Volume 30 (2012): Issue 4 (December 2012)

The influence of densification temperature and the size of powder particles on magnetization of remanence J r, coercivity J H c of hot-densified Nd-Fe-B magnets made of MQP-A quenched ribbons was investigated. A connection between magnetic properties, parameters of production process and microstructure of the magnet was confirmed. It was found that the remanence of isotropic magnet depends on temperature of hot densification process but is practically independent of the size of powder particles. On the other hand, the temperature of hot densification process and the size of powder particles have a substantial influence on coercivity.

Ion nitriding is an operation widely used in industry to harden materials surface. Nowadays, friction welding is one of the special welding methods used for welding the same or different kinds of materials. Especially in industry, it can be necessary to use materials after having operated them with different techniques or to use materials obtained by different manufacturing techniques. Investigating the mechanical and metallurgical properties of this kind of materials can be crucial. In this study, austenitic-stainless steel was used as an experimental material. Additionally, the samples of austenitic stainless steel with a diameter of 10 mm were joined by friction welding. The samples were subjected to ion nitriding process at 550 °C for 24 and 60 h. Then, tensile, fatigue, notch-impact and hardness tests were applied to the weldless and welded parts, and metallographic examinations were carried out. It was found that chromium and iron nitrides precipitated along the grain boundaries and in the middle of the grains. Spectrum patterns revealed that the most dominant phases resulted from the formation of CrN, Fe4N and Fe3N. However, the tests revealed that high temperature and longer time of ion nitriding caused a decrease in the values of fatigue and tensile strengths as well as in the notch-impact toughness in the ion nitrided joints.

Duck eggs are one of the most versatile cooking ingredients in which residue eggshells are discarded. Raw duck eggshells were calcined at temperatures between 300 to 900 °C, for 1, 3, and 5 h. Both the raw and calcined duck eggshells were characterized by FTIR, STA, XRD, XRF, TEM, BET, a particle size analyzer, and an impedance analyzer. The proper calcination conditions are: 900 °C and 1 h, yielding calcium oxide with a purity of 99.06 % w/w. The calcium carbonate of the rhombohedral form (CaCO3) transforms completely into the calcium oxide or lime of the face centered cubic form (CaO) at 900 °C, as shown by XRD diffraction patterns. The transmission electron microscopy (TEM) images of the calcium oxide reveal a moderately good dispersion of nearly uniform particles. The calcium oxide has a white color, a spherical shape, high porosity, and narrow particles size distribution. The percentage of ceramic yield of the calcium oxide is 53.53, as measured by STA (TG-DTA-DTG). The calcium oxide has a N2 adsorption-desorption isotherm indicating the meso-porosity range. The dielectric constant and the electrical conductivity of the calcined calcium oxide are 35 and 1:0×10−6(Ω·m)−1, respectively, at the frequency of 500 Hz.

Pulsed magnetron sputtering of metal targets in the presence of reactive gas is widely used to deposit compound materials. This method is very popular but still the aim of research is to obtain more stable and efficient processes. The standard procedure of compound thin film deposition is sputtering in so called reactive mode of magnetron work — sputtering of the target surface covered with the formed compound. The authors postulate that the problem of low deposition rate of reactive compounds can be solved if the magnetron source operates in the metallic mode or near the border of metallic and transient mode. Aluminium oxide thin films were deposited using high effective reactive pulsed magnetron sputtering. The main purpose of the research was electrical characterization of metal-compound-metal structures in the wide range of frequencies and determination of deposition technique influence on the thin film properties.

In this paper the results of fatigue strength tests of ceramic joints are presented. These tests have been performed on the samples subjected to thermal and vibration fatigue as well as on the reference samples without any additional loads. The main goal of the investigation was to determine the strength of hybrid ceramics joints using tensile testing machine. The experiment enabled evaluation of fatigue effects in the mentioned joints. Geometry of test samples has been designed according to FEM simulations, performed in ANSYS FEM environment. Thermal stress as well as the stress induced by vibrations have been analyzed in the designed model. In the experiments two types of ceramics have been used — LTCC green tape DP951 (DuPont) and alumina ceramic tape. The samples have been prepared by joining two sintered ceramic beams made of different types of material. The bonds have been realized utilizing low temperature glass or a layer of LTCC green tape.

One of the methods of achieving high packaging density of passive elements on the PCB is using the capacitors embedded in multilayer PCB. Test structures consisting of embedded capacitors were fabricated using the FaradFlex® capacitive internal layers. Impedance spectroscopy and equivalent circuit modelling was used to determine their electrical properties such as the capacitance, parasitic resistance and inductance. The use of several stages of accelerated ageing allowed us to test the durability of the structures. The results showed good quality stability of the embedded elements. The spatial distribution of the capacitance of the test structures on the surface of the PCB form was tested. The influence of the process parameters during lamination on the values of embedded capacitors was revealed.

Interfacial reactions between Ti/Al/Ni/Au metallization and GaN(cap)/AlGaN/GaN heterostructures at various annealing temperatures ranging from 715 to 865 °C were studied. Electrical current-voltage (I–V) characteristics, van der Pauw Hall mobility measurements and surface topography measurement with atomic force microscopy (AFM) were performed. The ohmic metallizations were annealed at various temperatures in a rapid thermal annealing system and the annealing time of 60 seconds was kept for all samples. To study the influence of the parameters of annealing process on the properties of the 2 dimensional electron gas (2DEG) the van der Pauw Hall mobility measurement was used. Interfacial reactions between the contact metals and heterostructures were analyzed through depth profiles of secondary ion mass spectroscopy. It was observed that transition from nonlinear to linear I-V behavior occurred after the annealing at 805 °C. For the studied samples, the most promising results were obtained for the annealing temperature of 805 °C. This temperatue ensured not only low contact resistance but also made possible to preserve the 2DEG.

ZrO2-Y2O3-Al2O3 nanocrystalline powders with different grain sizes have been synthesized using a chemical coprecipitation method. Nano-powders were compacted uniaxially and densified in a vacuum hot-pressing furnace. Density, pore size distribution, grain size and composition of the composites were determined by various techniques, including BET gas absorption, field-emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD). It has been shown that the porosity, grain and pore size of the ceramics can be controlled by the initial powder size and sintering temperature. Fully densified ceramics with narrow grain size distribution in the range of 100 ∼ 500 nm could be obtained.

In this paper, crack-free bulk thermoelectric material Zn4Sb3 was prepared rapidly by high pressure and high temperature (HPHT) method. Near a single-phase Zn4Sb3 specimen was obtained using nominal stoichiometric powder mixtures, which were indexed by powder X-ray diffraction. The temperature-dependent thermoelectric properties including the Seebeck coefficient and electrical resistivity were studied. The maximum power factor of Zn4Sb3 specimen prepared by HPHT reaches 10.8 μW/(cmK2) at 637 K, which is comparable to the published data. The results show that the HPHT offers potential processing route to produce the thermoelectric material Zn4Sb3 quickly and effectively.

We present first principles calculations of the electronic, structural and optical properties of the cubic perovskite CsSrF3 by using the full potential linearized augmented plane wave (FP-LAPW) plus local orbitals method with generalized gradient approximation (GGA) in the frame work of density functional theory. The calculated lattice constant is in a good agreement with the experimental result. The electronic band structure shows that the fundamental band gap is wide and direct at Γ point. The contribution of the different bands was analyzed from the total and partial density of states curves. The charge density plots show strong ionic bonding in Cs-F, ionic and weak covalent bonding between Sr and F. The calculated optical spectra viz., the dielectric function, optical reflectivity, absorption coefficient, real part of optical conductivity, refractive index, extinction coefficient and electron energy loss, are presented for the energy range of 0–30 eV.

Ceramic materials are interesting due to their properties such as chemical and thermal stability, corrosion resistance, biocompatibility, piezoelectricity, high dielectricity. Also, nano-sized materials may have properties different from the micro scale materials. Pyrosol method is an alternative method to obtain nanoscale particles. In this study alumina particles were prepared by pyrosol method using AlCl3 (0.1 M and 0.05 M) as precursor solutions. The particles were obtained by maintaining the temperature of 400 °C in the pyrolysis furnace. Then, the powders were heat treated at 1000 °C for 2 hours. The X-ray diffraction analyses indicated that the obtained nanoparticles were identified as a mixture of a and g crystalline alumina. Scanning electron microscopy images showed that the prepared Al2O3 nanoparticles obtained from the concentration of 0.05 M had smaller dimensions than those obtained from the concentration of 0.1 M. Images of transmission electron microscopy showed spherical particles with the median diameter approximately of 150 nm, using as precursor AlCl3 solution (0.05 M).

Thin films of non-stoichiometric indium antimonide (In0.66Sb0.34) have been deposited by electron beam evaporation technique on glass substrates at different substrate temperatures, (300–473 K). The films have polycrystalline nature with zinc blende structure. The decrease in electrical resistivity with increasing temperature shows semiconducting behavior. Hall measurements indicate that the films are of n-type. Optical transmission spectra of as deposited thin films have been measured at different substrate temperatures. All the electrical parameters i.e. electron mobility (µ), carrier concentration (n), resistivity (ρ), activation energy and band gap (E g) have been found to be temperature dependent. Suitable explanations are given in the paper.

Cysteine capped magnetite nanoparticles (10 to 20 nm) were synthesized via coprecipitation method under ultrasonic irradiation. The influence of pH value of the solution and cysteine addition on the size distribution and hydrodynamic size of nanoparticles were studied via TEM and PCS methods, respectively. The crystal structure and magnetic properties of the nanoparticles were characterized by XRD and VSM techniques, respectively. Coating density was calculated using TGA and TEM results. Cytotoxicity assessment performed by incubation of L929 cells, confirmed that ferrofluids are biocompatible. MRI studies conducted on rats demonstrated suitability of synthesized nanoparticles as contrast agents, especially for imaging of the lymph nodes.

The field electron emission from polycrystalline diamond/silicon and nitrogen-doped polycrystalline diamond/silicon structures obtained by HF CVD deposition method has been investigated. Electron emission currents from the samples were measured in a chamber at the pressure equal to 2·10−6 Pa in sphere-to-plane diode configuration with the 5 μm distance between electrodes. As expected, the results confirm the relation between the structure of diamond films and their emission properties. The type of silicon substrate also influences the value of emission currents and the diamond/n-Si heterostructures exhibit better electron emission than diamond/p-Si ones. The nitrogen doping significantly enhances the electron emission from the heterostructures and their emission parameters. The values of the threshold field between 2 V/μm and 3 V/μm were registered, the values of emission current close to 1 mA/cm2 at 5 V/μm for the nitrogen-doped films were obtained. The shape of current-voltage characteristics for nitrogen-doped polycrystalline films may be interpreted in terms of stochastic distribution of diameters of conducting channels which form the emission centers.

The synthesis of yellow baddeleyite pigments involves a phase transition of ZrO2, which facilitates the process of incorporating vanadium chromophore into its lattice, and occasionally also modifiers, such as oxides of trivalent elements, either chromophoric or non-chromophoric. Depending on the type of the modifier used — an oxide of indium, yttrium, gallium or thallium — the yellow-green colour of the two-component ZrO2-V pigment may be changed smoothly into yellow or yellow-orange. The temperature of the synthesis of ZrO2-based pigments ranges from 1050 °C to 1325 °C, depending on the properties of the substrates, such as a non-oxide form of the materials — ZrOCl2, Zr(SO4)2, certain additives (e.g. Bi2O3). Raising the synthesis temperature over 1400 °C has no visible effect on the colour quality of the pigment, as it depends on the concentration of the vanadium chromophore in the ZrO2 structure. The article describes the effect of the synthesis temperature and modifying additives on the effectiveness of incorporating vanadium into the ZrO2 lattice and, consequently, on the colour properties of the pigment.

Monophasic mullite samples doped with 0.002 M, 0.02 M, 0.1 M, 0.15 M and 0.2 M of NiCl2 were prepared via sol-gel technique. The prepared gels were dried, grinded, pressed into pellets and sintered at 400 °C, 800 °C, 1000 °C and 1300 °C. The electrical resistivity and activation energy of the composites have been measured and the variation of resistivity with concentration of the nickel ion doping has been investigated. The resistivity decreases with the concentration of nickel ions. X-ray analysis confirms the presence of Ni2+ ions in mullite. The Ni2+ ion, which substitutes Al3+ ion in the octahedral site of mullite structure, can be considered as an efficient factor in reducing the resistivity. The mullite unit cell parameters suggest predominant incorporation of NiCl2 in a glassy phase. The lowest activation energy of resistivity (E act) that was achieved is 1.22 eV at 0.02 M.

Epitaxial Lateral Overgrowth (ELO) is a method of epitaxial growth on a partially masked substrate. It can be a promising method for photovoltaic applications due to a possibility of producing thin and high quality silicon substrates. Since the mask prevents propagation of the substrate dislocations to the laterally overgrown parts of the ELO layer they are characterized by a lower dislocation density than the substrate. It means that it is possible to fabricate good quality solar cells on a poor quality Si substrate. The main goal of the research is to obtain a higher growth rate in the lateral direction than in the direction normal to the substrate. The epilayer growth kinetics depends on many technological factors, basically the growth temperature, the cooling rate, the solvent and the mask filling factor. For this reason the best way to achieve the goal is a computational analysis of the epitaxial layer growth process. This work presents a two-dimensional computational study of such a process of growth for different technological conditions. The computational model is based on the assumption of pure diffusion control growth.

NiFe2O4-PVDF composites in different ratios (10 %, 30 % and 50 %) were prepared in two steps. Firstly, fine nanosized NiFe2O4 powder was synthesized using the precursor solution method. Then the composites were made by hot-press technique. The presence of both the phases (ceramic and polymer) was confirmed by XRD micrographs. The average particle size of the composites varied from 18–23 nm. SEM micrographs showed that the ferrite particles were embedded in the polymer matrix. The saturation magnetization and the remanence showed an increasing trend with the increase in ferrite content while the coercivity remained almost constant. Impedance plot showed the presence of a single semicircle, which indicates the presence of bulk effect. The composites exhibited non-Debye relaxation. The bulk conductivity followed the Arrhenius type of behavior. The conduction mechanism was explained by the Vervey-de-Boer mechanism.