Synthesis of WC-Co Nanocomposites Using Polymer as Carbon Source
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prolong the reaction time, which can lead to particle coarsening and degradation of the nanostructure. The gas-phase carburization may also lead to excess uncombined, free carbon on the surface, which needs to be removed. In an attempt to minimize some of these difficulties, we have developed a new technique, which uses polyacrylonitrile precursor as an in situ carbon source. SYNTHESIS AND CHARACTERIZATION Ammonium tungstate solution with a pH - 8 (Solution A) was first prepared as shown in flowchart I. Solution A was then used to prepare WC-Co nanocomposites by two different procedures as outlined in Flowcharts II and II1. X-ray diffraction was used for phase identification and Transmission Electron Microscopy (TEM) to investigate particle morphology and size.
Ammonium Apply vacuum to
ITungstic acid (1)
remove excess NH3
tungstate solution pH>12
(pH
Ammonium tungstate solution =8 (Solution A)
Flowchart I: Preparation of ammonium tungstate solution with pH = 8.
Procedure 1 Synthesis of WC-Co nanocomposites was attempted first by Procedure 1 (Flowchart II). In step 3, 1.5 g of Black mass A was dispersed in 3 g of polyacrylonitrile in dimethylformamide (DMF). The black mass B obtained in step 4 could be ground easily into fine powder. Approximately 150 mg of Black mass B was then introduced into a tube furnace maintained at 900 OC in a flowing gas mixture of 90 % Ar-10 % H2. The sample was kept at this temperature for various duration, cooled to room temperature and examined by x-ray diffraction. It was found that for the quantities of the Black mass A and polyacrylonitrile chosen, a firing duration of 2 hr gave a nearly phase pure WC-Co nanocomposite. The x-ray diffraction pattern of this sample is shown in Fig. 1. The very weak unmarked peaks correspond to the impurity phase Co6W6C. The TEM micrograph of this sample is shown in Fig. 2, which indicates a particle size range of 50-80 nm. The results indicate that polyacrylonitrile can be used as an in situ source for carbon and the procedure developed is a viable approach for the synthesis of WC-Co nanocomposites. However, there are a few problems which need to be addressed: (1)Co(NO 3 )2 .6H 20 has a dehydration and decomposition temperature of about 55 oC, and the drying and decomposition involved in step 1 need to be carried out gently and carefully so that no compositional variations occur due to spurting out of the solution. (2) Mixing of ammonium tungstate and cobalt nitrate in step 1 leads to the formation of CoWO 4 , which may facilitate the formation of Co6W6 C initially. (3) The thermal degradation of polyacrylonitrile occurs over a range of temperatures beginning at around 464
Polyacrylonitrile Ploymer (2) DMF Sdecompose stir Clear Solution
Solution Al Add nitric acid to pH < 1.6, (1) add cobalt nitrate, dry & on a hot plate Black mass (A) Grind
I
1(3) Disperse (4) 1 Decompose on a hot plate Black mass (B) (5) Fire at 800 - 900 °C ( in Ar-H 2 mixture
Fwc-co I
Flowchart II: Synthesis of WC-Co nanocomposites by procedure 1
Conc. nitric acid
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