Effect of Additives on Thermal Conductivity of Si3N4 Ceramics

Compared with traditional ceramics, Si3N4 ceramics have the characteristics of high theoretical thermal conductivity, high thermal shock resistance, high oxidation resistance, high strength, and strong current carrying capacity. It is a potential high-speed circuit and high-power device for heat dissipation and heat dissipation. Sealing material. For applications in 5these fields, β-Si3N4 with a relatively stable structure and high thermal conductivity is an ideal material. However, β-Si3N4 powder is difficult to sinter as a raw material. Therefore, the prepared Si3N4 generally has a low density, and there are various defects in the crystal. The existence of these defects will cause interference and scattering of heat in the transfer process. Limits the application of β-Si3N4 ceramics. Studies have shown that the introduction of different additives can form a liquid phase at high temperatures, which can effectively reduce the firing temperature of the sample and increase the density. At the same time, it can also remove lattice oxygen, weaken the intercrystalline phase, and promote the α→β phase transition. Thereby improving the thermal conductivity and sintering performance of Si3N4 ceramics. Therefore, this article reviews the types of additives and their effects on the properties of Si3N4 ceramics and their mechanism. Trying to find an additive system for the preparation of high thermal conductivity Si3N4 ceramics with excellent comprehensive performance, hoping to provide help for the work and researchers engaged in the research on the thermal conductivity of Si3N4 ceramics.

Effect of the Deposition Time and Heating Temperature on the Structure of Chromium Silicides Synthesized by Pack Cementation Process

Transition metal silicides have attracted great interest for their potential use in optoelectronic devices, photovoltaic cells, and thermoelectric conversion elements because of their high melting point, high oxidation resistance, and satisfactory thermoelectric properties. This study focuses on the effect of the deposition time and the heating temperature on the morphology and structure of the chromium silicides synthesized by the pack cementation method. A series of experiments were carried out at various temperatures (1000–1150 °C) with different deposition times (15–120 min). The morphology and the chemical composition of the samples were determined using SEM with an EDS analyzer. The structure determination and phase identification were performed by XRD analysis. The examination of the as-formed materials was completed by performing thermal stability tests. The most suitable conditions for producing CrSi2 sample with satisfactory properties and simultaneously minimizing the cost and production time are listed. It was found that the sample synthesized at 1000 °C for 15 min during the chromizing step, in combination with the siliconizing step at 1000 °C for 60 min, presents the best thermal stability and these selected temperatures offer appropriate, economical, and repeatable results.

Structure and Properties of Zr-Mo-Si-B-(N) Hard Coatings Obtained by d.c. Magnetron Sputtering of ZrB2-MoSi2 Target

Coatings in a Zr-Mo-Si-B-N system were deposited by the magnetron sputtering of ZrB2-MoSi2 targets in argon and nitrogen. The structure of the coatings was investigated using scanning electron microscopy, X-ray diffraction, energy-dispersive spectroscopy, and glow-discharge optical emission spectroscopy. Mechanical and tribological properties were measured using nanoindentation and pin-on-disc testing. Oxidation resistance and oxidation kinetics were estimated via annealing in air at 1000–1500 °C and precision weight measurements. We found that the coatings deposited in Ar demonstrate a superior combination of properties, including hardness of 36 GPa, elastic recovery of 84%, a friction coefficient of 0.6, and oxidation resistance at temperatures up to 1200 °C. High oxidation resistance is realized due to the formation of the protective (SiO2 + ZrO2)/SiO2 oxide layer, which inhibits the diffusion of oxygen into the coating.

Three-Dimensional Modeling of Thermal-Mechanical Behavior of Accident Tolerant Fuels

Considering the safety issues of the traditional UO2-Zr fuel, a variety of accident-tolerant fuel (ATF) candidates have been proposed in recent years. Among the several ATFs, U3Si2, and UN are the two promising candidates for fuel materials owing to their high thermal conductivity and high uranium density. The FeCrAl alloy and the SiC/SiC composite material are the two promising candidates for cladding owing to their high oxidation resistance and high strength. In order to quantitatively evaluate the performance of ATFs, this study summarizes the physical models of typical ATF cladding materials (FeCrAl and SiC) and pellet materials (UN and U3Si2). Then a three-dimensional non-linear finite element method is applied to simulate the thermal-mechanical behavior of several typical fuel-cladding combinations, including UO2-FeCrAl, UN-FeCrAl, U3Si2-FeCrAl, U3Si2-Zr, and U3Si2-SiC. The important physical quantities, such as the fuel centerline temperature, the deformation of the pellet and the cladding as well as the pellet-cladding mechanical interaction (PCMI) were studied. The fission gas release model was also verified and improved.

Bending and Welding of New High Oxidation Material - Thor™ 115

Development of materials used in the power industry for the production of USC boilers poses new challenges. The introduction of new alloying agents intended at obtaining the best possible mechanical properties, including creep resistance, affects the fabricability of new steel grades. All new materials have to undergo a lot of tests, particularly as regards bending and welding processes, with the aim of enabling the development of technologies ensuring failure-free production and assembly of boiler components. Martensitic steels containing 9% Cr, used in the production of steam superheaters shall have good creep resistance and, at the same time, low oxidation resistance at a temperature above 600°C. In turn, steels with a 12% Cr content, for example, VM12-SHC or X20CrMoV12-1 are characterized by significantly higher oxidation resistance but have lower strength at higher temperatures, which translates to their limited application in the production of modern USC and A-USC boilers.X20CrMoV12-1 was withdrawn from most of the power plants across Europe and VM12-SHC was supposed to replace it, but unfortunately, it failed in regards of creep properties. To fulfill the gap a new creep strength-enhanced ferritic steel for service in supercritical and ultra-supercritical boiler applications was developed by Tenaris and named Thor™115 (Tenaris High Oxidation Resistance). This publication covers the experience obtained during first steps of fabrication which includes cold bending and TIG welding of homogenous joints.

Studies on the Oxidation Behavior and Microstructural Evolution of Two Nb-Modified HR3C Austenitic Steels under Pure Water Vapor at 650 °C

The steam oxidation behavior of three heterogeneous HR3C alloys was investigated at 650 °C comparatively. After a long-term oxidation process for 1000 h, the results demonstrated that the commercial HR3C alloy already exhibited a high oxidation resistance. However, the spallation resistance of the oxide scale was low during the initial oxidation period. The addition of a moderate amount of Nb into the alloy (1#HR3C) increased the oxidation resistance of the alloy. In addition, the improvement of the microstructural stability was substantially influenced by solid solution strengthening and fine grain strengthening. However, the addition of excessive Nb could significantly affect the growth model of the oxide scale and negatively affect the oxidation performance and microstructural evolution of the alloy (2#HR3C).