Mechanochemical synthesis of nanocrystalline lead selenide

Mechanochemical synthesis of lead selenide PbSe nanoparticles has been performed by high-energy milling of lead and selenium powder in a planetary ball mill Pulverisette 6 (Fritsch, Germany) and in an industrial eccentric vibratory mill ESM 654 (Siebtechnik GmbH, Germany). Structural properties of the synthesized lead selenide were characterized by X-ray diffraction, which confirms crystalline nature of PbSe nanoparticles (JCPDS 6-354). The average size of PbSe crystallites of 37 nm was calculated from XRD data by Williamson-Hall method. The methods of particle size distribution analysis, specific surface area measurement, scanning electron microscopy and transmission electron microscopy were used for characterization of surface, mean particle size, and morphology of PbSe. An application of industrial mill verified a possibility of the synthesis of a narrowband-gap semiconductor PbSe at ambient temperature and in a relatively short reaction time

Estimation of electrical resistivity of conductive materials of random shapes

Introduction. Electrical resistivity is an important material characteristic in the field of electrical engineering and material science. There are several methods that can be used to measure resistance, like the 4-wire method which relates the resistance to a voltage drop at a given current flow, but to define the resistivity from the resistance value requires an analytical expression for the given system which requires a sufficient mathematical apparatus for describing complicated shapes. Therefore we use finite element method computations to compute the resistivity of a metal material. This approach has been already used for different materials like concrete and aluminum in the past. We then compare this method with an analytical expression that due to intuition could approximate the solution sufficiently. After that, the same material is used again to test the electrical isotropy of the sample. Novelty. A method is developed by combining the results of experimental studies and the results of mathematical modelling of the process of determining the electrical conductivity of metals. The goal is to describe and employ a method of measuring the electrical resistivity of metal objects of random shapes. Using this method, it is possible to measure the resistivity of materials without the need to manufacture them into wires or ribbons. Methods. The solution to the problem was carried out by the finite element method via the COMSOL Multiphysics 5.6 simulation program in a cartesian coordinate system and the resistance between two points of the metal sample was measured by the 4-wire method. Results. A similar resistance value was obtained when the measuring terminals were placed in different places. The difference between them was within 1,5 % and the obtained values were close to the values given by the literature for the electrical resistivity of electrical steels. Terminal size influences the measured conductivity and a max error of 5,2 % was estimated. Practical value. A method of estimating the resistivity of materials without the need to manufacture them into specific shapes, like wires or ribbons, for which analytical expressions between resistivity and resistance are easily derived.Вступ. Питомий електричний опір є важливою характеристикою матеріалу в галузі електротехніки та матеріалознавства. Існує кілька методів, які можна використовувати для вимірювання опору, наприклад, 4-провідний метод, який пов’язує опір з падінням напруги при заданому струмі, але для визначення питомого опору за значенням опору потрібен аналітичний вираз для даної системи, який вимагає достатнього математичного апарату для опису складних форм. Тому ми використовуємо розрахунки методом скінченних елементів до розрахунку питомого опору металевого матеріалу. Цей підхід вже використовувався в минулому для різних матеріалів, таких як бетон та алюміній. Потім ми порівнюємо цей метод з аналітичним виразом, який завдяки інтуїції може достатньо апроксимувати рішення. Після цього матеріал знову використовується для перевірки електричної ізотропії зразка. Новизна. Розроблено метод шляхом поєднання результатів експериментальних досліджень та результатів математичного моделювання процесу визначення електропровідності металів. Мета – описати та застосувати метод вимірювання питомого електичного опору металевих предметів довільної форми. Використовуючи цей метод, можна вимірювати питомий опір матеріалів без необхідності виготовлення дротів або стрічок. Методи. Розв’язання задачі здійснювалося методом скінченних елементів за допомогою програми моделювання COMSOL Multiphysics 5.6 у декартової системі координат, а опір між двома точками металевого зразка вимірювався 4-провідним методом. Результати. Отримано аналогічне значення опору під час розміщення вимірювальних клем у різних місцях. Різниця між ними знаходилася в межах 1,5% і отримані значення були близькими до наведених у літературі значень електричного опору електротехнічних сталей. Розмір клеми впливає на провідність, що вимірюється, максимальна похибка становить 5,2 %. Практична цінність. Метод оцінки питомого опору матеріалів без необхідності надання їм певної форми, наприклад, дроту або стрічок, для якого легко отримати аналітичні вирази між питомим опором та опором

Današnje stanje međulonca s pokrivnim troskama u ljevaonici slabova

Tundish metallurgy development, which has been focused on steel cleanliness improvement in last few years, affected also tundish slag systems. Molten slags are the most suitable for both absorbing and refining functions of tundish cover slags, while a system of extremely low density does satisfy their isolating functions in the best way. Hence it is obvious, that it is not possible to assure the main functions using one sort of tundish covering slags. In addition, traditional acid slags, are being replaced with basic covering slags, or combination of both types. In this work, we have tried to investigate the operation of tundish covering slag under the condition of slab continuous casting. A genesis of covering slag from applied cover powders its refining functions have been investigated, as well as the changes of covering slag chemical composition during one tundish casting sequence.Razvoj metalurgije međulonca koji se usmjerio na unapređivanje čistoće čelika tijekom zadnjih nekoliko godina odrazio se i na vrste troske u međuloncu. Rastaljena troska je najpogodnija kako za funkcije apsorbiranja tako i za funkcije rafiniranja pokrovne troske međulonca sve dok sistem krajnje male gustoće zadovoljava na najbolji način njene izolacione funkcije. Iz toga je jasno, da nije moguće osigurati glavne funkcije uporabom samo jedne vrste pokrovnih troski međulonca. Osim toga, tradicionalne kisele troske zamjenjuju se bazičnim pokrovnim troskama, ili se koristi kombinacija obadva tipa. U ovom članku se nastojalo istražiti rad pokrovne troske međulonca u uvjetima kontinuiranog lijevanja slabova. Istraživano je kako se uporaba pokrovne troske razvijala iz uporabe pokrovnih prahova te njene funkcije rafiniranja kao i promjene njenog kemijskog sastava tijekom jednog ciklusa lijevanja međulonca

Pročišćavanje čelika optimiranjem konfiguracije međulonca

In continuous casting process, liquid steel flows through the tundish, which is supposed to operate as a continuous refining finisher. To provide a functionality of such a device, a pack of tundish metallurgy techniques must be applied, whose efficiency is conditioned especially with optimal symmetrical and dynamical melt flow. Flow optimizing can be achieved through the shaping of inside tundish configuration, using flow control devices such as turbulence inhibitors, impact pads, baffles, weirs, dams, etc. The theme of the present paper is the precise employment of a turbulence inhibitor, commercially known as TURBOSTOPTM, together with a pair of baffles & a flat impact pad in slab custer to improve steel cleanliness and a fluid flow phenomenon in a two strands tundish.Tijekom procesa kontinuiranog lijevanja tekući čelik prolazi kroz međulonac koji funkcionira kao kontinuirani uređaj za završnu doradu rafiniranjem. Za osiguranje funkcioniranja takvog uređaja mora se primijeniti tehnika metalurgije međulonca čija učinkovitost je uvjetovana naročito optimalnim simetrijskim i dinamičkim tečenjem taline. Optimalizacija protoka se može postići oblikovanjem unutrašnje konfiguracije međulonca uporabom takvih uređaja za kontrolu protoka kao što su inhibitori turbulencije, zadebljanje za ublažavanje udaraca, pregradne ploče, brane itd. Tema ovog rada je točna primjena inhibitora turbulencije poznatog pod trgovačkim nazivom TURBOSTOPTM, zajedno s parom skretnih pregrada i ravnim zadebljanjem za ublažavanje udaraca u lijevanom slabu radi poboljšanja čistoće čelika i fenomena toka fluida u dvije žile međulonca

Nanogranular MgB2 thin films on SiC buffered Si substrates prepared by in-situ method

MgB2 thin films were deposited on SiC buffered Si substrates by sequential electron beam evaporation of B-Mg bilayer followed by in-situ annealing. The application of a SiC buffer layer enables the maximum annealing temperature of 830 C. The Transmission Electron Microscopy analysis confirms the growth of a nanogranular MgB2 film and the presence of a Mg2Si compound at the surface of the film. The 150-200 nm thick films show a maximum zero resistance critical temperature TC0 above 37 K and a critical current density JC ~ 106 A/cm2 at 11K.Comment: 7 pages, 6 figures, submitted to Applied Physics Letter

Optical spectra obtained from amorphous films of rubrene: Evidence for predominance of twisted isomer

In order to investigate the optical properties of rubrene we study the vibronic progression of the first absorption band (lowest \pi -> \pi^* transition). We analyze the dielectric function of rubrene in solution and thin films using the displaced harmonic oscillator model and derive all relevant parameters of the vibronic progression. The findings are supplemented by density functional calculations using B3LYP hybrid functionals. Our theoretical results for the molecule in two different conformations, i.e. with a twisted or planar tetracene backbone, are in very good agreement with the experimental data obtained for rubrene in solution and thin films. Moreover, a simulation based on the monomer spectrum and the calculated transition energies of the two conformations indicates that the thin film spectrum of rubrene is dominated by the twisted isomer.Comment: 7 pages, 6 figure

Estimation of the thermal diffusivity in a large electroceramic body by an invere method

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.We investigate the temperature dependence of the thermal diffusivity for a large ceramic body of a cylindrical shape during firing up to 900 °C. The body was made of a ceramic material used in the production of electroporcelain insulators. We describe the corresponding heat transfer by the standard heat equation and solve the inverse problem by the Levenberg-Marquardt method. The results show that the method allows one to detect the physical-chemical processes occurring in the material during firing, namely, the liberation of physically bound water in the range up to 250 °C, the phase transformation of kaolinite into metakaolinite (dehydroxyla-tion) in the range ~ 450 °C – 650 °C, and solid-state sintering starting at ~ 700 °C.cf201