Stefania Stoleriu

Temperature dependence of tunability of Ba(SnxTi1−x)O3 ceramics

The electric field dependence of the dielectric constant (dc-tunability) as a function of temperature for a few compositions of Ba(Sn,Ti)O3 ceramics was investigated. Dense and homogeneous BaSnxTi1−xO3 ceramics (ferroelectric for x = 0.05 and x = 0.10, relaxor for x = 0.15 and 0.20) with high dielectric constant and low dielectric losses were prepared by solid state reaction. Their dc-tunability characteristics were investigated at a few temperatures from 20–120 °C, including the Curie range. The tunability data were discussed in terms of the Johnson model completed with a Langevin term that describes the “extrinsic” contribution to the nonlinear ɛ(E) dependences.

Analysis of the Structure and Morphology of Hydroxyapatite Nanopowder Obtained by Sol-Gel and Pirosol Methods

Nano-size hydroxyapatite is the main inorganic component of bone. There is a critical particle size that each property starts to change. Each property can be modified under controlled conditions. Multifunctional materials for applications, one must consider several properties change. Bone properties, are those which dictate the properties of hydroxyapatite, properties to be followed the synthetic materials to be used successfully as implants. Of these mention those related to surface materials, solubility and biocompatibility and their mechanical strength. Depending on the location and functionality of the implant have met some of these properties. The purpose of this study is to develop a synthetic hydroxyapatite at nano scale, using sol-gel and pyrosol method, for its use in medicine

Preparation of Alumina and Cordierite Based Porous Ceramic Materials

The present paper deals with the elaboration of high porosity ceramic materials. The ceramic powders used are alumina and cordierite, known for their good thermo – mechanical properties. The ceramic powders were prepared by a wet method, starting from soluble salts of the component oxides. For generating the porosity, an organic add was used (glucose). After sintering, the phase composition, the specific ceramic properties and the microstructure were investigated. The results are composition and preparation parameters dependent.

Feldspar Porcelain with High Mechanical Strength

There were studied three compositions, according to the masses vitrified between 1150° and 1250°C, placed in the leucite – mullite – silica subsystem. The reference sample (a mullite type one) was obtained from English kaolin and aplite (provided from “Philip’s Ravine”, Romania) and the other two (mullite – cristobalite type) were made from reference samples mixed with Romanian Uricani’s quartz (in proportions of 7.5% and 15%). It was used two different quartz grain size (a fine one and a coarse one). The samples were prepared through classical ceramic technology, by thermal treatment between 1150-1300°C with 2 hour soaking time. Through X rays diffraction analysis was confirmed the existence of theoretical mineralogical compounds: silica, mullite and orthose. The compressive strengths of samples thermally treated at 1250°C, 2h were between 98 and 183 MPa range. The highest value was observed for sample with 7.5% fine quartz (<100μm).

Defining Properties of Fly Ashes for their Usage in Silicate Binder Industry

Present paper deals with investigation regarding the most important characteristics of fly ashes (especially, Romanian fly ashes) for use in cement fabrication. These characteristics can be classified as: (i) morphological and surface characteristics; (ii) physical properties; (iii) compositional characteristics. It is pointed out the interdependence between these characteristics, reflected in mechanical (at grinding) and chemical (at Portland clinkers and Portland blended cements fabrication) behaviour of fly ashes. Introduction Fly ashes are industrial by-products, obtained in large quantities, and if not used they can become a dangerous environmental factor, with both ecological and economical consequences. Due to their composition and chemical activity, fly ashes exhibit an intrinsic potential for a wide use, including building materials industry (especially cements) [1-3]. The use of fly ashes in cement industry concerns three important aspects: (i) obtaining of unitary Portland cements, as raw material when replacing the clay component in the raw mixture preparation [1-4]; (ii) obtaining of blended Portland cements with pozzolana admixtures (especially fly ashes) [2,3]; (iii) obtaining of blended silicate cements free of Portland clinker – cementitious material (slag) – pozzolana (fly ashes) – activator binding systems [3]. The most important characteristics of fly ashes for use in silicate cements fabrication can be classified: (a) morphological and surface characteristics; (b) physical properties; (c) chemical properties. Experimental Considering previous aspects, the morphology of fly ash grains and their surface shape were studied. Specific surface area, grain size distribution of fly ashes, bulk density mass and grinding aptitude were determined as well as their oxide and phase composition. For a better understanding of the important influence of the fly ashes upon the studied properties, correlations between characteristics and behaviour of such cements made with fly ashes were established. Results and Discussion The microscopic evaluation of fly ashes points out their morphologic heterogeneity (an Hitachi electronic microscope was used). Most of researchers [1-5] define fly ashes as a mixture of grains having different shapes and colours. In this mixture one can distinguish full round spherical particles or with grooves having different depths or particles with holes and open shells. The surface of particles is, generally, smooth and shiny. The empty spherical grains (resulted from expanding the melt drops) were named cenospheres. The spherical particles with open cavities filled with smaller grains were named plerospheres. Figure 1. Fly ash spherical open particle containing smaller grains and surrounded by cenospheres and irregular shape grains [4] Figure 1 shows the fly ash morphology [4]. Beside spherical smooth particles can also appear coarse particles of Fe2O3 or Fe3O4, containing quartz particles, porous spherical or irregular grains and unburned coal particles. For a better grindability and a good behaviour in cement fabrication and use, fly ashes must contain a high proportion of cenosphere grains free of quartz and other crystalline minerals (i.e. mullite) with a low proportion of compact irregular grains and spherical course grains of Fe2O3 or Fe3O4 microcrystals. The grain size parameters, as measurable quantities, are important characteristics for fly ashes reactivity in solid-state reactions (at Portland cement fabrication) and for pozzolana activity of fly ashes (at hardening of Key Engineering Materials Online: 2004-05-15 ISSN: 1662-9795, Vols. 264-268, pp 2149-2152 doi:10.4028/www.scientific.net/KEM.264-268.2149