Volume 105, Issue 5

Ceramists are constantly looking for materials to be used as dielectric resonators in the telecommunication industry. These applications require materials with ∊ r ∼ 4 – 120, Q × f 0 > 10 000 GHz and τ f ∼ 0 ppm K −1 . Additionally, efforts are also underway to lower the sintering temperatures (e. g. ≤ 800 °C) to reduce processing and electrode costs. The simultaneous achievement of all the three properties mentioned above is difficult; nevertheless, some materials have been synthesized fulfilling the criteria for microwave applications. This study is an overview of various studies on materials for possible applications as microwave dielectrics and factors affecting their microwave properties. These factors include crystal structure, defects, fabrication route, and the type and concentration of substituents and additives.

This study is devoted to the investigation of the ability of Ti 48 Ni 52 alloys for production of effective work during thermal cycling over a narrow temperature range. Effective work output and efficiency were studied in Ti 48 Ni 52 alloy subjected to thermal cycling through the temperature range of the B2 ↔ R martensitic transformation under different stress regimes. The optimal stress regimes for production of effective work output and maximum efficiency were found. The data obtained have shown that under optimal stress conditions (cooling of the sample under a stress of 100 MPa and heating under a stress of 400 MPa), one may develop a device with effective work being equal to 1.5 MJ m −3 and an efficiency of 2.5 %. During repeated thermal cycling under the optimal stress regime the alloy accumulated 8 % irreversible strain per 30 cycles. Starting from the 20 th cycle the rate of irreversible strain accumulation was equal to ∼ 0.06 % per cycle.

Unidirectionally solidified porous Cu-24 wt.% Mn alloy is prepared by the Gasar process, using mold casting equipment under various conditions. The experimental process demonstrates the feasibility of fabricating a Gasar structure for alloys with a wide solidification temperature range, as well as a columnar dendritic structure. The effects of melt temperature, initial mold temperature, and partial gas pressures on the pore structure and microstructure of an alloy matrix were investigated. The growth and interruption of directional pores were analyzed based on the solidification mode to determine whether the fine-grained region atop directional pores stunted the growth of Gasar pores.

Direct chill (DC) casting is currently the most common casting practice employed in industry for Al alloys. In this study, an intermediate frequency electromagnetic (IFEM) field was applied during DC casting of Al-8 wt.%Si alloy. The results show that compared to conventional casting, the alloys processed under IFEM field exhibit superior refinement in the microstructure as well as enhancement of the mechanical properties. In addition, these improvements become more pronounced when the current is increased. The role of IFEM field in inducing forced convection, and the subsequent phenomena, such as achievement of a uniform temperature field, fragmentation of the developed dendrites and the promotion of their equiaxed growth during solidification, could be confirmed by experimental observations.

For the first time, the present study investigates the preparation of Al/Mg bimetal using Ni-based conversion plating and Ni–P electroless plating by means of diffusion bonding. In this study, AZ31 alloy was coated with an Ni-based interlayer by conversion plating and an Ni–P interlayer by electroless plating; the Al/Mg bimetallic materials were prepared by diffusion bonding in an air atmosphere. For conversion plating, the thickness of the Ni-based layer is several hundred nanometers. When heat-treated at 450 °C for 90 min, most of the interfaces between Al and AZ31 alloy bond together well. For electroless plating, the thickness of the Ni – P layer is about 10 μm for a plating time of 60 min. When heat-treated at 450 °C for 150 min, good bonding between pure Al and AZ31 can be obtained.

Nanocrystalline merwinite was successfully synthesized by mechanical activation of a mixture of magnesium hydroxide, calcium oxide and silica gel powders followed by heat treatment. The phase development and various physical properties were investigated as a function of milling time using X-ray diffraction, scanning electron microscopy, simultaneous thermal analysis, particle size analysis and transmission electron microscopy. The results showed that formation of merwinite in annealed powders was enhanced by increasing the milling time. Nanocrystalline merwinite powder with an average crystallite size of about 36 nm was obtained after 30 hours of ball milling and subsequent annealing in 900 °C for 1 hour.

In this work, powder metallurgy was performed for preparation of iron-based soft magnetic composites (SMCs). For this purpose, iron powders were coated with a thin alumina insulating layer using the sol–gel process, and then compacted for characterization. Microstructural analysis confirmed the formation of an alumina insulating layer with high thermal stability and uniform coverage on the powder surfaces. The alumina coating increased the magnetic permeability of the composites at high frequencies as a result of increased electrical resistivity and decreased eddy current loss. In addition, the different components of the total loss factor were calculated by means of a loss separation method. It was found that by addition of 8 wt.% insulation, the hysteresis loss coefficient increased from 2.7 × 10 −3 to 4 × 10 −3 while the eddy current loss coefficient decreased from 3.00 × 10 −6 to 1.53 × 10 −6 as compared with pure iron compacts.

A Pt/BST/NiFe/Cu multilayered capacitor was fabricated incorporating a polycrystalline Ba 0.5 Sr 0.5 TiO 3 (BST) film deposited using the pulsed laser deposition technique. Qualitative X-ray diffraction analysis confirmed a perovskite structure for the deposited BST dielectric films which were fired at various temperatures. No intermediate phase was discernable with a post-annealing temperature of 750 °C and highly crystallized thin film was obtained at a post-annealing temperature of 800 °C. The fabricated capacitor with a BST film thickness of 665 nm exhibited respectable electrical performance with a dielectric constant, k of 657, and a dielectric loss, tan δ = 0.0137 at room temperature at an applied frequency of 1 MHz. The recorded charge storage density and leakage current density were 4.6 μC cm −2 and 33 nA cm −2 , respectively, with ± 5 V bias.

The slurry erosion resistance of the high strength steel DP980 was investigated using a jet erosion tester. The cumulative volume loss of DP980 samples was about 32.6 mm 3 after 30 min erosion in a slurry containing 5.68 wt.% sand particles with an impact velocity of 13.01 m s −1 at an impact angle of 30°. With water quenching directly from austenite, the erosion resistance of DP980 samples was improved to about 2.0 times that of common low-carbon steel ASTM A283C and was also better than that of AISI 304 under identical test conditions. Heat treatment was very useful to improve the erosion resistance of the DP980 samples at the high impact velocity, but was adverse at the low impact velocity. Martensite islands in the DP steels could resist the microcutting and microploughing better than the base which improves the wear resistance, especially at high impact velocity.

In this research, nanocomposite scaffolds were prepared from gelatin/chitosan and hydroxyapatite via a biomimetic method. The prepared scaffolds were then evaluated by means of commonly used bulk techniques: X-ray diffraction, scanning electron microscopy and Fourier transform infra-red spectroscopy. In this study, the gelatin–chitosan/apatite nanocomposite scaffolds were fabricated using the applied biomimetic method accompanied by freeze drying. The apatite was formed using double diffusion within the gelatin/chitosan hydrogel at similar pH and temperature to the human body. The obtained nanocomposites were soaked in glutaraldehyde solution in order to enhance their mechanical properties and also make them insoluble in water. Diffusion of calcium and phosphate ions from lateral sides into the hydrogel middle caused formation of a white layered precipitate. Analysis of precipitates formed within the hydrogel middle for the samples showed that detected materials are composed of carbonated hydroxyapatite and amorphous dicalcium phosphate dihydrate. After incubation in simulated body fluid, the amorphous phase converted to crystalline hydroxyapatite, which process is similar to bone formation in the human body. Mechanical properties obtained for the scaffolds were comparable to spongy bone. Also in this work, MTT assay and alkaline phosphatase activity on the prepared scaffolds were evaluated and discussed. Finally, the obtained experimental results ascertained that the nanocomposite prepared with chitosan/gelatin (20/80) had appropriate properties for bone tissue engineering scaffold.

This study investigates the effects of rolling rate on the microstructure and mechanical properties of magnesium alloy sheets. AZ31 plates were rolled at speeds of 5, 10, 20 and 30 r · min −1 at room temperature. Optical microscopy, micro-hardness testing, and X-ray diffraction were carried out to characterize the material microstructure. Results show that the number of twin grains increases as the rolling rate increases. Simultaneously, the micro-hardness of the samples decreases, and the (0 001) basal texture is weakened. These results suggest the use of high rolling speed to practically reduce the formation of unfavorable basal texture in manufacturing magnesium alloy sheets.

The effects of post-weld heat treatment at a solution temperature of 540 °C for 5 hrs 30 mins followed by ageing at 190 °C for 6 hrs on microstructure and mechanical properties of friction stir welded dissimilar aluminium alloys A319 and AA6063 have been examined. Post-weld ageing results in improved mechanical properties. The effect of ageing on the morphology of the second phase particles of silicon, copper and iron in the stir zone has been studied. The hardness of the stir zone for both as-welded and post-weld ageing was found to be lower than that of AA6063 but higher than A319 base metal. Microstructural studies reveal that the A319 dominates the stir zone.

Ba 0.8 Sr 0.2 TiO 3 – Y 3 Fe 5 O 12 – YFeO 3 (BST-YIG-YIP) composites were prepared via the in-situ growth solid-state method. The composites were obtained by sintering green bodies composed of Ba 0.8 Sr 0.2 TiO 3 and Y 3 Fe 5 O 12 powders at 1 350 °C. The phase composition and microstructure of the composites were analyzed. The dielectric and magnetic properties of the composites were also investigated. The results show that the as-prepared BST-YIG-YIP composites have large dielectric constants, high permeabilities and high saturation magnetizations.

Zirconium-based metal–organic framework (Zr-MOF) was synthesized using a microwave-assisted modulated method in a short reaction time of 5 min. The Zr-MOF material was highly crystalline with well-defined octahedral shaped crystals, and it exhibited comparable hydrogen storage capacity to Zr-MOF of similar specific surface area synthesized using conventional methods with much longer synthesis time.