Enhanced compactness of oriented MFI zeolite films by heat treatment of seed layer

The achievement of compact and defect-free film structure is crucial for the application of b-oriented MFI zeolite film. In this work, a novel heat treatment technique was used to treat zeolite seeded substrates prior to secondary growth. The influence of the heat treatment parameters such as treat temperature and time on the final film morphology were systematically investigated. The relationship between film compactness and the parameters was established. Under the optimized treat temperature of 120?°C and treat time of 1?h, compact and uniform b-oriented MFI zeolite film was achieved. The applicability of the optimum heat treatment condition was validated by employing various film substrates.     

Synthesis of V-doped SiC powder for growth of semi-insulating SiC crystals

V-doped semi-insulating (VDSI) SiC crystal is a promising substrate for high-frequency electronic devices achieved using GaN epitaxial films. However, V doping in a SiC crystal is difficult to control owing to the different sublimation temperatures of VC and SiC. The amount of V changes depending on the growth sequence, which has been a significant concern in VDSI SiC substrates in terms of wafer reliability.In this study, therefore, we aimed to synthesize a single source by vaporizing Si, C, and V under the same conditions to improve the doping issue in VDSI SiC. We synthesized V-doped SiC powder as the starting material for VDSI SiC substrate based on thermodynamic modeling, and the synthesized powder was used to grow a VDSI SiC crystal via physical vapor transport.Finally, considering the homogeneous V concentration in the grown crystal, the synthesized V-doped SiC was observed to be effective to grow VDSI SiC independent of the growth sequence.     

Ultrasonic vibration-assisted laser engineering net shaping of ZrO2-Al2O3 bulk parts: Effects on crack suppression,microstructure, and mechanical properties

Laser additive manufactured zirconia-alumina ceramic (ZrO₂-Al₂O₃) parts demonstrate severe problems resulting from cracking and inhomogeneous material dispersion. To reduce these problems, we propose a novel ultrasonic vibration-assisted laser engineered net shaping (LENS) process for fabrication of bulk ZrO₂-Al₂O₃ parts. Results showed that the initiation of cracks and the crack propagation were suppressed in the parts fabricated by LENS process with ultrasonic vibration. For the parts fabricated without ultrasonic vibration, the sizes of cracks decreased with the increase of laser power. Scanning electron microscope analyses proved that the introduction of ultrasonic vibration was beneficial for grain refinement and uniform material dispersion. Due to the suppressed cracking, refined grains, and homogenized material dispersion, the parts fabricated with ultrasonic vibration demonstrated better mechanical properties (including higher microhardness, higher wear resistance, and better compressive properties), compared with the parts fabricated without ultrasonic vibration.     

Role of Nd-Ni on structural,spectral and dielectric properties of strontium-barium based nano-sized X-type ferrites

The influence of neodymium and nickel substitution on structural and dielectric parameters was investigated in strontium-barium X-type hexagonal ferrites having composition SrBaCu_2?xNi_xNd_yFe_28?yO₄₆ (x = 0, 0.2, 0.4, 0.6, 0.8, 1 and y = 0, 0.02, 0.04, 0.06, 0.08, 0.1). Sol-gel method was employed for synthesizing these hexagonal ferrites. The XRD plots of all studied materials which were annealed at 1250 °C show single phase characteristics. Lattice parameter ‘c’ increased as a consequence of larger radius of rare earth ion (Nd³⁺) as compared to (Fe³⁺), while lattice parameter ‘a’ showed very small variation. The cell volume was obtained in the range 2508.32–2523.75 (ų). The inclusion of Nd-Ni also affected X-ray density, bulk density and porosity. The FTIR spectroscopy indicated the particular absorption peaks of hexagonal ferrites and it was performed in the range of 500–700 cm^?1. On account of Nd-Ni doping, the dielectric constant, dielectric loss and AC-conductivity showed decreasing trend. The occupancy of Nd³⁺ ions at octahedral site impedes the valence alternation of Fe³⁺; therefore there was decrease in dielectric permittivity. Ac conductivity has been decreased from 9.14 to 6.49 (Ω cm)^?1 at frequency of 2.7 GHz. The Cole-Cole plots of synthesized materials noticeably revealed grain boundary contribution. The appearance of single semi-circle in impedance Cole-Cole graphs confirms the exceptional role of grain boundaries in the conduction process. The considerably lower dielectric parameters of investigated nano X-type ferrites propose their feasibility for high-frequency applications (phase shifters, dielectric resonators, stealth technology etc).     

pH dependent hydrothermal synthesis of Ca14Al10Zn6O35:0.15Mn4+ phosphor with enhanced photoluminescence performance and high thermal resistance for indoor plant growth lighting

Artificial light source for indoor cultivation has been vastly impeded by the lack of high far red emitting phosphors. Recently, Mn⁴⁺ activated phosphors were reported to be promising luminescent materials to solve above matter. In this study, controllable design of Ca₁₄Al₁₀Zn₆O₃₅:0.15Mn⁴⁺ (CAZO:0.15Mn⁴⁺) far red emitting phosphors was realized via pH assisted hydrothermal approach. The pure CAZO:0.15Mn⁴⁺ phosphors were obtained merely when the reaction pH was 1 or 2. Meanwhile, by adjusting the pH value of the reaction solution, far red emission CAZO:0.15Mn⁴⁺ phosphors with grains, sphere-like as well as aggregated bulk particles can be achieved at pH =?4, pH =?6 and pH =?10, respectively. Furthermore, the structures and morphologies depended photoluminescence (PL) performances of CAZO:0.15Mn⁴⁺ were checked. The best PL performance was found for the phosphor produced at pH =?6, while over acidic or alkaline conditions would lower the emission intensity. In addition, this phosphor also exhibit good thermal resistance which can maintain 78% initial intensity at 150?°C. The practical indoor tobacco cultivation demonstrated that CAZO:0.15Mn⁴⁺ obtained through this pH adjusted hydrothermal route is a promising phosphor for indoor plant growth lighting.     

Preparation of nanocrystalline nickel oxide from nickel hydroxide using spark plasma sintering and inverse Hall-Petch related densification

Nanocrystalline nickel oxide (NiO) was prepared from nickel hydroxide by Spark plasma sintering (SPS) and the mechanisms involved in the densification of NiO were studied. Reverse precipitated nickel hydroxide powders were SPS processed at 400, 600 and 700?°C with 70?MPa pressure. Pure NiO with 12?nm crystallite size formed after 400?°C sintering process. However NiO grains had grown to 18 and 38?nm after 600 and 700?°C sintering respectively. NiO pellets prepared using 600 and 700?°C SPS sintering schedules had relative densities of 83% and 94% respectively. Two displacement rate regimes were observed during densification of NiO in both 600 and 700?°C sintering processes. Decomposition of nickel hydroxide and particle sliding of NiO led to first displacement rate maximum while inverse Hall-Petch based plastic deformation facilitated densification during the constant second displacement rate regime. No densification occurred during sintering holding times indicating the limited role that diffusion played during densification.     

Oxygen vacancies as a link between the grain growth and grain boundary conductivity anomalies in titanium-rich strontium titanate

Titanium-rich (Sr/Ti?=?0.995) strontium titanate (ST) ceramics, air-sintered in a temperature range of 1400–1625?°C, were reported to possess anomalies in the grain growth and analogous anomalies in the grain boundary (GB) conductivity activation energy. However, these two interface-related phenomena, occurring at GBs, could not be associated with each other using a simple “brick-layer” model. In this work we revise the topic and advocate that the deviation from the model comes from the oxygen vacancies localized at GBs of the rapidly-cooled ST ceramics. To verify this, we annealed the ceramics in oxygen and performed their systematic and comparative analysis using impedance spectroscopy. A levelling-off in the GB conductivity activation energy, which increases for ≤1.24?eV, and a four-fold decrease in the GB permittivity are observed after annealing. Thus, we confirm a key role of oxygen vacancies in relation between the grain growth and GB conductivity anomalies of as-sintered Ti-rich ST ceramics.     

Effect of sintering aids on the densification and electrical properties of SiO2 –containing Ce0.8Sm0.2O1.9 ceramic

In this work, five different metal-oxide additives (metal?=?Ba, Co, Fe, Li, and Mn) were examined as sintering aids and SiO₂ impurity scavengers for Ce_0.8Sm_0.2O_1.9 (SDC). 2?mol% additives were loaded into the SDC with ～150?ppm (moderately impure) and ～2000?ppm (highly impure) SiO₂. Ba-, Co-, Fe- and Mn-oxides showed comparative sintering-aid effect on both moderately- and highly-impure SDC specimens, but the sintering-assisting effect of Li-oxide was completely neutralized in highly impure SDC. Regarding electrical property, the deleterious effect of 2000?ppm SiO₂ impurity on the grain-boundary conduction of SDC can be effectively alleviated by adding Ba-, Co-, Fe-, or Mn-oxides. Microstructure analysis revealed that Ba-oxide reacted directly with SiO₂ and consequently enhanced grain-boundary conduction. By contrast, with the addition of Co-, Fe-, and Mn-oxides, the improved grain-boundary conductions of impure SDC were related to the scavenging reactions between Si, Ca (another original impurity) and Sm components.     

Effect of sintering temperature on electrical properties of SiC/ZrB2 ceramics

SiC/20?wt% ZrB₂ composite ceramics were fabricated via pressureless solid phase sintering in argon atmosphere at different temperature. The effect of sintering temperature on microstructure, electrical properties and mechanical properties of SiC/ZrB₂ ceramics was investigated. Electrical resistivity exhibits twice significant decreases with increasing sintering temperature. The first decrease from 1900?°C to 2000?°C is attributed to the obvious decrease of continuous pore channels in as-sintered materials. The second decrease from 2100?°C to 2200?°C results from the improvement of carbon crystallization and the disappearance of amorphous layers enveloping ZrB₂ grains. Additionally, the increase of sintered density with increasing temperature caused greatly advance of flexural strength, elastic modulus and Vickers hardness. But excessive temperature is detrimental to flexural strength because of SiC grain growth.