First La2O2S infrared transparent ceramics

Lanthanum oxysulfide (La₂O₂S) was investigated as infrared transparent ceramic to benefit from stronger chemical bonds and superior mechanical performances to that of non-oxide benchmark infrared materials. La₂O₂S ceramics were processed by hot-pressing powders prepared by combustion synthesis followed by a sulfurization treatment. Powders and ceramics were characterised through various techniques (XRD, UV-Vis-IR spectroscopy, particle size analysis, SEM, Impulse Excitation Technique, microhardness and fracture toughness tests) to assess their purity, study their microstructure and determine their optical and mechanical properties. The study reports the first IR transmission spectra, Poisson’s ratio, Young’s and shear moduli and fracture toughness values of La₂O₂S polycrystalline ceramics. The ceramics showed transparency in the 2–11 µm range and their mechanical performances were all superior to that of commercial infrared ceramics. The best transmission (89% of the theoretical transmission) was measured at 7.3 µm for 1 mm-thick ceramics hot-pressed at 1200–1250 °C.     

KNbTeO6 transparent ceramics prepared by the combination of pressure-less sintering and pseudo hot isostatic pressing

KNbTeO₆ transparent ceramics were prepared by combining pressure-less sintering and pseudo-hot isostatic pressing (PHIP) of the synthesized submicron single-phase powder. The PHIP was conducted by wrapping coarse magnesium aluminate powders around the pre-sintered body in the spark plasma sintering (SPS) furnace. With an average grain size of 412 ± 23 nm, the in-line transmittance of transparent KNbTeO₆ ceramics reaches 80.25% at 2677 nm. By contrast, the density of the samples prepared by conventional SPS with the same sintering procedure is only 98.73%, and the highest in-line transmittance 64.25% occurs at 4976 nm. In particular, by investigating the sintering mechanism of PHIP, the improvement of microstructure and optical transmittance could be attributed to the plastic deformation caused by shear stress. The obtained ceramics exhibited excellent mechanical and dielectric properties, which was benefited from the novel sintering technology.     

Pressureless sintered Al4SiC4 ceramics with Y2O3 addition

Highly densified Al₄SiC₄ ceramics with a relative density of 96.1% were prepared by pressureless sintering using 2 wt% Y₂O₃ as additives. The densification mechanism, phase composition, microstructures and mechanical properties of Al₄SiC₄ ceramics were investigated. Y₂O₃ in-situ reacted with the oxygen impurities in Al₄SiC₄ powder to form a yttrium aluminate liquid phase during sintering, which promoted the densification and anisotropic grain growth. The final Al₄SiC₄ ceramics were composed of equiaxed grains and columnar grains, and presented a bimodal grain distribution. The mechanical properties of the pressureless sintered Al₄SiC₄ ceramics were better than those reported for hot pressed Al₄SiC₄, including a flexural strength of 369 ± 24 MPa, fracture toughness of 4.8 ± 0.1 MPa m^1/2 and Vickers hardness of 11.3 ± 0.2 GPa. Pressureless sintering of Al₄SiC₄ ceramics is of great significance for the development and practical application of Al₄SiC₄ ceramic parts, especially with big size and complex shape.     

Tailoring the electrochemical reduction kinetics of dual-phase BaCe0.5Fe0.5O3-δ cathode via incorporating Mo for IT-SOFCs

The BaCe_0.5Fe_0.5O_3-δ (BCF) cathode consists of the ion-electron mixed conducting phase BaCe_0.15Fe_0.85O_3-δ (BCF1585) and the proton-conducting phase BaCe_0.85Fe_0.15O_3-δ (BCF8515). In this paper, the electrochemical performance is improved by incorporating the high valence element Mo into the BCF and applied to intermediate-temperature solid oxide fuel cells (IT-SOFCs). High-temperature X-ray diffraction (HT-XRD) and O₂-temperature programmed desorption (O₂-TPD) results show that Mo doping enhances the structural stability of BCF. The X-ray photoelectron spectroscopy (XPS) results suggest that the introduction of Mo increases the amount of adsorbed oxygen and thus the oxygen reduction reaction (ORR) catalytic activity. Compared to BCF, the polarization impedance of BaCe_0.5Fe_0.45Mo_0.05O_3-δ (BCFM) at 800 °C is 0.154 Ω·cm², a reduction of 22 %. Meanwhile, the BCFM output power at 800 °C is 778.01 mW·cm⁻², an improvement of 32.17 %, and maintains a stable current density after 250 h at 0.7 V. The results demonstrate that Mo doping is an effective strategy to enhance the electrochemical performance of BCF.     

异质金属竞争配位用于混合配体型氯代异烟酸铀酰配合物的合成调控北大核心CSCD

本工作从基于2,6-二氯异烟酸(H-2,6-dcpca)和2,2′-联吡啶(2,2′-bpy)混合配体的卤代异烟酸铀酰配合物((UO;)(2,6-dcpca);(2,2′-bpy))出发,通过进一步引入第二金属中心(Ag;、Zn;和Cu;)进行竞争配位,实现了对这一混合配体型氯代异烟酸铀酰配合物的结构调控,并成功合成了六种新型混合配体型异质金属铀酰配合物。单晶结构分析表明,这些异质金属节点具有不同的配位行为,可以参与2,2′-bpy和[2,6-dcpca];配体的竞争配位并改变铀酰中心的配位环境,导致最终形成的水热产物具有不同铀酰配位模式和化学结构。同时,由于晶体结构的不同,这些化合物在超分子晶格中的弱相互作用和晶格堆积方式也表现出较大变化。通过第二金属中心的竞争配位策略,实现了混合配体型铀酰配合物中铀酰配位模式的调控及多种异质金属铀酰化合物的制备,证明了基于多重竞争配位的合成调控策略的可行性,并有望在未来用于更多的功能性锕系-有机杂化材料的合成。     

Pulsed sonoelectrodeposition of leaf-like Bi2WO6 film with exposed [001] facet as a photocathode with a remarkable high photovoltage in water splitting

Bi₂WO₆ is one of the promising triplet bismuth compound that has a layered structure with a high photocatalytic activity for photo-electrochemical (PEC) water splitting systems. Here, Bi₂WO₆ synthesizes by the sonoelectrochemical method in pulse mode of ultrasound. Unexpectedly, synthetic samples show photocathode rather than photoanode activity in PEC systems. Applying of this method creates a leaf-like morphology with exposed [001] crystal facet and controllable amount of surface defects. The co-existence of oxygen and metal vacancies play a significant role in suppressing charge recombination and enhancing charge transporting in photoelectrodes. The creation of high surface vacancies leads to change the conduction and valence band positions and cause hydrogen evolution by Bi₂WO₆ photoelectrodes. Another surprising result for the synthesized film by pulse mode is the creation of high photovoltage about 1.25 V that has a remarkable effect in suppressing charge recombination rate and proposed driving force for water reduction. Furthermore, the onset potential of the photoelectrodes improves and records high efficiencies (ABPE = 2.46% at −0.8 V and IPCE≈28% at 450 nm). The obtaining results introduces the sonoelectrochemical method as a promising method for the synthesis of highly efficient photoelectrodes.     

Microstructure of high Jc NdBa2Cu3Ox/Nd4Ba2Cu2O10 composite superconductor

Single crystalline NdBa₂Cu₃O_x (Nd123) superconductors with dispersed Nd₄Ba₂Cu₂O₁₀ (Nd422) particles were produced by the floating zone partial melting and solidification (FZPMS) method. The initial composition of the precursor material was Nd_1.8Ba_2.4Cu_3.4O_x with or without 0.1 wt% Pt addition. FZPMS was carried out in a low oxygen partial pressure atmosphere. Microstructure of quenched samples were investigated by optical microscopy, scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Furthermore, superconductive properties were measured by superconducting quantum interference device (SQUID). The results of SQUID measurements indicate that the critical temperature (T_c) of the samples with Pt addition so produced with the oxygen heat treatment (623 K for 300 h) was 94 K and the critical current density (J_c) was 2.3×10⁴ A/cm² at 77 K, 0.2 T.     

Experimental investigation on surface quality in ultrasonic-assisted honing of 304 stainless steel

Honing is a surface finishing technology, which plays an important role in improving the surface quality and working performance of workpieces. However, with the extensive application of hard-to-machine materials and the increasing requirement for the surface properties of workpieces, the performance of conventional honing (CH) is becoming more and more difficult to meet the production needs. The ultrasonic vibration provides a way to overcome the technological constraints of CH and improve the machined surface properties. To realise the ultrasonic vibration of honing stone, a slotted block horn was used as a vibration transmission component and a supporting component of honing stone in this study. Finite element method was used to analyse and optimise the block horn to improve the uniformity of the amplitude on the output surface, to provide stable and efficient vibration for the honing stone. In this study, the effects of processing parameters on surface roughness in CH and ultrasonic-assisted honing (UAH) of stainless steel SUS304 were studied. The results indicated that the application of ultrasonic vibration in honing made a significant contribution to the formation of a dense punctiform morphology and reduction of the defects of the workpiece surface. Moreover, interactions between ultrasonic vibration, speed of revolution, and honing time in UAH make the changes of surface roughness much different from the CH. On the whole, for UAH, it is a feasible approach to improve the surface roughness by using a smaller amplitude, increasing the speed of revolution appropriately, and extending honing time.