CC BY
22iSHS 2019
Moscow, Russia
W-Ag NANOCOMPOSITE PREPARATION BY COMBINING
SCS AND SHS
M. K. Zakaryan*ab, A. A. Baldryan", and S. L. Kharatyanab
aA.B. Nalbandyan Institute of Chemical Physics NAS RA, Yerevan, 0014 Armenia
bYerevan State University, Yerevan, 0025 Armenia *e-mail: [email protected]
DOI: 10.24411/9999-0014A-2019-10197
In recent years, tungsten (W)-based heavy alloys have received increased use in both commercial and industrial spheres. Most heavy alloys consist of W particles embedded in matrix of other metals or their alloys such as iron, nickel, silver or copper. In particular, W-Ag alloys may tend to replace as heat dissipation materials in the microelectronic devices that are prone to failure at high operating temperatures, as diverter plates in fusion reactors. They combine the arc erosion and welding resistance of tungsten with the excellent thermal and electrical conductivities of silver. The composite's thermal expansion coefficient can be adjusted by changing its composition to match those of ceramic materials used as substrates in semiconductor devices. The tungsten-silver composite may have various contents of silver, generally from 25 to 50% by weight [1]. In these cases, the W-Ag composite may be used in heat sinks and microwave absorbers in microelectronic devices. Powder metallurgy is the technique utilized to manufacture this material, but due to the mutual insolubility of W and Ag and to the poor wet ability of liquid Ag on W, sintering cannot easily produce dense and homogeneous structures [2, 3].
As there is no alloying between the silver and tungsten the properties of the composites depend on direct proportion their composition, the size, morphology, and distribution of phases within the composite, with finer particles giving improved performance [4].
In this work we report a new pathway for the preparation of Ag-W composite nanopowder/pseudoalloy by energy-saving combustion synthesis (CS) method. It is well known that the selection of the starting materials can highly contribute to enhancing the structure and properties of the final products. Regarding to that, fine precursor representing silver + tungsten oxide, was prepared by solution combustion synthesis (SCS) method using ammonium paratungstate ((NH4)10(H2W12O42)-4H2O) (APT) and silver nitrate (AgNO3) in the presence of citric acid (C6H8O7) as a reducer and ammonium nitrate (NH4NO3) as an auxiliary oxidizing agent to increase the reaction enthalpy. The solution prepared was heated on electrical heater for evaporation of water, after a viscous liquid is formed which is autoignited at ~ 150 °C accompanied by release of gas and smoke (Tmax ~ 600oC) (Fig. 1a). As a result, the initial reaction media being in the liquid state allows mixing the reactants on the molecular level, thus promoting the formation of nanocomposite with homogeneous composition and uniform distribution of phases (Fig. 1b).
Thus, the suggested approach of Ag:W = 1:1 composite powder preparation comprises the obtaining of homogeneous and nanostructured WO3-Ag precursor by SCS method and subsequent reduction of WO3 by conventional SHS with applying reaction's coupling approach [5].
Thermodynamic calculations were carried out to reveal the possibility of combustion in Ag-WO3-yMg-xC system and for determining optimal conditions for complete reduction, aimed to obtain W-Ag pseudoalloy.
XV International Symposium on Self-Propagating High-Temperature Synthesis
700 600 500 400 300 200 100
APT+12AgNO3+8C6H„O7+25NH4NO3
) T
30
60
90
120 t, s
Fig. 1. Combustion temperature profile of the SCS reaction (a) and XRD and SEM analyses results of obtained precursor (b).
In Fig. 2, the formation area for tungsten-silver bimetallic system was clearly marked depending on carbon and magnesium amounts. According to these results, the joint and complete reduction of silver and tungsten is attainable at magnesium and carbon amounts of y = 1.5-2 moles and x = 0.5-1.2 moles, respectively, within the temperature interval 1500-2200oC. Based on the results of thermodynamic calculations, experiments on magnesiocarbothermal reduction were carried out with the amount of magnesium corresponding to the low-temperature part of the optimal region (y = 1.5 mol) (Fig. 2). To find out optimum composition for preparing W-Ag composite material, the effect of carbon amount on the behaviour of combustion parameters (temperature and velocity), phase composition and morphology of product, a series of experiments were performed with changing carbon amount (Fig. 3). As it can be seen in Fig. 2, at x > 6.25 mol combustion limit was observed. All the products were exposed to XRD analyses, according to which WO3 + Ag + 1.5Mg + 1.2C composition (see Fig. 2) was selected as optimum for preparing W-Ag pseudoalloy. Byproduct magnesia was removed by acid treatment with hydrochloric acid (ro = 10%). The product after acid leaching represents W-Ag nanocomposite material.
C(C),mol
Fig. 2. Thermodynamic analysis results for the WO3-Ag-yMg-xC system, P = 0.5 MPa.
Fig. 3. Combustion temperature and velocity vs. carbon amount for the the WO3-Ag-1.5Mg-xC system, P = 0.5 MPa.
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■SHS 2019 Moscow, Russia
This work was supported by the Committee of Science MES of RA (Research grant 18A-1d12).
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