G. Seenivasan

Investigation of the thermal conductivity of selected compounds of gadolinium and lanthanum

Abstract As a part of the development of rare earth based ceramic materials for nuclear applications, the thermal conductivity of gadolinium zirconate, lanthanum zirconate and gadolinium aluminate was investigated employing the laser flash technique and covering a temperature range from 650 to 1400 K. Based on the temperature dependence of the thermal resistivity, the heat transport behaviour of these materials was briefly examined.

Investigation of the thermal conductivity of selected compounds of lanthanum, samarium and europium

Abstract As part of the development of rare earth based ceramic materials for a variety of applications, the thermal conductivities of lanthanum aluminate, samarium zirconate and europium zirconate were investigated, employing the laser flash technique and covering a temperature range from 650–1400 K. Based on the variation of the thermal resistivity as a function of temperature, the heat transport behaviour of these materials was briefly examined.

Thermal conductivity of rare earth–uranium ternary oxides of the type RE6UO12

The knowledge of thermophysical properties of the rare earth uranium ternary oxides of the type RE6UO12 (RE=La, Gd and Dy) is essential to understand the fuel performance during reactor operation and for modeling fuel behavior. Literature on the high temperature properties of this compound is not available and there is no report at all on the thermal conductivity of these compounds. Hence a study of thermal conductivity of this compound has been taken up. The compounds were synthesized by a solution combustion method using metal nitrates and urea. Thermal diffusivity of these compounds was measured by the laser flash method in the temperature range 673–1373 K. The specific heat data was computed using Neumann–Kopp’s law. Thermal conductivity was calculated using the measured thermal diffusivity value, density and specific heat data for different temperatures. The temperature dependence of thermal conductivity and the implication of structural aspects of these compounds on the data are discussed here.

Calorimetric study of selected NZP-type of ceramic materials

Sodium zirconium phosphate, NZP, ideally NaZr (PO ) , is a potential host matrix for nuclear waste. Thermophysical properties of 24 3 these materials are necessary to assess the thermal stability of these compounds. This paper presents the synthesis and calorimetric measurements of some NZP-type compounds containing alkaline earth elements. It also describes the indigenously developed room temperature drop calorimeter for low temperature calorimetric measurements. Calibration of the instrument is done with standard reference materials. Enthalpy increment of these materials is measured using this setup in the temperature range 373-873 K, and specific heat data are derived. Temperature dependence of enthalpy increment and specific heat of these materials, and the influence of the chemical composition on these properties are discussed.  2002 Elsevier Science B.V. All rights reserved.

Thermal conductivity of selected cermet materials

Cermets offer the benefits of both ceramics and metals. Due to the benefits associated with cermets, especially high temperature stability and thermal conductivity, there is renewed interest in their thermophysical properties. The present work describes thermal conductivity studies on selected cermets using laser flash technique in the temperature range 673-1373 K. This work describes the preparation and thermal conductivity measurement of urania-nickel cermets of composition 20 and 30 wt.% nickel by the solid-state reaction method. The thermal conductivity value is obtained as a product of the measured thermal diffusivity, estimated specific heat and room temperature density of these cermets. The theoretical value of the thermal conductivity is also computed for these cermets using different models for two-phase solid mixtures, such as parallel, series and Maxwell models. The calculated thermal conductivity, obtained using measured thermal diffusivity, of these cermets is compared with the theoretical values from the above models. Thermal conductivity integral values are also calculated and presented here.

A thermoanalytical study of the ignition behaviour of uranium dicarbide

Abstract A knowledge of the ignition behaviour of uranium and plutonium carbides is necessary to understand their high reactivity, arrive at procedures for their proper handling in various reactor and laboratory applications, and also to examine the conditions for a controlled oxidation of these carbides. This paper presents a study of the ignition behaviour of uranium dicarbide employing a thermoanalytical method. The onset of ignition of the carbide under various heating rates was investigated and the influence of the experimental variables was also examined.

Thermogravimetric study of the oxidation behaviour of uranium dicarbide

Systematic studies were carried out on the oxidation behaviour of uranium dicarbide in air in the temperature range 25 °C to 700 °C employing thermogravimetry, evolved gas detection X-ray diffraction and chemical analysis. The mechanistic aspects were investigated and the oxidation was found to proceed through reaction processes involving oxidation of uranium toα-UO3 with release of chemically bound carbon, reaction of released carbon leading to the formation of CO2 which gets liberated stepwise, decomposition ofα-UO3 to U3O8 and oxidation of free carbon to CO2. Examination of the thermoanalytical behaviour of sintered graphite, UO3, mixtures of graphite with UO3 and with U3O8 supported the above findings.ZusammenfassungDas Oxydationsverhalten von Urandikarbid in Luft im Temperaturbereich zwischen 25 und 700 °C wurde mittels Thermogravimetrie, Gasprodukteanalyse, Röntgendiffraktion und chemischer Analyse systematisch untersucht. Studien des Mechanismus ergaben, daß die Oxydation über Teilschritte abläuft, die folgende Schritte beinhalten: Oxydation von Uran zuα-UO3 unter Freisetzung chemisch gebundenen Kohlenstoffes, Reaktion des freigesetzten Kohlenstoffes unter Bildung von CO2, das schrittweise entweicht, Zerfall vonα-UO3 zu U3O8 und Oxydation von freiem Kohlenstoff zu CO2. Diese Feststellungen wurden durch Untersuchung des thermoanalytischen Verhaltens gesinterter Graphit- und UO3-Proben sowie Mischproben von Graphit mit UO3 bzw. U3O8 bekräftigt.РезюмеМетодами ТГ, обнаруже ния выделяющегося га за, рентгеноструктурны м и химическим анализом проведено системати ческое исследование окисле ния дикарбида урана в атмосфере воздуха и и нтервале температур 25–700°. Изучены общие аспект ы и найдено, что окисление протекает через реакционные пр оцессы, включающие окислени е урана доα-UO3 с выделением химическ и связанного углерод а, реакцию окисления углерода д о двуокиси углерода, выделяющейся ступен чато, разложениеα-UO3 д о U3O8 и реакцию окисления св ободного углерода до двуокиси углерода. Исследован ие термоаналитическог о поведения спëкшего ся графита, UO3, смесей граф ита с UO3 и U3O8 подтвержда ют вышеприведенные соо бражения.

Investigation of the Thermal Conductivity of Selected Rare Earth Aluminates

As a part of the thermophysical property studies on rare earth based oxide ceramics thermal conductivity of the aluminates of samarium and dysprosium was investigated employing laser flash technique and covering a temperature range from 673 to 1373 K. Heat transport behaviour was briefly examined by analysing the variation of thermal resistivity as a function of temperature.

Thermoanalytical study of the oxidation of selected metals with reference to the influence of heating rate

Abstract Study of the effect of heating rate on reaction temperature is important in order to understand the kinetic and mechanistic aspects of solid-gas reactions involving metals and alloys. The oxidation behaviour of selected metals has been studied by thermogravimetry as a function of temperature. The dependence of reaction temperature on heating rate has been examined and found to agree well with the generalised Urbanovici-Segal correlation. Nickel and tantalum were taken as typical metallic samples and oxidation of zirconium sponge has also been included in this study in view of our interest in this material.

Improved method of data acquisition and processing for the measurement of thermal diffusivity by the laser flash technique

Generally, in the laser flash technique, thermal diffusivity of any sample is calculated using the t1/2 value obtained by analyzing the transient signal. The t1/2 value, and thus the thermal diffusivity obtained in this way, will have a good amount of scatter in the data at low temperatures due to the weak signal and the pick up of external noise during the measurement. This necessitated suitable modifications in the data acquisition system to enhance the signal-to-noise ratio for the measurements carried out at low temperatures. This article presents the necessary modifications made in the existing data acquisition system. The measurements of thermal diffusivity of stainless-steel samples by the laser flash method and their results are discussed here.