CVD Reaction Research Articles

A revisit from CVD kinetics to CVD reactor: Investigating uniform growth mechanism of polysilicon in a reduction furnace

The uniform growth of polysilicon rods is essential for CVD process in the reduction furnace. Considering the complex interaction of CVD reaction and transport phenomena, the reaction engineering analysis of CVD reaction has been investigated and the requirements for the design and operation of CVD reactor are proposed concerning temperature and species concentrations. Based on the numerical simulation of CVD process in a reduction furnace with 12 pairs of rods, the non-uniform deposition is mainly attributed to the mismatching between the TCS transportation and consumption. Moreover, two measures have been attempted. One is to enhance axial mixing by increasing inlet velocity and the other is to reduce TCS consumption by reducing inlet TCS mole fraction and heat flux at the rod surface. Both measures have been confirmed beneficial for the uniform growth of the polysilicon rods.

Erosion resistance of ethylene propylene diene monomer insulations reinforced with precoated multi-walled carbon nanotubes

Condensed particles in the internal flow field of a solid rocket motor can cause serious erosion to thermal insulations under overload flight condition. Multi-walled carbon nanotubes (MWCNTs) can improve erosion resistance of insulations but at the cost of reduced thermal insulation performance. In this study, MWCNTs were precoated with pyrolytic carbon via chemical vapor deposition to reduce their thermal conductivity. Further, ethylene propylene diene monomer insulations reinforced with different contents of MWCNTs and precoated MWCNTs were prepared. Moreover, their ablation performances under simulated overload ablation conditions were tested and compared. Experimental results show that precoated MWCNTs can not only improve thermal insulation performance but also significantly increase particle erosion resistance of ethylene propylene diene monomer insulation materials. Charring ablation rate of the formula with 10phr precoated MWCNTs is 44.3% lower than that of basic formula and 41.6% lower than that of the formula with 3phr MWCNTs which has the same pure MWCNTs content. The char layers of MWCNTs reinforced formulas are more complete than basic formula, indicating that MWCNTs can improve char layer structure under particle erosion condition. The precoated MWCNTs can also promote the CVD reaction of pyrolysis gas in char layers during ablation, decreasing the porosity of char layers, which can increase the particle erosion resistance of insulation.

Resting coronary blood flow velocity profile predicts coronary flow reserve in HFpEF

Abstract Background Coronary microvascular disease (CMD) is prevalent in patients with heart failure with preserved ejection fraction (HFpEF). CMD can be assessed by coronary flow reserve (CFR) using transthoracic echocardiography (TTE). We hypothesised that the coronary Doppler flow profile in LAD at rest could reveal information about the downstream resistance in the vessel, where increased resistance is a sign of CMD. Purpose We aimed to measure features of the acceleration and deceleration of the LAD Doppler flow profile to investigate association with coronary and cardiac function. Methods CFR was assessed in 202 patients by TTE in the PROMIS-HFpEF-study. Detailed flow profile measurements were possible in 169 patients (84%) who constituted the study population. The coronary Doppler flow profiles were analysed with respect to acceleration time (corAT) and slope (corAS) and deceleration pressure half time (corPHT) (figure 1). Results The average age was 75±9 years and 55% were female. Atrial fibrillation (AF) was present in 53% and 62% were current or previous smokers. There was no significant difference in gender, age, BMI, blood pressure or heart rate in the CMD vs non-CMD group, but AF as well as a history of smoking was more prevalent in the CMD group, p=0.022 and 0.003 respectively. Further, there were no significant differences in neither corAT nor corAS between the two groups. However, patients with CMD had shorter corPHT of 268±64 ms compared to 298±67 ms, p=0.01. A longer corPHT was associated with increased TAPSE (R=0.205, p=0.007) and higher CFR (R=0.231, p=0.002). In a multivariable analysis adjusted for age, sex, BMI, SBP, reactive hyperemia index, HR, AF, diabetes, CVD, smoking, LVM and study site*, corPHT independently predicted CFR (table 1, p=0.016). Conclusion Short pressure half time, indicating a steep deceleration of the coronary Doppler signal at rest, may provide useful information for prediction of CFR determined by transthoracic ultrasound by reflecting the increased resistance in the coronary microvasculature associated with CMD. Funding Acknowledgement Type of funding source: Public hospital(s). Main funding source(s): Sahlgrenska University Hospital, Sponsor AstraZeneca

Nb-doped TiO2 coatings developed by high power impulse magnetron sputtering-chemical vapor deposition hybrid deposition process

Novel methods for the deposition of thin functional coatings, such as hybrid physical vapor deposition-chemical vapor deposition (CVD) technologies, have the potential to become an important means of overcoming the limitations of current processes, such as low deposition rates, associated with some sputtering processes, or limited material/precursor choices, associated with CVD processes. This work explores the potential of addressing these issues through the development of a hybrid system, which combines the latest magnetron sputtering technology, high power impulse magnetron sputtering (HiPIMS), with plasma enhanced chemical vapor deposition (PECVD) technology. This system seeks to overcome the limitations of each technique and provide a new, flexible deposition tool for functional films, such as transparent conductive oxides. In this system, the plasma generated by the magnetron provides a source of electrons to drive the CVD precursor decomposition and reaction chemistry in the PECVD process. Consequently, only one power supply is required. Thus, niobium-doped titania coatings were deposited on glass and Si wafer substrates by this hybrid HiPIMS-CVD technique. The TiO2 coatings were deposited by CVD from a titanium (IV) tetraisopropoxide precursor via the vapor drawn method. The HiPIMS process provided not only the source of the Nb metal dopant to the functional films but also sustained the low temperature CVD process by supplying energetic plasma particles. Furthermore, since HiPIMS deposition rates are very sensitive to magnetic field strength and the degree of unbalance, by using a magnetron with variable magnetic field strength, it was possible to adjust the dopant content of the film without adjusting the power applied to the magnetron target. The effect of processing parameters (pulse frequency, peak powers, precursor flow rates, operating pressure, etc.) on generating a stable HiPIMS discharge across the process envelope has been studied. The composition and microstructure of the deposited coatings have been investigated, in respect to variable process parameters, such as substrate temperature and operating pressure.

Recovery of waste gold for the synthesis of gold nanoparticles supported on radially aligned nanorutile: the growth of carbon nanomaterials.

Precious and expensive metals are lost each year through the discarding of old jewellery pieces and mine tailings. In this work, small amounts of gold were recovered by digestion with aqua regia from waste tailings. The recovered gold in the form of HAuCl4 was then used to deposit Au0 onto radially aligned nanorutile (RANR) to form a supported catalyst material. The support material, RANR, was synthesized using the hydrothermal technique whereas the deposition of gold was achieved using the deposition–precipitation with urea method at various loadings. Electron microscopy was used to show that the structure of the support is a sphere formed by multiple nanorods aligned in a radial structure. The Au nanoparticles were observed at the tips of the nanorods. It was confirmed by XRD that the support was indeed a rutile phase of TiO2 and that the Au nanoparticles had a face-centred cubic structure. The various catalysts were then used to synthesize carbon nanomaterials (CNMs) using the chemical vapour deposition technique. A parametric study varying the reaction temperature, duration and carbon source gas flow rate was conducted to study the effects these conditions have on the structural properties of the resulting CNMs. Here, it was found that mainly carbon nanofibers were formed and that the different reaction conditions influenced their graphicity, width, structure and thermal properties.

Gas Phase Synthesis and Deposition of Directly Fused Porphyrin Tapes: Reaction Mechanism and Central Metal Ion Effect.

The formation of directly fused porphyrin tapes attracted the interest of the scientific community because of their electronic and optical properties such as extremely low bandgap, NIR and two photon absorption. The synthesis of these compounds is tedious because of their low solubility and require the introduction of solubilizing substituents adding further synthetic efforts.[1] Moreover, these materials are hard to process and integrate into device while the solubilizing substituents hinder the π­-π interactions affecting the properties of the resulting materials.[1],[2] These drawbacks were circumvented with the oxidative Chemical Vapour Deposition (oCVD) of phenyl meso-substituted porphyrins, which allowed to achieve in a single step the synthesis, deposition and doping of polymeric fused porphyrins tapes thin films from commercially available chemicals.[2] Our approach allows the deposition of patterned films and is compatible with sensitive substrates such as paper or polymers (Figure 1a). Using Ni(II) 5,15–(diphenyl) porphyrin (NiDPP), 5,10,15–(triphenyl) porphyrin (NiP3P) and 5,10,15,20–(tetraphenyl) porphyrin (NiTPP), we elucidate the reaction active sites, the side reactions (Figure 1b) and the differences to solution based approaches via high resolution mass spectrometry, UV-Vis-NIR spectroscopy, electron paramagnetic resonance (EPR) and Density Functional Theory (DFT). We also investigate the effect of the central metal ion on the direct fusion reaction of Fe(III)Cl, Co(II), Ni(II), Cu(II), Pd(II), H2 and Zn(II), 5,15–(diphenyl) porphyrins in oCVD. Furthermore, the electrical, electronic and morphological properties of the prepared thin films were thoroughly investigated by means of X-ray Photoelectron Spectroscopy (XPS), Helium Ion Microscopy (HIM) and Conductive Atomic Force Microscopy (C-AFM). [1] T. Tanaka, A. Osuka – Conjugated porphyrin arrays: synthesis, properties and applications for functional materials – Chem Soc Rev. 2015, 44, 943. [2] G. Bengasi, K. Baba, G. Frache, P. Gratia, K. Heinze, N. D. Boscher – Conductive Fused Porphyrin Tapes on Sensitive Substrates by a Chemical Vapor Deposition Approach – Angew. Chem. Int. Ed. 2018, DOI: 10.1002/anie.201814034. Figure 1) a) The chemical vapour deposition reaction of NiDPP and FeCl3 yields a homogeneous and conductive coating also on sensitive and bendable substrates which evidences the suitability for modern optoelectronic devices. b) Reaction schemes for the CVD reaction of NiDPP and FeCl3 and side reactions observed during the deposition (chlorination, cyclization of the phenyl ring). Figure 1

Controlled Growth of Metal Nanoparticles on Multi-Walled Carbon Nanotubes for Chemical Sensing Applications

We have grown nanoparticles of gold, silver, copper, cobalt and nickel on the surface of carbon nanotubes which are deposited by CVD on gold wires bearing a nickel catalyst layer. A study of the CVD reaction as a function of time revealed that one-minute and ten-minute deposition times yielded qualitatively similar coatings. Metal nanoparticles were grown from electroplating solutions by using the nanotube-coated wire as the working electrode and running a galvanostatic, electrochemical deposition. For gold depositions, we were able to show that varying the conditions of current, plating bath concentration and deposition time allowed control of the size of the deposited nanoparticles within the range of 10 – 150 nm. The morphology of the gold nanoparticles was approximately spherical, except under unique conditions where spiked spheroids, also known as nano-urchins, were observed. By using the gold nanoparticle-decorated CNT electrode as one plate in a parallel-plate capacitor, we were able to show a change in electrical properties, such as capacitance, by comparing voltammograms before and after exposure of the electrode to a dilute solution of organic thiols. This demonstrates the potential utility of the electrode structure as a chemical detector. The two SEM images below show typical results. On the left, CVD carbon nanotube growth prior to electrodeposition of metal nanoparticles; note the presence of nickel catalyst particles at nanotube tips. On the right, carbon nanotube sample after electrodeposition of gold nanoparticles. Figure 1

Fabrication of ZrN Barrier Coatings for U-Mo Microspheres Via Fluidized Bed Chemical Vapor Deposition Using a Metalorganic Precursor

A fluidized bed chemical vapor deposition (FB-CVD) system with an inverted reactor was used to develop barrier coatings of zirconium nitride on the surface of U 8 wt% Mo microspheres in order to examine the resulting coating structure and composition, as well as to evaluate the effects of certain process parameters on the coating properties. The process utilized the metal-organic precursor tetrakis(dimethylamino)zirconium heated to 51°C ± 2°C. Time-dependent studies analyzed the coating produced after 2 to 8 days of operation at 100 mL/min precursor carrier flow rate and 500 mL/min fluidization flow rate. Flow-rate–dependent studies produced coated samples using a total flow rate of 600 mL/min, with the precursor carrier flow rate ranging from 100 to 300 mL/min and the fluidization flow rate adjusted accordingly. Ultrahigh-purity argon was used for the precursor carrier and fluidization gas. The CVD reaction was carried out at 280°C ± 10°C with precursor transport tubes heated to 60°C to 75°C. The zirconium-based coatings were manufactured up to 2.2 ± 0.3 µm thick after 2 days of deposition. The estimated coating thickness was not significantly impacted by extending the duration of the deposition process or increasing the precursor carrier gas flow rate. Imaging of the microsphere cross sections provided evidence of uranium oxide, zirconium oxide, and zirconium nitride layers formed within the coating, with nitrogen content becoming more dominant farther from the bulk U-Mo surface. Certain modifications to the process would improve the results, including installing the system inside an inert atmosphere to limit oxygen contamination and higher resolution analyses to better determine coating structure, and finally, performance of a kinetic study of the coating process, particularly over the first 24 h in which the majority of the deposition was suspected to have occurred by enabling partial powder removal during FB-CVD operation to analyze samples from different points within a single experiment.

Reactive chemical vapor deposition of heteroepitaxial Ti1−xAlxN films

A novel methodology combining CVD experiments, nanoscale characterisation and reaction–diffusion modelling demonstrates Ti1−xAlxN epitaxial growth on single crystalline AlN films.

A Chemical Vapor Deposition Route to Epitaxial Superconducting NbTiN Thin Films

The deposition of epitaxial superconducting (Nb,Ti)N thin films is addressed with a new approach, using a chemical vapor deposition technique with in situ production of precursors. Both classic and reactive CVD process are optimized toward (i) the control of crystal structure of the deposited films and (ii) the control of stoichiometry and thus of superconducting properties. Films are chararcterized using thermodynamic, structural, and electrical characterization tools. Precession electron diffraction and electron backscatter diffraction techniques provide the phase and orientation mapping of the films down to the nanometer scale. We demonstrate that the cubic phase (structure with the most prominent superconducting properties) is the thermodynamically stable phase under classic CVD from 800 °C up to 1300 °C. The hexagonal (Nb,Ti)N is formed via a “nitridation like” process under reactive CVD, up to 1200 °C. The composition of the films is controlled by the chemistry of the gas phase, whereas the thicknes...

Hexagonal boron nitride hollow capsules with collapsed surfaces: Chemical vapor deposition with single-source precursor ammonium fluoroborate* *Project supported by the National Natural Science Foundation of China (Grant Nos. 51332005, 51372066, 51172060, 51202055, and 21103056).

SBA-15 (mesoporous SiO2) is used to stabilize and transfer F− in the NH4BF4 CVD reaction for the first time, and a large-scale crystalline h-BN phase can be prepared. We successfully fabricate hollow h-BN capsules with collapsed surfaces in our designed NH4BF4 CVD system. Optimum temperature conditions are obtained, and a detailed formation mechanism is further proposed. The successful SBA-15-assisted NH4BF4 CVD route is of importance and enriches the engineering technology in the h-BN single-source CVD reaction.

Decoration of Carbon Nanotubes by Semiconducting or Metallic Nanoparticles using Fluidized Bed Chemical Vapour Deposition

Multi-Walled Carbon Nanotubes (MWCNTs) have promising properties that make them potentially useful in a wide variety of applications. The decoration of MWCNTs by metallic or semiconducting nanoparticles aims to intensify some of their properties, in particular thermal and electrical conductivity. Fluidized Bed Chemical Vapour Deposition (FBCVD) is an efficient process to uniformly coat powders by various materials. The coating by SnO2, Fe and Si nanoparticles of MWCNTs (Graphistrength®) tangled in balls of 360 microns in mean diameter using the FBCVD process has been studied. The influence of some deposition parameters with and without oxidative pre-treatment is analysed on the nucleation and growth of nanoparticles. The various results obtained indicate that the intrinsic surface reactivity of MWCNTs is high enough for CVD precursors involving the formation of highly reactive unsaturated species such as silylene SiH2 formed from silane SiH4 pyrolysis in the case of Si deposition. But it must be enhanced for less reactive CVD precursors such as tin tetrachloride SnCl4 which needs the presence of oxygen-containing groups at the nanotube surface to allow Sn nucleation. So, provided the reactivity of the powder surface and that of the CVD precursors are well tuned, the FBCVD process can uniformly coat the outer surface of MWCNTs by metallic or semiconducting nanoparticles.

Drastic increase in catalyst productivity of nanoclay-supported CVD-grown carbon nanotubes by organo-modification

The organo-modification of montmorillonite (Mt) prior to catalyst insertion was proposed as a promising approach to increase carbon nanotube (CNT) yield on nanoclay (NC) supports in catalytic chemical deposition (CVD) reaction. The behavior of organo-modified montmorillonite (OMt) as the support in CVD reaction was compared with that of Mt. According to morphological images and Raman spectra, the carbon atoms form CNT in case of both Fe-loaded pristine Mt and OMt. Decreasing gas flow-rate and diminishing hydrogen fraction in growth atmosphere were found to favor improvement of CNT yield, whereas generally tended to deteriorate its quality. TGA results showed that the CNT content of OMt-supported hybrids were drastically increased compared to that of non-modified Mt. There was about two- to fifteen-fold increase in CNT yield depending on the CVD conditions. This observation was found correlated to the larger specific surface area (SSA) and d001-value of Fe-loaded OC. The evident advantages of OMt as catalyst support include efficient and scalable synthesis of high thermally stable NC/CNT hybrid at low cost.

Titanium Dioxide Thin Films Deposited by Electric Field‐Assisted CVD: Effect on Antimicrobial and Photocatalytic Properties**

Thin films of anatase titanium dioxide are deposited on fluorine‐doped tin oxide (FTO) glass substrates utilizing the electric field‐assisted aerosol (EA)CVD reaction of titanium isopropoxide in toluene at 450 °C. The as‐deposited films are characterized using scanning electron microscopy (SEM), X‐ray diffraction (XRD), Raman spectroscopy (RS), and UV‐vis spectroscopy. The photoactivity and antibacterial activity of the films are also assessed. The characterization analysis reveals that the use of an electric field affects the film microstructure, its preferential orientation, and the functional properties. XRD of the anatase films reveals that the application of electric fields causes a change in the preferential orientation of the films from (101) to (004) or (211) planes, depending on the strength of the applied field during the deposition.

Aerosol assisted chemical vapor deposition of conductive and photocatalytically active tantalum doped titanium dioxide films

This paper shows the aerosol assisted chemical vapour deposition of transparent, blue coloured and conductive tantalum doped titanium dioxide films from the CVD reaction of Ti(OEt)4 and Ta(OEt)5. Hall effect measurements showed the doped films to have excellent n-type electrical conductivity showing, to the best of our knowledge, the lowest reported sheet resistance ever recorded for Ta-doped TiO2 of 14 Ω sq−1. The Ta 6 atom% doped TiO2 film also showed the best electrical results with a charge carrier concentration of 1.60 × 1021 cm−3 and mobility of 1.44 cm2 V−1 s−1 making it a suitable electrode in photovoltaic devices. The doped films were multifunctional, showing good photocatalytic activity under UV-light illumination. XPS and XRD studies gave strong evidence that the Ta was entering the TiO2 lattice as Ta5+ and that a reduction of some Ti4+ to Ti3+ was observed.

High Temperature Behavior of Si3N4 and Yb2SiO5 Coated Carbon Fibers for Silicon‐Nitride CMC

For oxide‐free ceramic matrix composites (CMC), with Si3N4 matrix and carbon fiber reinforcement, for extreme high temperature applications, protective coatings of the C‐fibers are investigated. Two different coatings are compared: reactive CVD‐derived pure Si3N4 coatings to investigate C‐fiber‐matrix reactions and powder based Yb‐silicate coatings to reveal potential reactions with the Yb‐silicate additive serving as sintering aid for Si3N4. The reactivity toward carbon in nitrogen atmosphere is studied in the temperature interval from 20 °C up to 1700 °C. A new ceramic phase – an Yb‐carbido‐nitiridosilicate, Yb2Si4CN6–is found as product of carbothermal reduction of the Yb‐silicate. The carbothermal reduction occurs also with other RE‐silicates, RE = Yb, Er, Y, Gd, and Sm while SiC is found as reaction product on carbon fibers coated with pure Si3N4. The oxidation resistance of the coated fibers in air was investigated in the temperature interval up to 1000 °C, and the apparent activation energy of oxidation was analyzed based on DTA‐EGA results. The oxidation kinetic reveals a significant increase of onset point of oxidation temperature by up to 150 K for Si3N4 coated short carbon fibers obtained from the reactive CVD coating process. Such fibers have a high application potential for carbon‐fiber reinforced Si3N4‐CMC. The role of Yb2Si4CN6 as reinforcement for Si3N4‐CMC is discussed based on bond strength comparison of carbides (SiC), nitride silicates (SiAlON), and nitrides (Si3N4).

Characteristics of carbon films prepared by thermal chemical vapor deposition using camphor

The properties of carbon films prepared by thermal chemical vapor deposition (thermal CVD) using camphor are investigated. As the deposition temperature increases from 1098 to 1198K, the deposition rate follows the Arrhenius law with activation energy of 59.8kJ/mol. The possible reaction paths and intermediate species of this thermal CVD process are also considered. The product gases CH3, CH4, and C2H2 can be speculated as the main species for pyrolytic carbon deposition. The crystallinity and ordering degree of carbon films decrease with increasing the deposition temperature. Nevertheless, the sp2 carbon sites increase with increasing the deposition temperature, and results in the decrease of electrical resistivity and the increase of water contact angle. When the camphor weight changes from 0.06 to 0.50g, the CVD reaction is controlled by a process of half order. Moreover, as the camphor weight increases from 0.30 to 0.50g, the surfaces of carbon films are partially covered with spherical particles, the water contact angle substantially increases. Finally, the results of this work are compared to those of using CH4 and C2H2 as the precursor gases.

Efficient and reusable Co/nitrogen doped hollow carbon sphere catalysts for the aerobic oxidation of styrene

Monodispersed hollow carbon spheres (HCSs) were made by decomposition of gaseous CH3CN over silica Stöber spheres in a CVD reaction. The nitrogen doped HCSs (N=8.7%) were used as support for well dispersed Co3O4 nanoparticles (7–8nm; 1% and 2% Co). The Co/N-HCS samples were calcined at various temperatures. The materials calcined at 300°C for 3h were found to oxidize styrene to styrene oxide (styrene epoxidation) using dry air as oxidant (120°C/DMF) with 99% styrene conversion and 85% selectivity to styrene oxide; they could also be reused in repeat catalytic reactions.

Solution Reactivity Studies for Identification of Promising New ALD and Pulsed CVD Reaction Chemistries

Solution screening reactions are described for a range of copper(II) and nickel(II) precursors in combination with the co-reagents AlMe3, ZnEt2, BEt3, Si2Me6, Sn2Me6 and B2(Pin)2 (Pin = pinacolate). These screening reactions were used to identify particularly promising precursor/co reagent combinations for copper or nickel metal deposition, and subsequent ALD/pulsed-CVD studies are described for selected cases. NMR spectroscopic studies focused on elucidating the reaction pathways responsible for copper metal deposition from solution are also described. This manuscript reviews and expands upon previously reported work (B. Vidjayacoumar, D. J. H. Emslie, J. M. Blackwell and S. B. Clendenning et al., Chem. Mater., 2010, 22, 4844-4853 and 4854-4866).

Controllable synthesis of hollow α-Fe2O3 nanostructures, their growth mechanism, and the morphology-reserved conversion to magnetic Fe3O4/C nanocomposites

In this work we present a controllable synthesis of hollow nanostructures of α-Fe2O3 by a facile hydrothermal route using FeCl3·6H2O as the iron source and NH4H2PO4 as the shape-directing agent. We found that the morphology of α-Fe2O3 experienced a continuous change from thin nanospindles to flat nanodisks via finely tuning the phosphate ion concentration due to the selective absorption of phosphate ions onto the Fe2O3 nanostructures. At a given phosphate ion concentration, prolonging the reaction time led to the formation of hollow nanostructures (nanotubes, nanobeads, and nanorings) due to the dissolution and re-crystallization of Fe2O3. We also found that morphology-preserved conversion from α-Fe2O3 to magnetic Fe3O4/C can be achieved during the CVD reactions in C2H2. The Fe3O4/C nanocomposites with uniformly coated carbon layers exhibited morphology-dependent magnetic properties.