Growth and electron field emission properties of ultrananocrystalline diamond on silicon nanostructures

The electron field emission (EFE) properties of Si nanostructures (SiNS), such as Si nanorods (SiNR) and Si nanowire (SiNW) bundles were investigated. Additionally, ultrananocrystalline diamond (UNCD) growth on SiNS was carried out to improve the EFE properties of SiNS via forming a combined UNCD/SiNS structure. The EFE properties of SiNS were improved after the deposition of UNCD at specific growth conditions. The EFE performance of SiNR (turn-on field, E₀ = 5.3 V/μm and current density, J_e = 0.53 mA/cm² at an applied field of 15 V/μm) was better than SiNW bundles (turn-on field, E₀ = 10.9 V/μm and current density, J_e < 0.01 mA/cm² at an applied field of 15 V/μm). The improved EFE properties with turn-on field, E₀ = 4.7 V/μm, current density, J_e = 1.1 mA/cm² at an applied field of 15 V/μm was achieved for UNCD coated (UNCD grown for 60 min at 1200 W) SiNR. The EFE property of SiNW bundles was improved to a turn-on field, E₀ = 8.0 V/μm, and current density, J_e = 0.12 mA/cm² at an applied field of 15 V/μm (UNCD grown for 30 min at 1200 W).     

Highly conductive SiC ceramics containing Ti2CN

Highly conductive SiC ceramics were fabricated by sintering β-SiC and TiN powder mixture in N₂ atmosphere. SiC ceramics exhibited decreased electrical resistivity (ρ) with increasing TiN content. X-ray diffraction data indicated that the specimens consisted of β-SiC grains without a detectible secondary phase for low TiN content (≤2 vol%) but contained a Ti₂CN phase as the TiN content increased. The temperature-dependent resistivity ρ(T) of specimens revealed semiconductor-like behavior for TiN content up to 10 vol% and metal-like behavior above 20 vol%. For the specimen with TiN content of 15 vol%, ρ(T) remained almost constant (2.06 ± 0.01 × 10⁻³ Ω cm) in the 4–300 K range. The resistivity of metal-like specimens were as low as 3.5 × 10⁻⁴ Ω cm for TiN content of 20 vol%. For semiconductor-like specimens, ρ(T) was primarily affected by N donors in the β-SiC grains. Metal-like specimens were primarily affected by metallic Ti₂CN clusters.     

Magnetic field induced delocalization in multi-wall carbon nanotube-polystyrene composite at high fields

In well dispersed multi-wall carbon nanotube-polystyrene composite of 15 wt%, with room temperature conductivity of ∼5 S/cm and resistivity ratio [R_2 K/R_200 K] of ∼1.4, the temperature dependence of conductivity follows a power-law behavior. The conductivity increases with magnetic field for a wide range of temperature (2–200 K), and power-law fits to conductivity data show that localization length (ξ) increases with magnetic field, resulting in a large negative magnetoresistance (MR). At 50 T, the negative MR at 8 K is ∼13% and it shows a maximum at 90 K (∼25%). This unusually large negative MR indicates that the field is delocalizing the charge carriers even at higher temperatures, apart from the smaller weak localization contribution at T < 20 K. This field-induced delocalization mechanism of MR can provide insight into the intra and inter tube transport.     

Carbon nanotubes growth in AlPO4-5 zeolites: Evidence for density dependent field emission characteristics

We report on the correlation between the concentration of Fe-catalyst, doped in the aluminum phosphate (AlPO₄-5) zeolite and the resulting density of carbon nanotubes (CNTs) to obtain the optimum electron field emission conditions from the CNTs. Initially, AlPO₄-5 crystallites were impregnated, for a period of ∼ 10–60 min, in the Fe-catalyst solution and subjected to Electron Spectroscopy for Chemical Analysis (E.S.C.A.). The analysis revealed that the concentration of Fe-catalyst, C_Fe, was increased from ∼ 1.7% to ∼ 8.6%, respectively, with increase in impregnation time, I_T. The HRTEM results showed that Fe nano-clusters, with diameter ∼ 7–10 nm, were formed in the surface region of the crystallites. These crystallites were sprayed on the conducting substrates, under identical spraying conditions. SEM study revealed that the coverage of the crystallites on the substrates was ∼ 10³–10⁴ crystallites/cm². These substrates were subjected to direct current plasma enhanced chemical vapor deposition (dc-PECVD) process, to grow CNTs. The SEM micrographs were recorded for the CNT-grown substrates and the average areal density of CNTs, (σ_T)_av, on the crystallites (t/cm²) was estimated. The analysis indicated that (σ_T)_av increased from ∼ 6.24 ± 0.19 × 10¹⁰ to 2.04 ± 0.61 × 10¹¹ t/cm² with gradual increase in C_Fe. The field emission study of the samples revealed that the optimum values of the turn-on electric field, ∼ 3.69 V/μm and the field emission current density, ρ_d, ∼ 1.78 × 10³ μA/cm² were achieved for (σ_T)_av, ∼ 6.24 ± 0.19 × 10¹⁰ t/cm², at a concentration of Fe, C_Fe, ∼ 3.0%, encapsulated in the AlPO₄-5 crystallites.     

Long-term stability of a horizontally-aligned carbon nanotube field emission cathode coated with a metallic glass thin film

A horizontally-aligned carbon nanotube (HACNT) field emission cathode was coated with a metallic glass thin film (MGTF) to improve the stability of the field emission properties. HACNT field emission cathodes have previously been fabricated on glass substrates using composite plating and crack-formation techniques. A carbon nanotubes/nickel (CNTs/Ni) composite film is deposited onto a glass substrate at 80 °C by the composite plating technique alone. Cracks are then formed in the CNT/Ni composite film during 30 min heating at 300 °C, and HACNTs are exposed in the cracks. The field emission properties of the HACNT field emission cathode show a low turn-on electric field E_on of about 2.3 V/μm, a low threshold electric field E_th of about 4.7 V/μm at an emission current density of 1 mA/cm², and a stability time of 78 h. The degradation of the HACNT field emission cathode is prevented by using a MGTF-coating technique and superior long-term stability (i.e. >125 h, with 5 nm MGTF; >270 h, with 10 nm MGTF) for the MGTF/HACNT field emission cathode is achieved.     

The stability of the CNT/Ni field emission cathode fabricated by the composite plating method

A novel composite plating method has been developed for the fabrication of carbon nanotube/Ni (CNT/Ni) field emission cathode. The field emission properties of the initial CNT/Ni field emitter show a low turn-on electric field E_on of about 1.1 V/μm with an emission current density of 1 μA/cm², and a low threshold electric field E_th of about 1.7 V/μm with an emission current density of 1 mA/cm². After performing a stability test with a high emission current density in high vacuum, the corresponding microstructure and the degree of graphitization of the CNT/Ni field emitter were measured by using scanning electron microscopy and Raman spectroscopy. We found that the degree of graphitization slowly decreases with the duration time t_FE of the stability test, the size of small rod-like CNT/Ni composite structures in the film increases with t_FE, and obvious cracks appear in the film as t_FE is larger than 60 h. The degradation of the field emission properties may be explained by the Joule heating effect on the CNT/Ni field emitter under high emission current density.     

Effect of deposition parameters on the growth rate and dielectric properties of the Ba(SnxTi1−x)O3 thin films prepared by radio frequency magnetron sputtering

The growth rate and dielectric properties of the Ba(Sn_xTi_1−x)O₃ (BS_xT_1−x) thin films prepared by radio frequency (rf) magnetron sputtering at room temperature have been characterized as a function of deposition parameters. The BS_xT_1−x thin films are amorphous when deposited at low rf power (R_p = 100 and 125 W). The XRD result shows merely a single perovskite (BaTiO₃) structure and the intensity of reflection peaks increases with the rf power increasing from 125 to 150 and 175 W. When the BS_xT_1−x thin film is deposited at R_p = 150 W and room temperature, the deposition rate decreases with the increasing working pressure (W_p) and O₂/(O₂ + Ar) ratio (O_r). The refractive index of the BS_xT_1−x thin films is between 2.1 and 2.3, which shows that the variation of working pressure is not very significant. The dielectric constant of the BS_xT_1−x thin films increases with the R_p increasing from 100 to 150 W and decreases above 150 W. The leakage current density of the BS_0.15T_0.85 thin films nearly displays the ohmic behavior when the electric field is below 50 kV/cm. The conduction mechanism of the BS_0.15T_0.85 thin films involves the Schottky emission (SE) and Poole–Frenkel emission (PF) models. The BS_xT_1−x thin film shows a ferroelectric characteristic in the polarization-electric field plot.     

Field emission characteristics of CNFB-CNT hybrid material grown by one-step MPCVD

A hybrid material consisting of carbon nanotubes (CNTs) and carbon nanoflake balls (CNFBs) was successfully synthesized by microwave-plasma-assisted chemical vapor deposition using a H₂/CH₄/N₂ ratio of 4:1:2 at 80 Torr for 30 min. The precursor used was a sol-gel solution containing ferric nitrate, tetrabutyl titanate, and n-propanol. The carbon hybrid material (CNFB-CNT) exhibited excellent field emission properties, with its turn-on field being 1.77 V/μm. It also showed two field enhancement factors (1536 and 7932) for different electric fields. The emission current density of the hybrid remained higher than 0.65 mA/cm² for more than 50 h and was 0.82 mA/cm² even after 50 h of continuous emission. Further, the field emission properties of the CNFB-CNT hybrid were better than those of other single-structured carbon nanomaterials (CNTs, CNFs, or CNFBs). Therefore, the CNFB-CNT hybrid material should be a promising candidate for use in high-performance field emitters.     

Study of physical properties of cobalt substituted Pr0.7Ca0.3MnO3 ceramics

An investigation on single phase semiconducting polycrystalline Pr_0.7Ca_0.3MnO₃ and Pr_0.7Ca_0.3MnCo_0.1O₃ crystallized in the orthorhombic system with Pnma space group is reported. We found that σ_DC increases when introducing Co for T<110 K but for T>110 K, it decreases. Also, the contribution of hopping process in conduction mechanism was in agreement with the Jonscher law and Mott theory. Capacitance was extensively dependent on temperature and frequency. A dielectric transition was observed at T=150 K for the doped compound. The temperature dependence of dielectric permittivity is well described by Curie–Weiss law. The parameter of deviation from Curie–Weiss behavior to modified Curie–Weiss law is found to be ΔT_m=30 K. The substitution of Mn by Co was found to destroy the charge order state observed in the parent compound and to induce a ferromagnetic phase at low temperature. The cobalt-substituted sample exhibits a maximum value of magnetic entropy change |∆S^max|=3.2 J kg⁻¹ K⁻¹and a large relative cooling power with a maximum value of 301 J/kg under an applied field of 5 T. Technically, these large values make the prepared material very promising for magnetic refrigeration.     

Microstructures and EMI shielding properties of composite ceramics reinforced with carbon nanowires and nanowires-nanotubes hybrid

Carbon/ceramic composites are promising candidates as electromagnetic interference (EMI) shielding materials used at various harsh environments. The aim of present work is to prepare and investigate two kinds of composite ceramics reinforced with carbon nanowires (CNWs) and nanowires-nanotubes (CNWs-CNTs) hybrid, respectively. Results indicate that CNWs is highly curved and multi-defected, and CNWs-CNTs hybrid shows the best crystal structure at an optimal catalyst concentration of 5 wt%. When CNWs accounts for 5.15 wt%, the total shielding effectiveness (SE) of CNWs/Si₃N₄ reaches 25.0 dB with absorbed SE of 21.3 dB, meaning that 99.7% incident signal can be blocked, while it reaches 25.4 dB for CNWs-CNTs/Si₃N₄ as the carbon loading only increasing to 3.91 wt%. By contrast, CNWs/Si₃N₄ exhibits better electromagnetic attenuation capability with stronger absorption, mainly due to the unique microstructure of CNWs. Both of two composite ceramics have great potential to be designed as structural and multi-functional materials.     

Electron field emission properties on UNCD coated Si-nanowires

The electron field emission (EFE) properties of Si-nanowires (SiNW) were improved by coating a UNCD films on the SiNWs. The SiNWs were synthesized by an electroless metal deposition (EMD) process, whereas the UNCD films were deposited directly on bare SiNW templates using Ar-plasma based microwave plasma enhanced chemical vapor deposition (MPE–CVD) process. The electron field emission properties of thus made nano-emitters increase with MPE–CVD time interval for coating the UNCD films, attaining small turn-on field (E₀ = 6.4 V/μm) and large emission current density (J_e = 6.0 mA/cm² at 12.6 V/μm). This is presumably owing to the higher UNCD granulation density and better UNCD-to-Si electrical contact on SiNWs. The electron field emission behavior of these UNCD nanowires emitters is significantly better than the bare SiNW ((E₀)_SiNWs = 8.6 V/μm and (J_e)_SiNWs < 0.01 mA/cm² at the same applied field) and is comparable to those for carbon nanotubes.     

Time dependent growth of ZnO nanoflowers with enhanced field emission properties

Herein, we report the facile growth of ZnO nanoflowers composed of nanorods on silicon substrate by non-catalytic thermal evaporation process. The grown nanoflowers were examined in terms of their morphological, structural, optical and field emission properties. The detailed characterizations revealed that the nanoflowers are grown in high density, possessing well-crystallinity and exhibiting wurtzite hexagonal phase. The Raman-scattering spectrum shows a sharp optical-phonon E₂ mode at 437 cm⁻¹ which confirmed the wurtzite hexagonal phase for the grown nanoflowers. The room-temperature PL spectrum depict a strong ultraviolet emission at 381 nm, revealed good optical properties for the ZnO nanoflowers. The field emission studies revealed that a turn-on field for the ZnO nanoflowers based field emission device was 4.3 V/μm and the emission current density reached to 0.075 mA/cm² at an applied electric field of 7.2 V/μm and exhibit no saturation. The field enhancement factor ‘β’ for the fabricated device was estimated from the F-N plot and found to be ~2.75×10³. Finally, systematic time-dependent experiments were performed to determine the growth process for the formation of ZnO nanoflowers composed of nanorods.     

Formation of diamond-like BN phases under shock compression of graphite-like BN with different degree of structural ordering

The formation of diamond-like modifications of BN under high-temperature shock compression (P = 33 GPa and T up to 3500 K) has been studied. The powders of graphite-like BN with different degree of three-dimensional ordering of structure (D₃) were used as starting materials. To increase shock temperature and preserve dense phase formed, boron nitride powder mixed with alkali halide salt was compressed in cylindrical ampoule. The recovered samples were studied by X-ray diffraction and electron microscopy. It was revealed that the phase composition of shock compression products depends on the degree of three-dimensional ordering of initial graphite-like structure. Turbostratic BN with disordered structure (D₃ = 0) transformed mainly into cubic cBN by diffusion mechanism, whereas highly ordered graphite-like BN (D₃ = 0.95) transformed only into wurtzite wBN by martensitic mechanism. Both cubic and wurtzite dense modifications are formed from partly ordered initial structures (for which D₃ was 0.45 ÷ 0.7). The both diamond-like phases have nanocrystalline structure.     

Systematic examination of thermal,mechanical and dielectrical properties of aromatic polybenzoxazines

A series of bis-benzoxazines, prepared in high yield and purity using two synthetic procedures, is reported. Differential scanning calorimetry reveals similar temperatures for the onset of polymerisation (162–180 °C); the higher values representing monomers containing polar bridges or rigid backbones. Dynamic viscoelasticity data reveal glass transition temperatures for the polybenzoxazines ranging from 187 °C to 235 °C; a fluorinated polybenzoxazine consistently yields the highest T_g of the polymers studied. The latter is interesting since it is superior to many commercial benzoxazines with a relatively high T_g (235 °C), flexural modulus (5.0 GPa) and flexural strength (146.7 MPa), but coupled with a breaking strain (3.06%) that is uncharacteristically high for polybenzoxazines. The incorporation of fluorine results in a low dielectric loss properties (D_k = 3.71–4.12 at 10 MHz, D_f = 0.0109 – 0.0980 at 10 MHz), which are comparable with commercial polybenzoxazines, FR4 and aerospace epoxy resins and superior to commercial bismaleimides.     

Electron field emission from filtered arc deposited diamond-like carbon films using Au and Ti layers

In this work, tetrahedral diamond-like carbon (DLC) films are deposited on Si, Ti/Si and Au/Si substrates by a new plasma deposition technique — filtered arc deposition (FAD). Their electron field emission characteristics and fluorescent displays of the films are tested using a diode structure. It is shown that the substrate can markedly influence the emission behavior of DLC films. An emission current of 0.1 μA is detected at electric field E_DLC/Si=5.6 V/μm, E_DLC/Au/Si=14.3 V/μm, and E_DLC/Ti/Si=5.2 V/μm, respectively. At 14.3 V/μm, an emission current density J_DLC/Si=15.2 μA/cm², J_DLC/Au/Si=0.4 μA/cm², and J_DLC/Ti/Si=175 μA/cm² is achieved, respectively. It is believed that a thin TiC transition layer exists in the interface between the DLC film and Ti/Si substrate.     

Superconducting fluctuation and infrared absorption of Cd-substituted Tl0.9Bi0.1Sr1.8Yb0.2Ca1−xCdxCu1.99Fe0.01O7−δ ceramics

Tl_0.9Bi_0.1Sr_1.8Yb_0.2Ca_1?xCd_xCu_1.99Fe_0.01O_7?δ (x=0–0.4) ceramics were prepared using the conventional solid-state synthesis method to investigate the effects of Cd²⁺ substitutions on superconducting fluctuation behavior, SFB and structural properties of the Tl1212 derivatives. Electrical resistivity measurements showed all the samples exhibit metallic normal-state behavior with zero resistivity critical temperature, T_{c zero} increasing from 40.0 K (x=0) to 76.4 K (x=0.3) before decreasing to 74.8 K (x=0.4) with increasing Cd²⁺. FTIR analysis in conjunction with XRD results indicates improved inter-plane coupling that is evidenced in the form of decreased c-axis length and softening of the apical oxygen mode when Cd concentration was increased. In order to study the effect of Cd substitutions on SFB, excess conductivity analysis above T_c was performed using the Aslamazov–Larkin, AL model in conjunction with the Lawrence–Doniach, LD model. The results showed cross-over between two-dimensional, 2D to three-dimensional, 3D SFB transition for all the samples (x=0.1–0.4) with decreasing temperature. The highest transition temperature, T_2D–3D was observed at x=0.3 which showed both the longest coherence length, ξ_c(0) and the highest inter-plane coupling constant, J within the series. Our results suggest that the increase in J and lowering of anisotropy, γ resulted in enhancement of superconductivity in the compound.     

Electrical properties of a-C: Mo films produced by dual-cathode filtered cathodic arc plasma deposition

Molybdenum-containing amorphous carbon (a-C:Mo) thin films were prepared using a dual-cathode filtered cathodic arc plasma source with a molybdenum and a carbon (graphite) cathode. The Mo content in the films was controlled by varying the deposition pulse ratio of Mo and C. Film sheet resistance was measured in situ at process temperature, which was close to room temperature, as well as ex situ as a function of temperature (300–515 K) in ambient air. Film resistivity and electrical activation energy were derived for different Mo and C ratios and substrate bias. Film thickness was in the range 8–28 nm. Film resistivity varied from 3.55 × 10^− 4 Ω m to 2.27 × 10^− 6 Ω m when the Mo/C pulse ratio was increased from 0.05 to 0.4, with no substrate bias applied. With carbon-selective bias, the film resistivity was in the range of 4.59 × 10^− 2 and 4.05 Ω m at a Mo/C pulse ratio of 0.05. The electrical activation energy decreased from 3.80 × 10^− 2 to 3.36 × 10^− 4 eV when the Mo/C pulse ratio was increased in the absence of bias, and from 0.19 to 0.14 eV for carbon-selective bias conditions. The resistivity of the film shifts systematically with the amounts of Mo and upon application of substrate bias voltage. The intensity ratio of the Raman D-peak and G-peak (I_D/I_G) correlated with the pre-exponential factor (σ₀) which included charge carrier density and density of states.     

Influence of the bed geometry on the kinetics of the extraction of clove bud oil with supercritical CO2

There is a need for scientific research that evaluates the influence of important process variables on the scale up of supercritical technology. For supercritical fluid extraction (SFE), one of these variables is the extractor's bed geometry, which can be defined by the ratio of the bed height (H_B) to the bed diameter (D_B). A systematic study is needed to select suitable criteria that can be used to obtain similar extraction curves among beds with different geometries. In this study, maintaining a constant ratio of solvent mass to feed mass for two beds with 1-L volumes but different geometries (E-1: H_B/D_B = 7.1; E-2: H_B/D_B = 2.7) was confirmed as a successful scale up criterion. For constant values of the temperature, pressure and bed porosity, there is experimental evidence that the mass transfer rate is equal in the two beds when the solvent flow rate is high. When 0.6 kg of clove buds was packed in the beds, the extraction rates were 2.10 ± 0.08 and 2.3 ± 0.1 g extract/min for beds E-1 and E-2, respectively. However, when the solvent flow rate was lower, the extraction rates were 0.93 ± 0.06 and 1.12 ± 0.02 g extract/min for beds E-1 and E-2, respectively. This difference in behavior between the extraction beds is associated with the axial dispersion of the fluid, which is more pronounced when the H_B/D_B ratio is increased. Thin particles tend to compact in the beds with high H_B/D_B ratios, which shorten the solvent passage. Non-isothermal profiles and differences in chemical composition of the extracts were also observed: 17% more α-humulene and 9% more eugenol were extracted in E-1 and E-2, respectively.     

Magnetic and magnetocaloric properties of FeMnO3

FeMnO₃, prepared by mechano-synthesis method using high energy planetary ball mill, crystallizes in bixbyite type cubic structure in space group Ia3. Magnetic measurements show that the compound is ferrimagnetic at room temperature and on cooling undergoes antiferromagnetic ordering around, T_o~ 36 K. Magnetization measurements exhibit strong magnetic anisotropy in this system. Change in the magnetic entropy, also known as the magnetocaloric effect, has been observed in FeMnO₃ on subjecting the sample to a changing magnetic field. We found maximum change of magnetic entropy ~1.5 J/kg K, for a field change of 90 kOe at T~2T_o with relative cooling power of 50 J/kg.     

Role of spin-glass phase for magnetoresistance enhancement in nickel substituted lanthanum calcium manganite

Effect of Ni substitution in lanthanum calcium manganite (LCMO) has been investigated for change in magnetoresistance (MR). Scanning electron microscopy images revealed decrease in grain size from 3.72 µm to 0.55 µm by Ni substitution. Maximum increase in MR has been found 28% at low temperature (100 K) for x=0.10, Ni substitution at Mn site. Metal insulator transition temperature has been decreased from 253.2 K for x=0.0–90 K for x=0.10. Above x=0.10, Ni substitution no metal-insulator transition temperature appeared due to the presence of porosity in the samples. Ni substitution lowered the magnetic transition temperature from 255 K for x=0.0–125 K for x=0.25. Lowering of irreversible temperature (T_irr) from 250 K for x=0.0–135.4 K for x=0.20 has been obtained by zero field cooled (ZFC) and field cooled (FC) measurements confirm reduction of ferromagnetic clusters and spin-glass phase like behavior due to Ni presence. The spin-glass phase presence allows spin-polarized tunneling even at low magnetic field, which ultimately results in enhancement of MR at low temperature. Core level X-ray photoelectron spectroscopy measurements confirm Ni²⁺ charge state of Ni ions and increase in Mn⁴⁺/Mn³⁺ ratio with increasing Ni content. Increase in resistivity and weakening of ferromagnetism with Ni substitution at Mn site has been observed due to the reduction in grain size and dilution of double exchange interaction.