Ju-Liang He

Thermally induced stress voids in hermetically carbon-coated optical fibers with different coating thickness

The thermally induced stress voids in carbon-coated optical fibers are investigated. Ten samples of carbon-coated optical fibers with different coating thickness are prepared using the plasma-enhanced chemical vapor deposition method. The carbon structure exhibits a larger amount of graphite-like phase and a smaller amount of disordered diamond-like phase. After these fibers are immersed in the liquid nitrogen for 1 day, thermally induced stress voids propagated along the fiber axial direction are found. It is believed that stress voids are caused by the thermally induced maximum tangential stresses. The stress voids slightly decrease with increasing the coating thickness in the beginning, and then increase. The carbon-coated optical fiber with a coating thickness of approximately 255 nm induces the minimum number of stress voids. Nevertheless, the stress voids become larger and even in-line cracks when the coating thickness is equal to or larger than 800 nm.

Effect of coating thickness on thermal stresses in tungsten-coated optical fibers

This study investigates the effect of coating thickness on the thermal stresses in tungsten-coated optical fibers. Theoretical results indicate that the maximum normal stress in the tungsten coating decreases with increasing coating thickness. However, the maximum shear stress at the interface of the glass fiber and tungsten coating increases. Eight samples of tungsten-coated optical fibers with coating thicknesses of 58, 75, 101, 128, 158, 383, 557, and 1013 nm, respectively, are immersed in liquid nitrogen for one day. Experimental results show that thermal stresses will either break or delaminate the tungsten coating. The crack density decreases with increased coating thickness, while the delaminated area of tungsten coating increases. The theoretical results can explain the break and delamination of the tungsten coating in the optical fiber. To minimize the break and delamination of the tungsten coatings in the optical fibers, the optimal thickness of the tungsten coating is about 158 nm.

Carbon nitride films incorporated with metal by rf plasma enhanced chemical vapor deposition

Abstract Carbon-based coatings alloyed with metal elements by using various routes had been reported. This encouraged a study on the effect of metal alloying on the carbon nitride films deposited by using plasma- enhanced chemical vapor deposition (PECVD) which has some advantages over the other plasma-assisted processes. The sputtering phenomenon occurring on rf electrode enables the incorporation of metal to carbon nitride films. An attempt is to reveal microstructure and mechanical properties of the films deposited by using this synergetic process with N2 CH4 H2 premixed gas as precursors. Chromium, iron, 304 stainless steel and graphite were chosen as rf electrode materials. Experimental results show that carbon nitride films composed of amorphous matrix and microcrystalline β-C3N4 were deposited by using PECVD. Metal content of the deposited films linearly increases as the N2 gas fraction is increased, with a rate depending on the rf electrode material used. This highlights the sputtering effect of ionized N2 gas. The incorporated metals display in forms of oxide, carbide and amorphous matrix, which result in a change in microstructure and wear behavior. Chromium induces an adhesion wear and exhibits the highest friction coefficient, yet has the lowest wear rate.

Enhancement of bioactivity on medical polymer surface using high power impulse magnetron sputtered titanium dioxide film.

This study utilizes a novel technique, high power impulse magnetron sputtering (HIPIMS), which provides a higher ionization rate and ion bombardment energy than direct current magnetron sputtering (DCMS), to deposit high osteoblast compatible titanium dioxide (TiO2) coatings with anatase (A-TiO2) and rutile (R-TiO2) phases onto the biomedical polyetheretherketone (PEEK) polymer substrates at low temperature. The adhesions of TiO2 coatings that were fabricated using HIPIMS and DCMS were compared. The in vitro biocompatibility of these coatings was confirmed. The results reveal that HIPIMS can be used to prepare crystallinic columnar A-TiO2 and R-TiO2 coatings on PEEK substrate if the ratio of oxygen to argon is properly controlled. According to a tape adhesion test, the HIPIMS-TiO2 coatings had an adhesion grade of 5B even after they were immersed in simulated body fluid (SBF) environments for 28days. Scratch tests proved that HIPIMS-TiO2 coatings undergo cohesive failure. These results demonstrate that the adhesive force between HIPIMS-TiO2 coating/PEEK is stronger than that between DCMS-TiO2 coating/PEEK. After a long period (28days) of immersion in SBF, a bone-like crystallinic hydroxyapatite layer with a corresponding Ca/P stoichiometry was formed on both HIPIMS-TiO2. The osteoblast compatibility of HIPIMS-TiO2 exceeded that of the bare PEEK substrate. It is also noticeable that the R-TiO2 performed better in vitro than the A-TiO2 due to the formation of many negatively charged hydroxyl groups (-OH(-)) groups on R-TiO2 (110) surface. In summary, the HIPIMS-TiO2 coatings satisfied the requirements for osseointegration, suggesting the possibility of using HIPIMS to modify the PEEK surface with TiO2 for spinal implants.

Using Micro-Arc Oxidation and Alkali Etching to Produce a Nanoporous TiO2 Layer on Titanium Foil for Flexible Dye-Sensitized Solar Cell Application

To increase the specific surface area of a TiO2 layer synthesized by micro-arc oxidation (MAO), an alkali etching process was developed to form a nanoflaky structure in place of the existing microporous morphology of the MAO-TiO2 layer for dye-sensitized solar cell (DSSC) electrode application. An annealing treatment was also carried out to enhance the crystallinity of the nanofeatured TiO2 layer for a higher photovoltaic efficiency. Experimental results show that a 6-µm-thick crystalline porous TiO2 layer was fabricated on a Ti foil by MAO treatment, which consists of major amorphous and anatase phases with a minor rutile phase. As expected, the pores in the MAO-TiO2 layer exhibited micrometer-scale dimensions. The maximum photovoltaic efficiency realized in a device assembled with the MAO-TiO2 layer was only 0.061%. After alkali etching, a nanofeatured layer was developed over the MAO-TiO2 layer surface with numerous pores and nanoflakes of 50 nm size. These nanoflakes were uniformly distributed over the entire surface of the treated layer. The device assembled with the alkali-etched TiO2 layer exhibited an improved photovoltaic efficiency of 0.329%. This fivefold increase of the photovoltaic efficiency for the MAO-TiO2 layer indicates the effectiveness of enlarging the specific surface area by alkali etching. Furthermore, after postannealing, the crystallinity and fraction of the anatase phase in the overall TiO2 layer were enhanced. As a result, the photovoltaic efficiency ultimately reached 2.194%.