Fluorination Treatment Research Articles

CF4 plasma-fluorinated nano-SiC promotes the charge transfer in the interface of epoxy nanocomposites

Insulation failure and surface flashover of nanocomposites insulator caused by charge accumulation remain to be major hurdles in the application of high voltage equipment. In this study, we develop a plasma fluorination treatment method to graft fluorinated functional groups onto the surface of nano-SiC, in order to change the interfacial characteristics of the nanocomposites and promote charge transfer. Surface potential measurement and volume conductivity measurement are performed to evaluate charge transfer in the nanocomposites. When the concentration of fluorinated nano-SiC is 5 wt.%, the surface decay rate of the sample is larger than that of samples doped with less nanofiller, and the initial surface charge density is 79.8% that of the undoped sample. Furthermore, the fluorinated nano-SiC/epoxy nanocomposites show larger volume conductivity because of the introduction of CF3 functional groups, which influences the interfacial characteristics and promotes charge transfer in the interfacial regions of the nanocomposites. This work demonstrates the effect of charge transfer on surface flashover of the nanocomposites and the plasma fluorination treatment of nanoparticles is a novel approach to promote charge transfer and boost surface electrical performance of nanocomposites.

High-performance quasi-vertical GaN Schottky barrier diode with anode selective fluorine treatment

In this letter, we report a high performance GaN quasi-vertical Schottky barrier diode (SBD) on sapphire substrate with a planar anode selective fluorine treatment (SFT). The presented SBD with anode SFT–SBD exhibits a large forward current density over 2 kA cm−2 and a low differential specific on-resistance of 0.49 mΩ cm2. Compared to the conventional SBD, the leakage current of SFT–SBD is suppressed over 4 orders of magnitude, leading to the breakdown voltage (BV) dramatically improved from 78 to 145 V, but without severe degradation of the on-state performance. The simulation results indicate that the anode SFT can effectively reduce the path of leakage current at reverse bias, leading to the suppressed leakage current thus enhanced BV.

Tactical modification of pseudo-SILAR process for enhanced quantum-dot deposition on TiO2 and ZnO nanoparticles for solar energy applications

Pseudo successive ionic layer adsorption and reaction (p-SILAR) process is a unique route to develop quantum-dot (QD) sensitized nanocomposites (NCs) via QDs deposition on metal oxide nanoparticles (NPs). In this work, we have successfully increased the deposition of PbS and CdS QDs on TiO2 as well as ZnO NPs using a simple wet chemical modification of p-SILAR. An optimal fluorination treatment of both NPs with 0.1 M aqueous solution of NH4F was carried out under centrifugation. Consequent increase in deposition of PbS and CdS QDs was affirmed using scanning electron microscopy, EDX, UV–vis spectroscopy and XRD. Eventually, solar paint based photovoltaics and photocatalysis were carried out to show that modification of p-SILAR enabled TiO2-PbS NC to exhibit ˜30% increase (from 6.5 mA/cm2 to 8.5 mA/cm2) in the photocurrent density (JSC) of solar cells and more than 11% increase (from 81% to 92%) in the degradation of congo red (CR) dye.

Stand-off fabrication of irregularly shaped, multi-functional hydrophobic and antireflective metal surfaces using femtosecond laser filaments in air

Multi-functional metal surfaces with simultaneous superhydrophobic and antireflective properties have a wide range of industrial, medical, and defense applications. Rapid fabrication of such multifunctional surfaces on irregularly shaped metals and on a large scale has been a challenge. Here we demonstrate an approach for rapid, standoff fabrication of multi-functional surfaces with super-hydrophobic and antireflective properties on irregularly-shaped samples, by texturing them with femtosecond laser filaments in air, followed by chemical fluorination treatment. Two types of metals with a variety of curved surfaces have been processed at a standoff distance of 1 m: the type 304 stainless steel and the type Ti–6Al–4V titanium alloy. Through laser processing and fluorination, multi-functional surfaces with the water contact angle of over 150° and diffused reflectance below 9%, over the wavelength range from 240 to 2500 nm, have been realized on both metals. Our results suggest that this technique could enable cost-effective and large-scale fabrication of irregularly-shaped, multi-functional surfaces that can be useful for various applications.

Oxidation behaviour of the MAX-phases Ti2AlC and (Ti, Nb)2AlC at elevated temperatures with and without fluorine treatment

MAX-phase materials have shown great potential for different technical applications due to their mechanical properties. If the main group element is aluminium their excellent oxidation resistance also makes them attractive for several high temperature applications. As an example the thermodynamically stable MAX-phase Ti2AlC forms a thin, protective alumina layer in oxidising atmospheres at elevated temperatures. This alumina layer is formed due to the high Al activity within the material and prevents further attack by the environment. However, high temperature oxidation tests at 900 °C in air of “technical” Ti2AlC which is not pure single-phase Ti2AlC led to the formation of a non-continuous alumina scale which is intersected by a mixed TiO2/Al2O3 scale. Furthermore, internal oxidation was observed. This “technical” material consists of two phases namely Ti2AlC plus γ-TiAl due to the manufacturing route. Such γ-TiAl-grains are preferentially oxidised. This type of internal attack can be suppressed by a preceding fluorine treatment.

Fabrication of Super-hydrophobic Surfaces and Studies of Water Flow in Super-hydrophobic Micro-tubes

Hierarchical alveolate structures of micro-nano scale were prepared on stainless steel substrates with chemical etching. Fine papillae stand on protuberances are found. After fluorination treatments, the surfaces exhibit super-hydrophobic properties, with water contact angles of 163°. Stainless-steel micro-tubes with super-hydrophilic and super-hydrophobic surface were fabricated with chemical etching and chemical etching-fluorination treatment respectively. The effect of surface wettability on the fluid flow in micro-tubes was investigated with water as the working fluid. The experimental setup was designed in such a way that the investigation of the pressure drop and friction factor was possible. It is found that friction factor in super-hydrophilic micro-tube is in good agreement with Hagen-Poiseuille theory at low Reynolds number (Re<600), while higher than classical values at higher Reynolds number. The pressure drop of water flowing in super-hydrophobic micro-tube decreases compared with that in super-hydrophilic micro-tube, with the maximal decrease of 39%, which indicates the prospective utilization in process industry.

Enhanced electrochemical performance of LiNi0.8Co0.15Al0.05O2 cathode by reducing lithium residue with low-temperature fluorination treatment

LiNi0.8Co0.15Al0.05O2 is considered as a representative layer-structured cathode material. Decreasing residual lithium (LiOH and Li2CO3) on its surface is the main challenge to improve cycling performance for commercial application. In this study, a dry and low-temperature fluorination (350 °C) method was proposed to modify LiNi0.8Co0.15Al0.05O2 cathode. The effects of fluorine content on the microstructure, surface composition and electrochemical properties of the cathode were extensively studied. The results showed that fluorination had multi-functions: (i) decreasing the lithium residues and generating amorphous LiF coating layer on the particle surface, (ii) restraining the diffusion of metal ions conveyed in the electrolyte, particularly at high temperature, (iii) retarding the increase of charge transfer impedance, and (iv) maintaining the structural stability of electrode during extended cycles. Electrochemical test results indicated that the sample fluorinated with 1.0 mol% F exhibited excellent cycling performance with the capacity retention of 93.9% after 100 cycles at 1 C, showing dramatical improvement compared with that of the pristine cathode (86.2%). This designed low-temperature dry fluorination process is expected to be practically applicable in other nickel-rich cathodes.

Novel insulator with interfacial σ-FGM for DC compact gaseous insulated pipeline

Electric field optimization plays a significant role in the insulation design of gaseous insulated pipeline (GIL) for the DC power system. Functionally graded material (FGM) with spatial distribution of conductivity σ (σ-FGM) provides a potential method to uniform the electric field and thus reduces flashovers to some extent. However, researches at present are nearly at a standstill for the complicated fabrication techniques and huge bulk conduction losses. In this paper, a novel cone-type insulator with surface conductivity gradient was proposed based on direct fluorination treatment. Theoretical analysis shows that the electric field distortion near the high voltage electrode can be effectively suppressed with increasing the thickness and conductivity of the fluorinated layer. Epoxy cone-type insulators with homogeneous and graded surface conductivity were then prepared by different fluorination treatments to verify the effectiveness of σ-FGM design. As for the homogeneously fluorinated cone-type insulators, the DC flashover characteristics were improved with the rise of fluorination time. Especially, the flashover voltage of the insulators with σ-FGM layers can be raised by up to 36.3 % compared with that of the traditional ones, owing to a more uniform electric field distribution. Theoretical and experimental results prove that the insulator with surface σ-FGM is a promising choice for DC compact GIL.

Surface charge decay of epoxy resin treated by AP-DBD deposition and direct fluorination

Both atmospheric pressure dielectric barrier discharge (AP-DBD) deposition and direct fluorination are effective methods to modify the surface electrical properties of polymer insulation. To investigate the surface charge decay properties of Al 2 O 3 -filled epoxy resin (ER) prepared by AP-DBD and direct fluorination treatments, before and after drying, samples were studied under various treatment parameters. The experimental results show that charge decay rates for AP-DBD and fluorinated samples are more than 90% after 5 days in ambient air. The corresponding bulk and surface resistivity is reduced when the charge decay rate increases. After a longer time, the surface charge on the fluorinated samples decays faster than those on the AP-DBD samples before drying. The hydrophilicity of the fluorinated samples significantly affects the surface charge decay after drying. After 100 days of storage, the decay rates for the fluorinated samples decrease with increased storage time (less than 9% after 100 days) while for AP-DBD treated samples the decay rates exceed 24%. The surface resistivity of the AP-DBD samples is always lower than that of the fluorinated ones. Therefore, for a long storage time, AP-DBD treatment may provide a better solution to the surface charge decay of ER than direct fluorination treatment, after drying.

P‐11: Carrier Concentration Reduction by Fluorine Doping in P‐Type SnO Thin‐Film Transistors

Presently reported is the use of plasma fluorination treatment based on tetrafluoromethane to enhance the performance of a p‐type SnO thin‐film transistor (TFT). The improved performance metrics include a larger on/off current ratio, a smaller subthreshold swing and a reduction of an originally large positive turn‐on voltage. The effects of such fluorination treatment were also investigated using a host of thin‐film characterization techniques and their origin most plausibly attributed to both the significant reduction of the carrier concentration from ~2 × 1019 cm−3 to ~4.3 × 1016 cm−3 and the passivation of the defects. It is demonstrated that fluorination is an effective technique for making higher performance p‐type metal‐oxide TFTs.

Normally-off AlGaN/GaN MIS-HEMT with low gate leakage current using a hydrofluoric acid pre-treatment

We demonstrate the electrical performances of an AlGaN/GaN metal–insulator–semiconductor high electron mobility transistor (MIS-HEMT) with low gate leakage current (Ig). A low gate leakage current as low as the order of 10−11 A/mm was achieved from normally-off MIS-HEMT device (Vth = 2.16 V) with a partially recessed gate, fluorine treatment, and ALD Al2O3 gate dielectric layer. The gate leakage current decrease is attributed to the pre-treatment of the gate region with hydrofluoric acid (HF) and deionized water (DI) solution, which acts to remove the native oxide layer and thus decrease interface traps. X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) analyses demonstrate that the AlGaN surfaces are modified such that the surface roughness and native oxide introduced by the treatments used to achieve normally-off operation are remedied with the use of the pre-treatment.

Interaction of Perfluorinated Fluids with Fluorine in Gas-Liquid Reactor

Process of fluorination of perfluorodecalin and perfluoro-1,3-dimethyl cyclohexane with fluorine in a gas-fluid reactor with a high-speed stirrer was experimentally studied. It was shown that the fluorination reaction rate decreases in the process of fluorine treatment for 2 h and then stabilizes, being at 80°C 0.5 and 1.3 µg s–1 L–1 for perfluoro-1,3-dimethylcyclohexane and perfluorodecalin, respectively. A comparison of the fluorination rates of perfluorodecalin, perfluoro-1,3-dimethylcyclohexane, and fozhalin (main substance perfluorotripropylamine) made it possible to formulate recommendations on choosing a perfluorinated fluid as a medium for studying the fluorination of organic compounds with fluorine in a gas-fluid reactor, for which the loss of fluorine in the destructive fluorination of a fluid will be on the order of 10−1%.

Enhanced surface insulation and depressed dielectric constant for Al2O3/epoxy composites through plasma fluorination of filler

Al2O3, an insulating ceramic with excellent mechanical and thermal properties, has been a kind of ideal fillers for composites. However, due to its poor charge transport capacity, composites doped with Al2O3 have a bad surface insulation property. In this study, nano-Al2O3 particles were modified by plasma fluorination using dielectric barrier discharge, and then doped into epoxy resin to prepare composites. In the preparation stage, the plasma treatment with and without fluorine were differentiated to determine whether fluorine or plasma itself had an influence on the microstructure of Al2O3. The changes of dielectric and surface insulation properties of composite materials before and after filler fluorination were studied. The variation of dielectric constant as a function of filler content has presented a Z-shape curve. Both larger particle diameter and fluorination treatment can reduce the value of dielectric constant, and lead the curve to a monotonous increasing trend. A model of dielectric distribution by the finite element method was constructed to explain the variation. Before fluorination treatment, the flashover voltage as a function of filler content presents a multi-inflection variation. After fluorination, the surface charge dissipation capacity together with flashover voltage were enhanced, and the fluorination treatment could increase the flashover voltage by up to 34.7% of the pure epoxy. The modification process and test results are of guiding significance for improving the insulation properties of epoxy composites.

Atmospheric pressure plasmas and direct fluorination treatment of Al2O3-filled epoxy resin: A comparison of surface charge dissipation

In this paper, three methods, including dielectric barrier discharge (DBD) etching and deposition, and direct fluorination, were used for the surface modification of Al2O3-filled epoxy resin (Al2O3-ER) insulators to improve surface charge dissipation properties. The surface charge dissipation properties of the Al2O3-ER samples after these three treatments were compared. Firstly, the experimental results showed that DBD deposition reduced the most accumulated surface charges among the three treatments. The surface charge decay rates after DBD deposition and direct fluorination exceeded 98%. Then, the surface morphology, chemical components, and electrical parameters before and after the treatments were analyzed. Among the above three treatments, the surface roughness of the DBD deposited sample was the smallest, which facilitated the reduction of surface charge accumulation. The increase of the surface conductivity of the DBD deposited sample accelerated surface charge dissipation. Furthermore, the ageing effects of these three treatments were investigated. After five days of storage, it showed no significant difference between the dissipation properties of the DBD etched and the untreated samples due to the reorientation of polar groups towards the interface. However, there was only subtle changes in the surface charge decay rates of those DBD deposited and fluorinated samples, indicating that no obvious ageing effects were observed after DBD deposition and direct fluorination. The introduction of SiOx and C-Fn functional groups improved the anti-ageing properties of the DBD deposited and fluorinated samples, respectively. These comparative results can stimulate the development of eco-friendly plasma deposition techniques for the surface modification of Al2O3-ER insulators.

Electrospun CuS/PVP Nanowires and Superior Near-Infrared Filtration Efficiency for Thermal Shielding Applications

Selective filtration of near-infrared (NIR) regions is of primary importance to energy saving via thermal shielding. However, uniform coating of highly effective nanomaterials on flexible substrates remains very challenging. Here, we introduce new material processing and fabrication methodologies that manufacture electrospun copper sulfide/polyvinylpyrrolidone (CuS/PVP) nanowires for enhanced thermal shielding efficiency. Electrospinning offers well-dispersed CuS nanoparticles in a thermal shielding film, which is not achievable in typical solution coating processes. Directly deposited CuS/PVP nanowires on a flexible polymer membrane are enabled by a fluorination treatment that decreases the interfacial electrostatic repulsion. Monitoring of in situ temperature change of a box-shielded, CuS/PVP nanowire film demonstrates excellent NIR shielding efficiency (87.15%). Direct integration of the film with a model car and exposure to direct sunlight demonstrates about twice-higher shielding efficiency than commercial tungsten oxide films. Overall, the comprehensive study of nanomaterial preparation, surface treatment, and integration techniques allows the fabrication of highly flexible and reliable thermal shielding films.

Investigation of a Hybrid Approach for Normally-Off GaN HEMTs Using Fluorine Treatment and Recess Etch Techniques

A hybrid approach for obtaining normally off high electron mobility transistors (HEMTs) combining fluorine treatment, recess etch techniques, and AlGaN buffer was studied. The effects of process variations (recess etch depth and fluorine treatment duration) and epitaxial differences (AlGaN and carbon doped GaN buffers) on the DC characteristics of the normally off HEMTs were investigated. Two different epitaxial structures and three different process variations were compared. Epitaxial structures prepared with an AlGaN buffer showed a higher threshold voltage ( $V_{\mathrm {th}} = + 3.59 ~\text {V}$ ) than those prepared with a GaN buffer ( $V_{\mathrm {th}} = +1.85 ~\text {V}$ ).

Improved flame retardant performance of cellulose fibers following fluorine gas treatment

To improve the flame retardant performance of cellulose fibers, fluorine functional groups were introduced under various controlled fluorination conditions. The properties of the fluorinated cellulose fibers were analyzed by X-ray photoelectron spectroscopy and a thermogravimetric analysis. The fluorine functional group content in the fluorinated cellulose fibers increased with an increase in the fluorination temperature. However, the fluorination reaction increased the char yield and decreased the rate of degradation of the cellulose fibers by introducing donors, enabling the formation of a thick and compact char layer. Therefore, the flame retardant properties of cellulose fibers were improved following the fluorination treatment.

Effect of calcination temperature and fluorination treatment on NiF2-AlF3 catalysts for dehydrofluorination of 1, 1, 1, 2-tetrafluoroethane to synthesize trifluoroethylene

High-performance NiF2-AlF3 fluoride catalysts for the catalytic dehydrofluorination of 1, 1, 1, 2-tetrafluoroethane (CF3CH2F) were prepared by impregnation and fluorination methods. The effect of calcination temperature and vapor-phase fluorination on the properties of NiF2-AlF3 catalysts were investigated by BET, SEM, XRD, UV-DRS, Raman, IR, TG and XPS. By increasing the calcination temperature, the NiO species diffused from the surface into the inside bulk phase of the alumina support and the inverse spinel NiAl2O4 was formed at a calcination temperature up to 600 °C. Vapor-phase fluorination can improve the stability of the catalytic dehydrofluorination. The unfluorinated NiAl2O4 affected the surface area and acidity of NiF2-AlF3 catalyst. The acid sites of catalysts were investigated by py-IR, disclosing that the affinity of Lewis acid sites toward activity of catalysts. In addition, it was found that the weak and medium Lewis acid sites derived from NiF2-AlF3 complex phase are active centers for catalyzing dehydrofluorination of CF3CH2F to trifluoroethylene (CF2=CHF). The highest activity was obtained over a fluorinated catalyst calcined at 500 °C, with a reaction rate of 2.13 mmol min−1 gcat.−1 and a trifluoroethylene selectivity of 99%, highlighting a good prospect for the commercial application.

Complementary Schottky diode formation with carbon buffer and p-doped single layer graphene on intrinsic SiC via fluorine intercalation

A practical application of graphene is in transistors and diodes fabricated through processes compatible with integrated circuit fabrication processes that are currently used. In this paper, a highly controlled gas phase fluorination treatment (using XeF2) of an intrinsic Si-terminated SiC (i-SiC) substrate and a (6√3ⅹ6√3)R30° carbon buffer layer is shown to effectively convert the buffer layer to p-doped SLG (p-SLG), which is decoupled from the i-SiC substrate through F intercalation. The electrical properties of two diode structures, (1) metal/SiC with buffer layer and (2) p-SLG/SiC, were investigated considering the bias-dependent carrier injection at each interface. The analysis results suggest that the diode turn-on for each diode is due to carrier injection from the metal or p-SLG to the i-SiC substrate, with an exponential modulation of the thermionic injection driven by the image barrier lowering effect. A complementary SLG-based SiC diode formation scheme is demonstrated, as hole injection from p-SLG is the origin of positive bias diode turn-on in the second diode type, whereas the diode having metal/SiC with buffer structure showed negative bias turn-on.

Autoclaving-induced in-situ grown hierarchical structures for construction of superhydrophobic surfaces: A new route to fabricate antifouling coatings

A simple fabrication method was developed to prepare a superhydrophobic aluminum surface. The approach involved the in-situ formation of a micro/nano-structure and fluorination treatment on an aluminum surface. Based on the combination of micro/nanostructures and low-energy compositions, the as-prepared surface became super-repellent toward deionized water and biological liquids including egg white and egg yolk. A water contact angle of 160° and a contact angle hysteresis of 1.8° were observed on the fluorinated aluminum surface with hierarchical structures. Meanwhile, the surface presented excellent antifouling performance against marine diatom Phaeodactylum tricornutum by the air layer captured within the micro/nanostructures. The results indicated that the structures played a critical role in the formation of a superhydrophobic surface with excellent antifouling capability. The work provides a simple and highly efficient method to fabricate superhydrophobic aluminum surfaces with exceptional antifouling performance.