Search

A Ka -band AlGaN/GaN metal–insulator–semiconductor high-electron-mobility transistor (MIS-HEMT) with SiN gate dielectric grown by plasma-enhanced atomic layer deposition (PEALD) is demonstrated. The gate reverse leakage current in a controlled HEMT is reduced by about three orders of magnitude by the PEALD-SiN, contributing to an improved breakdown voltage of 92.5 V (source–drain separation of $2.0~\mu \text{m}$ ) in the fabricated MIS-HEMTs. The MIS-HEMTs also feature small threshold voltage hysteresis in dc transfer and capacitance–voltage characterizations, suggesting a good PEALD-SiN/AlGaN interface. The fabricated MIS-HEMTs deliver a high output power density of 7.16 W/mm and a peak power-added efficiency of 60.3%, respectively, at ${V}_{\text {DS}} =30$ V and ${V}_{\text {DS}} =10$ V in pulse mode at 28 GHz. PEALD-SiN could be a compelling gate dielectric for fabrication of high-power and high-efficiency MIS-HEMTs.

Low damage atomic layer etching (ALE) gate recess is developed for fabrication of millimeter-wave AlGaN/GaN high-electron-mobility transistors (HEMTs). Plasma ion induced bombardments to the AlGaN barrier is effectively suppressed by the ALE recess, contributing to a well-controlled recessed surface morphology. The suppressed lattice damage to AlGaN/GaN heterostructure is also reflected by a significantly reduced gate leakage as well as an invisible threshold voltage shift associated with damage induced traps. With a 0.15- $\mu \text{m}$ T-gate fabrication technology, a high power-gain cutoff frequency ${f}_{\text {MAX}}$ of 205 GHz has been achieved. The ALE-recessed AlGaN/GaN HEMTs exhibits a record high power-added-efficiency (PAE) of 43.6% at 40 GHz in a continuous-wave mode. The associated gain and output power density are also remarkably improved compared with controlled HEMTs with conventional gate recess process.

In this letter, a high-temperature (HT) gate recess technique is implemented into the fabrication of high-performance millimeter-wave AlGaN/GaN high-electron-mobility transistors (HEMTs). By virtue of the low damage feature of the HT recess, a high extrinsic transconductance of 422 mS/mm, a three-terminal breakdown voltage of 134 V with a source–drain separation of 2.4 $\mu \text{m}$ , and a remarkable low Schottky gate leakage current of $1.6\times 10^{-6}$ A/mm at ${V}_{\textsf {GS}} = -\textsf {60}$ V are achieved, which is at least two orders of magnitude lower than the HEMTs with gate recessed at room temperature. By employing a 0.2- $\mu \text{m}$ T-shaped gate technology, a current-gain cutoff frequency ${f}_{T}$ of 81 GHz and a unit-power-gain frequency ${f}_{\textsf {MAX}}$ of 194 GHz are realized. The current collapse in the HT-recessed AlGaN/GaN HEMTs is significantly suppressed, contributing to a record high power-added-efficiency of 42.8%, as well as high output power density of 5.1 W/mm at 35 GHz in a continuous-wave mode.

Superior iron-based Fischer-Tropsch synthesis (FTS) catalysts (Fe/NMCs) were developed by impregnating high amount of iron (40 wt%) over nitrogen-rich mesoporous carbons (NMCs) with high porosities. The large pore volumes and specific surface areas of NMCs realized the high loadings and proper dispersions of iron while the nitrogen-containing groups enhanced the basicities of the catalysts. Weak metal-support interactions were observed in Fe/NMCs, which improved the reduction, carburization of iron, and further the FTS activities. FTS tests indicated a medium amount of nitrogen (≤8.3 wt%) did not show obvious effect on the activity of the catalyst, but a much higher amount of nitrogen (≥16.5 wt%) could result in a significant decrease of activity. Nitrogen-containing groups could effectively suppress the methane selectivity and improve the lower olefins selectivity in the FTS reaction. The hydrocarbon productivity of the Fe/NMCs was up to 0.62 g HC/(h·g cat.) at 260 °C, 1 MPa, and H2/CO ratio of 1, much higher than that of any other iron catalyst reported under similar conditions. As a result of the superior performance, Fe/NMCs could be one of ideal candidates for iron-based FTS catalysts in the future.

Iron-based Fischer-Tropsch synthesis (FTS) has received renewable interests due to the gradual depletion of crude oil resources and its flexible product adjustment from lower olefins to long-chain hydrocarbons. However, one of the main challenges of iron-based FTS catalysts is the relative lower activity or productivity (per gram catalyst) at low or middle temperature. In this work, ultra-active iron-based FTS catalysts (Fe/SMC) were developed by loading iron over spherical mesoporous carbon (SMC) with huge porosity. The large pore volume (2.22 cm 3 /g) and high specific surface area (767 m 2 /g) of SMC allowed the high iron loading (up to 50 wt%) with proper dispersion. Weak C-Fe interaction was observed in the Fe/SMC catalysts, which was favorable for the reduction of iron and its further carburization to form more active sites of FTS. Activity tests showed that the Fe/SMC catalysts exhibited ultra-high activities in FTS. With the increase of iron loadings (30–50 wt%), the FTS rates (FTY, mmol CO/(h·g cat.)) of Fe/SMC catalysts increased due to the larger iron surfaces exposed at higher iron loadings. Small amount of K promoter significantly enhanced the FTY and apparent turnover frequency (TOF, S −1 ) of the Fe/SMC catalysts. The hydrocarbon productivity of K-promoted catalyst (40FeK/SMC) was up to 0.91 g HC/(h·g cat.) (260 °C, H 2 /CO = 1, 2 MPa), much higher than that of any other supported or unsupported iron-based FTS catalysts reported at similar reaction conditions. The results obtained in this work will open a new avenue for the development of highly active iron-based FTS catalysts.

In this contribution, the efficient hydrogenation of dimethyl oxalate (DMO) to ethanol was achieved over a series of cerium promoted Cu/SiO2 (xCe-Cu/SiO2) catalysts prepared by a urea-assisted gelation approach. As a promoter, Ce played a crucial role in improving the structure, properties and catalytic performance of the Cu/SiO2 catalysts. The structure and chemical properties of the synthesized catalysts were characterized by XRD, FT-IR, in situ XRD, TEM, STEM-EDX mapping, N2O titration, H2-TPR, H2-TPD, XPS, XAES, etc. The characterization results disclosed that the addition of a Ce promoter to the Cu/SiO2 catalysts remarkably increased the Cu dispersion and retarded the sintering of small-sized Cu species. The strong interaction between the Ce promoter and Cu species substantially changed the redox properties of xCe-Cu/SiO2 catalysts and made the Cu2+ species easy to reduce. In addition, the activation ability of H2 species was significantly improved in the xCe-Cu/SiO2 catalysts, as evidenced by H2-TPD studies. Among these synthesized catalysts, the 1Ce-Cu/SiO2 catalyst with a 1.0 wt% Ce loading exhibited the highest catalytic activity (100% DMO conversion), ethanol selectivity (91.8%) and stability. This enhancement of catalytic activity, ethanol selectivity and stability is very promising for the development of an alternative route for the production of ethanol by hydrogenation of DMO from syngas.

Mesoporous Ni-Mg-Al and Ni-Al catalysts were facilely synthesized via evaporation-induced self-assembly (EISA) method and employed for coupled reforming reaction consisting of dry reforming of methane (DRM) and partial oxidation of methane (POM) to produce syngas (H 2 + CO). The Ni-Mg-Al and Ni-Al catalysts with encapsulated nickel nanoparticles were directly synthesized in one-pot way. For comparison, Ni/Al 2 O 3 as reference catalyst was also prepared by general impregnation method. Characterization by BET, XRD and H 2 -chemisorption revealed that the Ni-Mg-Al catalyst owned larger surface area and higher Ni dispersion as well as smaller metallic Ni particles size compared to Ni-Al and Ni/Al 2 O 3 catalysts. CO 2 -TPD demonstrated that the Ni-Mg-Al catalyst presented stronger basicity due to Mg incorporation. H 2 -TPR confirmed that the reduction of Ni-based species to Ni 0 was performed in high temperature due to the formed NiAl 2 O 4 phase. Activity tests indicated that this Ni-Mg-Al catalyst, due to its excellent physicochemical property, exhibited higher CH 4 conversion, H 2 selectivity, and H 2 /CO ratio in the coupled DRM-POM reaction. XRD, SEM and TG-DTA analyses of the used catalysts disclosed that the Mg-modified Ni-Mg-Al catalyst for syngas production using coupled DRM-POM reaction exhibited robust resistance to coke deposition. Consequently, by the synergistic cooperation between the coupled DRM-POM reaction and Mg-modified Ni-Mg-Al catalyst, high catalytic activity and stability could be accomplished to produce syngas.

Silicalite-1 zeolite coating was coated on the honeycomb-like monolithic SiC foam via hydrothermal synthesis method. The as-made Silicalite-1-coated SiC (S-1/SiC) was used as catalyst support to prepare a novel supported Ni monolith catalyst (Ni/S-1/SiC) by impregnation method. The Ni/S-1/SiC monolith catalyst was characterized by X-ray diffraction (XRD), N 2 physical adsorption, scanning electron microscope (SEM), thermogravimetry (TG), etc. The analysis results of XRD and SEM disclosed that Silicalite-1 coating successfully grew in situ on the SiC surface under the employed hydrothermal conditions. The Ni/S-1/SiC monolith catalyst was employed for syngas production in a combined reforming reaction consisting of CO 2 dry reforming of methane (CDRM) and partial oxidation of CH 4 (POM). Activity results indicated that the Ni/S-1/SiC monolith catalyst exhibited better catalytic activity than Silicalite-1 supported Ni catalyst (Ni/S-1) and SiC supported Ni (Ni/SiC) catalyst. The Silicalite-1 coating on the Ni/S-1/SiC catalyst combined the SiC support and Ni-based species tightly, leading to the enhanced catalytic performance of Ni/S-1/SiC catalyst. The stability test indicated that the Ni/S-1/SiC monolith catalyst also presented excellent stability and strong resistance to carbon deposition, which was attributed to the high heat conductivity of SiC, the presence of Silicalite-1 being coated on SiC as well as the combined reforming reaction of CDRM-POM. Furthermore, the H 2 /CO molar ratio of syngas generated in the combined CDRM-POM reaction over Ni/S-1/SiC catalyst could be tuned facilely by adjusting the proportion of O 2 coexisting in feed gas.

A silicon carbide (SiC) foam monolith decorated with a carbon nanofibers (CNFs) layer was employed as the catalyst support for Ni-based catalyst preparation, used for the CO2 dry reforming of methane (DRM) reaction. The loading amount of CNFs on the SiC foam monolith was 6.6 wt.%, which obviously increased the surface area of the pristine SiC foam from 4 m2/g to 24 m2/g. The prepared CNFs layer strongly attached to the pristine SiC surface and was considerably stable even after 100 h time on stream (TOS) DRM reaction. The CNFs decorated SiC composite support provided more anchorage sites for improving the dispersion of the Ni particles and enhanced the metal-support interaction compared to the pristine SiC support. Compared with other catalysts such as Ni/SiC and Ni/CNFs, the Ni/CNFs-SiC catalyst exhibited not only the highest activity but also remarkable stability during DRM reaction. The XPS and SEM-EDS results showed that the carbon deposition over the nickel surface of Ni/CNFs-SiC catalyst was much less than those of Ni/SiC and Ni/CNFs catalysts. In addition, the XRD analysis verified that almost no sintering of nickel particle was detected over the Ni/CNFs-SiC catalyst, which was prepared with CNFs-SiC composite as catalyst support, even after 100 h TOS DRM reaction at 750 °C.

Benzene as an important raw material for production of industrial chemicals is generally synthesized from petroleum and coal tar. Here, we realized the synthesis of benzene from greenhouse CO2 and H2 with two connected reactors by a tandem catalysis reaction comprising CO2 methanation and CH4 aromatization. The Ni/SiO2 catalyst loaded in the first reactor was used to convert CO2 to CH4, and the formed CH4 was sequentially converted to benzene on the Mo/HZSM-5 catalyst in the second reactor. The benzene formation rate reaches 0.68 μmol g−1 min−1 accompanied by a high CO2 conversion of 92%. This concept will provide a new pathway for the direct synthesis of benzene and CO2 utilization.

Heteroatom doping is a promising approach to improve the properties of carbon materials for customized applications. Herein, a series of Cu catalysts supported on boron-doped carbon nanotubes (Cu/xB-CNTs) were prepared for the hydrogenation of dimethyl oxalate (DMO) to ethanol. The structure and chemical properties of boron-doped catalysts were characterized by XRD, TEM, N2 O pulse adsorption, CO chemisorption, H2 temperature-programmed reduction, and NH3 temperature-programmed desorption, which revealed that doping boron into CNT supports improved the Cu dispersion, strengthened the interaction of Cu species with the CNT support, introduced more surface acid sites, and increased the surface area of Cu0 and especially Cu+ sites. Consequently, the catalytic activity and stability of the catalysts were greatly enhanced by boron doping. 100 % DMO conversion and 78.1 % ethanol selectivity could be achieved over the Cu/1B-CNTs catalyst, the ethanol selectivity of which was almost 1.7 times higher than that of the catalyst without boron doping. These results suggest that doping CNTs with boron is an efficient approach to improve the catalytic performance of CNT-based catalysts for hydrogenation of DMO. The boron-doped CNT-based catalyst with improved ethanol selectivity and catalytic stability will be helpful in the development of efficient Cu catalysts supported on non-silica materials for selective hydrogenation of DMO to ethanol.

An obvious increase in the gate leakage current has been commonly observed in GaN HEMTs, after Plasma-enhanced chemical vapor deposition (PECVD) SiN passivation has been observed to obviously increase. This paper presents an Al/SiN stack layer passivation structure. The high gate leakage current in GaN HEMTs caused by the PECVD SiN passivation is distinctly reduced by 2 to 3 orders of magnitude by introducing a thin Al layer. It is mainly attributed to the Al layer blocking and minimizing the damage for the (Al)GaN surface in the build-up of the luminance process of PECVD SiN and then reducing the surface trap density. TEM mapping and SRIM software simulation reveal that neither damage nor inter-diffusion is demonstrated at the Al/AlGaN interface, where a continuous crystalline region is observed. The moderate current collapse suppression and 32.8% improvement in VBR are achieved in GaN HEMTs with Al/SiN passivation.

This paper concentrates on the impact of SiN passivation layer deposited by plasma-enhanced chemical vapor deposition (PECVD) on the Schottky characteristics in GaN high electron mobility transistors (HEMTs). Three types of SiN layers with different deposition conditions were deposited on GaN HEMTs. Atomic force microscope (AFM), capacitance–voltage (C–V), and Fourier transform infrared (FTIR) measurement were used to analyze the surface morphology, the electrical characterization, and the chemical bonding of SiN thin films, respectively. The better surface morphology was achieved from the device with lower gate leakage current. The fixed positive charge Q f was extracted from C–V curves of Al/SiN/Si structures and quite different density of trap states (in the order of magnitude of 1011–1012 cm−2) was observed. It was found that the least trap states were in accordance with the lowest gate leakage current. Furthermore, the chemical bonds and the %H in Si–H and N–H were figured from FTIR measurement, demonstrating an increase in the density of Q f with the increasing %H in N–H. It reveals that the effect of SiN passivation can be improved in GaN-based HEMTs by modulating %H in Si–H and N–H, thus achieving a better Schottky characteristics.