MPCVD diamond coating of Si3N4–TiN electroconductive composite substrates

A study of microwave plasma (MPCVD) diamond deposition on Si₃N₄–TiN composites with different TiN amounts (0–30 vol.% TiN) is performed. These ceramic composites are requested in order to obtain a suitable material to be cut by electrodischarge machining (EDM), aiming their use as substrates for cutting tools and tribological components. TiN is an electrical conductor, contrarily to Si₃N₄, but it is characterized by a higher thermal expansion coefficient value than Si₃N₄ and diamond. The estimated thermal stresses are found to be low and tensile (0.90 GPa) when using the monolithic Si₃N₄ substrate, but compressive for the Si₃N₄–TiN composites, and even relatively high in magnitude (− 1.9 GPa) for the Si₃N₄–30 vol.% TiN composite. Brale indentation assessed the adhesion strength of diamond on the different substrate grades. Optimal behaviour (very low residual stress; no film delamination under 1000 N) is observed for the Si₃N₄–9 vol.% TiN substrate, corresponding to the lowest thermal mismatch and minimal residual stress magnitude.     

Spreading Codes Generator for Wireless CDMA Networks

The prime characteristic of spread spectrum modulated signals is that their bandwidth is greater than the information rate. In this way a redundancy is introduced that allows the severe levels of inteference inherent in the transmission of digital information over radio and satellite links to be overcome. Current spread spectrum applications are primarily in military communications; nevertheless, there is growing interest in this technique for third generation mobile radio networks (UMTS, FPLTS, etc.) with open discussion regarding the practicality of using a multiple access system based on spread spectrum techniques (CDMA). However, in order to support as many users in the same bandwidth as other multiple access techniques such as TDMA or FDMA, it is important how to generate large families of sequences that present low cross-correlation. The aim of this paper is to describe a spreading codes generator that can produce a large number of PN sequences with good properties of auto- and cross-correlation. Moreover, the codes generated shows high unpredictability and good statistical behaviour. This also allows the implementation of some features that are common on military networks such as message privacy (increasingly important in commercial networks) without additional cost. The structure presented shows itself to be advantageous for high speed generation of codes at a low cost, low power consumption (allowing longer life for batteries), small size and simplicity of implementation, essential ingredients for commercial equipment. Another attractive feature is its structural parallelism, useful in VLSI implementations. All of these features render it potentially suitable for the implementation of channel bandwidth sharing systems in future wireless personal communications networks.     

Effect of gravity on visco-elastic surface waves in solids involving time rate of strain and stress of first order

The aim of the present paper is to study the effect of gravity on visco-elastic surface waves in solids. The wave velocity equations are deduced from Biot’s theory of initial stress on the assumption that gravity creates a type of initial stress — hydrostatic in nature. Resulting equations are used to investigate surface waves of the Rayleigh, Love and Stoneley types. Results are in good agreement with corresponding classical results when gravity and viscosity are neglected.     

Red clays from Central and Eastern Crete: geochemical and mineralogical properties in view of provenance studies on ancient ceramics

The existing geochemical diversity of clay deposits consists the underlying assumption for the determination of provenance of archaeological ceramics on the basis of their chemical composition. This assumption was tested on Quarternary red clayey alluviums from Central and Eastern Crete (Greece), an area rich in archaeological pottery and very popular for ceramics provenance studies. This type of clay has been used for pottery production on the island since the antiquity and it is still in use by traditional workshops. Altogether, 17 samples of clayey raw materials were collected from 7 different locations in Central and Eastern Crete, and characterised by chemistry, mineralogy and petrography. The study was focussed on the internal variability within deposits and the diversity among different deposits.It was possible to distinguish all seven deposits from each other on the basis of their chemical composition, taking into account the available mineralogical and petrographical information, although some deposits presented compositional relations. Compared to Neogene clay deposits in the same area, the variability among the examined red clayey deposits was clearly higher and exceeded by far the intra-deposit variability.     

Direct Methods Optimised for Solving Crystal Structure by Powder Diffraction Data: Limits,Strategies, and Prospects

The ab-initio crystal structure solution by powder diffraction data requires great efforts because of the collapse of the experimental information onto the one dimensional 2θ axis of the pattern. Different strategies will be described aiming at improving the process of extraction of the integrated intensities from the experimental pattern in order to make more straightforward the structure solution process by direct methods. Particular attention will be devoted to the EXPO program. Some of its performance will be analysed and results will be shown.     

Uses of Ozone in a Three-Phase System for Water Treatment: Ozone Adsorption

A new technique of using ozone for water treatment is presented. This new technique consists of using a three-step-process composed firstly of ozone adsorption on an appropriate adsorbent, secondly water treatment, and thirdly regeneration of the adsorbent. Results regarding ozone adsorption (the first step) are presented in this paper. Different types of silica gel and a type of TiO₂ have been tested for ozone adsorption. It was found that the physical characteristics of the silica gel affect its capacity for ozone. Titanium dioxide has shown ozone decomposition instead of adsorption as it contains Lewis acid sites. An exponential decrease of the silica gel capacity with its moisture content has been found. Linear isotherms in the range of ozone concentrations less than 100?g/m³ NTP have been found. A particle diffusion model with linear equilibrium isotherm has been used to model the breakthrough curves in fixed bed columns.     

Quantitative x-ray photoelectron spectroscopy analysis of alkali ions / zirconia systems

Zirconia-supported alkali ions, containing Na, from 0.74 to 3.61 wt.%, and K, from 0.91 to 4.76 wt.%, have been studied by X-Ray Photoelectron Spectroscopy, X-ray diffraction and chemical analyses. The results show that the alkali ions remain dispersed on the zirconia surface. After treatment at 773 K in air, 5h, the Na containing samples maintain a good dispersion at low Na content, whereas for the more concentrated samples, agglomeration and incorporation in solid solution have been detected. The K based system shows neither agglomeration nor incorporation in solid solution of potassium ions. The XPS intensity ratios are compared with calculated values, according to different models, to account for the changes in surface morphology.     

CONSTRUCTION OF ADMISSIBLE OPERATION CONDITION DOMAIN FOR WELL-STIRRED REACTORS DURING THEIR DESIGN AND CONTROL STAGES

The work examines the problem of constructing an admissible operation condition domain for a well-stirred reactor which would satisfy restrictions in conversion, selectivity, heat removal and stability. The domain is very important and must be known during the reactor design and control stages because it yields visual information for analyzing the reactor operations and makes it possible to find the variation intervals of decision variables, necessary to calculate and optimize the reactor. Being known, the admissible operation condition domain makes it possible to estimate the limiting restrictions and to choose the necessary reserves. The domain has completely been studied analytically for chemical processes with arbitrary kinetics.     

Design of statistical databases: A methodology for the conceptual step

The aim of the conceptual step in database design is to describe data involving in the application in a formal and abstract way, without any concern to the specific model and language chosen for the implementation. In statistical applications, data are described at different levels of aggregation, from elementary facts of the reality to complex aggregations such as classifications, time series, indexes. The paper describes a methodology for conceptual design of statistical databases that provides the designer suitable strategies for defining such different levels of aggregation starting from user requirements, and checking the completeness, coherence and minimality of the conceptual schema at the different levels. The methodology makes use of two data models for the representation of data: for elementary data the Entity-Relationship model, widely used in database applications, and for summary data a new model is proposed, designed to be an effective trade-off between expressive power and simplicity of use.     

Utilization of DTA in determination of crystallization mechanism in SiO2–Al2O3–CaO–MgO(R2O) glasses in presence of various nuclei

The effect of various nuclei on crystallization mechanism of SiO₂–Al₂O₃–CaO–MgO(R₂O) glasses were investigated by differential thermal analysis (DTA) through Matusita, Marotta and modified Kissinger methods. The Avrami constant, n, and the activation energy for crystallization of the most promising specimens containing Cr₂O₃, Fe₂O₃ and TiO₂ in the single, double and triple nuclei series were determined. According to the results the Avrami constants derived from the Marotta method were more consistent with the other experimental observation. While glasses containing TiO₂ as the single nucleant represents surface crystallization and those containing Cr₂O₃ or Fe₂O₃ one-dimensional bulk crystallization, the crystallization mechanism of specimens containing both Cr₂O₃ and Fe₂O₃ and also the glasses containing the triple nuclei, are bulk and two dimensional.