Papers by Keyword: Hydrofluoric Acid (HF)

Abstract: In this study, we developed aA novel abrasive-free polishing method called the catalyst-referred etching (CARE) has been developed. CARE can chemically remove SiC chemically with using an etching agent activated by a catalyst. Platinum and hydrofluoric (HF) acid are used for the planarization of SiC substrates as a catalyst and etchant, respectively. CARE can produce an atomically flat surface of 4H–SiC (0001) with a root-mean-square roughness of less than <0.1 nm, regardless of the cut-off angle. However, the mechanism of CARE has hasis not yet been clarified to date. In this study, to clarify the mechanism, KF and NH4F are added to the etchant to clarify the mechanism. The An investigation of removal rate revealeds that the removal rate is proportional to [HF]×([F^- ]+[〖HF_2〗^- ]), and it is shown that both the HF molecule and fluorine ions (F− and HF2−) arethe reactive species of the CARE process are both HF molecule and fluorine ions (F- and HF2-).

Abstract: In order to improve the corrosion resistance, the samples made of Mg-Zn-Zr alloy were immersed in 20% or 40% hydrofluoric acid (HF) aqueous solutions for different intervals to prepare magnesium fluoride (MgF₂) coating on the surface. By comparing the surface morphologies, the samples immersed in 20% HF solution for 6 h on which fine particles in nanoscale covered was selected for the further study. Immersion and electrochemical tests showed that the dense MgF₂ coating would improve the corrosion resistance of Mg-Zn-Zr alloy. The corrosion current density (i_corr) decreased from 2.10 μA·cm^-2 to 0.05 μA·cm^-2. The influence of HF treatment on the cytocompatibility was evaluated in vitro. There were significant differences in the cell number between the naked and coated samples after culturing for 3 and 5 days (p<0.05). All the results demonstrate that HF treatment is a promising approach to improve the corrosion resistance and in vitro biocompatibility of Mg-Zn-Zr alloy used as intravascular stents.

Abstract: We report a damage-free and efficient planarization process for silicon carbide (SiC) using platinum as a catalyst in hydrofluoric acid (HF) solution. In previous studies, 4H-SiC (0001) on-axis wafers were planarized by this process and an extremely flat surface was obtained. However, electronic device substrates require off-axis wafers. In the present study, 4H-SiC (0001) 8° off-axis Si-face wafers were planarized using a Pt catalyst plate and HF solution. In the first trial using these wafers, the surface roughness worsened and a diagonal pattern was observed by phase-shift interference microscopy. The pattern seemed to have been formed when the Pt plate morphology was transcribed onto the wafer. The removal rate of the 8° off-axis Si-face wafer is much greater than that of the on-axis Si-face wafer. Thus, we concluded that the use of a smoother catalyst plate would be necessary to obtain a smooth 8° off-axis Si-face wafer surface. Improving the Pt plate morphology by hand lapping also improved the surface roughness of the processed wafer as compared with the preprocessed surface. The maximum height of the surface irregularity (peak-to-valley, P-V) and root-mean-square roughness were improved to 0.513 nm and 0.044 nm, respectively, as determined by atomic force microscopy (2×2 μm2).

Abstract: The characterization of the early reaction steps of the corrosion process in liquid media represents a tremendous challenge within the field of chemical analysis. In response to this, some known instrumental methods like AFM, SIMS and SNMS have proven themselves useful. Within the present work on the subject, we will introduce an optimized direct chemical etching technique which allows the quantification of the primary removed glass surface layers. It is based on the established etching procedure using hydrofluoric acid in combination with other mineral acids. The above mentioned method is modified and appropriately extended by enhancing the precision of the selected exposure time with the help of an automatic dipping device. Other improvements include the minimization of the dissolution rate by lowering the test temperature to – 10°C and also the decrease of the aqueous activity by adding alcohol instead of water into the acid solutions. Furthermore, by applying sequential etching with low concetrations of HF and HNO3, very small removal depths within the range below 10 nm per step are obtained. The equivalent components from the dissolved surface layers of the examined float glass are determined by subsequent chemical analysis of the etchants via AAS and ICP-OES. Their dissolution character can be well distinguished through a comparison between the gravimetric mass loss qgravim and the so-called normalized mass release per surface area qsol, calculated from the chemical composition of the corrosive solutions.

Abstract: We report the damage-free planarization of 4H-SiC (0001) wafers using a new planarization technique we named CAtalyst-Referred Etching (CARE). The CARE setup equipped with a polishing pad made of a catalyst is almost the same as a lapping setup. Since the catalyst generates reactive species that activate only when they are next to the catalyst surface, SiC can be chemically removed in contact with the catalyst surface with a pressure noticeably lower than that in a conventional polishing process. The processed surfaces were observed by optical interferometry and AFM. These observations presented a marked reduction in surface roughness. A step-terrace structure was observed with a step height of approximately 3み, corresponding to one-bilayer thickness of Si and C, in the AFM images. To estimate the crystallographic properties of the CARE-processed surface, the surfaces were observed by cross-sectional TEM. The TEM images showed that a more crystallographically well-ordered surface was realized in comparison with the conventional CMP-processed surface.

Abstract: Surface characteristics of anodic titania formed by electrochemical method and effects of electrolytic mediums on microstructures of anodic TiO2 layer were investigated. The titania film formed in sulfuric acid or mixture electrolyte with sulfuric and phosphoric acids shows porous cell structures. But the films formed in sulfuric acid represent smaller cell structures than those formed in the mixture electrolyte, and the change in microstructural parameters is associated with electrolytes. However, when titanium substrate was anodized in HF, surface morphologies and structures of anodic titania film greatly changed, forming self-organized TiO2 nanotubes.

Abstract: TiO2 coatings on titanium alloy substrates were prepared by atmospheric plasma spraying using commercial nano-powders. Then, as-sprayed coatings were treated using 10% hydrofluoric acid (HF) at room temperature for 30 seconds. As-sprayed and HF-treated titania coatings were soaked in simulated body fluid to investigate the formation of apatite on their surface. Field-emission scanning electron microscopy was used to observe the surface morphologies, and the phase composition of the as-sprayed coating and apatite were analyzed by X-ray diffraction and energy-dispersive X-ray spectrometry. As-sprayed titania coating is composed of rutile, anatase, and a small quantity of Ti3O5. It exhibited excellent adhesion between the TiO2 coatings and titanium alloy substrates, and the bonding strength was about 38 MPa. After in vitro experiment, a new substance containing calcium and phosphate was formed on the surface of HF-treated TiO2 coatings after being soaked in SBF, while the new substance was not formed on the surface of as-sprayed TiO2 coatings. The results indicated that the bioactivity can be induced to the surface of plasma sprayed TiO2 coatings by hydrofluoric acid treatment.