CH4 H2 Gas Mixture Research Articles

Effect of Nitrogen on Growth and Optical Properties of Single-Crystal Diamond Synthesized by Chemical Vapor Deposition.

Concurrently achieving high growth rate and high quality in single-crystal diamonds (SCDs) is significantly challenging. The growth rate of SCDs synthesized by microwave plasma chemical vapor deposition (MPCVD) was enhanced by introducing N2 into the typical CH4-H2 gas mixtures. The impact of nitrogen vacancy (NV) center concentration on growth rate, surface morphology, and lattice binding structure was investigated. The SCDs were characterized through Raman spectroscopy, photoluminescence (PL) spectroscopy, and X-ray photoelectron spectroscopy. It was found that the saturation growth rate was increased up to 45 μm/h by incorporating 0.8-1.2% N2 into the gas atmosphere, which is 4.5 times higher than the case without nitrogen addition. Nitrogen addition altered the growth mode from step-flow to bidimensional nucleation, leading to clustered steps and a rough surface morphology, followed by macroscopically pyramidal hillock formation. The elevation of nitrogen content results in a simultaneous escalation of internal stress and defects. XPS analysis confirmed chemical bonding between nitrogen and carbon, as well as non-diamond carbon phase formation at 0.8% of nitrogen doping. Furthermore, the emission intensity of NV-related defects from PL spectra changed synchronously with N2 concentrations (0-1.5%) during diamond growth, indicating that the formation of NV centers activated the diamond lattice and facilitated nitrogen incorporation into it, thereby accelerating chemical reaction rates for achieving high-growth-rate SCDs.

Percolation-assisted coating of metal-organic frameworks on porous substrates

In the past two decades, significant advancements have been made toward thin film fabrication of metal-organic frameworks (MOFs) on porous solid substrates for gas separation and catalysis applications. To enhance the physical and chemical stability of the films, various top-down and bottom-up approaches have been implemented. While both approaches have some advantages, they are mostly limited due to a lack of adhesion with the substrate, cracking of the films, and non-uniform coverages. Therefore, there is a need for improvement in the coating processes for the fabrication of robust, uniform, and scalable films. Here, we develop a novel percolation-assisted coating (PAC) process that combines top-down and bottom-up approaches in a continuous-flow microfluidic device to deposit HKUST-1 on a porous substrate, yielding controlled film thicknesses and mass loading. The PAC process is optimized using a Multiphysics modeling approach to design the microfluidic reactor, which can be readily scaled up and deployed to fabricate MOF films on various porous substrates. The desired thickness in the range of 10–200 μm of the MOF film can be achieved by controlling the residence time and temperature of the reaction mixture. The synthesized film is characterized for physical adhesion using sonication, film coverage using scanning electron microscopy, and porosity using a porosimeter. The performance of synthesized films is benchmarked for the effective separation of 50 vol% CH4–H2 gas mixture with the separation factor of 4. The microfluidic device is also applicable to synthesize films of various MOFs like MOF-5, MOF-505 (also reported here), UIO-66, etc., over a wide range of porous substrates. Lastly, we propose a multi-chamber design of the microfluidic device for high-throughput screening of thin-film growth using the PAC process.

CVD synthesis of multi-layered polycrystalline diamond films with reduced roughness using time-limited injections of N2 gas

Multi-layered polycrystalline diamond (PCD) films were synthesized using microwave plasma-assisted chemical vapor deposition (CVD) with periodical addition (injections) of N2 gas to the standard CH4-H2 gas mixture. The aim of such an approach was to reduce the roughness of the films while preserving the overall high quality and phase purity of the PCD material. The thicknesses of the films were in the range of 5 to 51 μm, while the number of layers was from 1 to 15. The introduction of even the smallest amount of nitrogen leads to a significant (more than 2-fold) increase in the growth rate of PCD films. Optimized injection regimes allowed the reduction of the relative roughness (Sq/thickness) of the PCD films by more than 3 times in comparison with standard microcrystalline diamond film grown under similar conditions without N2 addition. The proposed method of periodic injection of N2 during growth restricted the formation of continuous NCD layers, which improved the overall sp3/sp2 ratio in comparison with standard multi-layered MCD/NCD materials. The obtained multi-layered PCD materials with reduced roughness may be used for the formation of protective and hard covers, optical coatings, electrochemical and thermal management applications. Prime novelty statementThe multi-layered PCD films were synthesized in a microwave plasma CVD in regimes with short-term periodic N2 injections in CH4-H2 process gas; the average growth rate is doubled owing to the pulsed nitrogen injection, while the surface roughness is reduced by more than 3 times in comparison with a standard (no N2 injection) microcrystalline film.

Single crystal diamond growth by MPCVD at subatmospheric pressures

Microwave plasma assisted chemical vapor deposition (MPCVD) is the established technique to produce high quality single crystal diamond (SCD). While typical pressures for SCD growth regimes in methane-hydrogen plasma are currently within 100–300 Torr, a transition to much high pressures promises enhanced growth rates. Here, we report on successful SCD synthesis by MPCVD in CH4-H2 gas mixtures at pressures up to 600 Tорр. A strong change of the plasma shape and volume (the latter shrinks by 10 times at fixed MW power) with pressure rise from 100 to 600 Torr was observed, still keeping the plasma stable. The record high absorbed MW power density of ≈1800 W/cm3 was achieved at 600 Torr. Optical emission spectroscopy (OES) was used for the plasma analysis via monitoring emission intensities of radicals Hα, C2 and CH. The gas temperature Tg determined from analysis of rotational fine structure of OES Swan transitions of dimer C2 (516 nm) turned out to be essentially constant ∼ 3100 ± 150 K over the pressure range explored. The diamond growth rate is found to increase by an order of magnitude with pressure to achieve 57 μm/h at 500 Torr at relatively low (4%) CH4 concentration, as measured in situ using low-coherence interferometry, but declined at further pressure increase. The produced films were characterized with SEM, XRD, Raman and photoluminescence spectroscopy, and a high/moderate quality of the obtained material was confirmed.

Synthesis of Ti(C, O, N) from ilmenite at low temperature by a novel reducing and carbonitriding approach

The clean and effective utilization of titanium-bearing minerals has challenged the titanium industry all over the world. In order to realize the high-efficiency, clean, and high value-added comprehensive utilization of ilmenite concentrate, a novel process has been proposed in this study by reducing and carbonitriding ilmenite with the CH4-N2-H2 gas mixture at low temperature. Carbonitride performance and mechanism have been investigated experimentally and theoretically. The obtained results showed that the reaction process could be divided into three stages: formation of metallic iron, reduction of titanium oxide to titanium suboxides, and formation of Ti(C, O, N). The metallic iron congregated at the first two stages, but dispersed once the Ti(C, O, N) formed. The effects of both reaction temperature and preoxidation treatment on the reaction have been studied as well. It was found that the increase of temperature was conducive to the formation of Ti(C,O), and the ilmenite could be reduced completely to Ti(C,O) at 1170°C for 8 hours. The preoxidation treatment could improve the kinetics of reduction. At 1170°C, the introduction of N2 could apparently increase the reduction rate, with the complete reduction time decreasing from 8 hours in CH4-H2 gas mixture to 3 hours in CH4-N2-H2 gas mixture. The proposed novel process has been assessed and it showed many potential advantages and feasibility.

Structure of Diamond Films Grown Using High-Speed Flow of a Thermally Activated CH4-H2 Gas Mixture.

Diamond films are advanced engineering materials for various industrial applications requiring a coating material with extremely high thermal conductivity and low electrical conductivity. An approach for the synthesis of diamond films via high-speed jet deposition of thermally activated gas has been applied. In this method, spatially separated high-speed flows of methane and hydrogen were thermally activated, and methyl and hydrogen radicals were deposited on heated molybdenum substrates. The morphology and structure of three diamond films were studied, which were synthesized at a heating power of 900, 1700, or 1800 W, methane flow rate of 10 or 30 sccm, hydrogen flow rate of 1500 or 3500 sccm, and duration of the synthesis from 1.5 to 3 h.The morphology and electronic state of the carbon on the surface and in the bulk of the obtained films were analyzed by scanning electron microscopy, Raman scattering, X-ray photoelectron, and near-edge X-ray absorption fine structure spectroscopies. The diamond micro-crystals with a thick oxidized amorphous sp2-carbon coating were grown at a heating power of 900 W and a hydrogen flow rate of 1500 sccm. The quality of the crystals was improved, and the growth rate of the diamond film was increased seven times when the heating power was 1700–1800 W and the methane and hydrogen flow rates were 30 and 3500 sccm, respectively. Defective octahedral diamond crystals of 30 μm in size with a thin sp2-carbon surface layer were synthesized on a Mo substrate heated at 1273 K for 1.5 h. When the synthesis duration was doubled, and the substrate temperature was decreased to 1073 K, the denser film with rhombic-dodecahedron diamond crystals was grown. In this case, the thinnest hydrogenated sp2-carbon coating was detected on the surface of the diamond crystals.

Maintenance of Plasma Layer in Intersected Microwave Beams in the Range of Frequency from 10 to 60 GHz: Analysis and Simulation

The paper presents the results of theoretical analysis and numerical modeling of physical processes occurring in microwave discharge supported in two intersecting wave beams in H2–CH4 gas mixture. The transformation rules are defined that allow transferring the results of self-consistent solution of system of equations describing the state of microwave discharge for selected parameters (gas pressure, microwave radiation power and frequency, reactor and wave beams geometry) to other parameters. The following parameters ranges are considered: the medium gas pressure range (100–600 Torr), the radiation power from 1 to 30 kW and radiation frequencies from 10 to 60 GHz. The results of numerical simulation confirm the proposed approach and allow comparing the energy efficiency of the systems in terms of their practical application. The optimal parameters for diamond deposition in the considered plasma-chemical reactor in given range are discussed. The limits of applicability of the microwave discharge model and the proposed transformation rules are discussed.

Reduction of perovskite-geikielite by methane–hydrogen gas mixture: Thermodynamic analysis and experimental results

The present study investigated the reduction and carbonization behavior of perovskite-geikielite by using CH4–H2 gas mixture, which aimed to recycle the titanium resource in titanium-bearing blast furnace slag. Thermodynamic phase equilibrium analysis in the CaTiO3–CH4–H2, MgTiO3–CH4–H2 and CaTiO3–MgTiO3–CH4–H2 systems was performed by FactSage. The analysis indicated that CaTiO3 could be ultimately reduced and carbonized to TiCxOy by CH4–H2 gas mixture at temperature above 1300 °C, while the temperature for reduction of MgTiO3 was above 1200 °C. MgTiO3 is found to be reduced prior to CaTiO3 in the CaTiO3–MgTiO3–CH4–H2 systems. The reduction experiments of perovskite-geikielite were carried out in a temperature range of 1300 °C to 1450 °C in a flowing CH4–H2 atmosphere. Experimental results showed that perovskite-geikielite could be reduced completely to TiCxOy at 1400 °C and above after 8 h reduction, while geikielite was reduced prior to perovskite. The reduction and carbonization extent of samples increased with increasing time and temperature when in a low temperature range (below 1400 °C). However, a much higher temperature (above 1400 °C) would hinder further carbonization of TiCxOy for the production of deposited carbon. Addition of iron oxides to sample strongly facilitated the reduction reaction, and even promoted the reduction of titanium to be completed at only 1300 °C after 8 h reduction. The gas-solid reaction provided a low temperature method to reduce and carbonize perovskite and geikielite, and thus it provided a possible way to use titanium-bearing blast furnace slag.

Laser monitor for imaging single crystal diamond growth in H2-CH4 microwave plasma

We used an active optical system referred to as ‘laser monitor’ based on a copper bromide vapor brightness amplifier for imaging a diamond crystal surface during its synthesis in a microwave plasma in CH4-H2 gas mixture. The approach allows observation of an entire crystal without interrupting the chemical vapor deposition process. It is demonstrated that the broadband plasma background radiation with the brightness temperature of about 3000 K does not interfere with object real-time monitoring at the laser wavelength of 510.6 nm. High quality images obtained using both passive (laser illumination) and active (laser monitor) methods provide information about the surface relief of the growing crystal with resolution of a few tens of micrometer. Each frame is formed through one pulse (about 30 ns), with the maximum frame rate being 20,000 frames/sec.

Thermodynamic and experimental study on the reduction and carbonization of TiO 2 through gas‐solid reaction

Titanium oxycarbide (TiCxOy) has attracted much attention recently as a soluble anode used in the USTB (University of Science and Technology Beijing) or MER (Materials and Electrochemical Research) molten-salt electrolysis process to produce sponge titanium. The present study aimed to investigate the production of titanium oxycarbide from TiO2 by using CH4–H2 gas mixture, and both thermodynamic and experimental studies were carried out. First, thermodynamic analysis in the TiO2–CH4–H2 system was performed by the method of equilibrium calculation. The analysis indicated that TiO2 could be ultimately reduced and carbonized to TiCxOy by CH4 and H2 gas at temperature above 1200°C, and the increase of temperature was theoretically favorable to the final carbonization extent of TiCxOy. The experimental process involved the reduction of both pure TiO2 powder and TiO2 with additives using methane–hydrogen gas mixture in the temperature range from 1200°C to 1450°C. Experimental results showed that pure TiO2 could be reduced completely to TiCxOy at 1350°C and above after 8-hour reduction. It was also found that the reduction extent of TiO2 increased with increasing time and temperature when at a low temperature range; however, a much higher temperature (above 1350°C) would hinder further carbonization for the production of deposited carbon. Addition of iron oxides to TiO2 powders strongly facilitated the reduction reaction, even prompting the reaction of TiO2 to TiCxOy completely at only 1200°C after 8-hour reduction. The gas-solid reaction provided a simple and quick way to produce titanium oxycarbide.

Effect of the ZrCl4 static vaporiser system and deposition time on growth characteristics of chemical vapour deposited zirconium carbide layers

ZrC layers were deposited from ZrCl4–Ar–CH4–H2 gas mixture in a home-built vertical wall chemical vapour deposition system within the deposition time range 0.5–2.5 h. The flow behaviour of ZrCl4 from the static vaporiser system to reaction chamber as a function of time was studied. To investigate the microstructure evolution and the growth characteristics of ZrC layers with deposition time, the growth rate, microstructure, morphology and composition were analysed. The layer thickness increased with deposition time all through; however, its growth rate increased up to 1.0 h and thereafter declined. The X-ray diffraction (XRD) analysis showed both ZrC and carbon peaks. The intensity of the carbon peaks followed a non-linear trend with deposition time. The average crystallite size and the number of crystallites per unit volume of the layers increased with deposition time. The orientation of crystallographic plane also varied with the deposition time. At short deposition times, the Raman spectra showed the acoustic and optic branches indicating that the ZrC deposited contained carbon vacancies. The D and G peaks of carbon increased as the deposition time increased, an indication of free carbon in the deposited layers. At short deposition times, the surface morphology of the layers was relatively flat and smooth. The particle size and agglomerations also increased with time.

A study on the carbonization and alloying process of MnO2 by methane-hydrogen gas mixture in the presence of Fe2O3

Carbonization of pure MnO2 by CH4-H2 gas mixture with various CH4/(CH4+H2) molar ratios in the temperature range of 900–1250°C had been performed in our previous work. This study further investigated the carbonization and alloying process of MnO2 by CH4-H2 gas mixture in the presence of Fe2O3 at 900–1300°C using XRD, optical microstructure, SEM-EDS and phase diagram analyses. Carbonization test of pure Fe2O3 was also conducted to understand the effect of Fe2O3 on the carbonization of MnO2 more explicitly. It was found that Fe2O3 could be carbonized to Fe3C at 900–1150°C. The Fe2O3 briquettes began to melt at 1100°C and a large amount of Fe-C melts appeared as the temperature went up to 1150°C. When the MnO2+25 wt%Fe2O3 briquettes were reduced at 900–1000°C in CH4-H2 gas mixture, the Fe2O3 was preferentially carbonized to Fe-C phase while the MnO2 was only reduced to MnO. The formation of Fe-C phase was beneficial to the carbonization of MnO since the carbonization temperature of MnO can decrease from 1100°C to 1000°C. The carbonization degree of MnO was dramatically accelerated over 1100°C due to the transformation of Fe-C phase from solid state to melts. Morphology analysis indicated that the MnO core was wrapped by the shell of Fe-C melts. The alloying process of Mn-Fe-C was conducted through the elements migration of Fe and Mn between the core and shell. At 1300°C, the Mn-Fe-C alloy turned to melt and aggregated since the highest melting temperature of the Mn-C-25 wt% Fe system was only 1274°C. Finally, a ferromanganese crude alloy with 68–72wt% Mn, 20–25wt% Fe and 6.0–8.0wt% C was obtained when the MnO2+25wt% Fe2O3 briquettes were carbonized by CH4-H2 gas mixture at 1300°C for 60min.

Thermodynamic Analysis and Reduction of MnO2 by Methane–Hydrogen Gas Mixture

Methane and hydrogen reduction has emerged as promising technology for clean and efficient metallurgy. To understand the reduction process of MnO2 by CH4-H2 gas mixture, the effects of various CH4/(CH4 + H2) molar ratios (0–1) and temperatures (1100°C–1250°C) on the reduction of pure MnO2 were researched. Characterization of the solid carbon derived from the decomposition of CH4 gas was also determined. Thermodynamic analysis indicated that the MnO2 was first reduced to MnO by CH4-H2 gas mixture, and then MnO was reduced to Mn7C3 according to this reaction, 7MnO(s) + 10CH4(g) = Mn7C3(s) + 7CO(g) + 20H2(g). Experimental results showed that under the CH4/(CH4 + H2) molar ratio of 0.2, the reduction extent of MnO2 increased with the rising of temperatures. Nevertheless, the deep reduction of MnO to Mn7C3 was retarded even by increasing the temperature and prolonging the time, which was a result of the formation of deposited solid carbon. The free carbon contents in the reduced samples increased with the increasing of the CH4/(CH4 + H2) molar ratio and reduction time. The reduction of MnO2 and formation of carbon can be regulated by adjusting the CH4/(CH4 + H2) molar ratio in a gas mixture. For example, when MnO2 was reduced by 50 vol.%CH4-50%N2, large amounts of MnO and Mn7C3 particles were wrapped by the platelike and rodlike carbon with nanoscale. Yet, the wrapping phenomenon cannot be observed under 10 vol.%CH4-40 vol.%H2-50 vol.%N2.

Express in situ measurement of epitaxial CVD diamond film growth kinetics

We used a low-coherence interferometry for precise continuous in situ measurements of thickness and growth rate of epitaxial single crystal diamond layers in microwave plasma CVD in H2-CH4 gas mixtures in a broad range of substrate temperatures Ts (750–1150°C) and CH4 concentrations (1–13%). Rich growth kinetics is collected in a single experiment by depositing about 60 layers on one (100) Ib HPHT diamond substrate in different regimes (the substrate temperature was controlled by the microwave power) at fixed pressure P=130Torr, without the plasma switch-off. The growth rate is found to follow Arrhenius dependence with activation energy Ea=11.1±1.0kcal/mol. By appropriate choice of the substrate temperature the growth rate can be significantly enhanced. The growth rate as high as 82μm/h is achieved by optimizing the temperature and gas composition. At low CH4 content (1%) growth competes with etching by atomic hydrogen, the etching dominating at high Ts (>1000°C in the present conditions). The etching rate in pure H2 plasma was measured and activation energy Ea=9.8±0.8kcal/mol was deduced. Gas temperature Tg in the plasma core evaluated from optical emission spectra for dimer C2 (Swan band), was found to be either constant or slightly and monotonically increasing with absorbed power, whereas the absorbed microwave power density shows a decreasing, although slight, trend. This suggests the temperature depended surface reactions to play a major role in the diamond growth kinetics under variable microwave power. Raman mapping of cross-section of the produced multilayered sample confirmed high quality of diamond structure over all the deposition regimes explored.

A switching decentralized and distributed extended Kalman filter for pressure swing adsorption processes

A continuous-discrete Distributed and Decentralized Switching Kalman Filter (DDSKF) is designed for estimation of spatial profiles in Pressure Swing Adsorption (PSA) processes. The introduced observer is an integral part of the control strategy of hybrid systems in general and PSA systems in particular. A reduced order model is developed based on the mechanistic model of the process. The sensors are optimally located and observability of the process is studied. The proposed observer is used to estimate the spatial profiles of various states of a two-bed, six-step PSA system used for production of pure H2 from a H2–CH4 gas mixture. The spatial profiles of the system have been estimated using the proposed observer quite accurately and rapidly based on noise corrupted measured temperature and pressure of a few points in the process.

Influence of Al content on diamond nucleation and growth on Fe-Al alloys in CH4-H2 plasma atmosphere

Fe-Al alloys with different Al contents have been exposed to a microwave plasma activated CH4-H2 gas mixture at 670 °C. An intermediate graphite layer formed on Fe-10Al substrate retarded diamond nucleation and deteriorated the interfacial adhesion, and severe metal dusting and carburization occurred. Diamond nucleation on Fe-20Al was enhanced by inhibited graphitization and cementite formation. Only Fe3AlCx-type precipitates were observed. On Fe-27Al and Fe-40Al alloys, graphitization and substrate carburization were completely prevented, producing enhanced nucleation and interfacial adhesion of diamond. The improved carburization resistance and diamond deposition with increasing Al contents is closely related to the formation of an alumina-rich layer on the alloy surface.

Synthesis of single crystal diamond by microwave plasma assisted chemical vapor deposition with in situ low-coherence interferometric control of growth rate

We performed synthesis of single crystal (SC) diamond by microwave plasma chemical vapor deposition in methane-enriched H2–CH4 gas mixtures, and achieved growth rates more than 30μm/h, without adding nitrogen in reaction mixture. A low-coherence interferometry (LCI) was employed for precise measurements of the thickness and growth rate of the epitaxial diamond layers in the course of the process. The performance of this in situ technique is demonstrated by continuously monitoring the SC diamond thickness in a single growth run upon variation of CH4 percentage in steps, up to 17%, without switching off the plasma, to produce a “multilayer” diamond film. In addition, etching rate of diamond in pure hydrogen plasma has been evaluated with the same method. The LCI technique allows quick collection of growth kinetics data upon systematic variation of a selected process parameter for the growth optimization.

Reduction of Quartz to Silicon Monoxide by Methane-Hydrogen Mixtures

The reduction of quartz was studied isothermally in a fluidized bed reactor using continuously flowing methane-hydrogen gas mixture in the temperature range from 1623 K to 1773 K (1350 °C to 1500 °C). The CO content in the off-gas was measured online using an infrared gas analyzer. The main phases of the reduced samples identified by XRD analysis were quartz and cristobalite. Significant weight loss in the reduction process indicated that the reduction products were SiO and CO. Reduction of SiO2 to SiO by methane starts with adsorption and dissociation of CH4 on the silica surface. The high carbon activity in the CH4-H2 gas mixture provided a strongly reducing condition. At 1623 K (1350 °C), the reduction was very slow. The rate and extent of reduction increased with the increasing temperature to 1723 K (1450 °C). A further increase in temperature to 1773 K (1500 °C) resulted in a decrease in the rate and extent of reduction. An increase in the gas flow rate from 0.4 to 0.8 NL/min and an increase in the methane content in the CH4-H2 gas mixture from 0 to 5 vol pct facilitated the reduction. Methane content in the gas mixture should be maintained at less than 5 vol pct in order to suppress methane cracking.

Experimental comparison of two different ilmenites in fluidized bed and fixed bed chemical-looping combustion

Carbon capture for storage or reuse is seen as one possibility to lower the emissions of anthropogenic CO2. Chemical-looping combustion (CLC) is a promising second generation technique to capture CO2 from thermochemical fuel conversion processes, such as combustion for power generation. The technical implementation of CLC can be conducted using different arrangement approaches based on fluidized or fixed bed reactor designs. When it comes to conversion of solid fuels, the lifetime of an oxygen carrier in a CLC system may be limited by side reactions with fuel ash, or by carryover losses. Therefore, low cost oxygen carriers are preferred. In that context, ilmenite, an iron titanium oxide has been the subject of a number of studies. Here, two ilmenite samples from the Lac Tio mine, Quebec, Canada, extracted from different preparation states were evaluated during activation at 900°C using a CO–CH4–H2 gas mixture diluted in N2. The experiments were carried out both under fluidized bed and fixed bed conditions to point out qualitative differences. Generally fixed bed reduction indicated higher gas conversion although coarser particles were used. However, if oxidation is carried out in a fixed bed, particle sintering may be a substantial problem.

Low temperature diamond growth by linear antenna plasma CVD over large area

AbstractRecently, there is a great effort to increase the deposition area and decrease the process temperature for diamond growth which will enlarge its applications including use of temperature sensitive substrates. In this work, we report on the large area (20 × 30 cm2) and low temperature (250 °C) polycrystalline diamond growth by pulsed linear antenna microwave plasma system. The influence of substrate temperature varied from 250 to 680 °C, as controlled by the table heater and/or by microwave power, is studied. It was found that the growth rate, film morphology and diamond to non‐diamond phases (sp3/sp2 carbon bonds) are influenced by the growth temperature, as confirmed by SEM and Raman measurements. The surface chemistry and growth processes were studied in terms of activation energies (Ea) calculated from Arrhenius plots. The activation energies of growth processes were very low (1.7 and 7.8 kcal mol−1) indicating an energetically favourable growth process from the CO2CH4H2 gas mixture. In addition, from activation energies two different growth regimes were observed at low and high temperatures, indicating different growth mechanism.