Waste heat recovery is significant for improving energy utilization, reducing carbon emissions, and neutrality. The gravity heat pipe (GHP) has excellent thermal performance due to the cyclic phase transformation of the working fluid. As an important thermal management device for waste heat recovery, the heat transport capacity of GHP improves, the efficiency and performance of the waste heat recovery increase, and more wasted heat can be stored more quickly. Nano-diamond has the highest thermal conductivity and can be dispersed in water to form a diamond nanofluid, enhancing the thermal performance of GHP. In contrast, the study on the heat transfer behavior of the diamond nanofluid in GHP is insufficient. Besides, the influences of filling ratio (FR), mass fraction (MF), and heat flux on thermal performance are in demand for further study. In this article, the heat transfer behavior is investigated by studying the flow patterns of diamond nanofluids. The influences of filling ratio and mass fraction on flow patterns are analyzed. An orthogonal experiment is conducted; the heat flux has the most significant effect on the thermal performance, followed by the filling ratio and mass fraction. The thermal performance is the best when the optimal parameters (FR = 20%, MF = 1 w.t.%) are selected under a heat flux of 20 × 104 W/m2. The equivalent heat transfer coefficient reaches 3485 W/(m2·°C). This article can achieve a deeper understanding of the diamond nanofluid heat transfer mechanism in GHP and enhance the thermal performance of GHP for better waste heat recovery.

Carbon, the fourth most abundant element in the Universe, possesses numerous allotropes with diverse bonding character (sp1-, sp2- and sp3-hybridized bonds) and structural motif of the constituting atoms. In particular, the carbon materials with a fully or nearly 100% sp3-hybridized strong C-C bonds often lead to excellent mechanical properties, chemical stability, thermal and optical properties, such as crystalline diamond and diamond-like amorphous carbon (DLC). In this review, we systematically summarize the synthesis, microstructure, mechanical properties, thermal and optical properties of ultrahard carbon materials with current experimental results on nano-polycrystalline diamond (NPD), nanotwinned diamond (NTD), micro-grained polycrystalline diamond (MPD), and amorphous diamond/carbon. In addition, we discuss the difference of spectra of XRD, Raman and EELS between various nanocrystalline diamond powder and ultrahard carbon materials. Finally, we provide our insights into the future development and applications in the research of ultrahard carbon bulk materials by high-pressure and high-temperature techniques according to the current advantages, limitations and challenges in the experiment.

Investigation of germanium gate hydrogen-terminated (H-terminated) diamond field effect transistor (FET) with Al2O3 dielectric layer has been successfully performed. The device demonstrates a normally-on characteristics, whose maximum drain-source current density, threshold voltage, maximum transconductance, on/off ratio, subthreshold swing, capacitance, carrier density, saturation carrier mobility, fixed charge density and interface state density are of −37.3 mA/mm, 0.22 V, 6.42 mS/mm, 108, 134 mV/dec, 0.33 μF/cm2, 9.83 × 1012 cm−2, 97.9 cm2/V·s, 7.63 × 1012 cm−2 and 2.56 × 1012 cm−2·eV−1, respectively. This work is significant to the development of H-terminated diamond FET.

Diamond and graphene are considered to be one of the most promising thermal interface materials (TIMs) for electronic devices benefited from their highest thermal conductivity in the natural world. However, orientated fabrication of high thermal conductivity diamond and graphene hybrid arrays with three dimensions (3 D) thermal conductive networks are still problematic. Here, we used a unique one-step microwave plasma chemical vapor deposition, n-butylamine, as the liquid source to prepare a novel high thermal conductivity 3 D vertical diamond/graphene (VDG) hybrid arrays films. The orientated 3 D thermal conduction path of the VDG is regulated by the growth temperature, and the through-plane thermal conductivity value of the VDG700 films up to 97 W m−1 K−1. In the actual TIM performance measurement, the system cooling efficiency with our VDG as TIM is higher than the state-of-the-art commercial TIM, demonstrating the superior ability to solve the inter-facial heat transfer issues in electronic systems.