Thermal conductivity of diamond composites sintered under high pressures

Thermal conductivity at room temperature of diamond composites of two types: with a diamond skeleton and with diamond grains imbedded in a non-diamond matrix was evaluated in dependence of the diamond grain size (d) varied from a ten of microns to 500 μm. The thermal conductivity of the compacts with diamond skeleton obtained in the Cu–diamond system at high pressure of 8 GPa strongly increases with diamond particles size approaching the maximum value of 9 W/cm K at d ≈ 200 μm. The compacts sintered in the Cu–Ti–diamond, Al–Si–diamond and Si–diamond systems at lower pressure (2 GPa) are formed predominantly owing to the presence of the binder. It was found for these conditions that the thermal conductivity is less sensitive to the diamond grain size, reaching the value of 6 W/cm K for the composites with SiC–Si matrix.     

The effect of boron doping on the morphology and growth rate of micron diamond powders synthesized by HFCVD method

Micron diamond powders are mostly fabricated by crushing the large-sized HPHT diamonds, but they are usually of low quality with the irregular shapes, sharp edges, and protruding points. In the present work, the powders are regrown using HFCVD technique, with the purpose of eliminating their morphological imperfections and developing CVD diamond powders with regular shapes, cubo-octahedral morphology, and smooth surfaces. First, the conventional powders that served as seeds are dispersed evenly on a silicon wafer with a low incidence of agglomeration by the seeding method, based on which a great many of crystals begin to grow simultaneously but independently. The HFCVD system is operating at a low carbon concentration (1.3–1.4%), high substrate temperature (900 °C) and active pressure (4500 Pa) to inhibit the films growth. Acetone, hydrogen gas, and trimethyl borate (C₃H₉BO₃) are used as source materials. Subsequently, the experiments are conducted to grow the micron diamonds with varying amounts of trimethyl borate in the gas mixture. The studies establish the relationships between the boron concentration and the growth rate, surface morphology and purity of CVD micron diamonds. The results show that the addition of trimethyl borate apparently increases the homoepitaxial growth rate by a factor of 1–2, and helps in the formation of single crystals with the euhedral diamond faces and smooth surfaces. However, a heavy boron doping (5000 ppm) readily leads to the growth of ploy-crystals or even films.     

Formation of fibrous structures on diamond by hydrogen plasma treatment under DC bias

Diamond films and powders were treated in microwave plasma of hydrogen at 1.6 torr under a negative DC bias of 200 V for 3–6 h. As a result, a fibrous structure was formed on the diamond surface along the direction normal to the surface, while diamond was simultaneously etched at a rate of approximately 1 μm/h. For a 3-h processing, the fiber diameter near the top end was ≤50 nm and the fiber lengths were 2–3 μm. Transmission electron microscopy (TEM) indicated that the fibers consisted of randomly oriented nanocrystals with diameters of <10 nm. The lattice spacings in the nanocrystals, obtained from TEM images, were 0.22 and 0.25 nm, and hence the structures were assigned to a strongly distorted diamond. An overgrowth of diamond on the fibrous structure resulted in an almost uniform coating of the fibers with crystal-faceted diamond.     

Explosive oxidation of HPHT diamond particles

The thermal oxidation of high pressure and high temperature (HPHT) diamond powders, of between 0.75 and 1.5 μm diameters, has been studied in dry oxygen. Under conditions of poor thermal transfer between the powders and the substrate extremely rapid oxidation has been observed. The onset temperature of the latter ‘explosions’ occurs between 514 and 576 °C, at 1 atm O₂ pressure, where bulk diamond is known to oxidise relatively slowly. A model has been developed in which the exothermicity of the surface oxidation reaction is not dissipated when the sample is in poor thermal contact with the support. A runaway ‘explosion’ occurs in samples of high surface area/volume particulate diamond arising from the high activation energy of the oxidation kinetics of diamond coupled with poor thermal contact. Improving the thermal contact allows diamond powders with the diameters of about 1 μm to be stable even at temperatures up to 700 °C.     

Dependence of the diamond coating adhesion on the microstructure of SiC-based substrates

Diamond thin films were deposited on the SiC–30TiC–10Cr₃C₂ substrates by using the microwave plasma CVD method and the effect of microstructural morphology of substrate was examined on the diamond–substrate adhesion strength. Two types of substrate were prepared by hot-pressing, one with small and equiaxed β-SiC grains and the other with large and elongated α-SiC grains. The SiC–30TiC–10Cr₃C₂ substrate surfaces were chemically etched to remove (Ti,Cr)C matrix phase by Murakami solution. Better diamond–substrate bonding was obtained on the substrate composed of large, elongated grains. Vickers indentation results indicated that mechanical interlocking between elongated and protruded grains on the etched surface and diamond thin film resulted in an increase in adhesion strength.     

Characterization of capacitance–voltage features of Ni/diamond Schottky diodes on oxidized boron-doped homoepitaxial diamond film

The electrical properties of Ni/diamond Schottky diodes fabricated on oxidized boron-doped homoepitaxial diamond film have been studied in order to investigate the electrical behavior of diamond-based electronic devices. The current–voltage (I–V) characteristics of the Ni–Schottky contacts to the boron-doped homoepitaxial diamond film show excellent rectification properties. The capacitance–voltage (C–V) features of the Ni/diamond Schottky diodes were characterized in the frequency range from 10⁻³ to 2×10⁵ Hz. The C–V measurements indicate that the space charge density (N_I) and built-in potential (V_d) values are approximately 6.0×10¹⁶ cm⁻³ and 1.25 V, respectively, and show weak frequency dependence in the range from 10⁻³ to 10⁴ Hz. Capacitance–frequency measurement at zero bias indicated that the degrading capacitance at high frequency (>10⁴ Hz) is primarily due to the high series resistance of the homoepitaxial diamond film.     

Preparation strategy for low-stress and uniform SiC-on-diamond wafer: A silicon nitride dielectric layer

Reducing the self-heating of SiC- and GaN/SiC-based high-powered devices by integrating diamond films offers promising performance enhancement of these devices. However, such a reduction strategy faces serious problems, such as diamond nucleation on SiC and stress accumulation greater than 10 GPa. In this work, a SiN_x dielectric layer (～50 nm) was coated onto the C polar face of a 4H–SiC wafer using microwave plasma chemical vapor deposition (MPCVD) to improve direct dense diamond nucleation and growth, significantly reduce the stress, and build Si–C(SiC)?Si?C(diamond) bond bridges. This SiN_x thin layer, prepared by activating Si ions under Ar/N plasma during magnetron sputtering, gave rise to local Si₃N₄ crystal features and a low effective work function (EWF) for promoting surface dipoles with electronegative carbon-containing groups. In the H plasma environment during diamond growth, the local Si₃N₄ crystal was amorphized, and the N atoms escaped as a result of atomic H and the high temperature. At the same time, C atoms diffused into the SiN_x and formed C–Si bonds (49.7% of the total C bonds) by replacing N–Si and Si–Si, thus increasing the direct nucleation density of the diamond grains. The diamond thin film grew rapidly and uniformly, with a grain size of approximately 2 μm in mixed orientation, and the stress of the 2-inch SiC-on-diamond wafer was extremely low (to ～0.1–0.2 GPa). In comparison, partially connected diamond grains (>10 μm) on the bare SiC in the preferential (110) orientation resulted in a film with twin-grain features and significant stress, which was associated with the hexagonal lattice interface of 4H–SiC. These results are considered the material and surface/interface bases for actively controlling wafer fabrication and enhancing the heat dissipation of SiC and GaN/SiC electronics.     

Fabrication and properties of lateral p–i–p structures using single-crystalline CVD diamond layers for high electric field applications

Chemical-vapor-deposited diamond p–i–p structures have been fabricated on homoepitaxially grown single-crystalline layers using focused ion beam etching in order to investigate carrier transport properties of diamond in high electric fields more than 10⁶ V/cm. The examined structures included a 200-nm-thick intrinsic (undoped) diamond region laterally sandwiched between two p-type (B-doped) diamond regions. At high fields above ≈3×10⁷ V/cm, I–V characteristics of the lateral p–i–p structure revealed abnormal increases in current. The observed performances can be explained more reasonably in terms of impact ionization events from the valence band to the conduction band in the intrinsic diamond than in terms of direct Fowler–Nordheim tunneling events of electrons from the valence band of the negatively biased p diamond to the conduction band of the intrinsic diamond.     

Application of the dusty plasma method for preparation of diamond ceramics

Diamond particles 3–7 μm in size sustained in plasma in a high-dispersion state were coated with cobalt by magnetron sputtering. The relative concentration of cobalt in obtained powders was 2–3 mass. %. Sintering the diamond powders with the cobalt coating under the pressure of 8 GPa and the temperatures of 2000–2100 K resulted in the production of homogeneous specimens having the density of 3.6 ± 0.1 g cm^− 3. The produced diamond compacts demonstrated high values of the ultrasonic wave propagation velocity and elastic moduli.     

Channel mobility evaluation for diamond MOSFETs using gate-to-channel capacitance measurement

In this work, the channel mobility (μ_ch) of diamond metal-oxide-semiconductor field-effect transistors (MOSFETs) with hole accumulation layer channels was evaluated from the gate-to-channel capacitance and drain conductance for the first time. The FET structure was utilized for the capacitance–voltage (C–V) measurement, and the gate-to-channel capacitance (C_GC) under the forward bias condition was proportional to the gate area, as in the case of Si MOSFETs. For the accurate evaluation of the drain conductance, diamond MOSFETs were fabricated on IIa-type diamond films with low boron concentrations (< 10¹⁴cm^{− 3}). In a 60-μm gate-length diamond MOSFET, a μ_ch of 145cm²/Vs was obtained, which is comparable to that of a SiC inversion layer.     

Structural and vibrational properties of the 6H diamond: First-principles study

The 6H diamond is the first successfully synthesized diamond polytype, which are not found in nature. We have performed the investigations on structural properties and vibrational properties of the 6H diamond with a first-principles method and we also have identified the vibration normal modes of the 6H diamond with group theory. The structural properties of the 6H diamond are quite similar to the cubic diamond (3C diamond) and the lonsdaleite (2H diamond) as expected. The vibrational properties of the 6H diamond are more complicated than the 3C and 2H diamonds. At Γ point of the first Brillouin zone of the 6H diamond, there are 15 Raman active modes with nine different frequencies, and six IR active modes with four different frequencies. However, there are no IR active modes in the 3C and 2H diamonds. Therefore, the IR active modes of the 6H diamond can provide the indication modes for distinguishing the 6H diamond from the 2H and 3C diamonds. For Raman active modes, the frequencies of the E_1g² and E_2g² modes of the 6H diamond are much smaller than the Raman frequencies of the 2H and 3C diamonds; consequently, it also can contribute to distinguishing the 6H diamond from the 2H and 3C diamonds. The frequencies of two Raman active modes of the 6H diamond–the A_1g² mode and the E_1g¹ mode–agree very well with the experimental results.     

Modification of mesoporous structure of silver-doped bioactive glass with antibacterial properties for bone tissue applications

Silver-containing mesoporous bioglasses powders with SiO₂–CaO–P₂O₅–Ag₂O composition have been successfully synthesized by sol-gel and evaporation-induced self-assembly (EISA) methods in presence of various amounts of surfactant (Pluronic-F127). The morphology and crystal structure of the powders were characterized by scanning electron microscopy (SEM) and X-ray diffraction spectroscopy (XRD). Also, the textural properties of samples have been evaluated by adsorption-desorption, Langmuir and BET methods. Accordingly, powders had a smooth surface morphology with cubic mesoporous structure and a desired surface area and pore volume. The in vitro bioactivity was assessed by SEM, XRD and Fourier transform infrared spectroscopy (FTIR) analyses. All samples enhance the formation of HA after soaking the material in simulated body fluid (SBF) solution. The antibacterial property of samples was evaluated to investigate the effect of silver content in chemical composition. The results showed an adequate antibacterial activity of the samples against Escherichia coli, Salmonella and Listeria monocytogenes.     

Tribological studies of electroless nickel/diamond composite coatings on steels

In this study, a commercially available process of electroless nickel plating with co-deposited diamond powders was applied to a steel substrate as an intermediate layer prior to diamond deposition by MPECVD. The diamond films show excellent adherence, since they are strongly bounded to the diamond particles, deeply anchored into the electroless plated nickel matrix. A synergism effect of electroless nickel plating and MPECVD diamond growth are discussed. The electroless nickel plate which can be hardened itself by the precipitated phosphide phases after the heat treatment is an efficient diffusion barrier against the inter-diffusion of iron from the steel substrate and carbon from CH₄. A more continuous and smoother diamond film can be formed on the outermost surface. The results of tribotesting indicated that each step in the process of composite formation significantly lowers the friction coefficient (μ), especially the secondary layer of electroless nickel plate (~ 1 μm) is particularly effective and possesses a steadily low value of μ, which has promise for tribological applications. The secondary nickel layer could enhance the adherence of diamonds in the metal matrix, and be responsible for the better continuity of the top diamond film.     

Simulations on effects of granular morphology on single-electron and optical spectra of nano- and micro-crystalline AlN and diamond

In the present study, effects of geometry of the granular morphology on particularities of the N(E) distributions and optical absorption spectrum in nominally undoped nano- and micro-crystalline AlN and diamond are simulated within a framework of a semi-empirical adiabatic ‘Generalized Skettrup Model’ (GSM). Obtained simulation results reveal, that alterations in the grain sizes mostly affect relatively deep (i.e. defect) intra-grain states, while band tail electron states are influenced pretty weakly. Amendments in a shape of the crystalline grains also cause considerable alterations in the intra-grain single-electron spectrum of the poly-crystalline layers and powders of AlN and diamond, though such effects are not as strong as the ones that originated from the grain size changes in the range from nano-meters to microns. Results of our semi-empirical simulations match well with corresponding experimental data, previously reported for single-micro- and nano-crystalline AlN and diamond, and might give physically transparent guidelines for adjustment optical and electronic properties of such materials for their various applications.     

Raman scattering and X-ray diffraction investigations of sol–gel derived SBN powders

Sol–gel derived Sr_xBa_1-xNb₂O₆ (SBN) powders were prepared at an annealing temperature of 1200°C. Their structural changes were characterized by Raman spectroscopy and X-ray diffractometry. Changes in the peak position and the relative intensity of the Raman spectra and X-ray patterns were examined for different values of x. Our results suggest that the SBN powders consist of a mixture of orthorhombic phase BaNb₂O₆ (BN) and tetragonal tungsten-bronze phase (TTB) SBN powders for x<0·5, and orthorhombic phase SrNb₂O₆ (SN) and TTB phase SBN powders for x>0·5. Pure TTB type SBN powders with x≠0·5, however, were obtained at higher annealing temperatures. A possible formation route of the sol–gel derived SBN powder is discussed.     

Sintering behavior of the Ti(C,N)-based cermets with graphite or diamond additives

Combining DSC/TG–QMS analysis and dilatometry experiments at constant heating rates, the sintering behaviors of the Ti(C,N)-based cermets with carbon additives were studied. It is found that the additives of diamond or graphite could promote the interactions among raw powders during solid-state sintering. The outgassing behavior and endothermic effect were enhanced, thereby resulting in the increasing densification activation energy. The negative activation energy during the liquid phase sintering indicates that the dissolution–precipitation process occurs easily and provides a rapid densification path. The dissolution–precipitation process for cermets with .6-wt% graphite or .6-wt% diamond additive became more efficient than for cermets without additional carbon additive. The white-core/gray-rim grains were clustered together in the cermets with .6-wt% graphite additive, whereas they were distributed evenly in the cermets with .6-wt% diamond additive. Moreover, the average sizes of ceramic grains in the cermets without additional carbon additive, and with .6-wt% graphite or .6-wt% diamond additives, were .453, and .517, or .525 μm, respectively.     

Ranking proposed models for attaining surface free energy of powders using contact angle measurements

This study is an attempt to evaluate the applicability of various proposed mathematical models to calculate the surface free energy of commercially available powders. The capillary rise experiments were employed to achieve the contact angle between 15 powders and seven corresponding liquids by means of the modified Lucas–Washburn's equation. The surface free energy of powders was then calculated using different models inclusive of Owens/Wendt, harmonic mean, van Oss et al., combined mean (i.e. the combination of Owens/Wendt and harmonic mean models) and Li/Neumann models. Mathematical approaches were used to assess the accuracy of the calculated surface free energy and its components for different powders. A series of first-, second- and third-order functions as well as an exponential one were developed and put to test for one-, two- and three-parameter variables of liquid surface tension. Unfortunately, all such functions did not perform well in correctly estimating the contact angles of the liquid/powder systems (i.e. r² range being 0.48–0.68 and PF/3 range being 114–312). On the other hand, a series of trained artificial neural networks (ANNs) comparatively gave good correlations, predicting with unsurpassed accuracy the contact angles of the same corresponding liquid/powder systems (i.e. r² range being 0.93–0.94 and PF/3 range being 30–55). Therefore, the attained and tested ANNs were used further to provide the surface free energy of the 15 powders. In addition, the ANNs were also employed to rank the surface free energies of powders as well as their corresponding components as calculated by other models. The results showed that the geometric mean model was able to calculate the surface free energy of powders with more accuracy than all the other models.     

Nanocrystaline solid solution CeO2–Bi2O3

Nanocrystalline powders of solid solution CeO₂–Bi₂O₃ were synthesized by self-propagating room temperature reaction (SPRT) procedure with composition (Ce_1?xBi_xO_2?δ where the x = 0.1–0.5). X-ray diffraction analyses show that for x < 0.50 a solid solution with fluorite structure is formed. Rietveld's structure refinement method was applied to characterize prepared powders and its microstructure (size–strain). The lattice parameters increase according to Vegard's rule with increasing of Bi concentration. The average crystallite size is about 2–3 nm. Spectroscopic ellipsometry and Raman scattering measurements were used to characterize the samples at room temperature. The Raman measurements demonstrated electron molecular vibrational coupling and increase of oxygen vacancy concentration whereas increase of Bi content provokes a small decrease of optical absorption edge in comparison with pure ceria. Specific surface area of obtained powders was measured by Brunauer–Emmet–Teller (BET) method.     

Wettability studies of reactive brazing alloys on CVD diamond plates

Wettability of both the diamond and the insert surfaces by the filler metal in CVD diamond brazed-on cutting tools is a key condition for good brazing strength. The brazing process of CVD diamond thick plates still has to be improved, namely on the influence of the brazing alloy composition and of the substrate surface finishing quality in wettability. In this study, contact angle measurements were performed in a dedicated high vacuum furnace coupled with a video recording system. Diamond films with different thickness (75<t<300 μm), and thus having distinct grain sizes and roughness, were grown with fixed conditions by the MPCVD technique on Si substrates and chemically detached for wettability experiments. Roughness parameters were evaluated by profilometry and AFM, which was used to observe the grown diamond surfaces of the self-standing films. The reactive Ag–Cu–Ti brazing system was investigated. Results showed a very good wettability in the temperature range 800–850°C, namely for the diamond surface where a minimal contact angle of 10° was reached. A Ti-rich thin reaction layer (0.5–0.8 μm) was detected at the drop side of the substrate/brazing alloy interface in both substrate materials, proving the affinity of Ti to carbon. The influence of the diamond roughness on the contact angle θ_R is notable, obeying a linear dependence of the type cosθ_R=cosθ₀+k cosθ₀·(R_a/G)², where R_a and G, the grain size, are related to asperity height and width, respectively. This relationship is based on the well-known Wenzel equation that correlates the real contact angle to the surface area increasing with roughness.     

Microstructure and performance of brazed diamond segments with NiCr–x(CuCe) composite alloys

Novel multi-layer brazed diamond segments were fabricated using NiCr–x(CuCe) composite alloys. Differential scanning calorimetry curves of the composite alloys were measured and analysed. The microstructures of the alloy segments and surface topographies of the brazed diamond segments were characterised. Performance tests of the alloy segments and brazed diamond segments were performed. The undercooling degree of the Ni–Cr alloy in the composite alloy increased with the Cu–Ce alloy addition, which led to coarse NiCu-rich regions and Ni₃Si phases. A brazed diamond segment with a 5% Cu–Ce alloy addition exhibited the highest wear resistance and machining performance and the best surface morphology after a wear test. An excessive Cu–Ce alloy addition led to a rapid decrease in wear resistance of the brazed diamond segment owing to the large number of coarse NiCu-rich phases falling off from the composite alloy. The mechanism of the reduction in thermal damage to diamonds by the Cu–Ce alloy is elucidated. Initially the Cu–Ce particles melted and mainly Ni atoms diffused into the Cu–Ce liquid, thereby leading to the formation of NiCu-rich regions and Ce₂Ni₇ and CeNi₂ phases, which in turn promoted the diffusion. The melting temperature of the Ni–Cr composite alloy was significantly reduced by the addition of the Cu–Ce alloy.