Improvement of field-emission-lamp characteristics using nitrogen-doped carbon nanocoils

Carbon nanocoils (CNCs) synthesized using thermal pyrolysis chemical vapor deposition on 304 stainless steel wire substrates were used as the cathodes of field emission lamps (FELs). The effects of the growth temperature on the FE performances were studied, and we observed that uniform and dense CNCs that are suitable for use as FE cathodes can be synthesized at 600 °C. We also found that a nitrogen doping post-treatment can significantly improve the FE efficiency of the CNCs. When doped at 200 °C with a nitrogen flow rate of 500 sccm for 30 min, the nitrogen content of the CNC surface could reach 4.9 wt.%. ESCA analysis indicates that the doped nitrogen atoms formed CN_x bonding and increased the sp² clusters in the CNCs. The turn-on voltage was reduced from 2.1 V/μm to 1.4 V/μm, and the β value increased considerably from 2465 to 3241 after N-doping post-treatment. The bulb-type FELs using our N-doped CNC cathodes showed a good luminous efficiency as high as 75 lm/W at 8 kV.     

Cellulose nanocrystals (CNCs) reinforced acrylic pressure- sensitive adhesives (PSAs) prepared via miniemulsion polymerization

In this study, poly (n-butyl acrylate-co-2-ethyl hexyl acrylate) (P(nBA-co-2EHA)) pressure sensitive adhesives (PSAs) were successfully synthesized in the presence of cellulose nanocrystals (CNCs) via in-situ miniemulsion polymerization. First, the CNCs were prepared via acid hydrolysis of cellulose microcrystals (CMCs) at various temperatures, 42–54 °C, and characterized using atomic force microscopy (AFM), field emission scanning electron microscopy (FESEM), Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) apparatus. The results showed that hydrolysis of MCCs at 45 °C resulted in CNCs with a well-defined aspect ratio, average length of 206 nm and thickness of 3.9 nm, and the highest crystallinity. Adding further CNC to the copolymer adhesive enhanced the mean particle size of the composite latex and decreased the glass-rubber transition temperature (T_g) of the copolymer matrix. Molecular weights and viscoelastic properties of the resultant PSAs were determined using gel permeation chromatography (GPC) and dynamic mechanical thermal analysis (DMTA), respectively. Adhesion performance of the neat and reinforced PSAs containing 1–5 wt% CNCs was evaluated at room temperature. The results showed that the incorporation of CNCs up to 4 wt% in the copolymer adhesive increased the shear resistance by 518%, peel strength by 176% and probe tack by 13%, while further addition, 5 wt%, lowered the adhesion performance due to a lack of surface wettability.     

Properties of DLC films deposited by an oxyacetylene flame

Diamond-like carbon (DLC) films were obtained by spinning a tungsten carbide substrate at a high speed using an oxyacetylene flame. The films deposited at a typical experimental condition of substrate temperature of 810°C, rotation of 600 rpm and 3 h deposition time, exhibited an uniform, very smooth, hard and glassy surface covering the entire exposed face of the substrate. These films were identified as DLC by their characteristic broad Raman spectra centered at 1554 cm⁻¹ and micro-Vicker's hardness >3400 kg mm⁻². For substrate temperatures <800°C the film started losing the uniform glassy surface and the hardness deteriorated. For temperatures >950°C the film was still hard and shiny, but black in color. DLC films were also obtained in a wide range of speeds of rotation (300–750 rpm), as long as the temperature remained close to 850°C.     

Properties of Si3N4–TiN composites fabricated by spark plasma sintering by using a mixture of Si3N4 and Ti powders

Si₃N₄–TiN composites were successfully fabricated via planetary ball milling of 70 mass% Si₃N₄ and 30 mass% Ti powders, followed by spark plasma sintering (SPS) at 1250–1350 °C. The sintering mechanism for SPS was a hybrid of dissolution–reprecipitation and viscous flow. The electrical resistivity decreased with increasing sintering temperature up to a minimum at 1250 °C and then increased with the increasing sintering temperature. The composites prepared by SPS at 1250–1350 °C could be easily machined by electrical discharge machining. Composite prepared by SPS at 1300 °C showed a high hardness (17.78 GPa) and a good machinability.     

Preparation and characterization of mesoporous silicon oxycarbide ceramics without free carbon from polysiloxane

Mesoporous silicon oxycarbide ceramics without free carbon were prepared by pyrolysis of cross-linked polysiloxane at different temperatures (1300–1450 °C) followed by post treatments. The post treatments comprised two steps (HF etching and oxidation at 650 °C in air). The sample pyrolyzed at 1300 °C after post treatments exhibits the largest specific surface area (SSA) reaching up to 204 m²/g and the biggest total pore volume (0.58 cm³/g) with an average pore size of 11.4 nm. Increasing pyrolysis temperature will lead a quick decline of SSA and total pore volume. The thermal stability of pore structure of the sample pyrolyzed at 1300 °C with post treatments was investigated in air. The SSA and total pore volume almost keeps the same up to 750 °C, and subsequently decreases with a high speed. The most possible reason is the pores are severely closed by viscous flow of SiO₂ produced from SiC nanocrystallites.     

Structure and dielectric properties of barium titanate thin films for capacitor applications

BaTiO₃ is a typical ferroelectric material with high relative permittivity and has been used for various applications, such as multilayer ceramic capacitors (MLCCs). With the tendency of miniaturization of MLCCs, the thin films of BaTiO₃ have been required. In this work, BaTiO₃ thin films have been deposited on Pt-coated Si substrates by RF magnetron sputtering under different deposition conditions. The films deposited at the substrate temperature from 550 °C–750 °C show a pure tetragonal perovskite structure. The films deposited at 550 °C–625  °C exhibit (111) preferential orientation, and change to (110) preferential orientation when deposited above 650 °C. The film morphologies vary with working pressure and substrate temperature. The film deposited at 625 °C and 4.5 Pa has the relative permittivity of 630 and the loss tangent of 2% at 10 kHz.     

Dielectric properties and electrical behaviors of tunable Bi1.5MgNb1.5O7 thin films

The Bi_1.5MgNb_1.5O₇ (BMN) thin films were prepared on Au-coated Si substrates by rf magnetron sputtering. We systematically investigated the structure, dielectric properties and voltage tunable property of the films with different annealing temperatures. The relationships of leakage current and breakdown bias field with annealing temperature were firstly studied and a possible explanation was proposed. The deposited BMN thin films had a cubic pyrochlore phase when annealed at 550 °C or higher. With the increasing of annealing temperature, the dielectric constant and tunability also went up. BMN thin films annealed at 750 °C exhibited moderate dielectric constant of 106 and low dielectric loss of 0.003–0.007 between 10 kHz and 10 MHz. The maximum tunability of 50% was achieved at a bias field of 2 MV/cm. However, thin films annealed at 750 °C had lower breakdown bias field and higher leakage current density than films annealed below 750 °C. The excellent physical and electrical properties make BMN thin films promising for potential tunable capacitor applications.     

High temperature elemental losses and mineralogical changes in common biomass ashes

The elemental losses from ashes of common biomass fuels (rice straw, wheat straw, and wood) were determined as a function of temperature from 525 °C to below 1525 °C, within the respective melting intervals. The experimental procedure was chosen to approach equilibrium conditions in an oxidizing atmosphere for the specific ash and temperature conditions. All experiments were conducted in air and used the ashes produced initially at temperatures of 525 °C as reactants. Losses during the initial ashing at 525 °C were negligible, except for a K₂O loss of 26% for wood and a Cl loss of 20% for wheat straw. Potassium losses are positively correlated with temperature for all fuel ashes. The K₂O loss for wood ash commences at 900–1000 °C. Carbonate is detected in the wood ashes to about 700–800 °C and thus cannot explain the retention of K₂O in the ashes to 1000 °C. Other crystalline phases detected in the wood ashes (pericline and larnite) contain little or no potassium. Petrographic examinations of high temperature, wood ash products have failed to reveal potassium bearing carbonates, sulfates, or silicates. The release of potassium, thus, appears to be unrelated to the breakdown of potassium-bearing crystalline phases. The straw ashes show restricted potassium loss compared to wood ash. The potassium content declines for both straw ashes from about 750 °C. Cristobalite appears in the straw ashes at about 700–750 °C and is replaced by tridymite in the rice straw ash from about 1100 °C. Sylvite (KCl) disappears completely above 1000 °C. The Cl content starts to decline at about 700 °C, approximately at the same temperature as potassium, suggesting that the breakdown of sylvite is responsible for the losses. The K–Cl relations demonstrate that about 50% of K (atomic basis) released from breakdown of sylvite is retained in the ash. The presence of chlorine in the ash is, therefore, best attributed to the presence of sylvite. Potassium is easily accommodated in the silicate melt formed at temperatures perhaps as low as 700–800 °C from dehydration, recrystallization, and partial melting of amorphous components. Loss of potassium persists for ashes without remaining sylvite and points to the importance of release of potassium from partial melt at temperatures within the melting interval for the fuel ashes.     

Application of BaO–CaO–Al2O3–B2O3–SiO2 glass–ceramic seals in large size planar IT-SOFC

BaO–CaO–Al₂O₃–B₂O₃–SiO₂ (BCAS) glass–ceramics can be used as sealant for large size planar anode-supported solid oxide fuel cells (SOFCs). BCAS glass–ceramics after heat treatment for different times were characterized by means of thermal dilatometer, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that the coefficients of thermal expansion (CTE) of BCAS glass–ceramics are 11.4×10⁻⁶ K⁻¹, 11.3×10⁻⁶ K⁻¹ and 11.2×10⁻⁶ K⁻¹ after heated at 750 °C for 0 h, 50 h, and 100 h, respectively. The CTE of BCAS matches that of YSZ, Ni–YSZ and the interconnection of SOFC. Needle-like barium silicate, barium calcium silicate and hexacelsian are crystallized in the BCAS glass after heat-treatment for above 50 h at 750 °C. The glass–ceramics green tape prepared by aqueous tape casting can be directly applied in sealing the cell of SOFCs with 10 cm×10 cm. The open circuit voltage (OCV) of the cell keeps 1.19 V after running for 280 h at 750 °C and thermal cycling 10 times from 750 °C to room temperature. The maximum power density is 0.42 W/cm² using pure H₂ as fuel and air as oxidation gas. SEM images show no cracks or pores exist in the interface of BCAS glass–ceramics and the cell.     

Comparative study of the structure and microstructure of PAN-based nano- and micro-carbon fibers

The work presents results on the manufacture and comparative assessment of the structure and microstructure parameters of polyacrylonitrile polymer (PAN)-based carbon nano- and micro-fibers. Using the same polymer solution, PAN nano- and microfibers were obtained. The PAN nanofibers were obtained by electrospinning, and microfibers were spun using the conventional solution-spinning method. The PAN-based fiber precursors were annealed to 1000 °C, 2000 °C and to 2800 °C. Using X-ray diffraction and Raman spectroscopy, the structural and microstructural parameters of both types of carbon fibers were examined. The morphology of PAN nanofibers and carbon nanofibers (CNF) were studied by SEM. Both types of ex-PAN carbon fibers (nano and micro) have similar the c-axis spacing (d₀₀₂) values and crystallite sizes after heat treatment to 2000 °C presenting turbostratic structure. HR-TEM images of low temperature CNF show uniform microstructure with the misoriented small carbon crystallites along the fiber axis. The ratio of the integrated intensities of the D and G peaks for carbon nanofibers after heat treatment at 2000 °C was distinctly higher in comparison to carbon microfibers (CF). After additional annealing the fibers to 2800 °C a better structural ordering show CNF. The crystallite sizes (L_c, L_a) in CNF were distinctly higher in comparison to the crystallites in CF. CF consist of two carbon components, whereas CNF contain three carbon components varying in structural and microstructural parameters. One of carbon phases in CNF was found to have the interlayer spacing close to graphite, i.e. d₀₀₂=0.335 nm.     

Dielectric and magnetic properties of BiFeO3 ceramics prepared by hydrothermal synthesis

Single-phase BiFeO₃ powders were prepared at a temperature of 200 °C by a hydrothermal synthesis. BiFeO₃ ceramics were prepared with the powders by a conventional ceramic process. The BiFeO₃ ceramics with no impurity phase were prepared at the sintering temperature of 650–800 °C. The dense microstructure was observed in the BiFeO₃ ceramics sintered at a temperature of 700 °C and higher. BiFeO₃ ceramics show linear M–H curves in low H, which are antiferromagnetic behaviors. The dielectric dispersion was observed at the frequency range of 10 kHz to 1 MHz in the BiFeO₃ ceramic sintered at 700 °C or lower. The dielectric constant and loss of the BiFeO₃ ceramics sintered at 750 °C or higher were about 85 and 0.4 at 100 kHz, respectively.     

Effect of temperature on structural properties of Aloe vera (Aloe barbadensis Miller) gel and Weibull distribution for modelling drying process

Aloe vera (Aloe barbadensis Miller) gel was dried at five inlet temperatures 50, 60, 70, 80 and 90 °C, in a convective dryer with a constant air flow of 2.0 ± 0.2 m/s. Rehydration ratio, water holding capacity, texture, microstructure and total polysaccharide content were evaluated. Drying kinetics was estimated using the Weibull distribution (r² > 0.97 and Chi-square < 0.0009). Values of scale and shape parameters ranged from 90.94 to 341.06 (min) and 1.43 to 1.49, respectively. Furthermore, the influence of temperature on the model parameters as well as on the quality attributes was analysed using a least significant difference test (p-value < 0.05). These effects were more evident for the long drying period (e.g. 810 min at 50 °C). However, minor alterations in the structural properties and total polysaccharide content were produced at drying temperatures of 60–70 °C, resulting in a high quality gel.     

Characterization of Mg doped ZnO nanocrystallites prepared via electrospinning

Undoped and Mg doped ZnO nanofibers with different doping concentrations were successfully synthesized using the electrospinning technique. The nanofiber structures were calcined at 300 °C, 400 °C, 500 °C, and 600 °C respectively. It was observed that the nanofibers turned into a nanoparticular structure at the calcining temperature of 400 °C. The nanoceramic mats were characterized by the Fourier transform infrared-attenuated total reflectance spectroscopy and by the scanning electron microscopy. The electronic band transitions of as-deposited and calcined films were identified by the evaluation of the photoluminescence measurements at room temperature. It was observed that the exitonic transition energy of the ZnO nanostructure blue-shifted to a high energy value with an increasing Mg doping ratio. In order to estimate the decomposition temperature of the nanofibers turning into a nanoparticular structure, the nanofiber structure was calcined at temperatures between 300 °C and 400 °C, the temperature ramp being 20 °C. The evaluation of the emission spectra of the calcined structures show that the decomposition of electrospun nanofibers started at 320 °C. In addition, band gap energies of the samples were determined by the transmittance measurement of the samples and by the UV–VIS spectrophotometer at the room temperature.     

Oxidation protection of carbon fibers by coating with alumina and/or titania using atomic layer deposition

Alumina and titania coatings were deposited by atomic layer deposition onto carbon fibers at temperatures of 200 °C or below and reduced pressure. The coatings were smooth, uniform and conformed to the fiber surface. Thermogravimetric analysis (TGA) revealed that the coatings improved the oxidation resistance of the carbon fibers: the oxidation onset temperature of uncoated fibers and fibers coated with 66 nm of alumina was 630 °C. For fibers coated with 20 nm of titania it was 550 °C. Double layer coatings by 50 nm of alumina followed by 13 nm of titania yielded an oxidation onset temperature of 660 °C, while changing the order of the layers, i.e., coating fibers first with 20 nm of titania followed by 30 nm of alumina yielded an oxidation onset temperature of 750 °C. These TGA results were confirmed by a set of additional oxidation experiments conducted at a fixed temperature of 550 °C using a tube furnace in air. In this latter set of additional experiments, the times needed for a complete oxidation of the above mentioned samples were 8 h, 12 h, 10 h, 13 h, and 30 h, respectively.     

Synthesis of glass–ceramics in the CaO–MgO–SiO2 system with B2O3, P2O5, Na2O and CaF2 additives

Glass–ceramics based on the CaO–MgO–SiO₂ system with limited amount of additives (B₂O₃, P₂O₅, Na₂O and CaF₂) were prepared. All the investigated compositions were melted at 1400 °C for 1 h and quenched in air or water to obtain transparent bulk or frit glass, respectively. Raman spectroscopy revealed that the main constituents of the glass network are the silicates Q¹ and Q² units. Scanning electron microscopy (SEM) analysis confirmed liquid–liquid phase separation and that the glasses are prone to surface crystallization. Glass–ceramics were produced via sintering and crystallization of glass-powder compacts made of milled glass-frit (mean particle size 11–15 μm). Densification started at 620–625 °C and was almost complete at 700 °C. Crystallization occurred at temperatures >700 °C. Highly dense and crystalline materials, predominantly composed of diopisde and wollastonite together with small amounts of akermanite and residual glassy phase, were obtained after heat treatment at 750 °C and 800 °C. The glass–ceramics prepared at 800 °C exhibited bending strength of 116–141 MPa, Vickers microhardness of 4.53–4.65 GPa and thermal expansion coefficient (100–500 °C) of 9.4–10.8 × 10⁻⁶ K⁻¹.     

Poly(N,N-dimethylaminoethyl methacrylate)/graphene oxide hybrid hydrogels: pH and temperature sensitivities and Cr(VI) adsorption

Different amounts of graphene oxide (GO) were incorporated to N,N-dimethylaminoethyl methacrylate (DMAEMA), fabricating a series of pH and temperature dual sensitive PDMAEMA/GO hybrid hydrogels by in situ polymerization. Their microscopic network structures as well as swelling properties and Cr(VI) adsorption were characterized. The equilibrium swelling ratios (ESR) of hydrogels increased significantly with 0.5 wt% GO feeding of DMAEMA amount, and then decreased with further GO loading increasing. All hydrogels showed obvious deswelling when pH value of swelling mediums increased from 5 to 10 gradually. At pH 7, hydrogels revealed slight ESR increment with temperature up to 50 °C, above which obvious deswelling occurred. In pH 8 buffer, 0.5 wt% of GO loading triggered lower critical solution temperature (LCST) to decrease by 3 °C, and 2–7 °C increment was observed when 1–6 wt% of GO was loaded, as compared with that of GO-free PDMAEMA hydrogel. Cr(VI) adsorption of hydrogels was also improved by the introduction of GO to some extent, and the maximum Cr(VI) adsorption of 180 mg/g was realized, indicating that the obtained PDMAEMA/GO hybrid hydrogels possess excellent adsorption performance.     

Mechanical behavior of zirconium diboride–silicon carbide ceramics at elevated temperature in air

The mechanical properties of zirconium diboride–silicon carbide (ZrB₂–SiC) ceramics were characterized from room temperature up to 1600 °C in air. ZrB₂ containing nominally 30 vol% SiC was hot pressed to full density at 1950 °C using B₄C as a sintering aid. After hot pressing, the composition was determined to be 68.5 vol% ZrB₂, 29.5 vol% SiC, and 2.0 vol% B₄C using image analysis. The average ZrB₂ grain size was 1.9 μm. The average SiC particles size was 1.2 μm, but the SiC particles formed larger clusters. The room temperature flexural strength was 680 MPa and strength increased to 750 MPa at 800 °C. Strength decreased to ～360 MPa at 1500 °C and 1600 °C. The elastic modulus at room temperature was 510 GPa. Modulus decreased nearly linearly with temperature to 210 GPa at 1500 °C, with a more rapid decrease to 110 GPa at 1600 °C. The fracture toughness was 3.6 MPa·m^½ at room temperature, increased to 4.8 MPa·m^½ at 800 °C, and then decreased linearly to 3.3 MPa·m^½ at 1600 °C. The strength was controlled by the SiC cluster size up to 1000 °C, and oxidation damage above 1200 °C.     

Rapid liquefaction of Longkou lignite coal by using a tubular reactor under methane atmosphere

Rapid liquefaction of Longkou lignite coal under methane atmosphere was studied in a novel laboratory scale tubular reactor. Experiments were performed at a temperature ranging from 400 to 800 °C, a residence time ranging from 4.5 to 11.2 s and 10–15 MPa (without catalyst). Reactions were also carried out under a nitrogen gas atmosphere at the same reaction conditions. The results indicate that there are synergistic effects between coal and methane at temperatures higher than 600 °C, and the temperature and residence time are the main factors influencing the coal conversion and products distribution. The oil yields reach a maximum of 21.97 (wt.% daf) at 750 °C during 9.0 s.     

In-situ thermo-mechanical testing of fly ash geopolymer concretes made with quartz and expanded clay aggregates

The mechanical and microstructural properties of geopolymer concretes were assessed before, during and after high temperature exposure in order to better understand the engineering properties of the material. Fly ash based geopolymer concretes with either quartz aggregate or expanded clay aggregate were exposed to various temperatures up to 750 °C using a thermo-mechanical testing apparatus. Microstructural investigations were also undertaken to better understand the measured changes in the mechanical properties. It was found that dehydration of capillary water caused cracking and strength losses at temperatures ≤ 300 °C, an effect that was more severe in the quartz aggregate geopolymer due to its lower permeability. At higher temperatures (T ≥ 500 °C) sintering promoted strength increases which enabled both concrete types to yield significant strength advantages over conventional materials. Stress–mechanical strain curves, which form the basis of the fire design of concrete structures, are reported.     

Microstructural evolution of multi-walled carbon nanotubes in the presence of mixture of silicon and silica powders at high temperatures

Microstructural evolution of multi-walled carbon nanotubes (MWCNTs) in the presence of mixture of silicon and silica powders in a coke bed is studied in the temperature range of 1000–1500 °C by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and thermogravimetry–differential scanning calorimetry (TG–DSC). The results showed that a thin amorphous SiO₂ coating was formed on the surface of MWCNTs at the temperature below 1300 °C. With the increase of the treated temperature, the coating became thicker, 3–7 nm in thickness at 1400 °C and a maximum of 10 nm at 1500 °C. Meanwhile, SiC nanowires and SiC nanocrystals around Ni catalyst at the tip of MWCNTs were formed at 1400 °C and 1500 °C, which were related to the vapor–vapor (V–V) and vapor–liquid–solid (V–L–S) reactions between SiO (g) and CO (g) or C (s), respectively. The oxidation resistance of all the treated MWCNTs was better than that of as-received ones. The oxidation peak temperature reached 804.2 °C for the treated MWCNTs, much higher than 652.2 °C for as-received ones.