On the Structural and Chemical Homogeneity of Spark Plasma Sintered Tungsten

Abstract

Tungsten-based materials are the prime candidate plasma-facing materials for future fusion reactors, such as DEMO. Spark plasma sintering is a prospective fabrication technology with several advantageous features. The concurrent application of electric current, temperature and pressure enhances the sintering process, allowing for lower temperatures and shorter sintering times than traditional powder metallurgy processes. This in turn helps to avoid excessive grain growth and phase segregation in W-alloys. This study is focused on several factors that may influence the homogeneity of the sintered compacts-namely the diffusion of carbon from the graphite die, purity of the powder and sintering conditions. The following characteristics of spark plasma-sintered tungsten compacts were studied: composition (especially carbon and oxygen content), porosity, mechanical properties (hardness and fracture strength), and thermal diffusivity. The effects of the abovementioned processing factors were quantified, and local variations of selected properties were assessed. plasma sintering (SPS, also called Field Assisted Sintering Technology, FAST) is a progressive technique to consolidate various materials, metals as well as ceramics, glass etc. Relatively low temperatures, high heating rates and short processing times-compared to traditional sintering techniques-are generally favorable for the retention of fine grains. In SPS, the sintered material in the form of powder is introduced into a die and, subsequently, high-intensity (pulsed or steady) electric currents up to several thousand A and uniaxial pressure up to hundreds MPa are applied. The die and the powder are heated by Joule heating. Graphite foil is usually inserted between the powder and the die in order to eliminate undesirable bonding of sintered material to the die. Depending on the parameters and the sintered material, temperature can reach up to~2400 • C and with suitable parameters, fully dense samples can be obtained [5][6][7]. Sintering parameters have a considerable effect on the resulting microstructure, homogeneity and other properties of the samples. Basic variables characterizing the sintering process are: (i) sintering temperature, (ii) applied pressure, and (iii) duration of the holding time at the sintering temperature. The results are also sensitive to the heating rate, manner of pressure application, die dimensions, sintering atmosphere etc.

Discovering tungsten-based composites as plasma facing materials for future high-duty cycle nuclear fusion reactors

“…As shown in this work and previous research 39 , 40 , oxygen impurities present a consistent challenge in both types of SPS-fabricated W composites. In the W-Zr precursors, the presence of zirconium oxide in the sample potentially limits the dissolution of Zr placeholders, which is essential for producing porous W. In the DSW composites, the oxidation of carbide dispersoids significantly reduces the material′s thermal conductivity.…”

“…According to Eq. ( 1 ), the thickness is well above the heat propagation distance for W in the range of power fluxes used for this study 40 . It should be noted that the thermal conductivity of pure W is used to estimate this distance and this value is lower for DS-W due to the presence of oxides and carbides 44 .…”

Irradiation-induced hardening in fusion relevant tungsten grades with different initial microstructures

“…IGP and ATW have, respectively, carrot-and pancake-like shape of grains with D50 being several tens of μm. D50 of W-0.5ZrC and FG is much smaller because of the specific production route [16,29].…”

Microstructure and Electrical Property of Ex-Situ and In-Situ Copper Titanium Carbide Nanocomposites

“…At a sintering temperature of 700 • C, the microhardness values of ex-situ composites containing 1 and 3 vol. % TiC decreased due to higher occurrence of recrystallization [26]. The microhardness of in-situ Cu-TiH2-C composites increased significantly from 175.8 to 206.5 HV when the sintering temperature increased from 600 to 950 • C. At a higher sintering temperature, the reaction between TiH2 and C was supported thermodynamically.…”