Sintering and characterization of fully dense aluminium nitride ceramics

Aluminium nitride ceramics with no sintering additives could be densified to close to theoretical density (99.6% theoretical) by pressureless sintering of tape-cast green sheets at 1900 °C for 8 h. The thermal conductivity and bending strength of the specimens were 114 Wm^–1 K^–1 and 240 MPa, respectively. The effect of Y₂O₃ additive on sinterability, thermal conductivity and microstructure of aluminium nitride ceramics was investigated. Thermal conductivity increased with increasing amount of Y₂O₃ additive, sintering temperature and holding time at the sintering temperature. Samples with a thermal conductivity up to 258 Wm^–1 K^–1 were fabricated by elimination of the grain-boundary phase.     

Thermal conductivity of calcium-doped aluminium nitride ceramics

Aluminium nitride ceramics were prepared with the addition of up to 12wt% of calcium oxide as a sintering aid. Both the oxygen and the calcium content of the samples decreased during sintering with increasing sintering temperature and soaking time. Higher amounts of calcium oxide resulted in higher thermal conductivities, with values up to 142 W m^–1 K^–1. Moderate sintering temperatures, short temperature soaking times and the use of inexpensive Ca-based sintering additives should enable the production of aluminium nitride ceramics with sufficiently high thermal conductivity at relatively low cost.     

Hot-pressed AlN-Cu metal matrix composites and their thermal properties

AlN-Cu metal matrix composites containing AlN volume fractions between 0.1 and 0.5 were fabricated firstly by liquid phase sintering of AlN using Y₂O₃ as a sintering aid and then by hot pressing the powder mixtures of sintered AlN and Cu at 1050°C with a pressure of 40 MPa under flowing nitrogen. With Y₂O₃ additions of 1.5 to 10 wt%, the densification of AlN could be achieved by liquid phase sintering at 1900°C for 3 h and subsequently slow cooling. The sintered AlN showed a maximum thermal conductivity of 166 W/m/K at a Y₂O₃ level of 6 wt%. Dense AlN-Cu composites with AlN contents up to 40 vol% were achieved by hot pressing. The thermal conductivity and the coefficient of the thermal expansion (CTE) of the composites decreased with increasing AlN volume fractions, giving typical values of 235 W/m/K and 12.6 × 10^–6/K at an AlN content of 40 vol%.     

Influence of the additives and processing conditions on the characteristics of dense SnO2-based ceramics

Three different SnO₂-based powder mixtures, containing 2 wt% CuO as sintering aid and Sb₂O₃ in amounts from 0 to 4 wt% as activator of the electrical conductivity, were sintered to high density at temperatures in the range 1000–1400°C and soaking times from 1 to 6 h. Densification behaviour and microstructure development are strongly dependent on the presence of CuO, that gives rise to a liquid phase, and on Sb₂O₃ that retards the liquid phase formation and hinders grain growth. Cu and Sb cations can enter s.s. in the SnO₂ network with different oxidation states and in different positions, depending on the sintering conditions. The characteristics of the grain boundary phase, of the SnO₂ solid solutions and their modification depending on thermal treatments were analyzed. The electrical resistivity values varied in a wide range from 10^–1 to 10⁴ cm, depending on starting composition and processing conditions: in terms of the final density and of the electrical conductivity, the optimal sintering conditions were found to be 1200°C, for 1–3 h. The electrical resistivity was related to the microstructural features, particularly to the characteristics of the resulting SnO₂-based solid solutions.     

Effect of ZrSiO2 and MgO additions on reaction sintering and properties of Al2TiO5-based materials

Two sets of Al₂TiO₅-based composites were prepared by reaction sintering of (a) Al₂O₃/TiO₂/ZrSiO₄ and (b)Al₂O₃/TiO₂/ZrSiO₄/MgO powder mixtures. The influence of the variation of ZrSiO₄ content (0 to 10wt%) and the addition of 2 wt% MgO on the reaction-sintering process, microstructure, mechanical and thermal properties, were evaluated. ZrSiO₄ addition shifted the Al₂TiO₅ formation to higher temperatures, whereas MgO accelerated both Al₂TiO₅ formation and ZrSiO₄ decomposition. The presence of ZrSiO₄ and an excess of Al₂O₃ generated a dispersion of ZrO₂ and mullite particles in the grain boundaries and enhanced simultaneously the densification process. After sintering in the temperature range 1350 to 1500 ° C, the obtained composites exhibited significantly higher bending strength than the monophasic aluminium titanate (up to 80 M Pa). Al₂TiO₅ (80wt%)-mullite-ZrO₂ composites which combined good mechanical strength (55MPa), low thermal expansion (_20–1000C < 1 × 10^–6 K^–1) and excellent thermal stability were obtained by reaction and sintering of powder mixtures containing both ZrSiO₄ and MgO.     

The study of AlN ceramics sintered at superhigh pressure with belt-type press technology

The effects of Y₂O₃ content, sintering time, sintering temperature, sintering pressure on thermal conductivity of AlN ceramics had been studied. X-ray diffraction (XRD), scanning electron microscope (SEM), laser conductometer and laser granularity dimension analysis measurer were respectively used to measure the phases, microstructure, thermal conductivity and particle size distribution of the samples. These studies reveal that the Y₂O₃ is an effective sintering addtive, and the best conditions of sintering are that the pressure is 5.15× 10⁹ Pa, the temperature is 1700∘C and the sintering time is 115 min. Under these conditions, the sintered body has reasonable structure and its thermal conductivity is 200 w/(m⋅k).     

Sintering studies on submicrometre-sized Y-Ba-Cu-oxide powder

The isothermal sintering behaviour of submicrometre-sized (<50 nm) powders of single-phase YBa₂Cu₃O _x (123) and unreacted stoichiometric mixture of submicrometre-sized (<50 nm) powders of BaCO₃, Y₂O₃ and CuO (which on calcination at 1173 K gives YBa₂Cu₃O _x ) was investigated through dilatometry under different sintering atmospheres. The sintering rate of the powder compacts was impeded by the presence of oxygen. The activation energies,Q, of sintering were determined to be 1218 kJ mol^–1 in argon, 1593 kJ mor^–1 in air and 2142 kJ mol^–1 in oxygen. A decrease in the apparent sintered density with increasing oxygen partial pressure was also observed. X-ray diffraction and thermal analyses (thermogravimetry and differential thermal analysis) showed no reaction during sintering of the single-phase product. Pellets fabricated from uncalcined powder exhibit two stages of sintering, one between 1073 and 1173 K having an activation energyQ=627kJ mol^–1, and a second one above 1173 K withQ=383.7 kJ mol^–1. A.c. susceptibility, resistivity and critical current density were determined as a function of the temperature of the sintered samples.     

Thermal properties of MoSi2 and SiC whisker-reinforced MoSi2

The heat capacity, thermal conductivity and coefficient of thermal expansion of MoSi₂ and 18 vol % SiC whisker-reinforced MoSi₂ were investigated as a function of temperature. The materials were prepared by hot isostatic pressing between 1650 and 1700 °C, the hold time at temperature being 4 h. The heat capacity of MoSi₂ showed an increase from about 0.44 Wsg^–11K^–1 at room temperature to 0.53 at 700 °C. Whisker reinforcement increased heat capacity by about 10%. Thermal conductivity exhibited a decreasing trend from 0.63 Wcm^–1 K^–1 at room temperature to 0.28 Wem^–1 K^–1 at 1400°C. Whiskers reduced conductivity by about 10%. The thermal expansion coefficient increased from 7.42 °C^–1 between room temperature and 200 °C to 9.13 °C^–1 between room temperature and 1200 °C. There was a 10% decrease resulting from the whiskers. The measured data are compared with literature values. The trends in the data and their potential implications for high-temperature aerospace applications of MoSi₂ are discussed.     

Effect of TiO2 addition on the sintering behavior, hardness and fracture toughness of an ultrafine alumina

The effect of TiO₂ addition (0–4.0 wt%) on the sintering behavior and mechanical properties of an ultrafine (150 nm) alumina–5 wt% zirconia powder has been investigated. TiO₂ is a beneficial additive, resulting in lower sintering temperature and higher sintered density. The grain growth is shown to be enhanced simultaneously after TiO₂ addition. At higher amounts of TiO₂ addition and at higher sintering temperature, the formation of secondary phases of ZrTiO₄ or Al₂TiO₅, which has lower elastic moduli compared with alumina, reduces the hardness of the sintered bulk from 19.3 to 17.9 GPa. The lager grain size and the transition of fracture mode to fully intergranular due to the TiO₂ addition seem to improve the toughness from 4.4 to 5.2 MPa m^1/2.     

Microstructure and properties of spark plasma sintered AlN ceramics

Spark plasma sintering (SPS) is a newly developed technique that enables poorly sinterable aluminum nitride (AlN) powder to be fully densified. It is addressed that pure AlN sintered by SPS has relatively low thermal conductivity. In this work, SPS of AlN ceramic was carried out with Y₂O₃, Sm₂O₃ and Li₂O as sintering aids. Effects of additives on AlN densification, microstructure and properties were investigated. Addition of sintering aids accelerated the densification, lowered AlN sintering temperature and was advantageous to improve properties of AlN ceramic. Thermal conductivity and strength were found to be greatly improved with the present of Sm₂O₃ as sintering additive, with a thermal conductivity value about 131 Wm⁻¹K⁻¹ and bending strength about 330 MPa for the 2 wt% Sm₂O₃-doped AlN sample SPS at 1,780 °C for 5 min. XRD measurement revealed that additives had no obvious effect on the AlN lattice parameters. Observation by SEM showed that AlN ceramics prepared by SPS method manifested quite homogeneous microstructure. However, AlN grain sizes and shapes, location of secondary phases varied with the additives. The thermal conductivity of AlN ceramics was mainly affected by the additives through their effects on the growth of AlN grain and the location of liquid phases.     

The solid state bonding of nickel to alumina

The paper describes the effect of sintering atmosphere (argon, hydrogen and vacuum), sintering temperatures (700 to 1300° C), sintering pressure (1/2 to 6 tsi [7.7 to 92.4 MN. m^–2]) and sintering time (1/2 to 24 h) on the room temperature shear bond strength developed between nickel powder compacts and alumina single crystals: bond strengths of 3 to 11×10³ psi (20.7 to 75.9 MN. m^–2) were developed and are satisfactory for composite strengthening. The spinel, NiAl₂O₄ was detected at the nickel/alumina interface. Heat treatment at 1100° C for 300 h resulted in gross chemical attack, but without degeneration of the bond strength. The variations in shear strength observed are discussed in terms of the nickel grain size and porosity and the differential thermal expansion of the two components.     

Fabrication of cellular structure composite material from recycled soda-lime glass and phlogopite mica powders

A homogeneous composite material with different physical structures has been fabricated from recycled colourless soda-lime glass powders and phlogopite-type mica powders by mixing the two powder components and sintering the mixture at a temperature above 850° C for a period of 30 min or longer. The physical structure of the composite material can be fabricated into either a cellular structure consisting of both closed and open cells or a highly densified ceramic body. The cellular structure composite material is found to have a compressive strength of about 0.877 MN m^–2 and thermal conductivity values in the range of 0.290 to 0.306 W m^–1 °C^–1 when measured at temperatures in the range of 25 to 100° C. The highly densified composite material, on the other hand, is found to have a compressive strength of about 53.0 MN m^–2 and thermal conductivity values in the range of 0.198 to 0.250 W m^–1 °C^–1. The composite material, when compared with other common building materials, is found to be potential material for construction applications because of its superior mechanical and thermal properties.     

Potential use of only Yb2O3 in producing dense Si3N4 ceramics with high thermal conductivity by gas pressure sintering

Yb₂O₃ is an efficient sintering additive for enhancing not only thermal conductivity but also the high-temperature mechanical properties of Si₃N₄ ceramics. Here we report the fabrication of dense Si₃N₄ ceramics with high thermal conductivity by the gas pressure sintering of α-Si₃N₄ powder compacts, using only Yb₂O₃ as an additive, at 1900 °C under a nitrogen pressure of 1 MPa. The effects of Yb₂O₃ content, sample packing condition and sintering time on the densification, microstructure and thermal conductivity were investigated. Curves of the density plotted against the Yb₂O₃ content exhibited a characteristic ‘N’ shape with a local minimum at 3 mol% Yb₂O₃ and nearly complete densification below and above this concentration. The effects of the sample packing condition on the densification, microstructure and thermal conductivity strongly depended on the Yb₂O₃ content. The embedded condition led to more complete densification but also to a decrease in thermal conductivity from 119 to 94 W m^-1 K⁻¹ upon 1 mol% Yb₂O₃ addition. The sample packing condition had little effect on the density and thermal conductivity (102–106 W m⁻¹ K⁻¹) at 7 mol% Yb₂O₃. The thermal conductivity value was strongly related to the microstructure.     

Fabrication and characterization of YBa2Cu3O7−x /Sn composite superconductors

YBa₂Cu₃O_7–x /Sn composites containing 5, 10, 15, and 20 wt% Sn were prepared by conventional powder metallurgy and pressureless sintering at 950 °C. Sn caused degradation of the ceramic superconductor by its chemical reaction with Ba, whose extent increased with Sn content. Composites with between 5 and 10 wt% Sn preserved their superconducting critical temperature,T _c, those with higher Sn content exhibited non-zero residual resistivity at low temperatures. Room-temperature resistivity and its thermal coefficient also increased with Sn content. The brittle behaviour of the composites is attributed to oxidation of Sn during sintering.     

Crystal,magnetic and electric behaviour of CoMnxFe2−xO4 cubic ferrites

Six powder samples of CoMn _x Fe_2–x O₄ (O x 1) were synthesized by using a ceramic sintering technique. X-ray powder diffraction patterns were obtained and confirmed the presence of single-phase spinel structure with no evidence of impurities. Lattice constants were determined. Differential thermal analysis measurements showed no variation in crystal phase with temperature. AC conductivity measurements at the temperature range (300–950 K) and for the frequency range (10²–10⁵ Hz) were also carried out and the Néel transition temperatures for all samples were determined. Mössbauer effect patterns revealed magnetic ordering for all compositions at room temperature. The obtained spectra were successfully analysed into two Zeeman sextets, could be attributed to the tetrahedral and octahedral sites. The different Mössbauer effect parameters were deduced and discussed. Neutron diffraction measurements were also performed where oxygen parameters and cation distributions were determined, and magnetic structure were studied. All obtained results from the different techniques support the Néel model of ferrimagnetism for such compounds.     

Thermal and dielectric properties of zirconyl phosphate compact

Experimental results are presented on the measurements of thermal expansion (up to 1500°C), thermal conductivity (up to 1000°C), dielectric constant (up to 450 °C) and tan (up to 800 °C) of zirconyl phosphate compacts obtained by sintering at 1600°C. The thermal expansion coefficient of the samples at the temperature below 1100°C was less than 1.7 × 10^–6°C^–1. The samples showed a definite shrinkage at temperatures of 1110 and 1470°C due to the phase transformations. The expansion at 1500°C was less than that at 1100°C probably because of the phase transformation. The thermal conductivity at room temperature was a very small value (0.0046 to 0.0065 cal s^–1 cm°C^–1 cm^–2). The dielectric constant was close to 9. The value of tan° (–0.0001) measured is one of the lowest values for ceramic materials.     

Composite adsorbent of CaCl2 and expanded graphite for adsorption ice maker on fishing boats

Adsorption performances and thermal conductivity were tested for three types of adsorbent: Pure CaCl₂ powder, simple composite adsorbent and consolidated composite adsorbent. The simple composite adsorbents show better adsorption performance because the additive of expanded graphite in CaCl₂ powder has restrained the agglomeration phenomenon in adsorption process and improved the adsorption performance of CaCl₂. The consolidated composite adsorbent are suitable to be used as adsorbent for ice maker on fishing boats because they have higher thermal conductivity, larger volumetric cooling capacity, higher SCP values and better anti-sway performance than simple composite adsorbents. Thermal conductivity of the consolidated composite adsorbent is 6.5–9.8 W m⁻¹ K⁻¹ depending on the molding pressure, ranging from 5 to 15 MPa, which is about 32 times higher than the thermal conductivity of CaCl₂ powder. The volumetric cooling capacity of consolidated composite adsorbent is about 52% higher than the best result obtained for CaCl₂ at the evaporating temperature of −10 °C. The SCP of the consolidated adsorbent increases of about 353% than CaCl₂ powder from simulation results at T_ad=30 °C and T_ev=−10 °C. The consolidated composite adsorbents have good anti-sway performance and they are not easy to be scattered out when the fishing boats sway on the sea.     

Phase formation, sintering behavior, and electrical characteristics of NASICON compounds

The dependence of phase formation, sintering behavior, and electrical characteristics of Sodium Superionic Conductor (NASICON) compounds on sintering temperature, time, and cooling process was investigated. In the von Alpen-type composition Na_3.2Zr_1.3Si_2.2P_0.8O_10.5, ZrO₂ second phase is in thermal equilibrium with crystalline NASICON and liquid phase above 1320°C, and when cooled through 1260–1320°C, the crystalline NASICON was formed by reaction between the ZrO₂ second phase and the liquid phase. Maximum relative densities of 96 and 91% were obtained for compositions Na₃Zr₂Si₂PO₁₂ and Na_3.2Zr_1.3Si_2.2P_0.8O_10.5, respectively. For these compositions, the maximum ionic conductivity and the minimum migration barrier height were 0.45 ohm^–1cm^–1 and 0.07 eV, respectively. The migration barrier height of the high temperature form (space group: R3c) is about 30–40% of that of the low temperature form (space group: C2/c). Ionic conductivity increases with increasing sinterability, and a considerably large amount of glass phase in Na_3.2Zr_1.3Si_2.2P_0.8O_10.5 ceramics significantly lowers ionic conductivity above the transition temperature.     

High thermal expansion KAlSiO4 ceramic

Orthorhombic kalsilite (KAlSiO₄) was prepared by solid-state reaction from K₂CO₃, Al₂O₃, and SiO₂. The axial thermal expansion coefficients of the orthorhombic kalsilite were 1.6×10^–5°C^–1 for the a-axis, 1.6×10^–5°C^–1 for the b-axis, 2.8×10^–5°C^–1 for the c-axis, and 2.0×10^–5°C^–1 for the average from room temperature to 1000°C. A high thermal expansion ceramic consisting of the orthorhombic kalsilite was prepared by sintering. The densification was promoted by adding Li₂CO₃. The KAlSiO₄ ceramic sintered at 1200°C for 2 h with 5 wt% Li₂CO₃ had a bending strength of 65 MPa and linear thermal expansion coefficient of 2.2×10^–5 °C^–1 from room temperature to 600°C.     

Preparation and properties of ALCON (Al28C6O21N6) ceramics

The synthesis of Al₂₈C₆O₂₁N₆ powder (ALCON), starting from the binary compounds is described. The powder is resistant to oxidation in air up to 760°C. From the prepared powder, fully dense ceramics have successfully been prepared using hot pressing. The as-prepared ceramics had a thermal conductivity of 20 W m^–1 K^–1. Experiments showed that it is also possible to prepare ALCON ceramics by reactive hot-pressing, starting from Al₂O₃, AlN and Al₄C₃. Further optimization is expected to raise the thermal conductivity significantly. The strength, about 300 MPa, is similar to that of AlN. The thermal expansion coefficient of 4.8 × 10^–6K^–1 closely matches that of silicon, making application of ALCON ceramics as heat sinks an interesting possibility.