Two-step pulsed electric current sintering of transparent Al2O3 ceramics

Abstract

Fully densified Al₂O₃ ceramics with fine grain size were obtained by pulsed electric current sintering through a two-step heating profile (referred to as TS-PECS). Highly transparent Al₂O₃ polycrystals with fine grain size (400 nm) were successfully fabricated by the TS-PECS process, namely, sintering at 1000°C for 1 h and followed at 1200°C for 20 min under uniaxial pressure of 100 MPa. Effects of the first step temperature and heating rate were discussed for bulk density, grain size and transparency. The temperature in the first step strongly affects densification and grain growth of Al₂O₃. On the other hand, heating rate, even of 100 K min^?1, in TS-PECS does not give significant influences on densification and grain growth of Al₂O₃. Inline transmittance at 640 nm in wavelength normalised to 1 mm in thickness is increased by decreasing heating rate even in TS-PECS.     

Binding Mode and Structure–Activity Relationships around Direct Inhibitors of the Nrf2–Keap1 Complex

An X‐ray crystal structure of Kelch‐like ECH‐associated protein (Keap1) co‐crystallised with (1S,2R)‐2‐[(1S)‐1‐[(1,3‐dioxo‐2,3‐dihydro‐1H‐isoindol‐2‐yl)methyl]‐1,2,3,4‐tetrahydroisoquinolin‐2‐carbonyl]cyclohexane‐1‐carboxylic acid (compound (S,R,S)‐ 1 a ) was obtained. This X‐ray crystal structure provides breakthrough experimental evidence for the true binding mode of the hit compound (S,R,S)‐ 1 a , as the ligand orientation was found to differ from that of the initial docking model, which was available at the start of the project. Crystallographic elucidation of this binding mode helped to focus and drive the drug design process more effectively and efficiently.     

Fabrication and NO2 gas sensing performance of TeO2-core/CuO-shell heterostructure nanorod sensors

TeO₂-nanostructured sensors are seldom reported compared to other metal oxide semiconductor materials such as ZnO, In₂O₃, TiO₂, Ga₂O₃, etc. TeO₂/CuO core-shell nanorods were fabricated by thermal evaporation of Te powder followed by sputter deposition of CuO. Scanning electron microscopy and X-ray diffraction showed that each nanorod consisted of a single crystal TeO₂ core and a polycrystalline CuO shell with a thickness of approximately 7 nm. The TeO₂/CuO core-shell one-dimensional (1D) nanostructures exhibited a bamboo leaf-like morphology. The core-shell nanorods were 100 to 300 nm in diameter and up to 30 μm in length. The multiple networked TeO₂/CuO core-shell nanorod sensor showed responses of 142% to 425% to 0.5- to 10-ppm NO₂ at 150°C. These responses were stronger than or comparable to those of many other metal oxide nanostructures, suggesting that TeO₂ is also a promising sensor material. The responses of the core-shell nanorods were 1.2 to 2.1 times higher than those of pristine TeO₂ nanorods over the same NO₂ concentration range. The underlying mechanism for the enhanced NO₂ sensing properties of the core-shell nanorod sensor can be explained by the potential barrier-controlled carrier transport mechanism.

PACS

61.46. + w; 07.07.Df; 73.22.-f     

Low-temperature growth of highly crystalline β-Ga2O3 nanowires by solid-source chemical vapor deposition

Growing Ga₂O₃ dielectric materials at a moderately low temperature is important for the further development of high-mobility III-V semiconductor-based nanoelectronics. Here, β-Ga₂O₃ nanowires are successfully synthesized at a relatively low temperature of 610°C by solid-source chemical vapor deposition employing GaAs powders as the source material, which is in a distinct contrast to the typical synthesis temperature of above 1,000°C as reported by other methods. In this work, the prepared β-Ga₂O₃ nanowires are mainly composed of Ga and O elements with an atomic ratio of approximately 2:3. Importantly, they are highly crystalline in the monoclinic structure with varied growth orientations in low-index planes. The bandgap of the β-Ga₂O₃ nanowires is determined to be 251 nm (approximately 4.94 eV), in good accordance with the literature. Also, electrical characterization reveals that the individual nanowire has a resistivity of up to 8.5 × 10⁷ Ω cm, when fabricated in the configuration of parallel arrays, further indicating the promise of growing these highly insulating Ga₂O₃ materials in this III-V nanowire-compatible growth condition.

PACS

77.55.D; 61.46.Km; 78.40.Fy     

Structural and electronic properties of germanene/MoS2 monolayer and silicene/MoS2 monolayer superlattices

Superlattice provides a new approach to enrich the class of materials with novel properties. Here, we report the structural and electronic properties of superlattices made with alternate stacking of two-dimensional hexagonal germanene (or silicene) and a MoS₂ monolayer using the first principles approach. The results are compared with those of graphene/MoS₂ superlattice. The distortions of the geometry of germanene, silicene, and MoS₂ layers due to the formation of the superlattices are all relatively small, resulting from the relatively weak interactions between the stacking layers. Our results show that both the germanene/MoS₂ and silicene/MoS₂ superlattices are manifestly metallic, with the linear bands around the Dirac points of the pristine germanene and silicene seem to be preserved. However, small band gaps are opened up at the Dirac points for both the superlattices due to the symmetry breaking in the germanene and silicene layers caused by the introduction of the MoS₂ sheets. Moreover, charge transfer happened mainly within the germanene (or silicene) and the MoS₂ layers (intra-layer transfer), as well as some part of the intermediate regions between the germanene (or silicene) and the MoS₂ layers (inter-layer transfer), suggesting more than just the van der Waals interactions between the stacking sheets in the superlattices.     

CdS quantum dot-sensitized solar cells based on nano-branched TiO2 arrays

Nano-branched rutile TiO₂ nanorod arrays were grown on F:SnO₂ conductive glass (FTO) by a facile, two-step wet chemical synthesis process at low temperature. The length of the nanobranches was tailored by controlling the growth time, after which CdS quantum dots were deposited on the nano-branched TiO₂ arrays using the successive ionic layer adsorption and reaction method to make a photoanode for quantum dot-sensitized solar cells (QDSCs). The photovoltaic properties of the CdS-sensitized nano-branched TiO₂ solar cells were studied systematically. A short-circuit current intensity of approximately 7 mA/cm² and a light-to-electricity conversion efficiency of 0.95% were recorded for cells based on optimized nano-branched TiO₂ arrays, indicating an increase of 138% compared to those based on unbranched TiO₂ nanorod arrays. The improved performance is attributed to a markedly enlarged surface area provided by the nanobranches and better electron conductivity in the one-dimensional, well-aligned TiO₂ nanorod trunks.     

Microwave fabrication of Cu2ZnSnS4 nanoparticle and its visible light photocatalytic properties

Cu₂ZnSnS₄ nanoparticle with an average diameter of approximately 31 nm has been successfully synthesized by a time effective microwave fabrication method. The crystal structure, surface morphology, and microstructure of the Cu₂ZnSnS₄ nanoparticle were characterized. Moreover, the visible light photocatalytic ability of the Cu₂ZnSnS₄ nanoparticle toward degradation of methylene blue (MB) was also studied. About 30% of MB was degraded after 240 min irradiation when employing Cu₂ZnSnS₄ nanoparticle as a photocatalyst. However, almost all MB was decomposed after 90 min irradiation when introducing a small amount of H₂O₂ as a co-photocatalyst. The enhancement of the photocatalytic performance was attributed to the synergetic effect between the Cu₂ZnSnS₄ nanoparticle and H₂O₂. The detailed photocatalytic degradation mechanism of MB by the Cu₂ZnSnS₄ was further proposed.     

A novel hierarchical Pt- and FTO-free counter electrode for dye-sensitized solar cell

A novel hierarchical Pt- and FTO-free counter electrode (CE) for the dye-sensitized solar cell (DSSC) was prepared by spin coating the mixture of TiO₂ nanoparticles and poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) solution onto the glass substrate. Compared with traditional Pt/FTO CE, the cost of the new CE is dramatically reduced by the application of bilayer TiO₂-PEDOT:PSS/PEDOT:PSS film and the glass substrate. The sheet resistance of this composite film is 35 Ω sq⁻¹ and is low enough to be used as an electrode. The surface morphologies of TiO₂-PEDOT:PSS layer and modified PEDOT:PSS layer were characterized by scanning electron microscope, which shows that the former had larger surface areas than the latter. Electrochemical impedance spectra and Tafel polarization curves prove that the catalytic activity of TiO₂-PEDOT:PSS/PEDOT:PSS/glass CE is higher than that of PEDOT:PSS/FTO CE and is similar to Pt/FTO CE''s. This new fabricated device with TiO₂-PEDOT:PSS/PEDOT:PSS/glass CE achieves a high power conversion efficiency (PCE) of 4.67%, reaching 91.39% of DSSC with Pt/FTO CE (5.11%).     

Optimization of the dye-sensitized solar cell performance by mechanical compression

In this study, the P25 titanium dioxide (TiO₂) nanoparticle (NP) thin film was coated on the fluorine-doped tin oxide (FTO) glass substrate by a doctor blade method. The film then compressed mechanically to be the photoanode of dye-sensitized solar cells (DSSCs). Various compression pressures on TiO₂ NP film were tested to optimize the performance of DSSCs. The mechanical compression reduces TiO₂ inter-particle distance improving the electron transport efficiency. The UV–vis spectrophotometer and electrochemical impedance spectroscopy (EIS) were employed to quantify the light-harvesting efficiency and the charge transport impedance at various interfaces in DSSC, respectively. The incident photon-to-current conversion efficiency was also monitored. The results show that when the DSSC fabricated by the TiO₂ NP thin film compressed at pressure of 279 kg/cm², the minimum resistance of 9.38 Ω at dye/TiO₂ NP/electrolyte interfaces, the maximum short-circuit photocurrent density of 15.11 mA/cm², and the photoelectric conversion efficiency of 5.94% were observed. Compared to the DSSC fabricated by the non-compression of TiO₂ NP thin film, the overall conversion efficiency is improved over 19.5%. The study proves that under suitable compression pressure the performance of DSSC can be optimized.     

Advanced Cu chemical displacement technique for SiO2-based electrochemical metallization ReRAM application

This study investigates an advanced copper (Cu) chemical displacement technique (CDT) with varying the chemical displacement time for fabricating Cu/SiO₂-stacked resistive random-access memory (ReRAM). Compared with other Cu deposition methods, this CDT easily controls the interface of the Cu-insulator, the switching layer thickness, and the immunity of the Cu etching process, assisting the 1-transistor-1-ReRAM (1T-1R) structure and system-on-chip integration. The modulated shape of the Cu-SiO₂ interface and the thickness of the SiO₂ layer obtained by CDT-based Cu deposition on SiO₂ were confirmed by scanning electron microscopy and atomic force microscopy. The CDT-fabricated Cu/SiO₂-stacked ReRAM exhibited lower operation voltages and more stable data retention characteristics than the control Cu/SiO₂-stacked sample. As the Cu CDT processing time increased, the forming and set voltages of the CDT-fabricated Cu/SiO₂-stacked ReRAM decreased. Conversely, decreasing the processing time reduced the on-state current and reset voltage while increasing the endurance switching cycle time. Therefore, the switching characteristics were easily modulated by Cu CDT, yielding a high performance electrochemical metallization (ECM)-type ReRAM.