Surface Modification of Diamonds in Diamond/Al-matrix composite
Abstract. Fe- or Mn-powders were mixed with the diamonds and the mixtures were heat treated under different gas atmospheres like hydrogen or argon gas and at varying temperatures in order to roughen the surface of the diamond particles. Subsequently the reaction layers are removed by the addition of aqueous solutions of HCl. The modified surface structure after the etching process is investigated by electron microscopy showing increased roughening of the formerly flat faces of the diamond particles with increasing heat treatment temperature. After drying the diamonds, composites were prepared by gas pressure infiltration with pure Al. For treatments in the temperature range from 750 to 850°C the thermal conductivity can be improved by up to 20 percent compared to composites based on un-treated powders.
Introduction
The aiming for improved heat sink materials exhibiting highest thermal conductivities at tailored coefficients of thermal expansion motivates the search for improved composite materials and for improved interfaces between any matrix and highly conductive inclusion phases. Diamond MMCs are amongst the most promising candidates to fulfill all the challenging requirements. In order to make use of the full potential of the inclusions, the interface must be carefully addressed. Research so far has mainly focused on using carefully adjusted matrix alloys or on the use of suitable coatings of the inclusions [1, 2, 3]. For this, the diamonds can be either coated with carbide forming elements like B, Si or Ti, or formation of a strong interface can be conferred by using alloys containing reactive elements like Fe, Ni, Mo, B, Si, Cr etc., the kind of element mainly depends on the used matrix metal.
A rather rarely employed approach is to increase the specific surface area between the matrix and the inclusions to improve mechanical adhesion and heat flow. Since the diamond surfaces exhibit quite smooth and plane faces the specific surface for a given inclusion size is rather low. In order to increase the specific surface the faces of the diamond must be structured or roughened. To obtain a rough surface on the synthetic cubed-octahedral diamond grains, a thermal etching method at 700-1100°C in air at atmospheric pressure has been applied [4]. Such oxidizing conditions resulted in a preferable attack of the {111} face of the diamonds, as the etch rate is here higher than that of the {100} face, resulting in an increased surface area of a diamond grain. It has been shown that on etching at 900°C for 15 min, surfaces become fully covered with clear etch pits and the surface area shows maximum value. Etching with mixtures of gases at temperatures of 800- 1400°C has shown that O and water vapor are the relevant constituents of air in the formation of well-defined etch pits. Surface graphite formation below 1400°C is also considered [5]. The mechanism of formation is essentially a chemical reaction and not connected with a purely physical phase change.
Others report the use of NaNO3 or KNO3 for etching diamonds [6], whereas also a controlled reaction between carbide forming elements like Mn, Co or Fe can be used [7, 8]. To increase the surface of the diamond grains to at least double the natural area, the grains are embedded in metal particles at >700°C with passage of a stream of H2 or H2-containing gas. The carbon (diamond) diffuses into the metal and reacts with the metal-activated hydrogen progressively and creates pinpoint pores. After this roughening the diamond grain may be cleaned with acid. In this contribution we have used fine Fe and Mn powder as reactive environment to expose the diamond. After cleaning of the diamond from the carbides formed at high temperature, the diamond powder is packed in composites made by gas pressure assisted infiltration of liquid aluminium into the powder beds. The changes in diamond surface structure and of the metal diamond interface are quantified by electron microscopy and by the thermal conductivity of the resulting composites, respectively.
Experimental The method used for the roughening of the original smooth surface of the diamonds comprises the mixture of the respective diamond powders (Qiming Superabrasive Materials, Henan, China) with fine Fe-, or Mn-powders (BASF carbonyl iron powder CN, Mn-powder: Merck) at a weight ratio of diamond:metal = 1:1 followed by a heat treatment under inert (Ar gas or N2) or reducing gasatmospheres (H2 gas) in a muffle furnace. Subsequently, the reacted mixtures are etched by the addition of boiling aqueous solutions of HCl in order to completely remove the reaction layers and therefore end up with overall attacked – “etched” – diamond surfaces.
The heat treatment procedure for the diamonds was varied regarding time, temperature and atmosphere. Various isotherm heat treatments in the range between 700°C and 900°C were selected, either applying Ar- or H2-atmosphere during processing. The isothermal holding time was selected as 1h for temperatures of 700°C, 750°C and 800°C, and 1.5 hours for 850° and 900°C. The diamonds selected were of MBD4 and JR1 quality and had a grit size of 70/80 (i.e. average particle diameter 200 µm) or 500/600 (only MBD4; i.e. average particle diameter 30-35 µm). The crystals of JR1 are irregularly flaky and needle-like with rough surfaces, whereas the MBD4 quality contains more perfect crystals with smooth surfaces. The diamonds were subsequently gas pressure infiltrated with pure Al. To this end the diamond powder was filled and packed to roughly 60 vol.- pct into a cavity machined in a graphite mould that had the slightly conical shape of a sample for thermal conductivity, i.e. a height of 36 mm and a diameter of 13.3-13.7 mm. The graphite mould was inserted in a graphite coated alumina crucible and an ingot of 4N aluminium was placed on top. The crucible was inserted in a cold wall reactor equipped with an induction coil and a graphite susceptor. After having slowly pulled a vacuum down to below 0.1 mbar, the insert was heated to 750°C at a rate of 100-200 K/hour. After stabilization at the infiltration temperature for 30 min argon gas pressure was applied to 0.75 MPa. The infiltrated powder was solidified under pressure. Thermal conductivity measurements at room temperature were performed by a steady-state; equal heat flow method in which the temperature gradients of a reference and the sample connected in series is compared.
“Etched” diamonds were characterized by scanning electron microscopy SEM (Quanta 200 Mk2). Reactions between the Fe and Mn-powders and the diamonds were studied in detail by using a combined Differential Thermal/Thermogravimetry Analysis (DTA/TGA) equipment by heating up the mixtures under Ar gas atmosphere (H2 gas atmosphere was not applicable) and simultaneous detection of evolving gases from reactions by mass spectroscopy MS (Netzsch STA 449C + QMS 403C: heating rate 20 K·min-1).
Results and Discussion reflects the results for a reaction of the diamonds mixed with Fe-powders and heated under Ar gas atmosphere up to 900°C. The dashed curve represents the TG signal (mass loss); the bold line corresponds to the DTA signal [µV/mg]. The DTA signal indicates a reaction at approx. 750°C, and the TGA measurement gives a mass loss at 690°C (mass change -0.15%). The simultaneous recording of the isotope m44 (CO2), m28 (CO or N2), m32 (S or O2), m18 (H2O), m15 (organic compounds or CH3) and m12 (C) by mass spectroscopic measurements indicate evolving gases by reactions during heating. The results show a distinct reaction starting at approximately 470°C, as well as a very pronounced one at about 680°C, with its maximum at about 730°C, both characterized by evolving CO/CO2 gases (m44 and m28). m12 (C) and m32 (O2) give a simultaneous signal, but give no further relevant information. Therefore both – together with m18 and m15, which also show no relevant signals
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