Type Of Chemical Vapor Deposition Research Articles

Stabilization of Platinum Catalyst Surface-Treated by Atomic Layer Deposition of Cobalt for Polymer Electrolyte Membrane Fuel Cells

Polymer electrolyte membrane fuel cells (PEMFCs) are attracting attention as next-generation energy-conversion devices because they are environmentally friendly and have high energy-conversion efficiencies. A membrane electrode assembly (MEA), composed of an anode, membrane, and cathode, is the most important part of a PEMFC where electrochemical reactions occur. A Pt/C catalyst, wherein Pt is supported on a carbon, is used as the catalyst layer for the anode and cathode. However, as Pt is a precious metal, current research is focused on reducing its loading and improving durability of PEMFCs. This study focused on improving the durability of catalysts by suppressing agglomeration, one of the degradation mechanisms of Pt catalysts. A protective layer of Co metal was deposited using the atomic layer deposition (ALD) process to improve the durability of catalysts. The ALD process is a type of chemical vapor deposition (CVD) process. This process is suitable for the PT/C catalyst as it allows for the deposition of a uniform thin film even on the porous catalyst layer. If the protective layer is thick, it may block the active site of the catalyst, resulting in decreased activity and reduced performance. However, ALD can form a thin protective layer because it can be controlled at the atomic level by adjusting the number of cycles. After fabricating a single cell using ALD Co-Pt/C as a catalyst for the cathode, The U.S. Department of Energy's electrocatalyst durability protocol was used for an accelerated degradation test (ADT). After Co-ALD was used as a cathode for 10 and 20 cycles, the active area of the Pt catalyst particles was reduced owing to the metal Co protective layer. The initial performance and electrochemical surface area (ECSA) of Co-ALD 10, 20 cycles decreased compared to the case of using Co-ALD 0 cycle. However, after ADT, the performance and ECSA reduction rate rather decreased, confirming that the co-protective layer through the plasma-enhanced atomic layer deposition (PEALD) process can improve the durability of the Pt/C catalyst. To identify the cause of the durability improvement, the cathode catalyst particles before and after ADT were compared through high resolution transmission electron microscopy (HR-TEM) analysis. As a result, in the HR-TEM images after ADT, the Pt catalyst particles were relatively uniformly distributed on the carbon support in the Co-Pt/C catalyst compared to Co-ALD 0 cycle. In addition, it was confirmed that the size of the Pt catalyst particles of Co-Pt/C was smaller than that of bare Pt/C (Co-ALD 0 cycle). It was concluded that when the Co protective layer was deposited on the Pt/C cathode, the protective layer suppressed the agglomeration of Pt catalyst particles, therefore, the increase in Pt particle size improved compared to the Co-ALD 0 cycle, and it was revealed that durability improved by suppressing catalyst degradation.

Enhanced gas sensing capabilities of diamond layers using Au nanoparticles

The nanocrystalline diamond (NCD) film reveals a unique combination of physical, chemical, and optoelectronic properties, which makes it a promising material for various sensing applications. To improve a gas sensor's response, selectivity, or reproducibility, its surface is often modified with specific terminations, functional groups or (bio)molecules, thin films, etc. In this work, the NCD surface modification was achieved by a) layer morphology variation using two different types of chemical vapor deposition (CVD) systems, b) top surface termination (H-NCD and O-NCD), and c) Au nanoparticles (Au NPs). The properties of each structure are measured, compared and subsequently evaluated. The electrical properties (resistance changes) are measured for two types of active gas (oxidizing gas NO2 and reducing gas NH3) in a temperature range from 22 °C to 125 °C. Neutral synthetic air (80 % nitrogen and 20 % oxygen) was applied for flushing and resetting the sensors. Thin film fabrication, surface analysis (scanning electron microscopy and Raman spectroscopy), and measurement of electrical properties are described. Surface morphology greatly influences gas response because a large active surface area (higher roughness or 3D-like surface) enhances interaction with gas molecules. While the termination of the NCD with hydrogen is essential for the functionality of the gas sensor, the Au NPs further enhanced the dynamic response of the sensor and magnitude.

Enhancement of impact resistance of CVD diamond coating by multilayer strategy

Poor impact resistance of diamond coating has limited its wide application. In this work, a experimental analysis was conducted to explore the effect of multilayer strategy on impact behaviour of diamond coating. Multilayer diamond coating facilitated the enhancement of impact resistance. Thereafter, four types of chemical vapor deposition coating were deposited to further verify the impact resistance of multilayer diamond coating. Scanning electron microscopy, Raman spectrometer and X-ray diffraction equipment were used to characterize the diamond coatings before and after the impact, respectively. The results showed that MCD/NCD coating had the smallest impact scar. Furthermore, it could be concluded that the multilayer strategy was beneficial for coating to improve the impact resistance.

Active IrO2 and NiO Thin Films Prepared by Atomic Layer Deposition for Oxygen Evolution Reaction

Atomic layer deposition (ALD) is a special type of chemical vapor deposition (CVD) technique that can grow uniformed thin films on a substrate through alternate self-limiting surface reactions. Recently, the application of these thin film materials to catalytic systems has begun to attract much attention, and the capacity to deposit these catalytic films in a highly controlled manner continues to gain importance. In this study, IrO2 and NiO thin films (approximately 25 to 60 nm) were deposited on industrial Ni expanded mesh as an anode for alkaline water electrolysis. Different ALD operating parameters such as the total number of deposition cycles, sublimation and deposition temperatures, and precursors pulse and purge lengths were varied to determine their effects on the structure and the electrochemical performance of the thin film materials. Results from the electrochemical tests (6 M KOH, 80 °C, up to 10 kA/m2) showed the catalytic activity of the samples. Oxygen overpotential values (ηO2) were 20 to 60 mV lower than the bare Ni expanded mesh. In summary, the study has demonstrated the feasibility of using the ALD technique to deposit uniformed and electroactive thin films on industrial metallic substrates as anodes for alkaline water electrolysis.

Thermal Management of High-Power Switching Transistors Using Thick CVD-Grown Graphene Nanomaterial

Graphene is a promising material for thermal management. Heating in power electronics is a severe issue since heat generation can lead to rise in device temperature which can cause performance degradation and device failure. However, current approaches are not so efficient because of the extremely small thermal mass of monolayer or few layers graphene. Here we show a new type of chemical vapor deposition (CVD)–grown thick graphene network with in-plane thermal conductivity of 1100 W/mK on ceramic substrates for thermal management of power switching devices. The demonstration of graphene network's thermal management capabilities was carried out on a 10-Ω resistor module and a 1.2-kV SiC mosfet package module. The results show that the thermal management performance using graphene coated on a low cost Al2O3 direct-bonded copper (DBC) is significantly superior to the industry standard, costlier AlN DBC substrates. Specifically, the temperature of hot spots is lower by at least 25 °C when using CVD-grown graphene networks for thermal management. Numerical simulations suggest that the high in-plane thermal conductivity of thick graphene networks helps in quick transfer of heat away from the device. These demonstrations represent a transformative change on thermal management of WBG high-power switching devices.

Fabrication of Thin-Film Yttria-Stabilized Zirconia Electrolyte by Aerosol-Assisted Chemical Vapor Deposition

In this work, fully dense yttria stabilized zirconia (YSZ) electrolyte films under thickness of 1 μm are successfully fabricated on the anode-supported solid oxide fuel cells (SOFCs) by aerosol assisted chemical vapor deposition (AACVD), which is a type of chemical vapor deposition but conducted in an open atmospheric air not requiring expensive vacuum processes. SOFCs with thin film AACVD YSZ electrolytes marked 720 mW/cm2 at 650°C as the maximum power in our test with open circuit voltage over 1 V. We expect that this result will contribute to cost-effective production of high performance low temperature SOFCs.

In vivo preliminary evaluation of bone-microcrystalline and bone-nanocrystalline diamond interfaces

Chemical vapor deposited diamond is a new potential biomedical material which has the advantage of chemical inertness, extreme hardness and low coefficient of friction, among others. In orthopedics and maxillofacial surgery, these properties could improve implant performance, reducing metallic corrosion, particle wear, inflammatory reactions and bone loss. In the present study, two types of chemical vapor deposition (CVD) diamonds have been analyzed: microcrystalline diamonds (MD) and nanocrystalline diamonds (ND), both produced by hot-filament chemical vapor deposition. The diamond tubes were previously characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and Raman scattering spectroscopy (RSS). The aim of this study was to verify the interface between bone and MD and ND, surgically implanted in the femoral diaphysis of Wistar rats, after 4 and 8 weeks. The outcome was evaluated by scanning electron and optical microscopy using a semi quantitative method. The results suggest that nanocrystalline diamonds (ND) elicits a richer biological response than microcrystalline diamonds (MD) when in interaction with bone.

Surface Chemistry for Molecular Layer Deposition of Organic and Hybrid Organic−Inorganic Polymers

The fabrication of many devices in modern technology requires techniques for growing thin films. As devices miniaturize, manufacturers will need to control thin film growth at the atomic level. Because many devices have challenging morphologies, thin films must be able to coat conformally on structures with high aspect ratios. Techniques based on atomic layer deposition (ALD), a special type of chemical vapor deposition, allow for the growth of ultra-thin and conformal films of inorganic materials using sequential, self-limiting reactions. Molecular layer deposition (MLD) methods extend this strategy to include organic and hybrid organic-inorganic polymeric materials. In this Account, we provide an overview of the surface chemistry for the MLD of organic and hybrid organic-inorganic polymers and examine a variety of surface chemistry strategies for growing polymer thin films. Previously, surface chemistry for the MLD of organic polymers such as polyamides and polyimides has used two-step AB reaction cycles using homo-bifunctional reactants. However, these reagents can react twice and eliminate active sites on the growing polymer surface. To avoid this problem, we can employ alternative precursors for MLD based on hetero-bifunctional reactants and ring-opening reactions. We can also use surface activation or protected chemical functional groups. In addition, we can combine the reactants for ALD and MLD to grow hybrid organic-inorganic polymers that should display interesting properties. For example, using trimethylaluminum (TMA) and various diols as reactants, we can achieve the MLD of alucone organic-inorganic polymers. We can alter the chemical and physical properties of these organic-inorganic polymers by varying the organic constituent in the diol or blending the alucone MLD films with purely inorganic ALD films to build a nanocomposite or nanolaminate. The combination of ALD and MLD reactants enlarges the number of possible sequential self-limiting surface reactions for film growth. Extensions to three-step ABC reaction cycles also offer many advantages to avoid the use of homo-bifunctional reactants and incorporate new functionality in the thin film. The advances in ALD have helped technological development in many areas, including semiconductor processing and magnetic disk-drive manufacturing. We expect that the advances in MLD will lead to innovations in polymeric thin-film products. Although there are remaining challenges, effective surface chemistry strategies are being developed for MLD that offer the opportunity for future advances in materials and device fabrication.

A Calculation Method of the Deposition Profiles in Chemical Vapor Deposition Reactors Using Basis Functions

AbstractIn this paper, we introduced a novel calculation method to reproduce deposition profiles in various types of Chemical Vapor Deposition (CVD) reactors. Robust and accurate calculations along with reduced computing cost were achieved by the method. Boundary value problems for estimating diffusion-reaction equations by iterations of numerical integrations were changed into problems of finding the linear combinations consisted of a few "basis functions", which are inherent in the reactors. The coefficients of the linear combinations were optimized by Genetic Algorithms (GA). We could demonstrate the validity of the proposed method using various examples of the reaction mechanisms and conditions.

Stable trapping of electrons and holes in deposited insulating oxides: Al2O3, ZrO2, and HfO2

Charge trapping in high-permittivity metal oxides (Al2O3, ZrO2, and HfO2) grown on (100)Si using various types of chemical vapor deposition (CVD) was studied using generation of electron-hole pairs in the oxide by 10 eV photons. For most of the CVD methods, thin (≈5 nm) oxide films exhibit positive charging suggesting hole trapping as most efficient charge trapping process. Negative charge is observed only in as-deposited nitrogen-containing films grown from Hf(NO3)4. The trapped positive charge depends only weakly on the HfO2 thickness indicating that holes are trapped in a silicon oxide interlayer grown between the Si and HfO2 during deposition, which is further affirmed by enhanced positive charging after additional oxidation of the samples at high temperatures. The work function of the metal electrode material has a large influence on hole trapping in thin oxides, indicating electron exchange between the metal and defect states in the oxide. In addition, trapping of positive charge correlates with liberation of atomic hydrogen during electron-hole pair generation pointing to a considerable contribution of protonic species to the charge. Electron trapping increases with thickness of oxide layers and appears strongly sensitive to both the method of oxide deposition and post-deposition processing. This suggests the electron traps to be related to defects and/or impurities incorporated into the oxide film during deposition.

Thermal conductivity enhancement in cutting tools by chemical vapor deposition diamond coating

Two different types of chemical vapor deposition (CVD) diamond-coated tools have been investigated: (i) 300–1000 μm films brazed to WC-6 wt.% Co substrates; and (ii) 70-μm-thick films directly deposited on Si3N4–SiC composites. Thermal diffusivity perpendicular to the tool surface was measured up to 1000 °C by the laser flash method. Multiple-layer heat conduction models were used to appraise the effective thermal diffusivity of the film/substrate system. The thermal conductivity of the brazed samples increased up to 65% with respect to the hard metal substrate, evidencing a strong dependence on the diamond thickness. In the directly coated Si3N4 based tool substrates, the thermal diffusivity and the mean thermal conductivity increased up to ∼10%.

Deposition of TiN films using a thermal plasma reactive forming technology

TiN films have been deposited on graphite substrates by a novel reactive plasma technique. In this process, NH 3 and TiCl 4 are reacted in the jet of a plasma torch and are deposited onto a substrate forming TiN films at a high growth rate (above 2 μm min −1). It was found that the process parameters strongly affect the microstructure, thickness, density, and growth rate of the TiN films. These parameters include N : Ti ratio, temperature of substrate, spray distance and radial position away from the impingement point. In general, it was found that thick and dense coatings were deposited when the N : Ti ratio was kept low and the substrate temperature was high. Very dense and thin TiN coatings were deposited 3–4 cm away from the impingement point. These coatings are believed to be the result of a gas phase (chemical vapour deposition type) reaction. The limitations and advantages of this technique are also discussed in this paper.

Direct Determination of Stress Levels in Sputtered Films by means of X-Ray Diffraction Topographic Method

An X-ray topographic method was effectively used to determine stress values as well as the stress nature of thin sputtered films deposited on silicon single-crystal wafers. This method has a simple mechanism, and the stress values can be determined directly from X-ray topographic images without any knowledge of the elastic constants of films. This technique was applied to a determination of the stress levels in iron films and iron nitride films deposited on Si wafers. An iron film sputtered under a low Ar gas pressure has been found to be in a compressive state, while an iron nitride film is in a tensile state. This technique may serve as a monitor for estimating stress levels in such deposited films as new types of chemical vapor deposition (CVD) and physical vapor deposition (PVD) films whose elastic constants are imprecisely known.

Advances in deposition processes for passivation films

A review is presented of deposition processes for forming dielectric passivation layers on silicon semiconductor devices, with emphasis on recent advances in this field. Materials of prime interest are thin-film dielectrics used as secondary passivants deposited by several types of chemical vapor deposition (CVD) processes. The fundamental principles, deposition parameters, advantages, applicability, and limitations of the various types of CVD for dielectric passivation films are described and compared. The CVD processes discussed include low- and high-temperature CVD at normal pressure, and the relatively new technologies about to emerge for large-scale applications−low-pressure CVD at low and high temperatures, and plasma-enhanced CVD.