La-B on low-temperature sintering of BaO-0.15ZnO-4TiO2 dielectric ceramic

In the present work, the synergistic effect of Li₂O-B₂O₃-SiO₂ glass (LBS) and La₂O₃-B₂O₃ (LaB) addition on the wetting behavior, activation energy, phase composition, microstructure and microwave dielectric properties of the BaO-0.15ZnO-4TiO₂ ceramic (Ba-Zn-Ti) has been investigated to understand the low temperature mechanism of the sintering kinetic. The results show that co-doping with LBS and LaB decreases the activation energy and therefore accelerates the sintering process of the Ba-Zn-Ti due to the liquid phase formed at a low sintering temperature. Moreover, the co-doping of LBS and LaB restrains the appearance of BaTi(BO₃)₂ caused by the reaction of Ba-Zn-Ti and LBS, which has a deleterious effect on the densification and microwave dielectric properties of Ba-Zn-Ti. With LBS and LaB co-doped, the sintering temperature of Ba-Zn-Ti significantly dropped from 1150 °C to 850–900 °C and remained excellent microwave dielectric properties.     

Diamond dispersed Si3N4 composites obtained by pulsed electric current sintering

30 vol.% 2 and 30 μm diamond dispersed Si₃N₄ matrix composites were prepared by pulsed electric current sintering (PECS) for 4 min at 100 MPa in the 1550–1750 °C range. The densification behaviour, microstructure, Si₃N₄ phase transformation and stiffness of the composites were assessed, as well as the thermal stability of the dispersed diamond phase. Monolithic Si₃N₄ with 4 wt% Al₂O₃ and 5 wt% Y₂O₃ sintering additives was fully densified at 1550 °C for 4 min and 60 MPa. The densification and α to β-Si₃N₄ transformation were substantially suppressed upon adding 30 vol.% diamond particles. Diamond graphitisation in the Si₃N₄ matrix was closely correlated to the sintering temperature and grit size. The dispersed coarse grained diamonds significantly improved the fracture toughness of the diamond composite, whereas the Vickers hardness was comparable to that of the Si₃N₄ matrix ceramic. The Elastic modulus measurements were found to be an excellent tool to assess diamond graphitisation in a Si₃N₄ matrix.     

Fabrication and properties of Y2O3 transparent ceramic by sintering aid combinations

Transparent Y₂O₃ ceramics were fabricated by the solid-state reaction and vacuum sintering method using La₂O₃, ZrO₂ and Al₂O₃ as sintering aids. The microstructure of the Y₂O₃ ceramics sintered from 1550 °C to 1800 °C for 8 h were analyzed by SEM. The sintering process of the Y₂O₃ transparent ceramics was optimized. The results showed that when the samples were sintered at 1800 °C for 8 h under vacuum, the average grain sizes of the ceramics were about 3.5 µm. Furthermore, the transmittance of Y₂O₃ ceramic sintered at 1800 °C for 8 h was 82.1% at the wavelength around the 1100 nm (1 mm thickness), which was close to its theoretical value. Moreover, the refractive index of the Y₂O₃ transparent ceramic in the temperature range from 30 °C to 400 °C were measured by the spectroscopic ellipsometry method.     

Synthesis and characterization of LTCC compositions with middle permittivity based on CaO-B2O3-SiO2 glass/CaTiO3 system

CaTiO₃ ceramics with the addition of CaO-B₂O₃-SiO₂ (CBS) glass (45–55 wt%) composites were sintered at 830 °C, 850 °C, 875 °C and 900 °C. To illustrate influence mechanism of the different glass contents and sintering temperatures on the properties of the composites, we focused on the multiple performances of the composites by employing different qualitative and quantitative instruments. Composites with 50 wt% glass sintered at 875 °C presented fairly ideal performance: the bulk density was 3.20 g/cm³, the dielectric constant was 25.7 and the dielectric loss was 0.0009 at 7 GHz. Micro-Structure analysis of the composites showed a dense and pore-less microstructure except for few pores with size around 1 μm. In addition, the composite could meet the shrinkage requirement of Ag electrodes and could not possibly react with Ag electrodes any more. This makes them suitable for various dielectric applications at low sintering temperature.     

Spark plasma sintering of Sm2O3-doped aluminum nitride

The high sintering temperature required for aluminum nitride (AlN) at typically 1800 °C, is an impediment to its development as an engineering material. Spark plasma sintering (SPS) of AlN is carried out with samarium oxide (Sm₂O₃) as sintering additive at a sintering temperature as low as 1500–1600 °C. The effect of sintering temperature and SPS cycle on the microstructure and performance of AlN is studied. There appears to be a direct correlation between SPS temperature and number of repeated SPS sintering cycle per sample with the density of the final sintered sample. The addition of Sm₂O₃ as a sintering aid (1 and 3 wt.%) improves the properties and density of AlN noticeably. Thermal conductivity of AlN samples improves with increase in number of SPS cycle (maximum of 2) and sintering temperature (up to 1600 °C). Thermal conductivity is found to be greatly improved with the presence of Sm₂O₃ as sintering additive, with a thermal conductivity value about 118 W m⁻¹ K⁻¹) for the 3 wt.% Sm₂O₃-doped AlN sample SPS at 1500 °C for 3 min. Dielectric constant of the sintered AlN samples is dependent on the relative density of the samples. The number of repeated SPS cycle and sintering aid do not, however, cause significant elevation of the dielectric constant of the final sintered samples. Microstructures of the AlN samples show that, densification of AlN sample is effectively enhanced through increase in the operating SPS temperature and the employment of multiple SPS cycles. Addition of Sm₂O₃ greatly improves the densification of AlN sample while maintaining a fine grain structure. The Sm₂O₃ dopant modifies the microstructures to decidedly faceted AlN grains, resulting in the flattening of AlN–AlN grain contacts.     

Low temperature sintering and microwave dielectric properties of Ba3Ti5Nb6O28 with ZnO–B2O3 glass additions for LTCC applications

The effects of ZnB₂O₄ glass additions on the sintering temperature and microwave dielectric properties of Ba₃Ti₅Nb₆O₂₈ have been investigated using dilatometer, X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and a network analyzer. The pure Ba₃Ti₅Nb₆O₂₈ system showed a high sintering temperature (1250 °C) and had the good microwave dielectric properties: Q × f of 10,600 GHz, ɛ_r of 37.0, τ_f of −12 ppm/°C. It was found that the addition of ZnB₂O₄ glass to Ba₃Ti₅Nb₆O₂₈ lowered the sintering temperature from 1250 to 925 °C. The reduced sintering temperature was attributed to the formation of ZnB₂O₄ liquid phase and B₂O₃-rich liquid phases. Also the addition of ZnB₂O₄ glass enhanced the microwave dielectric properties: Q × f of 19,100 GHz, ɛ_r of 36.6, τ_f of 5 ppm/°C. From XPS and XRD studies, these phenomena were explained in terms of the reduction of oxygen vacancies and the formation of secondary phases having the good microwave dielectric properties.     

Preparation and sintering of nanocrystalline ITO powders with different SnO2 content

The effect of SnO₂ content on the sintering behavior of nanocrystalline indium tin oxide (ITO) ceramics was examined. Nanocrystalline ITO powders with different SnO₂ content from 0 to 12 at.% were prepared by a coprecipitation method. The particle size of the ITO powders was in the range of 20–26 nm. The temperature that showed maximum densification increased as the content of SnO₂ increased. Since the solubility limit of SnO₂ in In₂O₃ is known to be about 6–8 at.%, the samples with 8 and 12 at.% Sn showed second phases after sintering. Various phase development processes of the second phases were observed, i.e., In₂SnO₅, which was observed at a low temperature, decomposed into In₂O₃ and SnO₂ at over 1000 °C, then synthesized again into In₄Sn₃O₁₂ at over 1300 °C. The densification behavior with respect to the SnO₂ content was explained from a viewpoint of the second phase development at different sintering temperatures.     

Reclamation of used SiC grinding powder and its sintering characteristics

A process of recycling used abrasive SiC powder after grinding Si wafer was proposed to raw powder for sintering. The used SiC powder could be successfully converted to composite powders consisting of SiC particle and Si₃N₄ whisker via a heat treatment in N₂ atmosphere, in which iron oxide acted as a catalyst in the vapor–liquid–solid (VLS) formation of Si₃N₄. With the addition of 3 mass% Al₂O₃ and 1 mass% Y₂O₃, the composite powders sintered at 1900 °C for 2 h exhibited a 3-point bending strength of 626 ± 48 MPa and a fracture toughness of 3.9 ± 0.1 MPa m^1/2, which were significantly enhanced as compared with those of using recovered powder merely composed of SiC particle. The strength and fracture toughness of the sintered material could be improved by optimization of chemical and heat treatment parameters and controlling the amount of sintering additives and hot pressing conditions.     

Low-temperature sintering and microwave dielectric properties of Ba5Nb4O15 with ZnB2O4 glass

The Influence of ZnB₂O₄ glass addition on the sintering temperature and microwave dielectric properties of Ba₅Nb₄O₁₅ has been investigated using dilatometry, X-ray diffraction, scanning electron microscopy and network analyzer. It was found that a small amount of glass addition to Ba₅Nb₄O₁₅ lowered the sintering temperature from 1400 to 900 °C. The reduced sintering temperature was attributed to the formation of ZnB₂O₄ liquid phase and B₂O₃-rich liquid phases such as Ba₃B₂O₆. The Ba₅Nb₄O₁₅ ceramics with ZnB₂O₄ glass, sintered at a low temperature, exhibited good microwave dielectric characteristics, i.e., a quality factor (Q × f) = 12,100 GHz, a relative dielectric constant (ɛ_r) = 40, a temperature coefficient of resonant frequency (τ_f) = 48 ppm/°C. The dielectric properties were discussed in terms of the densification of specimens and the influence of glassy phases such as Ba₃B₂O₆ and ZnB₂O₄.     

Effect of calcination temperature on the fabrication of transparent lutetium titanate by spark plasma sintering

Transparent lutetium titanate (Lu₂Ti₂O₇) bodies were fabricated by spark plasma sintering using Lu₂O₃ and TiO₂ powders calcined from 700 °C to 1200 °C. No solid-state reaction was identified after calcination at 700 °C, whereas single-phase Lu₂Ti₂O₇ powder was prepared at 1100 and 1200 °C. The calcination at 700 °C promoted densification at the early stages of sintering, whereas residual pores at grain boundaries resulted in Lu₂Ti₂O₇ bodies with low transparency. Low-density and opaque Lu₂Ti₂O₇ bodies formed owing to the coarsening of the powder calcined at 1200 °C. The Lu₂Ti₂O₇ body sintered using the powder calcined at the moderate temperature of 1100 °C had a density of 99.5% with the highest transmittances of 41% and 74% at wavelengths of 550 nm and 2000 nm, respectively.     

Densification and biocompatibility of sintering 3.0 mol% yttria-tetragonal ZrO2 polycrystal ceramics with x wt% Fe2O3 and 5.0 wt% mica powders additive

The densification and biocompatibility of sintered 3.0 mol% yttria-tetragonal zirconia polycrystal (3Y-TZP) ceramics, with X wt% Fe₂O₃ and 5.0 wt% mica powders (denoted by 3Y-TZP: X-5.0 wt% mica) have been studied. When the pellets of 3Y-TZP: X-5.0 wt% mica were sintered at 1300 °C for 1 h, the relative shrinkage increases from 19.20–19.43% with the X increased from 0.3 to 1.0. The relative shrinkage of pellets containing 1.0 wt% Fe₂O₃ (X=1.0) increased from 19.43–19.59% when sintering temperatures were raised from 1300 °C to 1450 °C. X-ray diffraction results show that the pellets of 3Y-TZP: X-5.0 wt% mica sintered at 1400 °C for 1 h only contained single phase of tetragonal ZrO₂ (t-ZrO₂). When the sintering temperature was higher than 1400 °C, the Vickers microhardness was greatest in the pellets with X=0.5. Within pellets with the same Fe₂O₃ content, the dominant wavelength (λ_d) was only slightly different for pellets sintered at 1300 °C and those sintered at 1450 °C. The results of the materials were evaluated in vitro cytotoxicity tests reveals that the powders and sintered pellets are safe materials. The oral mucosa irritation tests did not find erythema or histopathological change including normal epithelium, and was free from leucocyte infiltration, vascular congestion and oedema.     

Effect of temperature and heating rate on the sintering performance of SiC-Al2O3-Dy2O3 and SiC-Al2O3-Yb2O3 systems

Samples of SiC+10 vol%(Al₂O₃+Dy₂O₃) and SiC+10 vol%(Al₂O₃+Yb₂O₃) mixtures were obtained by cold isostatic pressing and sintered for one hour in a dilatometer at 1800 °C and 1900 °C, applying heating rates of 10, 20 and 30 °C/min. The results of the complete sintering cycle indicated that the heating rates do not significantly influence the shrinkage, but that temperature and total sintering time may be relevant factors. The compacts sintered at 1900 °C shrank on average 9% more than those sintered at 1800 °C, and it was found that the sintering time can be reduced by 40–50% at faster heating rates. The maximum shrinkage rates occurred at temperatures lower than those of the sintering thresholds for the two mixtures, two temperatures and three heating rates. It was also found that after formation of the liquid, the mechanisms of particle rearrangement and solution-precipitation were not as fast as reported in the literature, even at high heating rates, for example 30 °C/min, but they are responsible for much of the shrinkage occurring throughout the sintering cycle.