Synthesis of oxygen-free aluminium nitride ceramics

The aluminum nitride raw material in the form of powder was synthesized using the Self propagating High temperature Synthesis (SHS) method which provides no oxygen impurities. Then AIN powder was sintered to the full density without sintering additives and under a high pressure in a belt apparatus. For the AIN ceramics obtained the temperature dependences of the thermal diffusivity were measured with the laser-flash method. Finally we produced oxygen-free aluminium nitride ceramics with parameters comparable with theoretical data.     

The wetting and spontaneous infiltration of ceramics by molten copper

Infiltration trials have been conducted by filling Cu tubes withceramic powders and melting them under argon. No externalforces were applied; successful infiltration of the ceramicrelied solely upon favourable metal-ceramic wetting conditions.Oxides and covalently bonded compounds could not bespontaneously infiltrated but transition metal compounds such asNbC, Cr₃C₂, WC, NbB₂ and Cr₂N were. It was impossible toinfiltrate any ceramics when oxygen was present in the system.Contact angle data in the literature were found to predict, withfair reliability, infiltration events in Cu-ceramic systems.The good correlation is thought to be due to the ease with whichthe oxide film can be prevented from forming on molten Cu duringboth sessile droplet experiments and infiltration processing.     

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.     

Thermal conduction mechanism of aluminium nitride ceramics

Extremely large grain size AIN ceramics were produced by HIP sintering at an ultra-high temperature of 2773 K without reducing the oxygen content in order to determine experimentally whether the factor controlling thermal conductivity is either grain boundaries or the internal structure of the grains. The room-temperature thermal conductivity of the HIPed AIN with a grain size of 40 m was 155 Wm^–1 K^–1, and was almost equal to that of the normally sintered AIN with a grain size of 4 m. Therefore, thermal conductivity at room temperature is independent of AIN grain size, or the number and amount of grain-boundary phase for reasonably well-sintered AIN ceramics. The calculated phonon mean free path of sintered bodies was 10–30 nm at room temperature, which is too small to compare with the AIN grain size. Consequently, it is shown that the thermal conductivity of sintered AIN is controlled by the internal structure of the grains, such as oxygen solute atoms.     

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.     

Reduction of sodium-accelerated oxidation of silicon nitride ceramics by aluminium implantation

Norton NBD 200 silicon nitride ceramics were implanted with sodium to a dose of 7.0×10¹⁵cm^-2 at 72 keV (1 at% peak sodium content at 100 nm). The sodium-implanted samples were further implanted with aluminium to 7.3×10¹⁵cm^-2 at 87 keV (1 at% peak aluminium content at 100 nm). The implanted and unimplanted samples were oxidized in 1 atm dry oxygen at 1100 and 1300°C for 2–6 h. Profilometry and scanning electron microscopy measurements indicated that sodium implantation led to up to a two-fold increase in the oxidation rate of silicon nitride. The sodium effect was effectively neutralized when aluminium was co-implanted. The opposite effects of sodium and aluminium on the oxidation resistance of silicon nitride can be attributed to their different roles in modifying the structure and properties of the oxide formed. This revised version was published online in November 2006 with corrections to the Cover Date.     

Elastic properties of carbide, nitride, and boride ceramics with WC-type structures

The elastic constants (C _ij ) of boride, carbide, and nitride (RuB, WC, WN, and TaN) ceramics with WC-type structures have been calculated using the full-potential linearized augmented plane wave (FLAPW) method with an exchange-correlation potential in the generalized gradient approximation (GGA). Numerical estimates of elastic parameters of the corresponding polycrystalline ceramics (bulk compression modulus, shear modulus, Young’s modulus, Poisson’s ratio, and Lamé’s coefficients) of these ceramics are obtained and analyzed for the first time.     

Phonon scattering and thermal conduction mechanisms of sintered aluminium nitride ceramics

From thermal diffusivity measurements of sintered AIN at temperatures ranging from 100 to 1000 K, the phonon mean free path of AIN was calculated in order to investigate phonon scattering mechanisms. The calculated mean phonon scattering distance was increased with decreasing temperature. The mean phonon-defect scattering distances were respectively limited to about 50 nm at temperatures ranging from 100 to 270 K and about 30 nm at temperatures ranging from 100 to 700 K, for AIN specimens with a room-temperature thermal conductivity of 220 and 121 Wm^–1 K^–1 containing 0.1 and 1.4 wt % oxygen, respectively. These short phonon-defect scattering distances were considered to correspond to the separation of oxygen-related internal defects in AIN grains. Calculation of the mean phonon scattering frequencies indicated that the phonon scattering is dominated by phonon-defect scattering at temperatures below 270 K for an AIN specimen with an oxygen content of 0.1 wt %, and at temperatures below 350 K for an AIN specimen with an oxygen content of 1.4 wt %.     

Erosion mechanisms of aluminium nitride ceramics at different impact angles

In this paper, erosion wear behaviour of aluminium nitride (AlN) ceramics is studied. The influence of particle hardness and shape on erosion of the AlN surface is examined. The effect of varying the impingement angle on the weight loss and the roughness parameters of AlN ceramics testing sample is also determined. Therefore, erosive wear behaviour of AlN ceramics was investigated using SiC and SiO₂ particles as erodents, at following impact angles: 30°, 45°, 60°, 75° and 90°. Scanning electron microscopy (SEM) was used to analyze the eroded surfaces in order to determine erosion mechanisms. The roughness parameters (R_a, R_z and R_max), before and after erosion with SiO₂ and SiC particles at 30° and 90° angles of impingement, respectively, were determined using a profilometer. It was found that the impact angle is influencing the erosion wear of the AlN ceramics and maximum erosion takes place at impact angle of 90°. The results indicate that hard, angular SiC particles cause more damage than softer, more rounded SiO₂ particles.     

Pressure-assisted infiltration of molten aluminium into open cell ceramic foams: Experimental observations and infiltration modelling

Partial infiltration of molten Al into three different open cell ceramic foams has been effected using pressure-assisted vacuum investment casting equipment. For the maximum pressure difference (only 0.25 MPa) up to 90% of the open porosity could be filled, but since the ceramic foams were found to contain closed pores, the resulting composite densities were low. A simple modelling approach accurately predicts the infiltration behaviour and enables the processing conditions and architecture of the preforms to be optimised in respect of the maximum infiltration pressure available from the processing equipment.     

High temperature compression creep of aluminium nitride after immersion in molten stainless steel

The effect of immersion in molten steel on subsequent creep behaviour has been investigated in two AlN ceramics. Steady-state creep was not significantly affected but an enhanced initial creep rate was observed, especially when the surface layer had not been removed after immersion. In immersed samples, SEM revealed some open porosity and microcracks near the surface and particles containing steel constituents were found by TEM deep inside samples, in grain-boundary secondary phases. This revised version was published online in November 2006 with corrections to the Cover Date.     

Characterization of porous silicon nitride/silicon oxynitride composite ceramics produced by sol infiltration

Porous silicon nitride/silicon oxynitride composite ceramics were fabricated by silica sol infiltration of aqueous gelcasting prefabricated Si₃N₄ green compact. Silica was introduced by infiltration to increase the green density of specimens, so suitable properties with low shrinkage of ceramics were achieved during sintering at low temperature. Si₂N₂O was formed through reaction between Si₃N₄ and silica sol at a temperature above 1550 °C. Si₃N₄/Si₂N₂O composite ceramics with a low linear shrinkage of 1.3–5.7%, a superior strength of 95–180 MPa and a moderate dielectric constant of 4.0–5.0 (at 21–39 GHz) were obtained by varying infiltration cycle and sintering temperature.     

Interfacial strength and chemistry of additive-free silicon nitride ceramics brazed with aluminium

Two kinds of additive-free silicon nitride ceramics were brazed with aluminium; one was with as-ground faying surfaces and the other was with faying surfaces heat-treated at 1073K for 1.8 ksec in air. The heat-treatment of the silicon nitride ceramics formed a silicon oxynitride layer on the faying surfaces and increased the brazing strength of the joints. A silica-alumina non-crystalline layer and a β′-sialon layer were formed successively from the aluminium side at the interface of the joints. The heat-treatment which made the former layer thicker is a necessary process in making reliable, strong brazed joints.     

Wetting of aluminium oxide by molten aluminium and other metals

The sessile drop technique has been used to measure the contact angle of molten aluminium, aluminium-nickel and aluminium-copper alloys, copper and gold, with sapphire, ruby and recrystallised alumina. Measurements were madein vacuo, and as a function of time and temperature over the range 800 to 1500° C. Cinematography and time-lapse photography were used. At temperatures below 950° C, sessile drops of aluminium reached equilibrium only after a period of time which increased with decrease in temperature and could be in excess of one hour. A rapid increase in contact area occurred around 900° C. Above 1150° C drops of aluminium and of the aluminium alloys were observed to spread and contract repeatedly. Contractions were observed with both polycrystalline and single-crystal alumina, although they were much more pronounced with the latter, and were associated with the formation of a series of reaction rings on the plaque. Ruby and sapphire behaved similarly. The shape of the rings depended on the crystallographic orientation of the plaque: the reaction profile tended to terminate in certain low index directions. Neither contractions nor reaction was observed with copper or gold. The observations are discussed in terms of the combined effects of evaporation, chemical reactivity and interfacial tensions in the system.     

Wettability of aluminium nitride by tin–aluminium melts

The wettability of aluminium nitride by Sn–Al melts was studied by the sessile drop method in a vaccum of 2 × 10^–3 Pa at 1100 °C over the whole concentration region. The minimum interval on the contact-angle concentration dependence curve was observed at intermediate composition. For comparison, experiments were also performed on porous AlN. Wetting of porous nitride is worse than the dense nitride. The results have been analysed on the basis of the relation between wettability and the chemical interface reactivity in solid–liquid metal systems.     

Synthesis of cubic niobium nitride by reactive diffusion under nitrogen pressure

The synthesis of niobium nitride by reactive diffusion in a furnace at 1395-1475 °C and under nitrogen pressure in the range 2-25 MPa was investigated. In experiments, we used compacted Nb powder with a mean particle size of 43 μm. Phase transformations in the product as studied by electron probe microanalysis (EPMA) were found to proceed in the following order: Nb → α-Nb(N) → β-Nb₂N_1±x → γ-Nb₄N_3±x → δ-NbN_1±x. The size of niobium particles which could react with nitrogen to yield cubic niobium nitride was estimated (SEM analysis) from the dependence of the thickness Δ of the δ-NbN_1±x outer layer formed on the surface of Nb particles on the dwell time t_dw at 1460-1473 °C. It was shown that Δ grew nearly proportional to t_dw. At t_dw = 30 min and P(N₂) = 2 MPa, Δ was found to attain a value of about 15.5 μm. Prolonged heating (t_dw ≈ 60 min) was found to result in decomposition of the single-phase cubic niobium nitride into a two-phase (multiphase) product. This was confirmed by XRD data and magnetic measurements which showed the occurrence of two different critical temperatures T_c in the same sample. The maximum critical temperature T_c was found to attain a value of 15.6 K.