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In the present study, two types of diamond films with a diameter of 123 mm were deposited by DC arc plasma jet chemical vapor deposition (CVD) operating in the gas recycling mode. These two diamond films were found to possess peculiar grain family morphologies. The fracture strengths of the samples were 182.65 and 995.58 MPa, respectively. To investigate the possible reason for the large difference in the fracture strength between the samples, the surface and fracture morphologies of the two types of diamond films as well as their grain size were characterized. The results revealed that the sample with low fracture strength suffered from a typical intergranular fracture. Weak grain boundary, impurities distributed in the boundary, and low strength of diamond grains led to a decrease in the fracture strength. However, the sample with high fracture strength mainly suffered from transgranular fracture and the columnar grain exhibited numerous voids. Owing to the impact of twinning, the voids could be occupied, thus increasing the contact area between the adjacent grains. Moreover, large grain family on the growth side contained numerous small grains and twins. These small grains, twins on growth side along with the difference of stress on surfaces inhibit fracture strength from decreasing rapidly. All these features benefitted fracture strength. Moreover, when the sample was loaded, the tips located in the voids probably resulted in increase of risk of stress concentration, which might have led to initiation of crack origin and eventual fracture. [ABSTRACT FROM AUTHOR]

Diamond has attracted increasing attention because it has high thermal conductive. Here, a freestanding diamond thermal conductive film with a diameter of about 7 in. and an average thickness of 1.54 mm was prepared by DC arc plasma jet CVD with multi-stage magnetic fields. Specifically, the thickness uniformity was ensured by adding the magnetic field during the deposition of the freestanding diamond film. The morphology, structure and thermal conductivity of the diamond film were studied. The results showed that the expansion area of the DC arc plasma approached the surface of the substrate increasing duo to add the bottom magnetic field. Structural characterization revealed that the grain of the diamond film in the arc main were coarse, affected by the shape of DC arc plasma, while in the arc center and edge were fine. The main orientation of polycrystalline structure of the diamond film was the crystalline plane of (220). The Raman peaks of diamond film in different areas moved to a higher wavelength, indicating the existence of compressive stress inside the diamond film. The coarse columnar grain and a few grain boundaries were in the arc main. Therefore, the average value of the thermal conductivity (k ⊥) of diamond film measured in the arc main was 1728.9 ± 4.9 W/(m·k). Meanwhile, the thermal conductivity of the arc center and edge area was lower than that of the arc main. [Display omitted] • A freestanding diamond thermal conductive film with a diameter of about 7 in. was prepared. • DC arc plasma jet CVD with multi-stage magnetic fields was adopted. • The average value of the thermal conductivity (k ⊥) of diamond film measured in the arc main was 1728.9 ± 4.9 W/(m·k). [ABSTRACT FROM AUTHOR]

In this study, standard diamond bars were prepared using an ultra-thick diamond plate, and the bars had sizes consistent with the International Organization for Standardization (ISO) 14704:2016(E) three-point bending method for measuring the fracture strength. Ultra-thick diamond plate with dimensions of 125 mm (diameter) × 6.5 mm (thickness) were deposited using direct current arc plasma jet chemical vapour deposition. After cutting, grinding, and polishing, 12 diamond bars meeting the requirements of the ISO standard were prepared (36 mm × 4 mm × 3 mm) and tested for fracture strength using the cited ISO standard and elastic modulus using deflection measurements. The fracture strength of the ultra-thick diamond plate was found to be 580 ± 66 MPa. In addition, for comparison, the fracture strengths of standard ZnS, sapphire, and alumina were also analysed. Crucially, this is the first report of the fracture strength of standard-sized diamond plate. The effects of the defects and grain size on the crack formation and failure of the diamond film were analysed. The diamond's fracture strength was affected by the sample thickness, which caused the gradual accumulation of defects as this dimension increased, but not by its length and width. The elastic moduli of sapphire and alumina were compared. Overall, the measurement of the fracture strengths of ultra-thick diamond plate via the ISO standard is important for industrial applications. [ABSTRACT FROM AUTHOR]

The direct-current arc plasma jet method is advantageous for the rapid deposition of large-area self-supporting diamond films. However, dark defects tend to be generated during the growth process, significantly affecting the performance of the diamond. In this study, we use the direct-current arc plasma jet method to prepare transparent and translucent samples. Subsequently, we compare their optical, thermal, and mechanical properties and analyze the differences in their performances via microstructure and defect analyses. The results indicate that the transparent diamond samples exhibit excellent optical and thermal properties, with a transmittance of 68.4 % at a wavelength of 10.6 μm and a thermal conductivity similar to that of type-IIa single crystal diamond (i.e. 2002 W/(m·K)). Meanwhile, the translucent diamond exhibits outstanding mechanical properties, with a fracture strength of 984 MPa. Characterization results based on electron backscatter diffraction and X-ray computed tomography suggest that a preferred orientation along the (220) planes is conducive to the densification growth of the samples, thereby enhancing their optical and thermal properties. However, the generation of numerous twins results in interlocking between grains, which improves sample's mechanical properties but introduces porosity and impurities, thus deteriorating its optical and thermal performance. [Display omitted] • Optical and thermal diamond films were deposited by DC-arc plasma jet CVD with growth rates >6 μm/h. • Strong (220) texture and numerous twins can improve optical transmittance and fracture strength, respectively. • X-ray CT was used to reveal defects distribution and content within diamond samples. [ABSTRACT FROM AUTHOR]

Abstract As we know that more effective synthesis of diamond combined with physical and chemical properties of hydrogen termination in aqueous environment as well as device structure design can greatly facilitate the chemical and electrochemical applications of higher cost-performance diamond. For this purpose, the direct-current (DC) characteristics, surface reaction and reparation of a hydrogen-terminated DC arc jet plasma CVD polycrystalline diamond, which has a high cost-performance, were characterized by I-V experiments based on a FET-like structure device in multiple aqueous solutions. The variation trends of the I-V properties of device based on pH were similar in different aqueous solutions but could be affected by disparate ions (such as K+ ions) or organic molecules (such as citric acid radicals or a benzene ring). Especially, owing to the founded replacement of hydrogen termination with hydroxyl (OH), carboxyl radical (COOH) or carbon-oxygen bond (C O C) in mixed solution, i.e., KHP + H 2 SO 4 (and +NaOH) solutions, the resistance of the device was remarkably increased from 13.57 kΩ to 95.78 kΩ. However, the raised resistance of surface reacted diamond (SRD) can be reduced prominently by repairing hydrogen termination through negative potential sweeps (NPS) at a low negative potential (−1 to −3 V) if the SRD was introduced as an electrode in a strong inorganic acid. What's more, the NPS repaired device, which subsequently stored for four weeks, was more sensitive than the original hydrogen plasma-treated diamond in aqueous solution environments. This repaired result was coming out of NPS re-formed C H bonds with higher intensity. These findings will be the references of failure and reparation of diamond hydrogen termination in aqueous environment. Graphical abstract Image 1 Highlights • The device-used high quality free-standing polycrystalline diamond with high cost-performance was prepared by DC arc jet plasma CVD. • The variation trends of I-V characteristic based on pH in diverse solutions are similar but could be affected (even damaged) by disparate ions or organic molecules. • The raised surface resistance caused by reaction could be reduced via negative potential sweep at low negative potential. • The recovered device was stable and more sensitive in solution environment. [ABSTRACT FROM AUTHOR]

The growth process of carbon film on the surface of hydrocooling copper substrate in high-density direct current (DC) arc plasma was studied. Scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, nuclear magnetic resonance spectroscopy, and UV confocal Raman spectroscopy were used to analyze the structural constituents of the intermediate products generated at different growth periods. Results show that despite the remarkable heating effect of DC arc plasma to the copper substrate, one layer of cauliflower saturated carbon and double carbon bond as main component loose matter are first formed because of water-cooled copper substrate. With the increase in thickness of carbon film, the surface temperature gradually increases, the etching ability of the atomic hydrogen is enhanced and high concentration reactive units are maintained on the surface of carbon film. The cauliflower structure may effectively reduce the diamond nucleation barrier and promote the generation of nano-diamond. The diamond grains gradually grow up to the micron scale by the continuing influence of carbon-based active group and plasma. The growth pattern of carbon film on the hydrocooling copper substrate in high-density DC arc plasma may provide a new way to prepare large crack-free diamond films. [ABSTRACT FROM AUTHOR]

The addition of titanium (Ti) interlayer was verified to reduce the residual stress of diamond films by self-fracturing and facilitate the harvest of a crack-free free-standing diamond film prepared by direct current (DC) arc plasma jet. In this study, the evolution of the Ti interlayer between large-area diamond film and substrate was studied and modeled in detail. The evolution of the interlayer was found to be relevant to the distribution of the DC arc plasma, which can be divided into three areas (arc center, arc main, and arc edge). The formation rate of titanium carbide (TiC) in the arc main was faster than in the other two areas and resulted in the preferred generation of crack in the diamond film in the arc main during cooling. Sandwich structures were formed along with the growth of TiC until the complete transformation of the Ti interlayer. The interlayer released stress via self-fracture. Avoiding uneven fragile regions that formed locally in the interlayer and achieving cooperatively released stress are crucial for the preparation of large crack-free diamond films. [ABSTRACT FROM AUTHOR]

In this study, the effect of defects on fracture toughness in different directions was investigated. Two diamond films with a diameter of 122 mm and as-deposited thicknesses of 2.2 and 1.5 mm, respectively, were deposited by DC arc plasma jet chemical vapor deposition. To accurately obtain the fracture toughness, the films were ground and polished to 1.6 and 0.8 mm, respectively. The results indicate that many defects, including pores, are introduced into the films during the growth process, particularly on the side close to the growth surface in thicker film. The size of pores reaches micrometres, which affects the fracture toughness under loading in different directions. Due to the minimum grain size, both films exhibit a maximum toughness of 8.2 and 10.0 MPa·m1/2, respectively, with growth-surface cracks. For the thinner sample, the maximum value is followed by that of the edge-surface cracks. As the grain size of the edge surface falls between that of the growth surface and that of the nucleation surface, the results indicate that fracture toughness is affected by grain size. However, the number of pores near the growth side increases when the thickness exceeds 0.8 mm. Pores reduce the toughness, resulting in the minimum fracture toughness (6 MPa·m1/2) of the thicker diamond film with edge-surface laser cracks. The sample with an edge-surface sharp pre-crack has a similar fracture toughness of 5.6 MPa·m1/2. This study provides guidance for selecting the applied load direction. • Two thick diamond films with thicknesses 2.18 and 1.48 mm were deposited. • Fracture toughnesses of the diamond films in three directions were investigated. • Numerous pores were formed in the diamond films during orientation growth. • Due to the effect of pores, the toughness values among three directions differ. [ABSTRACT FROM AUTHOR]

It was predicted that diamond would play an irreplaceable role in thermal management technology at ultra-high heat flux levels owing to its outstanding thermal conductivity. In this work, an ultra-thick diamond plate, which was realized by direct-current (DC) arc jet plasma chemical vapor deposition (CVD) system, was fabricated into an all-diamond microchannel heat sink by laser. The convective heat transfer capacity, thermal resistance, device surface temperature and pressure drop of the all-diamond MCHS were systematically studied under the heat flux of 40–120 W/cm2 based on a single-phase heat transfer system, comparing to the commercially available aluminum MCHS. Under the assumption that the extreme service temperature of the device at 100 °C, the advantage of all-diamond MCHS in reducing fluid flow is evaluated. The results demonstrated that the heat transfer coefficient of the all-diamond MCHS was in the range of 5637.10–11447.20 W/m2‧K, which is 37%–73% higher than that of the aluminum MCHS with the same geometry. The volume flow of the fluid in the diamond MCHS is merely 40% of that in the aluminum MCHS could effectively reduce the equipment load owing to the significant advantage of the all-diamond MCHS in reducing the device surface temperature and thermal resistance. • An all-diamond MCHS was obtained on an ultra-thick CVD diamond plate. • Diamond MCHS has the higher fin efficiency due to its superior thermal conductivity. • The heat transfer coefficient of an all-diamond MCHS was 37%–73% higher than that of an aluminum MCHS. • Only 40% of volume flow rate of coolant for an aluminum MCHS operation can satisfy the requirement of an all-diamond MCHS. [ABSTRACT FROM AUTHOR]