Transient liquid-phase sintering of AlN ceramics with CaF2 additive

Transient liquid-phase (TLP) sintering of CaF₂ additive on the densification behaviors and microstructural development of AlN ceramics are investigated. It is found that 1 wt% CaF₂ can effectively promote densification process. Increasing content of CaF₂ results in finer grain size and slower densification during intermediate sintering stage. XRD results show that grain-boundary phase of CaAl₄O₇ is formed at 1150 °C from reactions of AlN–CaF₂–Al₂O₃. With further temperature increasing, the grain-boundary phases of CA2 and CaAl₁₂O₁₈, which were formed from the reaction between CaF₂ and oxide layers, experienced transformations firstly into CaAl₄O₇ above 1600 °C and into CaAl₂O₄ at higher temperature. SEM and TEM results show that formed grain-boundary phases can evaporate from sintering bodies during further soaking, leaving clean grain boundaries. The efficiency of TLP sintering mechanism is further manifested by the preparation of transparent AlN ceramics with good combination properties.     

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.     

Second-phase evolution and densification behavior of AlN with CaO–Y2O3–C multicomponent additive system

AlN compacts with different CaO–Y₂O₃–C mixtures were sintered between 1100 and 1850 °C to understand the effects of the in situ formed reducing atmosphere on the densification behavior and evolution of the second-phases. AlN with Y₂O₃ densified at 1750 °C, but the addition of C changed the second-phases evolution towards Y-rich phases that delayed the densification. For AlN containing CaO, the second-phases were little influenced by the reducing atmosphere, but the addition of C increased the evaporation of the second-phase compounds during sintering, limiting the densification due to the reduction of the liquid-phase fraction and the gas trapping inside the pores. AlN with CaO–Y₂O₃ mixtures could be completely densified at 1650 °C, but the addition of C inhibited the densification below this sintering temperature because liquid-phase had poor wetting and spreading characteristics and the second-phase a high melting point (>1800 °C).     

Reaction sintering of AlN–AlON composites

Sintering behavior of three different compositions in the AlN–Al₂O₃ system using Y₂O₃ as a sintering aid was investigated. Samples with various ratios of AlN/Al₂O₃ were sintered in nitrogen atmosphere using a gas pressure furnace in the temperature range 1750–1950 °C. The densification of the samples was studied by shrinkage and relative density measurements. Results showed that samples containing 1 and 70 wt.% alumina were sintered to near theoretical density at 1800 °C; whereas the sample with 20 wt.% alumina never reached densities higher than 93% in the temperature range considered. It was found that the AlN/Al₂O₃ ratio and the sintering temperature had a great influence on the microstructure and crystalline phases present in the samples, namely, AlN, γ-AlON, 27R, and YAG. In the sample with 20 wt.% alumina, porosity formation prevented further densification. These porosities were probably due to the release of oxygen during sintering.     

High-strength AlN ceramics by low-temperature sintering with CaZrO3–Y2O3 co-additives

Aluminum nitride (AlN) ceramics with the concurrent addition of CaZrO₃ and Y₂O₃ were sintered at 1450-1700 °C. The degree of densification, microstructure, flexural strength, and thermal conductivity of the resulting ceramics were evaluated with respect to their composition and sintering temperature. Specimens prepared using both additives could be sintered to almost full density at relatively low temperature (3 h at 1550 °C under nitrogen at ambient pressure); grain growth was suppressed by grain-boundary pinning, and high flexural strength over 630 MPa could be obtained. With two-step sintering process, the morphology of second phase was changed from interconnected structure to isolated structure; this two-step process limited grain growth and increased thermal conductivity. The highest thermal conductivity (156 Wm⁻¹ K⁻¹) was achieved by two-step sintering, and the ceramic showed moderate flexural strength (560 MPa).     

Structural,thermal and mechanical properties of aluminum nitride ceramics with CeO2 as a sintering aid

AlN ceramics have been prepared with CeO₂ as a sintering aid at a sintering temperature of 1900 °C. The effect of CeO₂ contents on the microstructure, density, thermal conductivity and hardness was investigated. Addition of CeO₂ exerted a significant effect on the densification of AlN ceramics and hence on the microstructure. Thermal conductivity of AlN ceramics increased with CeO₂ content and was greater than that of Y₂O₃-doped AlN ceramics at a similar sintering temperature. The resulting AlN ceramics with 1.50 wt% of CeO₂ had the highest relative density of 99.94%, thermal conductivity of 156 W m⁻¹ K⁻¹ and hardness of 72.46 kg/mm².     

The role of MgSiN2 during the sintering process of silicon nitride ceramic

The reaction process between MgSiN₂ SiO₂ and Si₃N₄ was investigated by analyzing the composition change of the powder mixture of 61 wt% MgSiN₂, 34 wt% SiO₂ and 5 wt% α-Si₃N₄ after heat treatment at different temperatures. The phase and chemical compositions of the grain boundary phase in the silicon nitride ceramic was analyzed by x-ray diffraction, transmission electron microscope, and energy-dispersive x-ray spectroscopy. The results demonstrated that MgSiN₂ reacted with the surface silica and Si₃N₄ to form Mg–Si–O–N liquid phase, which promoted the consolidation densification of silicon nitride powders through liquid-phase sintering mechanism. The amount of Mg–Si–O–N glass boundary phase using MgSiN₂ as additives is much less than that using the same amount of MgO additive, owing to the lower oxygen concentration and higher nitrogen content.     

Low-temperature sintering behavior of the nano-sized AlN powder achieved by super-fine grinding mill with Y2O3 and CaO additives

For low-temperature sintering, mixtures of AlN powder doped with 3.53 mass% Y₂O₃ and 0–2.0 mass% CaO as sintering additives were pulverized and dispersed in a vertical super-fine grinding mill with very small ZrO₂ beads. The particle sizes achieved ranged between 50 and 100 nm after grinding for 90 min. The mixtures were then fired at 1000–1500 °C for 0–6 h under nitrogen gas pressure of 0.1 MPa. All nano-sized powders showed pronounced densification from 1300 °C as revealed by shrinkage measurement. The larger amounts of sintering additives enhanced AlN sintering at lower temperatures. Densified AlN ceramics with very fine and uniform grains of 0.3–0.4 μm were obtained at a firing temperature of 1500 °C for 6 h.     

Densification of AlN using boron and carbon additives

The effects of boron and/or carbon on the densification of AlN were investigated. Sintering was promoted by adding the additives simultaneously. Thermo-chemical analysis indicated that a liquid phase composed of aluminum and boron were formed by the borothermal reduction of Al₂O₃ and AlN. Liquid phase sintering is believed to be the reason for the enhanced densification of AlN. The present investigation proposes a method to densify AlN using non-oxide sintering additives having high melting temperature (>2000 °C). The deterioration of AlN refractory is expected to be suppressed by preventing the formation of oxide grain boundary network which is vulnerable to corrosion caused by molten metal.     

Microwave sintering of AlN powder synthesized by a SHS method

Sintering of the AlN powder synthesized by a combustion synthesis method, which was developed recently by the present authors, was studied by using a microwave sintering technique. A single mode microwave cavity was used and an insulation package with a simple configuration was developed. A high sintering temperature (1900 °C or higher) and a stable and uniform heating were readily achieved. A temperature measurement technique using a thermocouple with extrapolation was established to obtain the sintering temperature. A percent theoretical density of 99.5% and a thermal conductivity of 186 W/m K were obtained for a specimen which was sintered at 1900 °C with a soaking time of 30 min and 3 wt.% of Y₂O₃ added. The effects of sintering aid (i.e., Y₂O₃) and sintering temperature on densification, microstructure and thermal conductivity of the sintered specimens were investigated.     

Sintering behaviour of a glass obtained from MSWI ash

The sintering behaviour of a glass obtained by Municipal Solid Waste Incinerator (MSWI) bottom ash (WG) was investigated and compared with a Na₂O–MgO–CaO–SiO₂ composition (CG). The sintering activation energy, E_sin, and the energy of viscous flow, E_η, were evaluated by dilatomeric measurements at different heating rates. The formation of crystalline phases was evaluated by Differential Thermal Analysis (DTA) and X-Ray Diffraction (XRD), and observed by Scanning Electron Microscopy (SEM) and Transition Electron Microscopy (TEM). In CG, the sintering started at ≈10¹³ dPa s viscosity and E_sin (245 kJ/mol) remains constant in the measured range of shrinkage, up to 9%. In WG the densification started at ≈10¹¹ dPa s, E_sin resulted to be 395 kJ/mol up to 5% shrinkage, 420 kJ/mol at 8% and 485 kJ/mol at 10% shrinkage. The sintering rate decreased due to the beginning of the pyroxene formation and the densification stopped in the temperature range 1073–1123 K after formation of 5 ± 3% and 13 ± 3% crystal phase, at 5 and 20 K/min, respectively. Higher densification and improved mechanical properties were obtained by applying the fast heating rate, i.e. 20 K/min.     

Phase,microstructure and sintering of aluminum nitride powder by the carbothermal reduction-nitridation process with Y2O3 addition

Aluminum nitride powders were synthesized by carbothermal reduction-nitridation method using Al(OH)₃, carbon black and Y₂O₃ as raw materials. The change of phase, microstructure and densification during the AlN synthesis and sintering process were investigated and the effects of Y₂O₃ was discussed. The results showed that Y₂O₃ reacted with Al₂O₃ to form yttrium aluminates of YAlO₃ (orthorhombic and hexagonal phases), Y₄Al₂O₉ and Y₃Al₅O₁₂ at the low temperature of 1350 °C. YAlO₃ could firstly be transformed into Y₂O₃ and then completely into YN when the firing temperature and holding time increased. However, YN could be oxidized into Y₂O₃ again after the carbon removal at 700 °C in the air atmosphere. There were two ways generating AlN when adding Y₂O₃ and the possible mechanism was proposed. Y₂O₃ from YN oxidation favored the densification of AlN ceramics because the liquid had better flowability and distribution in the sintering process at 1800 °C.     

Densification and mechanical properties of cBN–TiN–TiB2 composites prepared by spark plasma sintering of SiO2-coated cBN powder

cBN–TiN–TiB₂ composites were fabricated by spark plasma sintering at 1773–1973 K using cubic boron nitride (cBN) and SiO₂-coated cBN (cBN(SiO₂)) powders. The effect of SiO₂ coating, cBN content and sintering temperature on the phase composition, densification and mechanical properties of the composites was investigated. SiO₂ coating on cBN powder retarded the phase transformation of cBN in the composites up to 1873 K and facilitated viscous sintering that promoted the densification of the composites. Sintering at 1873 K, without the SiO₂ coating, caused the relative density and Vickers hardness of the composite to linearly decrease from 96.2% to 79.8% and from 25.3 to 4.4 GPa, respectively, whereas the cBN(SiO₂)–TiN–TiB₂ composites maintained high relative density (91.0–96.2%) and Vickers hardness (17.9–21.0 GPa) up to 50 vol% cBN. The cBN(SiO₂)–TiN–TiB₂ composites had high thermal conductivity (60 W m⁻¹ K⁻¹ at room temperature) comparable to the TiN–TiB₂ binary composite.     

Densification and grain growth kinetics of Ce0.9Gd0.1O1.95 in tape cast layers: The influence of porosity

The sintering behavior of Ce_0.9Gd_0.1O_1.95 (CGO) tape cast layers with different porosity was investigated by an extensive characterization of densification, microstructural evolution, and applying the constitutive laws of sintering. The densification of CGO tapes associates with grain coarsening process at the initial sintering stage at T < 1150 °C, which is mainly influenced by small pores and intrinsic characteristics of the starting powders. At the intermediate sintering stage, densification is remarkably influenced by large porosity. Moreover, the sintering constitutive laws indicate that increasing the initial porosity from 0.38 to 0.60, the densification at the late stage is thermally activated with typical activation energy values increasing from 367 to 578 kJ mol⁻¹. Similar effect of the porosity is observed for the thermally activated phenomena leading to grain growth in the CGO tapes. The analysis of sintering mechanisms reveals that the grain growth behavior at different porosity can be described using an identical master curve.     

Pressureless sintering of boron carbide with Cr3C2 as sintering additive

In this study, chromium carbide (Cr₃C₂) was selected as the sintering additive for the densification of boron carbide (B₄C). Cr₃C₂ can react with B₄C and form graphite and CrB₂ in situ, which is considered to be effective for the sintering of B₄C composites. The sintering behavior, microstructure development and mechanical properties of B₄C composites were studied. The density of B₄C composite increased with the increase of Cr₃C₂ content and sintering temperature. The formation of liquid phase could effectively improve the densification of B₄C composites. The abnormal grains began to appear at 2080 °C. The bending strength could reach 440 MPa for the 25 wt% and 30 wt% Cr₃C₂ samples after sintering at 2070 °C.     

Reactive sintering cBN-Ti-Al composites by spark plasma sintering

When synthesizing polycrystalline cubic boron nitride (PcBN) at normal pressure, cBN had a trend of hexagonal transformation, which reduces the hardness and strength of PcBN. The cBN-Ti-Al composite was prepared by spark plasma sintering with introducing Ti and Al to absorb hexagonal boron nitride (hBN) transformed from cBN. By the results of X-ray diffraction (XRD), Ti and Al reacted with BN and forming TiN, TiB₂, and AlN, which combined cBN as the binder by chemical bonding. The mechanical properties of the prepared composite increased as the increment of sintering temperature. The threshold temperature for preparing composite without hBN phase was at 1400 °C. The composite with optimal mechanical properties was prepared at 1400 °C, and the relative density, the bending strength, hardness, and fracture toughness were 98.9 ± 0.1%, 390.7 ± 4.4 MPa, 14.1 ± 0.5 GPa, and 7.6 ± 0.1 MPa·m^0.5, respectively.     

Microstructure of boron carbide pressureless sintered in an Ar atmosphere containing gaseous metal species

97.4% of theoretical density was obtained for boron carbide (B₄C) ceramics by heating up to 2226 °C in an Ar atmosphere containing gaseous Al and Si species without external pressure. Impurities and secondary phases in the sintered B₄C samples were examined by X-ray fluorescence and X-ray diffraction analyses respectively, which revealed that both Al and Si elements infiltrated into the green compacts and reacted with B₄C to form SiC, Al₄C₃ and Al₄SiC₄ during the sintering. Triple junctions observed in the polished surfaces of the densified samples were filled by the secondary phases, indicating formation of liquid phase during heating. Dilatometric measurements at a constant heating rate in the Ar gas with the metallic gas species demonstrated that the shrinkage started at around 1700 °C, which was the liquid-phase formation temperature for the system reported in the previous studies. It was supposed that the liquid phase might be responsible for the densification.     

High pressure sintering of cubic boron nitride compacts with Al and AlN

Cubic boron nitride (cBN) compacts, using 15 wt.% Al and 20 wt.% AlN respectively as additives, were sintered in the temperature range of 1300–1700 °C for 20 min under high pressure of 5.0 GPa. The hardness, microstructure, phase composition and cutting performance of the high pressure sintered samples were investigated. A liquid phase sintering and reaction process was observed in the cBN–Al system, which leads to the formation of AlN and AlB₂ as confirmed by X-ray diffraction (XRD) in the sintered compacts. Scanning electron microscopy (SEM) analysis shows that the samples have a homogeneous microstructure. The hardness decreases with increase of sintering temperature and reaches the highest Vickers hardness of 32.1 GPa at 1350 °C. While in the cBN–AlN system, AlN grains agglomerate heavily at temperature below ~ 1500 °C. As the sintering temperature increasing, Al₂O₃ appeared and the AlN agglomeration disappeared gradually. A highest cBN–AlN composite hardness of 29 GPa was achieved when sintered at 1600 °C. Turning tests showed that cBN compacts with 15 wt.% Al as the additive has a longer tool life as compared to that with 20 wt.% AlN. Our results indicated that cBN–Al system is more favourable to obtain well-sintered cBN compacts comparing with the cBN–AlN system.     

A mechanism for low-temperature sintering

We explain the basic mechanism of the low-temperature sintering called reactive liquid-phase sintering. The mechanism involves the presence of a low-temperature liquid phase that must be able to directly or indirectly accelerate a reaction with the matrix phase. The mechanism is explained in details for the case of the low-temperature sintering of BaTiO₃, which was sintered to more than 95% of relative density in 15 min at 820 °C. We have applied reactive liquid-phase sintering to a number of different compounds with very different crystal-chemistry characteristics, and managed to sinter them as much as 400 °C below their original sintering temperatures. A thorough understanding of this sintering mechanism makes it possible to closely control the sintering behavior.     

Dielectric and thermal properties of AlN ceramics

The effects of slow-cooling and annealing conditions on dielectric loss, thermal conductivity and microstructure of AlN ceramics were investigated. Y₂O₃ from 0.5 to 1.25 mol% at 0.25% increments was added as a sintering additive to AlN powder and pressureless sintering was carried out at 1900 °C for 2 h in a nitrogen flowing atmosphere. To improve the properties, AlN samples were slow-cooled at a rate of 1 °C min⁻¹ from 1900 to 1750 °C, subsequently cooled to 970 °C at a rate of 10 °C min⁻¹ and then annealed at the same temperature for 4 h. AlN and YAG (5Al₂O₃/3Y₂O₃) were the only identified phases from XRD. AlN doped with 0.5 and 0.75 mol% Y₂O₃ had a low loss of <2.0 × 10⁻³ and a high thermal conductivity of >160 W m⁻¹ °C⁻¹.