| Many mechanical applications ranging from abrasives and cutting tools to scratch-resistant coatings constantly demand superhard materials. Currently, only diamond and cubic boron nitride (c-BN) are used in industry as superhard materials. Although diamond, with the highest known hardness (HV～70 to 100 GPa), has been widely used in industry, there are limits for its broad applicability. For example, diamond can not be used to cut steel or other ferrous metals because the formation of iron carbide is detrimental during high-speed machining. Diamond synthesis using metallic catalysts generally requires high P-T conditions (>5 GPa,>1.000℃). Cubic boron nitride (c-BN), the second hardest material (HV～45 to 50 GPa), has higher thermal stability and chemical inertness, and can be utilized to cut steel or other ferrous metals. However, c-BN must be synthesized under extreme pressure (> 5 GPa) and temperature (>1500℃), making it an expensive alternative.In an attempt to meet industrial need for alternative superhard materials, two boron compounds, boron suboxide (B6O) crystal and cubic BC2N, were recently synthesized. While BeO crystal has a hardness of 45 GPa that rivals c-BN, cubic BC2N has a hardness between c-BN and diamond. These compounds, however, require extreme pressure to synthesize, exceeding 5 and 18 GPa for B60 and cubic BC2N, respectively. While these compounds have the desired hardness for industrial applications, the expensive high pressure conditions make them impractical.One way to overcome this problem is to design and synthesize new superhard materials; another way is to explore milder conditions for the synthesis of known superhard materials, such as B6O crystal and diamond. In this dissertation, we report the synthesis of diamond and B6O under milder conditions.Diamond can be synthesized using high pressure and high temperature method or chemical vapor deposition (CVD) method. Static high pressure technique, with pressure higher than 5 GPa, was used to synthesize diamond in industry. Many efforts have been done in order to search new method to synthesize diamond. Chen and Qian et al reported the synthesis of diamond by reduction of dense CO2 with metallic sodium at a temperature as low as 440℃in an autoclave. Sachdev et al suspected this result. Therefore, potassium (K) and sodium carbonate (Na2CO3) were used to synthesize diamond in order to study the feasibility of the synthesis of diamond using alkali metals and carbonate. Potassium and sodium carbonate reacted at 2 GPa and 500℃for 18 h and the recovered sample contained micrometer-size diamond. CO can not decompose to diamond and CO2 but graphite and CO2 under the conditions above. A possible process was discussed:Na2CO3 was decomposed on the existence of K and Pt, and generated CO2 reacted with K to synthesize diamond. This result confirms that diamond can be synthesized using alkali metals and carbonate and provides a new carbon source for the synthesis of diamond.Because of its short interatomic bond lengths and strongly covalent character, B6O displays a range of outstanding physical and chemical properties such as great hardness (rival that of cubic BN), low mass density, high thermal conductivity, high chemical inertness, and excellent wear resistance (similar to that of diamond). A wide range of microhardness (Hyfrom 31 to 38 GPa) was reported for poly crystalline boron suboxide sintered compacts, which is lower than the hardness of single B6O crystal (45GPa). It is known that higher crystallinity results in higher hardness. B6O can potentially be used as a substitute for diamond or c-BN in various mechanical applications requiring superhard material. B6O can be synthesized by oxidation of boron with boron oxide (B2O3), or zinc oxide and other oxidants at or near ambient pressure. These experiments were performed at high temperature and in an argon atmosphere or using hot press method. These boron suboxide materials have poor crystallinity and are generally oxygen deficient (B6Ox, x<0.9). Icosahedral B6O particles with high crystallinity were obtained from amorphous B and B2O3 mixtures reacted under extreme conditions (P>5GPa, T>1700℃), similar to diamond synthesis. While B6O has the desired hardness and excellent wear resistance for industrial applications, the expensive high pressure conditions make them impracticalHere we report the synthesis of B6O combining ball milling with high pressure methods. The starting materials, boron and milled h-BN, reacted at mild conditions (1 GPa and 1400℃for 2 h) and icosahedral B6O particles with an average size of 300 nm were obtained. However, it is very difficult to separate B6O and h-BN from the product and this method can not be utilized to prepare B6O. Milled h-BN oxidized in air and generated B2O3 reacted with boron to synthesize B6O.The starting materials, boron and milled boron oxide, reacted at 2 GPa and 1400℃for 6 h and icosahedral B6O particles with sizes ranging from 100 nm to 1.3μm and an average size of 500 nm were obtained. The size of the milled B2O3 was very small and thus its surface area increased greatly with decreasing particle size, resulting in increased contact between B2O3 and boron. Furthermore, the reaction mixture containing the melted in situ B2O3 may have a higher homogeneity, which would make the reaction easier to proceed when compared to the reaction mixture containing reagent B2O3. The factors that affect the synthesis of B6O are discussed, including pressure, temperature, time, reagent ratio and way of sample assembly. High temperature of 1400℃is favored for the growth of icosahedral B6O grains. High pressure applied during the synthesis of B6O can significantly increase the crystallinity of the products. Long reaction time was found to be detrimental for the growth of well-crystallized icosahedral B6O grains. The favored sample assembly is that the starting materials were encapsulated in an h-BN capsule layer upon layer, with milled B2O3 at both the bottom and top of the capsule. Hardness testing was preformed on a MTS Nanoindenter. However, the most B6O particles are smaller than 1μm, and thus a precise hardness value of icosahedral B6O grain can not be obtained. The load-displacement curve of B6O sample gave an average hardness of 32.3 GPa, which is consistent with previous reports. Thermogravimetric analysis was used to study the thermal stability of the recovered sample. B6O sample exhibited an oxidation resistance in air up to 1000℃and mild oxidation in the temperature range of 1000～1200℃, which was higher than the temperatures observed for diamond and rivaled that of c-BN.Boric acid and boron was used to synthesize B6O. Boric acid and boron reacted at 1 GPa and 1300℃for 2 h and homogeneous icosahedral B6O crystals with an average size of 100 nm were obtained. The factors that affect the synthesis of B6O are discussed. High temperature (1400℃) is unfavorable for the growth of icosahedral B6O. High pressure was found to significantly increase the crystallinity of B6O products but limit the growth of B6O particle. Long reaction time was found to be detrimental for the growth of well-crystallized icosahedral B6O particles. Icosahedral B6O particles were not found using excess H3BO3 in the reaction mixtures or encapsulating the starting materials in the capsule in layers. Because of the small size of the crystals and the error in the hardness testing, a precise hardness value can not be obtained from the nanoindenter. Alternatively, a scratch test was performed to determine the approximate hardness of the sample based on the Mohs hardness scale. B6O sample readily scratched the surface of WC, indicating that the B6O sample has a higher hardness than WC. B6O sample was stable in air up to～600℃and then slowly began to oxidize at higher temperatures up to 1000℃, which was higher than those observed for diamond. Compared with the temperature 1200℃above,1000℃is low, which indicates that the larger size, the higher thermal stability.Icosahedral B6O particles with high crystallinity were obtained under the milder synthesis conditions (1-2 GPa,1300-1400℃) in comparison to previous work (>5 GPa,>1700℃). Based on the milder synthetic conditions and its outstanding physical and chemical properties (high hardness and high thermal stability, etc.), B6O is very possible to be used as a substitute for diamond or c-BN in various mechanical applications requiring superhard material.
|
PDF Full Text Request