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The present study is aimed to distinguish the different influences of CaO and MgO, as well as Al2O3 and SiO2 on viscosity of CaO-MgO-Al2O3-SiO2 melts. It is found that for melt without Al2O3, viscosity increases monotonously as CaO is gradually replaced by MgO. The addition of Al2O3 leads to a complex variation of viscosity. In different composition ranges, viscosity may exhibit different variation tendencies as changing the relative contents of CaO and MgO while keeping contents of other components constant. It is also found that when replacing SiO2 by equivalent mole of Al2O3, (i) in CaOAl 2O3-SiO2 system, viscosity increases when Al2O3 content is small relative to CaO content (there are enough Ca2+ ions to charge compensate Al3+ ions), but decreases when CaO content is inadequate for charge compensation; (ii) in MgO-Al2O3-SiO2 system, viscosity always decreases as substituting Al2O3 for SiO2. The reason for this difference for two systems may be resulted from the weak charge compensation ability of Mg2+ ion for Al3+ ion relative to Ca2+ ion. All the viscosity variations can be interpreted well by our new proposed viscosity model.

Abstract: Various gases such as N2, O2, and CO2 have been introduced in the typical reaction atmosphere of CH4/H2 and proposed to improve the growth of chemical vapor deposited (CVD) single-crystal diamonds (SCDs). In this paper, we study the influence of a new adding gas nitrous oxide (N2O) on the growth rate, morphology, and optical properties of homoepitaxy (100) CVD SCDs. The reaction pressure (H2/CH4 flow rates) was fixed at 300Torr (750/90 in sccm) with the addition of a small amount of N2O gas varied at flow rates of 0, 2, 5, 8 and 10sccm. With the appropriate addition of N2O, the growth rate was increased up to 135μm/h and the surface roughness was decreased to around 2nm. Furthermore, adding N2O is favorable for inhibiting the generation of large anti-pyramidal pits on the top surface of SCDs, which generally appeared in the products synthesized in CH4/H2 ambient. The combined effect of the nitrogen- and oxygen-related radicals decomposed from N2O on the growth and properties of the CVD SCDs is discussed. As a result, the addition of N2O provides a new route to realize high-rate growth CVD SCDs instead of the traditional nitrogen. [Copyright &y& Elsevier]

Abstract: In this paper, we report the effect of gaseous carbon dioxide (CO2) introduced in the typical reaction atmosphere of CH4/H2/N2 (60/500/1.8 in sccm) on the growth rate, morphology and optical properties of homoepitaxy single crystal diamonds (SCDs) by microwave plasma chemical vapor deposition. The additional carbonaceous sources supplied by CO2 are favorable to increase the growth rate, and meanwhile, the oxygen related species generated would enhance the etching effect not only to eliminate the non-diamond phase of SCD but also to decrease the growth rate. The appropriate addition of CO2 can increase the high growth rate, decrease the surface roughness, and reduce the concentration of N-incorporation. It is demonstrated that adding CO2 strongly affects the contents of various reaction species in plasma, which would determine the growth features of CVD SCDs. [Copyright &y& Elsevier]