Improvement and characteristics of conical silicon emitters employing wet-dry etching

A number of new technologies require conical and sharp tips to serve as electron emitters in the vacuum microelectronics. In this paper, we improved radius of curvature, height and cone angle of emitters in order to reach the enhancement result of field enhancement factor (β). We developed a fabrication process to improve geometry of emitter by employing isotropic dry etching in pure SF₆ and a mixture of SF₆ and O₂ followed by thermal oxidation technique. We successfully achieved excellent conical emitters with 5–10 nm radius of curvature, 4.4 μm height, and 30° cone angle. The conical silicon emitters current–voltage characteristics shows that E_to = 4.8 V/μm (turn-on electric field) with current density of 10 μA/cm², and maximum current density J = 60.4 μA/cm² at E = 8.14 V/μm. This study may provide a practical guideline for design and fabrication of a high-performance silicon emitter used in various industrial applications.     

Neutron Radioscopic Measurement of Water Adsorption Coefficients in Aerogels

Abstract

In this work, neutron radioscopy was utilized to investigate water vapor uptake by a hydrophilic silica aerogel. Aerogel is an unusual porous material, produced by a sol-gel process that results in a solid material with a unique microstructure composed of nanometer-size particles and pores. Aerogels have an extraordinarily large internal surface area which is accessible via open pores, making them candidates for filters and gas adsorption media.

The water vapor deposition was modeled both analytically and computationally, and an estimate for adsorption coefficients for water vapor in aerogel yielded 1.08 × 10^-3 ± 2.58 × 10^-4 cm²/s. Initial tests to measure water vapor uptake from moist air were very successful. Dry air was bubbled through water and then flowed past an aerogel. The aerogel was shown to uptake the water vapor readily from moist air. After uptake, the aerogel dried out rapidly in dry air. This phenomenon was repeatable, indicating that the aerogel could be reused with little change in its sorption properties. Neutron radiography was shown to be an effective nondestructive method for evaluating the real-time movement of water vapor in aerogel, as deposition patterns can be analyzed quantitatively as a function of time and penetration distance into the aerogel.     

Improvements in the performances of In–Ga–Zn–O thin-film transistors on glass substrates by annealing treatment

Thin film transistors (TFTs) with amorphous InGaZnO (IGZO) channel layer were fabricated by radio frequency magnetron sputtering technique. The IGZO films show optical transparency over 80 % both before annealing and after annealing. It was found that performances of transistors with IGZO thin films annealed in air at 450 °C were significantly improved. Through annealing treatment, Saturation current of TFTs increased from 2.8 to 181 μA at bias of V_DS = 20, V_GS = 20 V, and saturation mobility is up from 1.49 to 15.8 cm² V^?1 s^?1. In addition, X-ray photoelectric spectroscopy (XPS) was performed to provide elemental information on the surface of the IGZO films before and after annealing. O1s XPS spectra of unannealed and annealed IGZO films indicated oxygen vacancy concentration decreased by annealing treatment.     

Preparation of TiO2 thick film by laser chemical vapor deposition method

A TiO₂ film was prepared on Pt/Ti/SiO₂/Si substrate by a laser chemical vapor deposition method. The rutile TiO₂ film with pyramidal grains and columnar cross-section was obtained at a high deposition rate (R _dep = 11.4 μm h^?1). At 300 K and 1 MHz, the dielectric constant (ε _r) and loss (tanδ) of the TiO₂ film were about 73.0 and 0.0069, respectively. The electrical properties of TiO₂ film were investigated by ac impedance spectroscopy over ranges of temperature (300–873 K) and frequency (10²–10⁷ Hz). The Cole–Cole plots between real and imaginary parts of the impedance (Z′ and Z′′) in the above frequency and temperature range suggested the presence of two relaxation regimes that were attributed to grain and grain boundary responses. The ionic conduction in the rutile TiO₂ film was dominated by the oxygen vacancies.     

Weak ferromagnetic polar phase in the BiFe1−xTixO3 multiferroics

Neutron powder diffraction and magnetization measurements of selected samples of the BiFe_1?xTi_xO₃ series were performed. Ti⁴⁺ substitution was shown to induce the appearance of weak ferromagnetism in the initial polar R3c phase stable at x ≤ 0.1. In the concentration range 0 ≤ x ≤ 0.1, room-temperature residual magnetization increases from 0 to 0.25 emu/g (the latter is characteristic of the field-induced weak ferromagnetic state in pure BiFeO₃). The calculated ferroelectric polarization decreases from ~70 μC/cm² (x = 0) to ~60 μC/cm² (x = 0.1) at room temperature. Magnetic ordering coexists with the large spontaneous polarization in a broad temperature range to make the BiFe_1?xTi_xO₃ (x → 0.1) perovskites promising for multiferroic applications.     

Cleanable Air Filter Transferring Moisture and Effectively Capturing PM2.5

The lethal danger of particulate matter (PM) pollution on health leads to the development of challenging individual protection materials that should ideally exhibit a high PM_2.5 purification efficiency, low air resistance, an important moisture‐vapor transmission rate (MVTR), and an easy‐to‐clean property. Herein, a cleanable air filter able to rapidly transfer moisture and efficiently capture PM_2.5 is designed by electrospinning superhydrophilic polyacrylonitrile/silicon‐dioxide fibers as the adsorption–desorption vector for moisture‐vapor, and hydrophobic polyvinylidene fluoride fibers as the repellent components to avoid the formation of capillary water under high humidity. The desorption rate of water molecules increases from 10 to 18 mg min^?1, while the diameters of polyacrylonitrile fibers reduce from 1.02 to 0.14 µm. Significantly, by introducing the hydroxyl on the surface of polyacrylonitrile nanofibers, rapid adsorption–desorption of the water molecules is observed. Moreover, by constructing a hydrophobic to super‐hydrophilic gradient structure, the MVTR increases from 10 346 to 14 066 g m^?2 d^?1. Interestingly, the prepared fibrous membranes is easy to clean. More importantly, benefiting from enhanced slip effect, the resultant fibrous membranes presented a low air resistance of 86 Pa. A field test in Shanghai shows that the air filter maintains stable PM_2.5 purification efficiency of 99.99% at high MVTR during haze event.     

Microstructure, dielectric and piezoelectric properties of La-modified Bi0.5(Na0.84K0.16)0.5TiO3 lead-free ceramics

Lead-free ceramics (Bi_1?xLa_x)_0.5(Na_0.84K_0.16)_0.5TiO₃ were prepared by a conventional ceramic technique and the effects of La doping and sintering temperature on the microstructure, ferroelectric and piezoelectric properties of the ceramics were studied. All the ceramics possess a pure perovskite structure and La³⁺ diffuses into the Bi_0.5(Na_0.84K_0.16)_0.5TiO₃ lattices to form a solid solution with a rhombohedral symmetry. The addition of La leads to the significant change in the grain morphology and size for the (Bi_1?xLa_x)_0.5(Na_0.84K_0.16)_0.5TiO₃ and a number of rod grains with the length of 10–50 μm and the diameter of 1–2 μm are observed in the ceramic with x = 0.04 sintered at 1,140 °C for 2 h. However, as sintering temperature increases to 1,160 °C, the rod grains disappears and the uniform and rectangular grains are observed in the ceramics with x = 0.04. As x increases from 0 to 0.06, the coercive field E _c of the ceramics decreases from 4.33 to 2.81 kV/mm and the remanent polarization P _r of the ceramics retains the high values of 25.9–27.7 μm/cm². The depolarization temperature T _d decreases from 154 to 50 °C with x increasing from 0 to 0.10. All the ceramics exhibit the diffusive phase transition at high temperature (280–320 °C). The ceramic with x = 0.04 sintered at 1,150 °C for 2 h exhibit the optimum piezoelectric properties, giving d ₃₃ = 165 pC/N and k _p = 32.9 %. The optimum sintering temperature is 1,150 °C at which the improved piezoelectric properties (d ₃₃ = 165 pC/N and k _p = 32.9 %) are obtained. At the high La³⁺ level (x = 0.10 and 0.12), the ceramics exhibit weak ferroelectricity (P _r = 13.0–21.2 μm/cm²) and thus possess poor piezoelectricity (d ₃₃ = 17–27 pC/N).     

Enhanced multiferroic and dielectric properties of Sr2+-doped BiFe0.94(Mn0.04Cr0.02)O3 thin films

Sr²⁺-doped B_1?xSr_xFe_0.94(Mn_0.04Cr_0.02)O₃ (B_1?xSr_xFMC, x = 0.00, 0.05 and 0.09) thin films were prepared on FTO/glass (SnO₂: F) substrates by using a sol–gel spin-coating method. X-ray diffraction (XRD), Rietveld refined XRD data and Raman scattering spectra illustrate a structural evolution from trigonal (R3c: H) to tetragonal (P4) occurs in B_1?xSr_xFMC thin films with the increase of Sr²⁺ concent. Superiorly multiferroic and dielectric properties were obtained in the B_1?xSr_xFMC thin films, e.g., large remanent polarization value, a high dielectric constant (P _r = 139.21 μC/cm² and ε_r = 396.7 for x = 0.09) and large saturation magnetization (M _s = 2.08 emu/cm³ for x = 0.05). The leakage current density of B_1?xSr_xFMC thin films is increased with increasing Sr²⁺ concentration, wherein the leakage current density of all the B_1?xSr_xFMC films is of the order of magnitude of 10^?5 A/cm², which are still lower than that in the pure BFO film (10^?3 A/cm²).     

Combined effect of substrate temperature and laser energy density on laser ablated BiFeO3 thin films

Bismuth ferrite (BiFeO₃) thin films were prepared on Pt(111)/Ti/SiO₂/Si substrates by laser ablation deposition at various substrate temperature (T _sub) and laser energy density (D _L). The combined effect of T _sub and D _L on crystalline phase, preferential orientation and surface morphology of the films were investigated. The microstructure and morphology of the films showed a strong dependence on T _sub and D _L simultaneously. BiFeO₃ thin films with single phase could be prepared at T _sub-D _L of 893 K?1 J/cm², 943 K?2 J/cm² and 963 K?3 J/cm², respectively. Film deposited at T _sub = 963 K and D _L = 3 J/cm² exhibited better ferroelectric property due to the combination of high (111) preferential orientation, dense surface morphology and less Fe²⁺ content.     

Lewis and Brønsted acids in super-acid catalyst SO<Subscript>4</Subscript><Superscript>2−</Superscript>/ZrO<Subscript>2</Subscript>–SiO<Subscript>2</Subscript>

Super-acid catalyst, SO₄ ^2?/ZrO₂–SiO₂, with high zirconium loading was synthesized and the nature of the surface acid was investigated by FT-IR of pyridine adsorption. With the increasing ZrO₂ content, the Lewis and Brønsted acid sites increased and reached the maximum when Zr/Si (molar ratio) = 1.3. The sample with Zr/Si = 1.3 showed the strongest IR adsorption band in the S=O stretching region (1,300–1,400 cm^?1). Pyrosulfate and monosulfate species existed on the surface of the catalysts and the acidic strength could be enhanced by induction effect of their S=O groups. And there were two kinds of Brønsted acid sites on the surface of the catalysts.     

Extremely Fast 1 μmol · mol−1 Water-Vapor Measurement by a 1 mm Diameter Spherical SAW Device

When a surface acoustic wave (SAW) is excited on a spherical surface, a naturally collimated SAW propagates around the equator hundreds of times. The propagation characteristics such as the velocity and amplitude are affected by adsorbed and/or reacted molecules on the surface, and the changes are accumulated by multiple turns of propagation. This enables highly sensitive detection of adsorbed molecules including water vapor. In this paper, the development and testing of a 1 mm diameter spherical SAW sensor, which is capable of measuring water vapor at concentrations well below 1 μmol · mol^?1H₂O in N₂, are reported. The rise time from 10% to 90% of the spherical SAW sensor to a step change from dry N₂ to 1 μmol · mol^?1H₂O in N₂ was approximately 15 s.     

Microwave Determination of Water Mole Fraction in Humid Gas Mixtures

A small volume (65?cm³) gold-plated quasi-spherical microwave resonator has been used to measure the water vapor mole fraction x _w of H₂O/N₂ and H₂O/air mixtures. This experimental technique exploits the high precision achievable in the determination of the cavity microwave resonance frequencies and is particularly sensitive to the presence of small concentrations of water vapor as a result of the high polarizability of this substance. The mixtures were prepared using the INRIM standard humidity generator for frost-point temperatures T _fp in the range between 241?K and 270?K and a commercial two-pressure humidity generator operated at a dew-point temperature between 272?K and 291?K. The experimental measurements compare favorably with the calculated molar fractions of the mixture supplied by the humidity generators, showing a normalized error lower than 0.8.     

Pulsed laser deposition of BiFeO3 thin films with large polarization on Pt(111)/Ti/SiO2/Si by controlling substrate temperature

Bismuth ferrite (BiFeO₃) thin films with large polarization were grown on Pt(111)/Ti/SiO₂/Si by optimizing the substrate temperatures (T _sub). The BiFeO₃ thin films were prepared by pulsed laser deposition and the effects of T _sub (T _sub = 853–913 K) on crystallization orientation, surface morphology and properties of the films were investigated. The microstructure and morphology of the films showed a strong dependence on T _sub. The film prepared at T _sub = 893 K had a relatively high degree of (111) preferential orientation and densely packed morphology. A large polarization with the maximum remanent polarization of 108 μC/cm² was obtained, which was due to the high degree of preferential orientation and the dense surface morphology at the optimum substrate temperature.     

Neutron Radioscopic Measurement of Water Adsorption Coefficients in Aerogels

In this work, neutron radioscopy was utilized to investigate water vapor uptake by a hydrophilic silica aerogel. Aerogel is an unusual porous material, produced by a sol-gel process that results in a solid material with a unique microstructure composed of nanometer-size particles and pores. Aerogels have an extraordinarily large internal surface area which is accessible via open pores, making them candidates for filters and gas adsorption media. The water vapor deposition was modeled both analytically and computationally, and an estimate for adsorption coefficients for water vapor in aerogel yielded 1.08 \times 10 ^-3 \pm 2.58 \times 10 ^-4 cm ² /s. Initial tests to measure water vapor uptake from moist air were very successful. Dry air was bubbled through water and then flowed past an aerogel. The aerogel was shown to uptake the water vapor readily from moist air. After uptake, the aerogel dried out rapidly in dry air. This phenomenon was repeatable, indicating that the aerogel could be reused with little change in its sorption properties. Neutron radiography was shown to be an effective nondestructive method for evaluating the real-time movement of water vapor in aerogel, as deposition patterns can be analyzed quantitatively as a function of time and penetration distance into the aerogel.     

Structure and electrical properties of K0.5Na0.5NbO3–SrTiO3 lead-free piezoelectric ceramics with LiSbO3 doping

Lead-free piezoelectric ceramics (1 ? x)K_0.5Na_0.5NbO₃–xSrTiO₃ with 6 mol% LiSbO₃ doping have been prepared by conventional solid state sintering technique at 1,125 °C for 3 h in air. The effects of the SrTiO₃ and LiSbO₃ on the phase structure and electrical properties of the ceramics were systematically investigated. All ceramic samples show a single phase perovskite structure with tetragonal symmetry when LiSbO₃ content was 6 mol% and SrTiO₃ content was 2–10 mol% by X-ray diffraction analysis and highly dense structure by SEM patterns. The ceramic with x = 0.04 exhibits optimum electrical properties at room temperature (d ₃₃ = 267 pC/N, k _p = 46 %, ε _r = 1,168, tanδ = 0.021, P _r = 30.3 μC/cm², E _C = 1.98 kV/mm), which suggests that the ceramic is a promising candidate material for lead-free piezoelectric ceramics.     

Deposition of luminescent a-SiN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>:H Films with SiH<Subscript>4</Subscript>–N<Subscript>2</Subscript> gas mixture by VHF–PECVD using novel impedance matching method

Hydrogenated amorphous silicon nitride (a-SiN _x :H) films are produced from a SiH₄–N₂ gas mixture by plasma enhanced chemical vapor deposition (PECVD) system with a newly developed impedance matching method at frequencies 13.6–150 MHz. An increase in the rf power from 35 to 350 mW/cm² at the highest frequency of 150 MHz increases the optical bandgap (E _opt) from 2.0 to 4.5 eV. Optical emission spectroscopy (OES) of the SiH₄–N₂ plasma shows that the emission intensity of SiH* (414 nm) is almost proportional to deposition rate. Films of a-SiN _x :H deposited at 150 MHz and 210 mW/cm² has an optical bandgap of E _opt ≈ 4.1 eV and emits visible photoluminescence (PL) at room temperature (RT).     

Stable controlled growth of 3D CuO/Cu nanoflowers by surfactant-free method for non-enzymatic hydrogen peroxide detection

Sensitive, convenient and rapid detection of hydrogen peroxide(H_2 O_2) is highly desirable in fields like fundamental biological research, food industries, and clinical environmental analysis. Herein, a hierarchical porous CuO/Cu flower-like active electrode material for non-enzymatic H_2 O_2 sensor was synthesized via a low-cost and one-step chemical oxidation of Cu powder in water bath without surfactants. In order to discuss the growth mechanism of the product, products with different growth time length were fabricated. The electro-catalysis of all products were first exhibited by cyclic-voltammetry,and the product under 6 h reaction shows the best result. The detailed electro-catalytic behaviors of the best product(under 6 h reaction) are characterized by cyclic-voltammetry and amperometry under alkaline conditions. The materials have high sensitivity of 103 μA mM~(-1) cm~(-2)(R~2= 0.9979), low detection limit of 2 μmol/L and wide concentration range(from 2 μmol/L to 19.4 mmol/L). Large specific surface area and stabled nanostructure enabled good features, such as stability and sensitivity for the H_2 O_2 determination.     

Adsorption kinetics, isotherm, and thermodynamics of Hg2+ to polyaniline/hexagonal mesoporous silica nanocomposite in water/wastewater

A nanocomposite of polyaniline/hexagonal mesoporous silica (PAN/HMS) was prepared and characterized by BET analysis, transmission electron microscopy, and FT-IR spectra. Batch adsorption results showed that PAN/HMS had high affinity to Hg²⁺ in aqueous solutions. Various factors affecting the adsorption capacity such as contact time, temperature, absorbent dosage, pH, and initial concentration of Hg²⁺ ions were investigated. The adsorption kinetics for the Hg²⁺ showed that the adsorption reached equilibrium within 8 min and adsorption rates followed the pseudo-second-order rate law, indicating chemical sorption as the rate-limiting step of the adsorption mechanism. Sorption of Hg²⁺ to PAN/HMS agreed well to the Langmuir adsorption model at different ionic strengths with the maximum adsorption capacity of 843 mg g^?1 (I = 1000 mg L^?1) at pH 10. Thermodynamic studies revealed that the adsorption of Hg²⁺ ions was spontaneous, exothermic processes with an increase of entropy. The recovery of the Hg²⁺ from the adsorbent was found to be more than 88 % using H₂SO₄ (0.1 M), and the ability of the absorbent to be reused was investigated.     

Crystallographic, thermoelectric, and mechanical properties of polycrystalline type-I Ba8Al16Si30-based clathrates

Crystallographic, thermoelectric, and mechanical properties of polycrystalline Ba₈Al₁₆Si₃₀-based samples with type-I clathrate structure prepared by combining arc melting and spark plasma sintering methods were investigated. The major phase of the samples was a type-I clathrate with an actual Al/Si ratio of ~15/31, strongly suggesting that framework deficiency was absent or was present in very low concentration in the samples. The Hall carrier concentration n of the samples was approximately 1 × 10²¹ cm^?3, which is lower than the values reported so far for the Ba₈Al₁₆Si₃₀ system. Other important material parameters of the samples were as follows: the density-of-states effective mass m* = 2.3m ₀, Hall mobility μ = 7.4 cm² V^?1 s^?1, and the lattice thermal conductivity κ _L = 1.2 W m^?1 K^?1. The thermoelectric figure of merit ZT reached approximately 0.4 (900 K) for a sample with n = 9.7 × 10²⁰ cm^?3. Simulation using the experimentally determined values of material parameters showed that ZT reached values >0.5 if the carrier concentration is optimized at about 3 × 10²⁰ cm^?3. Young’s, shear, and bulk moduli were estimated to be approximately 98, 39, and 117 GPa, respectively, and Poisson’s ratio was found to be 0.25 from the longitudinal and transverse velocities of sound, v _L = 6038 m/s and v _T = 3503 m/s, respectively, for a sample with ZT = 0.4. The coefficient of thermal expansion (CTE) ranged from approximately 8 × 10^?6 K^?1 to 10 × 10^?6 K^?1 (330–690 K), which is smaller than the values reported for Ba₈Ga₁₆Ge₃₀ and Sr₈Ga₁₆Ge₃₀ clathrates.     

Effects of High Pressure on the Physical Properties of MgB2

The synthesis of MgB₂-based materials under high pressure gave the possibility to suppress the evaporation of magnesium and to obtain near theoretically dense nanograined structures with high superconducting, thermal conducting, and mechanical characteristics: critical current densities of 1.8?C1.0×10⁶ A/cm² in the self-field and 10³ A/cm² in a magnetic field of 8 T at 20 K, 5?C3×10⁵ A/cm² in self-field at 30 K, the corresponding critical fields being H _c2=15 T at 22 K and irreversible fields H _irr=13 T at 20 K, and H _irr=3.5 T at 30 K, thermal conduction of 53±2 W/(m?K), the Vickers hardness H _V =10.12±0.2 GPa under a load of 148.8 N and the fracture toughness K _1C =7.6±2.0 MPa?m^0.5 under the same load, the Young modulus E=213 GPa. Estimation of quenching current and AC losses allowed the conclusion that high-pressure-prepared materials are promising for application in transformer-type fault current limiters working at 20?C30 K.