Chemical Vapor Deposition Process Parameters Research Articles

The Corrosion Characteristics of Graphite Coatings on Nickel Foam and Their Applications in High-Temperature Proton Exchange Membrane Fuel Cells

This study investigates the effectiveness of graphite coatings on nickel foam for use in high-temperature proton exchange membrane fuel cells (HT-PEMFCs). Nickel foam, known for its high porosity and good conductivity, serves as an advantageous alternative to traditional flow channels but faces corrosion challenges in HT-PEMFCs, especially from phosphoric acid leakage. Graphite coatings are used as the protection layer and compared with nitride coatings. Chemical vapor deposition process parameters are first optimized for graphite growth. Corrosion polarization curves are then measured using a high-temperature phosphoric acid three-electrode system simulating HT-PEMFC conditions, followed by an analysis of the surface morphology of the samples. Subsequently, the performance and durability of the nickel foams with different coatings are evaluated after being assembled in single fuel cells. The results highlight the superior corrosion resistance of graphite coating under HT-PEMFC conditions, demonstrating its performance over other coatings. Graphite-coated foams showed a significant reduction in corrosion current, approximately 58% lower than uncoated foams and 21% lower than the TiN coated foams. In practical applications, an HT-PEMFC single cell with graphite-coated nickel foam demonstrated a current density of 763 mA/cm2 at 0.6 V under conditions of 180 oC and 2 atm H2/air. The current density is 40% higher than that of the uncoated nickel foam. The results show notable long-term durability under constant current holding tests at 160 oC. The voltage decay rate in the graphite-coated cell was 48% lower compared to the cell with uncoated nickel forms.

Studies on the growth mechanism of aluminide coating on K444 alloy surface by chemical vapor deposition

The studies of the growth mechanism of aluminide coatings by chemical vapor deposition (CVD) is an important prerequisite for optimizing the thermal protection properties of aluminide coatings and improving the CVD process parameters; however, the current research on the growth mechanism of CVD aluminide coatings on nickel-based alloys is relatively limited, and there is a lack of systematic studies in this area. In this work, aluminide coatings were prepared on the surface of nickel-based superalloys by the CVD method, and elemental diffusion and microstructure effects between coating and substrate at different deposition temperatures and times, as well as the growth kinetics and formation mechanism of CVD aluminide coatings were investigated. The results showed that the coating prepared on the surface of K444 nickel-based high-temperature alloy by CVD was an externally diffused aluminide coating with a bilayer structure: the outer layer of the coating was the β-NiAl phase. The interdiffusion layer was mainly composed of nickel-aluminum compounds, carbides, and topologically close-packed phase. The growth of the CVD aluminide coating was mainly controlled by the grain boundary diffusion mechanism.

N-polar GaN evolution on nominally on-axis c-plane sapphire by MOCVD part-I: Growth optimization

The present work is part-I of a study that demonstrates the influence of metalorganic chemical vapor deposition process parameters on the evolution of gallium nitride (GaN) deposited in a two-step growth process on c-plane sapphire substrates with a particular emphasis on nitrogen-polar GaN (N-polar GaN). First, the temperature domains of sapphire nitridation that result in Ga-polar and N-polar GaN epilayers were identified. Then, the structural and morphological evolution of low-temperature GaN nucleation layers were examined at the observed temperature domains of nitridation. Subsequently, a series of experiments were performed to study the influence of Ga and N precursor flow rates and epilayer growth temperature on the surface evolution of N-polar GaN. The observed results were analyzed in terms of thermodynamic and kinetic parameters of GaN growth. This work reports high-surface quality N-polar GaN epilayers without surface hillocks, exhibiting surface roughness about 1.5 nm over 2 × 2 μm2 area and with good crystalline quality on nominally on-axis c-plane sapphire substrates at low epilayer growth temperatures.

Combined synthesis of carbon nanospheres and carbon nanotubes using thermal chemical vapor deposition process

Carbon spheres (CNSs) and carbon nanotubes (CNTs) are attracting more attention from researchers due to their excellent potential applications as adsorbents, catalyst carriers, and drug delivery. The synthesizing of CNTs and CNSs is a challenging and tedious process. A researcher has to produce different catalyst materials and precursors to synthesize CNTs and CNSs. Hence, an attempt has been made to combined in-situ synthesize of CNSs and CNTs in a single experiment. NiO/CuO/Al2O3 catalyst was used as a substrate. The thermal chemical vapor deposition (CVD) process with acetylene as a precursor gas was used to synthesize the CNSs and CNTs. The influence of process parameters and the growth mechanism of carbon nanomaterials were investigated. The obtained carbon nanomaterials samples were characterized using FESEM, HRTEM with SAED pattern, XRD analysis, Raman spectroscopy, and FTIR analysis. Results indicated that the catalyst morphology has a greater influence on the growth of CNTs and CNSs. The CVD process parameters play a vital role in deciding the structure of carbon nanomaterials. A schematic representation was proposed to understand the formation of carbon nanomaterials on top of the catalyst particles.

Carbon nanotubes, nanorings, and nanospheres: Synthesis and fabrication via chemical vapor deposition—a review

Research work published since 2010 on the synthesis of carbon nanotubes (CNTs), carbon nanorings (CNRs), and carbon nanospheres (CNSs) using the chemical vapor deposition (CVD) process is reviewed. The effect of CVD process parameters on carbon nanomaterials morphology, such as diameter, yield, and quality was reviewed. The mechanism of CVD formation and growth of carbon nanomaterials was reviewed in detail. The catalyst materials used to synthesize these carbon allotropies were discussed in detail. The formation mechanism of carbon nanomaterials indicates that the variables of the CVD process parameters that appear to control growth are not directly manipulated in the CVD process. Rather, it is determined by complex interactions between variables. The impact of CNTs, CNRs, and CNSs was discussed using the schematic representation. The researchers who published the articles in the CNTs, CNRs, and CNSs for the past decade were discussed. From the intensive literature review, it was found that as compared to CNTs there is not much research was undertaken in synthesizing spheres and rings. But spheres and rings are capable of performing better than CNTs structure in energy storage, and sensors.

Optimal design of CH4 pyrolysis in a commercial CVD reactor using support vector machines and Nelder-Mead algorithm

Chemical vapour deposition (CVD) of pyrocarbon (PyC) effectively fabricates advanced carbon materials. Controlling the nanotexture of PyC is critical for the desired application. Reactor operating conditions, including temperature, pressure, inlet flow rate, reactant concentration, govern thickness, and film uniformity. The optimal film performance can be obtained by selecting appropriate process conditions. However, the optimisation of the CVD reactor is challenging due to the highly nonlinear and multi-variable nature of the process. In this study, the support vector machine, a robust supervised learning algorithm and Nelder-Mead algorithm are coupled to optimise the CH4 pyrolysis in a commercial CVD reactor. To this end, a comprehensive CFD model for CH4 pyrolysis in a commercial CVD reactor is first constructed. The model accuracy is improved by considering temperature-dependent transport properties of gas mixture and incorporating the detailed gas and surface chemistry (14 species and 32 reactions). The deposition rate and film uniformity are then obtained using a parametric study. Subsequently, the support vector machine (SVM) is employed to deduce the correlation between the PyC growth rate/film uniformity and operating variables. It has been found that the accuracy of SVM is better than the linear regression model. Finally, SVM coupled with the Nelder-Mead algorithm is proposed to optimise the CVD process parameters to maximise the PyC film quality.

Effect of Pressure and Temperature on Microstructure of Self-Assembled Gradient AlxTi1−xN Coatings

The correlation between structural properties of Al-rich self-assembled nano-lamellar AlxTi1−xN coatings and process parameters used during their chemical vapor deposition (CVD) remains unexplored. For this article, two gradient AlxTi1−xN coatings were prepared by a stepwise increase in temperature and pressure in the ranges of 750–860 °C and 1.56 to 4.5 kPa during the depositions at a constant composition of the process gas mixture. The cross-sectional properties of the coatings were analyzed using X-ray nanodiffraction (CSnanoXRD) and electron microscopy. Experimental results indicate that the variation of the process parameters results in changes in microstructure, grain morphology, elastic strain, nanolamellae’s chemistry and bi-layer period. At temperatures of ~750–800 °C and pressures of 2.5–4.5 kPa, preferably cubic nanolamellar grains are formed, whose microstructure correlates with the build-up of tensile stresses, which become relaxed in coating regions filled with nanocrystalline grains. CSnanoXRD superlattice satellite reflections indicate the period of the cubic Al(Ti)N-Ti(Al)N bilayers, which changes from 6.7 to 9 nm due to the temperature increase from 750 to ~810 °C, while it remains nearly unaffected by the pressure variation. In summary, our study documents that CVD process parameters can be used to tune microstructural properties of self-assembled AlxTi1−xN nanolamellae as well as the coatings’ grain morphology.

Thermal Barrier Stability and Wear Behavior of CVD Deposited Aluminide Coatings for MAR 247 Nickel Superalloy.

In this paper, aluminide coatings of various thicknesses and microstructural uniformity obtained using chemical vapor deposition (CVD) were studied in detail. The optimized CVD process parameters of 1040 °C for 12 h in a protective hydrogen atmosphere enabled the production of high density and porosity-free aluminide coatings. These coatings were characterized by beneficial mechanical features including thermal stability, wear resistance and good adhesion strength to MAR 247 nickel superalloy substrate. The microstructure of the coating was characterized through scanning electron microscopy (SEM), X-ray energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) analysis. Mechanical properties and wear resistance of aluminide coatings were examined using microhardness, scratch test and standardized wear tests, respectively. Intermetallic phases from the Ni-Al system at specific thicknesses (20–30 µm), and the chemical and phase composition were successfully evaluated at optimized CVD process parameters. The optimization of the CVD process was verified to offer high performance coating properties including improved heat, adhesion and abrasion resistance.

Nanoscale control by chemically vapour-deposited polymers

Chemical vapour deposition (CVD) enables nanoscale control for the synthesis of high-purity polymer thin films. Ultrathin (<20 nm) and ultrasmooth (<1 nm r.m.s. roughness) layers of CVD polymers can conform to the geometry of the growth surface. This Review focuses on CVD polymerization methods adapted from solution chemistry for selectively forming different classes of macromolecules. The mechanistically based CVD approaches provide full retention of the monomer’s organic functional groups and thus provide a rational basis for designing and optimizing film characteristics for a diverse array of applications. These include conjugated polymers for energy storage, thin dielectrics for low-power, flexible devices and responsive hydrogels for controlled drug release. Systematic variation in the CVD process parameters provides remarkable control of π–π stacking in conducting polymers, mesh sizes in hydrogels, crystallographic texture, surface energy, permeation rates of molecules and ions, optoelectronic properties and switchable smart behaviour. The initial research focus on process fundamentals, including adsorption, reaction kinetics, mass transport and conformality, formed a strong basis for the recent rapid expansion of materials, applications and scale-up activities in multiple laboratories. The materials and approaches used in CVD polymerization are also extending into hybrid inorganic/organic materials and devices. Chemical vapour deposition (CVD) enables the synthesis of high-purity, pinhole-free and conformal polymer thin films. This Review discusses the recent breakthroughs in mechanistically based CVD polymerization processes and designing CVD polymers for a diverse array of applications.

Modeling and validation of chemical vapor deposition of tungsten for tungsten fiber reinforced tungsten composites

Tungsten is the most promising first wall material for nuclear fusion reactors. One disadvantage, however, is its intrinsic brittleness. Therefore, tungsten fiber reinforced tungsten (Wf/W) is developed for extrinsic toughening. Wf/W can be produced by chemical vapor deposition (CVD), e.g. by reducing WF6 with H2 using heated W-fibers as substrate. However, it still needs to be optimized regarding relative density and fiber volume fraction. The decisive factor is the tungsten deposition rate, which depends on the temperature and the partial pressures. For this dependence, however, there are controversial results in the literature. In this article, a new rate equation is presented, in which different literature equations are partially adapted and combined. It adjusts the WF6 reaction order between one and zero, depending on the temperature and the H2 and WF6 partial pressure. For validation, a simplified experimental setup with a single fiber was designed, which provides very well defined boundary conditions while varying the CVD process parameters heating temperature, pressure, gas flow rate and gas inlet composition. The experimental runs were simulated with COMSOL Multiphysics. The model was successfully validated by measurements of the WF6 consumption rates (< 2 to 100 %), deposited tungsten masses and spatially high-resolved tungsten deposition rates.

Effect of chemical vapor deposition parameters on the diameter of multi-walled carbon nanotubes

The multi-walled carbon nanotubes (MWCNTs) with controlled diameter distribution are useful in the fabrication of composite materials as reinforcement, due to their superior strength and toughness. Chemical vapor deposition (CVD) is a viable process to synthesize MWCNTs. In this investigation, an attempt has been made to study the effect of CVD process parameters (reaction temperature, flow rate of precursor gas, process time) on the mean diameter of MWCNTs. The MWCNTs of controlled diameter distribution was selectively grown on NiO catalyst supported by Al2O3 nano particles. The argon and acetylene were used as carrier and carbon precursor gas, respectively. The catalyst and carbon samples were characterized using field emission scanning electron microscopy, high-resolution transmission electron microscopy, Raman spectroscopy and thermogravimetric/differential thermal analysis. From this investigation, it is understood that the increase in reaction temperature and flow rate of precursor gas increased the mean diameter of MWCNTs but increase in process time decreased the diameter. The diameter distribution and quality of MWCNTs are strongly influenced by the diameter of the catalyst particles.

Chemical Vapor Deposition Growth of Bernal-Stacked Bilayer Graphene by Edge-Selective Etching with H2O

Bernal-stacked bilayer graphene is uniquely suited for application in electronic and photonic devices because of its tunable band structure. Even though chemical vapor deposition (CVD) is considered to be the method of choice to grow bilayer graphene, the direct synthesis of high-quality, large-area Bernal-stacked bilayer graphene on Cu foils is complicated by overcoming the self-limiting nature of graphene growth on Cu. Here, we report a facile H2O-assisted CVD process to grow bilayer graphene on Cu foils, where graphene growth is controlled by injecting intermittent pulses of H2O vapor using a pulse valve. By optimizing CVD process parameters fully covered large area graphene with bilayer coverage of 77 ± 3.6% and high AB stacking ratio of 93 ± 3% can be directly obtained on Cu foils, which presents a hole concentration and mobility of 4.5 × 1012 cm–2 and 1100 cm2 V–1 s–1, respectively, at room temperature. The H2O selectively etches graphene edges without damaging graphene facets, which slows down the ...

CVD-graphene growth on different polycrystalline transition metals

The chemical vapor deposition (CVD) graphene growth on two polycrystalline transition metals (Ni and Cu) was investigated in detail using Raman spectroscopy and optical microscopy as a way to synthesize graphene of the highest quality (i.e. uniform growth of monolayer graphene), which is considered a key issue for electronic devices. Key CVD process parameters (reaction temperature, CH4/H2flow rate ratio, total flow of gases (CH4+H2), reaction time) were optimized for both metals in order to obtain the highest graphene uniformity and quality. The conclusions previously reported in literature about the performance of low and high carbon solubility metals in the synthesis of graphene and their associated reaction mechanisms, i.e. surface depositionand precipitation on cooling, respectively, was not corroborated by the results obtained in this work. Under the optimal reaction conditions, a large percentage of monolayer graphene was obtained over the Ni foil since the carbon saturation was not complete, allowing carbon atoms to be stored in the bulk metal, which could diffuse forming high quality monolayer graphene at the surface. However, under the optimal reaction conditions, the formation of a non-uniform mixture of few layers and multilayer graphene on the Cu foil was related to the presence of an excess of active carbon atoms on the Cu surface.

Towards a general growth model for graphene CVD on transition metal catalysts.

The chemical vapour deposition (CVD) of graphene on three polycrystalline transition metal catalysts, Co, Ni and Cu, is systematically compared and a first-order growth model is proposed which can serve as a reference to optimize graphene growth on any elemental or alloy catalyst system. Simple thermodynamic considerations of carbon solubility are insufficient to capture even basic growth behaviour on these most commonly used catalyst materials, and it is shown that kinetic aspects such as carbon permeation have to be taken into account. Key CVD process parameters are discussed in this context and the results are anticipated to be highly useful for the design of future strategies for integrated graphene manufacture.

Influence of CVD Parameter on Structure of Graded SiC-C Coating

The influence of chemical vapor deposition process parameters including carrier gas composition, deposition temperature, and content of reactants on the structure of graded SiC-C coating is discussed on the basis of thermodynamic calculation in this paper. The addition of enough hydrogen into carrier gas is necessary for the fabrication graded SiC-C coating. The increase of deposition temperature benefits the control of composition in graded coating but the concentration of free Si and free C becomes high at a too high deposition temperature. A high concentration of reactants is preferred while more defects are apt to exist in coatings if the concentration of reactants is too high. The optimum CVD process parameters for graded SiC-C coating are: gradually changing the molar ratio of SiCl4 and CH4 from 0 to 1 when the concentration of CH4 in hydrogen is 1-2 vol%, and the deposition temperature is 1200-1500 °C.

Precise, Biomimetic Replication of the Multiscale Structure of Intestinal Basement Membrane Using Chemical Vapor Deposition

While it has been shown that cells respond to topographical cues, most studies of the influence of topography have been restricted to culture substrates with regular, single-scale features, such as grooves. In contrast, in vivo topography is highly complex, irregular, and multiscale. In this work, we demonstrate the use of chemical vapor deposition (CVD) on native tissue to fabricate a precise nonbiological replica of irregular macro-to-microscale biological topography. Specifically, the porcine intestinal basement membrane was decellularized and used as a template to create a silica replica from which tissue was removed to produce a free-standing topographically biomimetic silica film. Preservation of the crypt-villus structure (tens to hundreds of micrometers in scale), which is theorized to influence intestinal cell development and behavior, as well as the porosity of the native tissue membrane (1-5 μM in scale), was demonstrated; however, submicrometer topography appeared to be masked by ball-like structures believed to be a result of the CVD process. CVD process parameters, including reactor pressure and deposition temperature, were explored in efforts to enhance structural and mechanical integrity of the silica replica. A rigid inorganic replica can be used as a template for casting of biocompatible polymeric membranes; thus, this is the first step in fabricating cell culture substrates that precisely mimic their in vivo counterparts in terms of irregular, multiscale topography.

ChemInform Abstract: Low-Temperature Chemical Vapor Deposition Tungsten Carbide Coatings for Wear/Erosion Resistance.

This paper reports on a new chemical vapor deposition (CVD) process that has been developed to deposit hard coatings, containing tungsten carbide, at temperatures below 500{degrees} C. These coatings, which have been applied to both ferrous and nonferrous alloys, exhibit excellent resistance to wear and erosion. The coatings comprise a mixture of tungsten and the tungsten carbide, the latter being present as W{sub 2}C, W{sub 2}C + W{sub 3}C or W{sub 3}C. The coatings' composition and properties can be controlled by varying the CVD process parameters. The unique lamellar, fine-grained microstructures of these coatings contribute to their good tribological properties.

Contact etch stop a-SixNy:H layer: A key factor for single polysilicon flash memory data retention

The silicon nitride (a-SixNy:H) contact etch stop layer strongly affects data retention performances in single polysilicon nonvolatile memories by acting on the initial charge loss phenomenon. Its improvement has required an analysis of influent plasma enhanced chemical vapor deposition process parameters through a design of experiment approach. The a-SixNy:H physico-electrical analysis points out that silicon rich compositions especially of its interfacial layer must be avoided to reduce a-SixNy:H charge amount and as a result to improve the data retention. Indeed, the a-SixNy:H being near the floating gate, its charges modulation could act as a parasitic memory screening charges stored in the floating gate by capacitive effects.

Aligned carbon nanotubes fabricated by thermal CVD at atmospheric pressure using Co as catalyst with NH 3 as reactive gas

Co is used as a catalyst for chemical vapor deposition (CVD) of vertically aligned multi-walled carbon nanotubes (CNTs) in a tube furnace at atmospheric pressure. C 2H 2 and NH 3 were used for the carbon feedstock and reaction control, respectively. The CVD process parameters determine the chemical properties of the Co particles and subsequently the morphologies and field emission behavior of CNTs as they strongly depend upon the catalyst condition. The flow rate ratio of NH 3 to C 2H 2 is shown to be central to the synthesis of vertically aligned CNTs. Repeatable synthesis of vertically aligned CNTs at atmospheric pressure in a tube furnace is cost effective for large area deposition of such structures which may be used, for example, in vacuum field emission devices.

CVD growth of carbon nanotube bundle arrays

Recent discovery of enhanced field emission current intensity from arrays of bundles of carbon nanotubes (CNT) has prompted this investigation of the growth of CNT bundle arrays by metal-catalyzed chemical vapor deposition (CVD), in order to understand and control the growth of these arrays. CNT bundle array growth has been characterized as a function of array geometric parameters: the CNT bundle diameter and inter-bundle spacing. We find that CNT bundle array growth varies significantly with bundle size and spacing, which we suggest is due to the formation of a volatile molecular byproduct of ethylene decomposition that enhances CNT growth in areas with high concentrations of metal catalyst. We have also studied and optimized CNT growth with respect to a variety of CVD process parameters, in order to control the length of the resultant CNT bundles. We find that the length of the CNT can be reliably controlled by varying either the reaction time or the gas pressure. Such control over CNT bundle length will be crucial in the incorporation of these bundle arrays into high-intensity electron field emission devices.