Venceslav Vassilev

Glass formation in the GeSe2 (As2Se3)–Sb2Se3–CdTe systems

Abstract Glass-forming regions in GeSe 2 –Sb 2 Se 3 –CdTe (I) and As 2 Se 3 –Sb 2 Se 3 –CdTe (II) systems have been determined by visual, X-ray diffraction and electron microscope analyses. Glasses have been obtained in the GeSe 2 -rich region in system I and As 2 Se 3 -rich region in system II, respectively. Glassformation has been observed along the tie-lines GeSe 2 –Sb 2 Se 3 -line (30–100 mol% GeSe 2 ) and As 2 Se 3 –CdTe-line (45–100 mol% As 2 Se 3 ). In the binary GeSe 2 –CdTe and As 2 Se 3 –CdTe systems, glasses has found homogeneous in the concentration ranges 0–5 mol% CdTe (I) and 0–7 mol% CdTe (II), respectively. Up to 15 mol% CdTe are dissolved in the system GeSe 2 –Sb 2 Se 3 and in the system As 2 Se 3 –Sb 2 Se 3 , up to 12 mol% CdTe are dissolved. The basic physical–chemical characteristics of the glasses are investigated—density, microhardness, transformation temperature, crystallization temperature, melting temperature. A compositional dependence of these properties is shown. IR spectra of the samples are investigated.

Electrical and optical properties of thin films in the Ag2Te–CdTe system

Abstract Some optical and electrical properties of thin films in the Ag2Te–CdTe system are studied as a function of the cadmium content. In order to determine the optical band-gap, Eg, the transmission spectra of films are analysed. From the transmission and the refraction spectra, the relationship between the absorption coefficient α and the wavelength λ at different CdTe contents is derived. The temperature dependence of the electrical conductivity is measured and the thermal band-gap, Ea, is determined. The relation between these parameters and the composition of the samples is discussed.

Composites Containing Waste Materials

In the development of new materials, apart from the properties of the materials, the economic and ecological aspects are also of considerable importance. Traditional products whose manufacture requires expensive raw materials are being replaced with composite materials with alternative properties which display better qualities and perform in a better way. Composite materials are obtained from much cheaper components and possess significantly lower relative weight. Over the last few years different kinds of waste materials have been successfully utilized as filler in polymer composites with various applications. This not only reduces the production costs but also offers an opportunity for utilization of waste materials thereby reducing environmental pollution. Among them a large number of insulation composite materials with different practical application have been developed. Many of the composites are thermal, electrical or noise insulating materials based on thermosetting polymers (polyester and epoxy resins) which combine the required physical characteristics with good processing properties. For example polyester resins have a series of valuable properties suitable viscosity, ability to solidify at both room and high temperatures, high electric strength and dielectric ratings, high chemical stability and etc. Experimental research shows that the properties of the composites in this case are a function of both the curing degree of the polymer and the kind and properties of the filler. At the same time there are organic or inorganic waste materials generated by the industrial processes or human activity, which could successfully replace or at least reduce the amount of raw materials used as components in insulation materials. This chapter presents the outcomes of the experimental work aimed to evaluate the possibility for utilization of organic and inorganic industrial waste as fillers in polymer composites with thermaland electrical insulation and sound damping properties.

Thermodynamic Investigations of Chalcogenide Glasses from the GeSe2–Sb2Te3–CdTe System

Glassy samples from the GeSe2-Sb2Te3-CdTe system are synthesized. The thermal characteristics (glass-transition Tg, crystallization Tcr and melting Tm temperatures) are determined using differential thermal analysis. On the base of the values of these temperatures the glass forming ability of the glasses is determined by the Hruby’s criterion. The main thermodynamic parameters (enthalpy ΔH and entropy ΔS alternation) of the crystallization are calculated. The values of the ΔH and ΔS vary within 21.196 – 98.625 kJ/mol and 33.91 – 170.93 J/(mol.K), respectively. The influence of the composition on the investigated characteristics is analysed.

Fabrication of Porous and Dense Ceramics from Transitional Nano-Alumina

Porous alumina ceramics (density 0.75 TD) with a typical vermicular microstructure were obtained from transitional nano-alumina powder by cold isostatic pressing (P = 500 MPa) and sintering at non-isothermal conditions from RT to 1,500°C. Mechanical activation, realized by attriting, was used to reduce the α-Al2O3 transformation to a temperature of 1,038°C. Conventional pressing (P = 500 MPa) and sintering at 1,500°C were used to fabricate 0.96 TD dense alumina ceramics. Electrophoretic deposition was applied to the mechanically activated powder followed by isostatic pressing and sintering. Compacts with a density of 0.94 TD were obtained at 1,400°C/30 min. The microstructure was homogenous with grain sizes of 300 ± 100 nm.

Piezoelectric Crystal Sensor for Ammonia Detection

Piezoelectric quartz sensors (AT-cut, 5 MHz) with vacuum evaporated electrodes (Cr, Ni and Ag) were developed. Active NiCl2 and AgCl films were chemically deposited onto the Ni//quartz//Ni/Ag structures. The sensors were tested at ammonia concentrations of 6.63; 31.32; 99.73 and 320.47 μgcm−3 and their calibration functions were taken. It could be established that the sensor with an active Ag film deposited on a Ni electrode is most suitable for the detection of high ammonia concentrations (≥ 30 μgcm−3), while the NiCl2/Ni//quartz//Ni/AgCl structure shows the best characteristics for the detection of low NH3 concentrations (≤ 30 μgcm−3).

Dry etching of thin chalcogenide films

Fluorocarbon plasmas (pure and mixtures with Ar) were used to investigate the changes in the etching rate depending on the chalcogenide glasses composition and light exposure. The experiments were performed on modified commercial HZM-4 vacuum equipment in a diode electrode configuration. The surface microstructure of thin chalcogenide layers and its change after etching in CCl2F2 and CF4 plasmas were studied by SEM. The dependence of the composition of As-S-Ge, As-Se and multicomponent Ge-Se-Sb-Ag-I layers on the etching rate was discussed. The selective etching of some glasses observed after light exposure opens opportunities for deep structure processing applications.

Electrical and optical properties of Ag2−2xZnxTe thin films

Abstract Optical and electrical properties of Ag2−2xZnxTe thin films have been investigated. Optical band gap has been determined from the transmission spectra using Swanepoels method and Taucs procedure. The calculated values increase with ZnTe content from 0.08 to 0.46 eV. The thermal activation energy of the films with different ZnTe concentration is determined by investigating the temperature dependence of conductivity. The increase of ZnTe content leads to increase in the thermal band gap from 0.06 to 0.44 eV.

Phase equilibria in the InSb–SnTe system

Abstract The phase diagram of the InSb–SnTe system is studied using thermal analyses, X-ray diffraction and microstructure analyses, combined with density measurements. The diagram is an eutectic type with border solid solutions. Formation of metastable phase with composition InSb·SnTe was observed as a result of peritectoidal reaction at 420°C. The unit cell parameters of the new phase are determined as: a =7.1 A; b =5.1 A; c =5.1 A; α =91.7°; β =129.9° and γ =88.9°.