Hot-press Sintering Technique Research Articles

Obtention and characterizationof modified Bi2 Sr Nb2 O9 aurivillius-typeceramics

Abstract Aurivillius ferroelectric ceramics, based on the Bi2Sr Nb2O9 compound, have been developed by making isomorphic substitutions in the (Bi2O2)2+ layer. High density ceramics were obtained by the uniaxial hot-pressing sintering technique. The macroscopic characterization was carried out by measuring both the dielectric constant, in the frequency range from 100Hz to 15MHz at temperatures between 20°C and 850°C, and piezoelectric parameters. We found that increasing the amount of cation Bi3+ substitution results in higher transition temperature values. Samples showed a high electrical resistivity that made poling difficult. Anyway, we found an important piezoelectric activity in the samples with tellurium present in their composition, with d33 greater than 11pC/N.

Life prediction methods for low-cycle torsional fatigue: Curioni, S., Freddi, A. and Caligiana, G. Proc. Conf. Testing of Metals for Structures, Naples, Italy, 29–31 May 1990, pp 162–173

Functionally graded WC-TiC-Co cemented carbides with a Co content gradient and not comprising the η-phase were prepared employing hot-pressing sintering technique by introducing pre-designed both gradients of Co contents and WC grain sizes. The effects of initial hard phase particle size distribution and sintering parameters on the microstructure and mechanical properties of functionally graded cemented carbides were investigated by fabricating coarse-particle, fine-particle and hybrid-particle (combining coarse-particle and fine-particle) functionally graded cemented carbides employing hot-pressing sintering technique. Various hot sintering temperatures (1325 ℃, 1350 ℃, 1375 ℃ and 1400 ℃) and soaking times (15 min, 30 min, 45 min, 60 min) were tested in order to determine the optimal parameters of hybrid-particle functionally graded WC-TiC-Co cemented carbides with the assistance of VC and Cr3C2 as grain growth inhibitors and PVP as dispersant. The experimental results showed that excellent mechanical properties are achieved for sintering at 1375 ℃ soaked for 45 min with a hardness of 1986 kgf/mm2, a TRS of 1752 MPa and a fracture toughness of 13.05 MPa m1/2. Hybrid-particle functionally graded WC-TiC-Co cemented carbides generated a good liquid-phase sintering at 1375 ℃ soaked for 45 min. Specimens sintering for 1325 ℃ and 1350 ℃ maintained the pre-designed Co gradient, whereas the specimens sintering for 1375 ℃ maintained a smaller Co gradient and the specimen sintering for 1400 ℃ had no Co gradient. The excessive grain growth of fine-grained WC resulted the WC size difference disappear of coarse-grain and fine-grain, which weaken the Co phase migration pressure from surface layer to core layer. Furthermore, Co liquid phase distribution behavior is normally related with temperature, soaking time and pressure. The five-layer structure developed in this paper provides the surface with exceptional combination of high wear resistance and high toughness, which is favorable for the application as metal cutting tools.

Thermopower of RBa2Cu3O7-x (R=Y,Er).

Resistance and absolute thermopower of high-${\mathit{T}}_{\mathit{c}}$ oxide superconductors R${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$ (R=Y,Er) synthesized by a hot-press sintering technique have been measured in the temperature range of 4--300 K. Resistive behavior of all samples is found to be metallic. The thermopower for both compositions is positive above the superconducting transition temperature. The thermopower of as-prepared ${\mathrm{ErBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$ samples shows a weak temperature dependence with a negative slope near room temperature, which increases in magnitude as ${\mathit{T}}_{\mathit{c}}$ is approached from above. No strong thermopower peak just above ${\mathit{T}}_{\mathit{c}}$ has been observed either in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$ or ${\mathrm{ErBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$. Reannealing of an ${\mathrm{ErBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$ sample at 500 \ifmmode^\circ\else\textdegree\fi{}C for 12 h in flowing oxygen increases the magnitude of the thermoelectric power and the magnitude of its slope without affecting ${\mathit{T}}_{\mathit{c}}$. Comparison of the results with available theory indicates that there is no completely satisfactory theory of electrical transport in these superconducting oxides.

Microstructural Coarsening During Sintering of Boron Carbide

The sintering behavior of boron carbide was investigated with particular attention given to microstructure development at various stages in the sintering process. Hot‐pressing and pressureless sintering techniques were employed and the effects of heating rate, firing atmosphere, and composition were used to characterize the sintering behavior. Pressureless sintering at temperatures up to 2300°C produces only limited densification. Microstructural coarsening is responsible for this since it leads to conditions where densification is slow. Hot‐pressing and carbon additions suppressed coarsening and permitted densification to >95% of theoretical density.

A1N Substrates with High Thermal Conductivity

A new aluminum nitride (AIN) substrate, which has high thermal conductivity of 160 W/mK at room temperture, has been developed using the hot press sintering technique. The new AIN substrate has the following excellent characteristics. 1) The thermal conductivity is eight times as high as that of AI 2 O 3 at room temperature and is almost equal to that of 99.5 percent BeO at 150°C. 2) The thermal expansion coefficient is smaller than that of AI 2 O 3 and BeO, and is close to that of a silicon semiconductor chip. 3) The electrical properties are almost as good as those for AI 2 O 3 and BeO in the wide frequency range. 4) It not only has higher mechanical stength but also easier machinable property than AI 2 O 3 . It is characterized by its light transparency from visible light to the infrared wavelength region. It was proved that the new AIN substrate is able to be metallized with good adhesion strength by the conventional evaporating method and the conventional sputtering method. The new AIN was found to be applicable to three kinds of semiconductor devices: 1) silicon epitaxial transistor, 2) GaAIAs light emitting diode, and 3) InGaAsP laser diode. Also, another AIN substrate was developed using the normal sintering technique, which has high thermal conductivity of 140 W/mK at room temperature.