T. Baykara

Effective nonlinear GaSe crystal. Optical properties and applications

We present an overview of the current state of the literature and research performed by the authors of the present paper on the experimental and theoretical results on the structural-, optical-, nonlinear optical (NLO)-properties (including two-photon absorption (TPA) and the terahertz (THz) range of spectra) and practical applications of a highly anisotropic Gallium Selenide (GaSe) semiconductor with emphasis on the ɛ-GaSe. Physical properties of ɛ-GaSe are important to researchers and designers developing different devices by using this material. This crystal possesses an outstanding NLO properties: high optical birefringence Δn ∼ 0.3 at 700 nm; high transparency range (0.7−18.0 μm) with low absorption coefficient (α ≤ 0.3 cm−1); very high nonlinear susceptibility χ(2) (d22 ≈ 86 ± 17 pm/V, corresponding to (2.0 ± 0.4) × 10−7 esu) that is used for phase matched second harmonic generation (SHG) in a wide transparency range; high power threshold for optical damage; possibility to perform optical frequency conversion under phase-matching conditions in the near- to mid-IR and THz range of spectra, etc. The domain structure of crystal in connection with the NLO properties is discussed as studied by confocal Raman microscopy experiments. Perspectives for future research of GaSe are considered in the present article, which does not pretend to be one reflecting all existing papers on GaSe crystal and discussed subjects.

The effect of TiO2 and MnO2 on densification and microstructural development of alumina

Abstract The effect of MnO 2 and TiO 2 as additives during the densification and microstructural development of Al 2 O 3 has been investigated between the temperature range of 1250 and 1550 °C. Sintered density, average grain size, microhardness measurements and microstructural features were evaluated to correlate densification with grain growth and second phase formation. Each additive both singly and in combination, was found to increase the final sintered density and microhardness values of the samples. 3 wt% MnO 2 when added with 0.5–3 wt% TiO 2 lowers the sintering temperature to 1250 °C. Excessive grain growth and draining of second phase were observed in these samples that were sintered above 1350 °C.

Sintering behaviour of sepiolite

Abstract Sepiolite containing minerals from various regions of Turkey were processed to investigate their sintering behaviour over a wide temperature range. The sintering was studied by mercury porosimetry and nitrogen adsorption (BET). Microstructural features were investigated using SEM and TEM techniques. Physico-chemical and microstructural characterization of the sintered ceramic bodies demonstrated unusual pore structure (mean pore size of 0.02 micron within the range of 0.015–1.0 micron) retained even at temperatures as high as 1000 °C. A low linear shrinkage of 7.28% at 1100 °C indicates the potential promising applications in the field of filtering, molecular sieving and adsorption at higher temperatures.

Effect of additives on the densification and microstructural development of low-grade alumina powders

Abstract The densification and microstructural evolution of a metallurgical grade alumina which is produced as an electrofilter residue during the Bayer process was investigated using a variety of additives. TiO2, MnO2, SiO2 and MgO were used as sintering aids to enhance densification and microstructural development during sintering. A colloidal technique was applied during the processing for the homogeneous mixing of alumina powders and additives. The treatment of metallurgical low-grade alumina powders through a series of calcination, hot washing and recalcination is shown to improve the powder quality, resulting in microstructures with densities of 90–96.7% of the theoretical density with the sintering aids. It is found that the addition of 1% B2O3 improves the compactibility of particles, to yield higher green densities. An additive with the composition 1%TiO2-1.5%MnO2-2%SiO2-0.5%MgO results in a homogeneously-distributed finer grain size. Strength values in the 180–380 MPa range were obtained.

Microstructural and mechanical properties of injection moulded gas and water atomised 17-4 PH stainless steel powder

Abstract This paper describes the microstructural and mechanical properties of injection moulded 17-4 PH stainless steel gas and water atomised powder. Gas and water atomised stainless steel powders were injection moulded with wax based binder. The critical powder loading for injection moulding were 62·5 and 55 vol.-% for gas and water atomised powders respectively. Binder debinding was performed using solvent and thermal method. After dedinding the samples were sintered at different temperatures for 1 h in pure H2. Metallographic studies were conducted to determine to extend densification and the corresponding microstructural changes. The results show that gas atomised powder could be sintered to a maximum (98·7%) of theoretical density, and water atomised powder could be sintered to a maximum (97·08%) of theoretical density. Maximum tensile strength was obtained for gas atomised powder sintered at 1350°C. The tensile strength of the water atomised powder sintered at the same temperature was lower owing to higher porosity. Finally, mechanical tests show that the water atomised powder has lower mechanical properties than gas atomised powder.

The densification and microstructural development of Al2O3 with manganese oxide addition

Abstract The effect of MnO2 as an additive on the densification behaviour and microstructural evolution of α-Al2O3 has been investigated over a wide sintering regime. A colloidal technique was applied for the mixing of MnO2 in order to achieve a uniform distribution and homogeneous microstructures. Sintered density, average grain size and microstructural features were elaborated along with the microhardness and mechanical strength measurements. Manganese oxide was found to promote grain growth and an intergranular second phase formation was detected for additions over 0.5 wt% MnO2. Excessive grain growth and draining of the second phase were observed in the samples with 3 wt% MnO2 additions sintered above 1500 °C.

Densification of alumina at 1250 °C with MnO2 and TiO2 additives

Abstract The effects of MnO2 and TiO2 additions in 3 wt% MnO2 + (0.5, 1.5, 3.0) wt% TiO2 additive combinations on the sintering of α-Al2O3 have been investigated. The microstructural development along with the microhardness has been characterized. The Al2O3 + 3 wt% MnO2 + 0.5 wt% TiO2 system resulted in high densification (98% of the theoretical density) giving a uniform grained microstructure and yielding a hardness value of 23.14 GPa at a sintering temperature as low as 1250 °C.

Surface hardening of tungsten heavy alloys

Tungsten-based heavy alloys are produced by sintering from elemental W, Ni, Fe and Cu powder mixtures, where the tungsten content is in the range 80--98 weight%. The alloys, however, show relatively poor surface properties such as wear and corrosion resistance. In the present study, a relatively simple method was used for modification of heavy alloy surfaces. The method involved conversion of W to WC in a flowing CH{sub 4} + Ar gas atmosphere without the aid of plasma. Composition, microstructure and hardness of the treated surfaces of the alloy were studied.

Mechanical Properties of Powder Injection Molded Ni-Based Superalloys

This paper describes the microstructural and mechanical properties of powder injection moulded (PIM) Ni based superalloys. Ni-based superalloys were mixed with a polymeric binder (paraffin wax, carnauba wax, stearic acid and PP) and injection moulded as standard tensile bars. The critical powder loading for injection moulding were 62.5 vol. % for standard samples. Binder removal, debinding, was performed using a two-step solvent/thermal method. After debinding the samples were sintered at 1100°C and at 1285°C under high level vacuum. Metallographic studies were conducted to reveal the extent of densification and the corresponding microstructural changes. Ni 718, Ni 625 and Nimonic90 samples achieved high sintered densities via supersolidus liquid phase sintering. The mechanical properties of Ni 718, Ni 625 and Nimonic90 are determined and compared with the properties of previously reported studies on similar PIM materials.

A new ceramic thread-guide composition via low-pressure injection molding

Abstract Alumina based ceramic—ceramic composite thread-guide samples were fabricated using a low-pressure injection-molding technique. Major processing steps in this method are introduced, and binder development and binder removal studies of a specially formulated multi-component binder composition are described. It is illustrated that the wicking process is beneficial to shorten the processing time. Microstructural features after debinding, the sintering stage and application in field testing are outlined. The sample hardness was determined to be lower than that of pure alumina-based thread guides, allowing less wear damage to fibers. The results indicate that low-pressure injection molding can be employed as a potential forming technique for the fabrication of ceramic thread-guide components used in the textile industry.