Effect of Temperature Variation on the Characteristics of Microwave Power AlGaN/GaN MODFET

The prime motivation for developing the proposed model of AlGaN/GaN microwave power device is to demonstrate its inherent ability to operate at much higher temperature. An investigation of temperature model of a 1 μm gate AlGaN/GaN enhancement mode n-type modulation-doped field effect transistor (MODFET) is presented. An analytical temperature model based on modified charge control equations is developed. The proposed model handles higher voltages and show stable operation at higher temperatures. The investigated temperature range is from 100 °K–600 °K. The critical parameters of the proposed device are the maximum drain current (I_Dmax), the threshold voltage (V_th), the peak dc trans-conductance (g_m), and unity current gain cut-off frequency (f_T). The calculated values of f_T (10–70 GHz) at elevated temperature suggest that the operation of the proposed device has sufficiently high current handling capacity. The temperature effect on saturation current, cutoff frequency, and trans-conductance behavior predict the device behavior at elevated temperatures. The analysis and simulation results on the transport characteristics of the MODFET structure is compared with the previously measured experimental data at room temperature. The calculated critical parameters suggest that the proposed device could survive in extreme environments.     

Electrical and Thermal Transport Properties of n-type Bi6Cu2Se4O6 (2BiCuSeO + 2Bi2O2Se)

New thermoelectric materials, n-type Bi₆Cu₂Se₄O₆ oxyselenides, composed of well-known BiCuSeO and Bi₂O₂Se oxyselenides, are synthesized with a simple solid-state reaction. Electrical transport properties, microstructures, and elastic properties are investigated with an emphasis on thermal transport properties. Similar to Bi₂O₂Se, it is found that the halogen-doped Bi₆Cu₂Se₄O₆ possesses n-type conducting transports, which can be improved via Br/Cl doping. Compared with BiCuSeO and Bi₂O₂Se, an extremely low thermal conductivity can be observed in Bi₆Cu₂Se₄O₆. To reveal the origin of low thermal conductivity, elastic properties, sound velocity, Grüneisen parameter, and Debye temperature are evaluated. Importantly, the calculated phonon mean free path of Bi₆Cu₂Se₄O₆ is comparable to the interlayer distance for BiO─CuSe and BiO─Se layers, which is ascribed to the strong interlayer phonon scattering. Contributing from the outstanding low thermal conductivity and improved electrical transport properties, the maximum ZT ≈0.15 at 823 K and ≈0.11 at 873K are realized in n-type Bi₆Cu₂Se_3.2Br_0.8O₆ and Bi₆Cu₂Se_3.6Cl_0.4O₆, respectively, indicating the promising thermoelectric performance in n-type Bi₆Cu₂Se₄O₆ oxyselenides.     

A low temperature internal friction study of Y0.85Ca0.15Ba2Cu3O7−δ with different oxygen content

The ceramic sample of Y_0.85Ca_0.15Ba₂Cu₃O_7−δ was prepared by standard solid-state reaction method, and samples with different oxygen concentration were obtained by quenching from high temperature. The internal friction was measured using the vibrating reed method from liquid-nitrogen temperature to room temperature at kilohertz frequency. An internal friction peak was observed around 250 K in Y_0.85Ca_0.15Ba₂Cu₃O_7−δ quenched from 1023 K. The peak is related to the one observed around 220 K (labeled as P3 peak) in undoped YBa₂Cu₃O_7−δ (Y123). This result shows the dependence of P3 peak on carriers density and P3 peak has a strong correlation to the abnormal behavior of Y123 in the underdoped range. The variation of two low temperature thermal activated relaxation peaks (P1 and P2) on oxygen content were also investigated. And consistent explanations were given based on all recent researches.     

Synthesis,structural, optical and magnetic properties of Cu-doped ZnO nanorods prepared by a simple direct thermal decomposition route

Cu-doped ZnO nanorods (i.e. Cu = 1.75, 3.55, 5.17 and 6.39 at.%) have been successfully synthesized by simple, direct, thermal decomposition of zinc and copper acetates in air at 300 °C for 6 h. The prepared samples were characterized by X-ray diffraction (XRD) and transmission electron microscopy. The XRD results indicate that the 1.75 at.% Cu-doped ZnO sample has a pure phase with the ZnO wurtzite structure, while the impurity phases are detected with increasing Cu concentrations. It was found that the doping of Cu results in a reduction of the preparation temperature. The optical properties of the samples were also investigated by UV–visible spectroscopy and photoluminescence measurements. The results show that the Cu doping causes the change in energy-band structures and effectively adjusts the intensity of the luminescence properties of ZnO nanorods. X-ray photoelectron spectroscopy analysis implies that there are some oxygen vacancies in the samples and also indicates that all the doped samples are associated with the mixture of Cu ion states (Cu²⁺ and Cu¹⁺/Cu⁰). Magnetic measurements by vibrating sample magnetometry indicate that undoped ZnO is diamagnetic, whereas all of the Cu-doped ZnO samples exhibit room temperature ferromagnetic behavior. We suggest that Cu substitution and density of oxygen vacancies (V _o) may play a major role in the room temperature magnetism of the Cu-doped ZnO samples.     

Sodium dodecyl sulfate-assisted synthesis of CoWO4 nanorods

CoWO₄ nanorods were synthesized at 453 K for 12 h by a hydrothermal technology from Na₂WO₄ · 2H₂O and CoCl₂ · 6H₂O in the presence of sodium dodecyl sulfate (SDS). The as-synthesized CoWO₄ nanorods were characterized by various techniques of X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and X-ray detector. Luminescent properties of the samples were measured at room temperature. The results showed that CoWO₄ products are nanorods with diameters of about 20 nm, and lengths ranging between 100 and 200 nm. CoWO₄ nanorods display a very strong PL peak at 453 nm with the excitation wavelength 300 nm. The possible formation mechanism of CoWO₄ nanorods was suggested.     

Magnetic properties for the Cu2MnSnSe4 and Cu2FeSnSe4 compounds

Magnetic susceptibility χ measurements in the range from 2 to 300 K were carried out on samples of the Cu₂FeSnSe₄ and Cu₂MnSnSe₄ compounds. It was found that Cu₂FeSnSe₄ was antiferromagnetic showing ideal Curie-Weiss behavior with a Néel temperature T_N of about 19 K and Curie-Weiss temperature θ=−200 K, while for Cu₂MnSnSe₄ the behavior was spin-glass with a freezing temperature T_f of about 22 K and Curie-Weiss temperature θ=−25 K. The spin-glass order parameter q(T), determined from the susceptibility data, was found to be in agreement with the prediction of conventional spin-glass theory.     

Structure and magnetic properties of Zn<Subscript>0.9</Subscript>Co<Subscript>0.1</Subscript>O nanorods synthesized by a simple sol–gel method using metal acetylacetonate and poly(vinyl alcohol)

Room-temperature ferromagnetism was observed in Zn_0.9Co_0.1O nanorods with diameters and lengths of ∼100–200 nm and ∼200–1000 nm, respectively. Nanorods were synthesized by a simple sol–gel method using metal acetylacetonate powders of Zn and Co and poly(vinyl alcohol) gel. The XRD, FT-IR and SAED analyses indicated that the nanorods calcined at 873–1073 K have the pure ZnO wurtzite structure without any significant change in the structure affected by Co substitution. Optical absorption measurements showed absorption bands indicating the presence of Co²⁺ in substitution of Zn²⁺. The specific magnetization of the nanorods appeared to increase with a decrease in the lattice constant c of the wurtzite unit cell with the highest value being at 873 K calcination temperature. This magnetic behavior is similar to that of Zn_0.9Co_0.1O nanoparticles prepared by polymerizable precursor method. We suggest that this behavior might be related to hexagonal c-axis being favorable direction of magnetization in Co-doped ZnO and the 873 K (energy of 75 meV) being close to the exciton/donor binding energy of ZnO.     

Muonium states and dynamics in phosphorus and sulphur

Magnetic properties of single crystal Ba₂Cu₃O₄Cl₂ with Cu₃O₄ planes were investigated by temperature‐ and magnetic‐field‐dependent anisotropic magnetization M(T,H) and zero‐field‐μSR. Muon spin precession was observed below as well as above T=32 K where an anomaly in M(T) appears. At this temperature the frequency of the precession changes and the damping of it diverges. This is an evidence for a magnetic transition. We also found the temperature dependence of the H‐linear component of M(H) shows a maximum around T=80 K, which is similar to the behavior of low‐dimensional magnets with an antiferromagnetic interaction. This revised version was published online in July 2006 with corrections to the Cover Date.     

Superconductivity,lattice transformations,and electronic instabilities in CuxMo3S4

The variations of the superconducting transition temperature (T_c) and of the lattice transformation temperature (T_L) with composition are reported for the Chevrel-phase system Cu_xMo₃S₄ (1 ≤ × ≤ 2). A complex low temperature phase diagram was found, containing at least three distinct low temperature phases, each exhibiting its own characteristic T_c (11 K, 6.5 K, 4.5 K), T_L (280 K - 125 K), and electronic transport behavior. Pressure-induced transformations between these three phases were observed at pressures below 25 kbar. Temperature-induced lattice distortion for compositions near Cu₂Mo₃S₄ was found to increase the T_c above that (< 1.1 K) of the high temperature, higher symmetry rhombohedral phase. This is apparently the first time the latter behavior has been observed: in all previous studies of other materials, the opposite behavior was reported.     

Phase transition in Cu2+-doped RbH2 PO4 as observed by EPR

The electron paramagnetic resonance (EPR) spectra of Cu²⁺-doped RbH₂ PO₄ at elevated temperatures indicate a phase transition at 358 K. The EPR-silent state at this temperature is attributed to a so-called polymeric phase transition. After the transition when the temperature is lowered to 293 K, the EPR signal does not appear; therefore, the transition is irreversible. This result seems to be in agreement with the other observations. The EPR spectra for the sample indicate the presence of two sites for Cu²⁺, and the values of EPR parameters are in accord with the literature on Cu²⁺-doped single crystals. Any other phase transitions could not to be observed at low temperatures down to 113 K.     

Morphologically controlled preparation of CuO nanostructures under ultrasound irradiation and their evaluation as pseudocapacitor materials

Various morphologies of copper oxide (CuO) nanostructures have been synthesized by controlling the reaction parameters in a sonochemical assisted method without using any templates or surfactants. The effect of reaction parameters including molar ratio of the reactants, reaction temperature, ultrasound exposure time, and annealing temperature on the composition and morphology of the product(s) has been investigated. The prepared samples have been characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive X-ray (EDAX), and thermogravimetric analysis (TGA). It has been found that Cu₂(OH)₃NO₃ nanoplatelets are achieved in mild conditions which can be then converted to various morphologies of CuO nanostructures by either using high concentrations of OH⁻ (formation of nanorods), prolonging sonication irradiation (nanoparticles), or thermal treatment (nanospheres). Application of the prepared CuO nanostructures was evaluated as supercapacitive material in 1 M Na₂SO₄ solution using cyclic voltammetry (CV) in different potential scan rates ranging from 5 to 100 mV s⁻¹. The specific capacitance has been calculated using CV curves. It has been found that the pseudocapacitor performance of CuO can be tuned via employing morphologically controlled samples. Accordingly, the prolonged sonicated sample (nanoparticles) showed the high specific capacitance of 158 F.g⁻¹.     

Synthesis and characterization of Cu2ZnSnSe4 nanotube arrays on fluorine-doped tin oxide glass substrates

Highly ordered arrays of Cu₂ZnSnSe₄ nanotubes have been successfully synthesized on fluorine-doped tin oxide glass substrate using ZnO nanorod arrays as sacrificial templates. The structure, morphology and optical properties of the Cu₂ZnSnSe₄ arrays were characterized by X-ray diffraction, Raman spectrometry, field-emission scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray, and UV–Vis absorption spectroscopy. The diameter and length of the Cu₂ZnSnSe₄ nanotubes can be adjusted by tuning the diameter and length of the ZnO nanorods. In addition, the effect of the length on the performance of the photoelectrochemical cells was also investigated.     

Photoelectric properties of ZnO: In nanorods/SiO2/Si heterostructure assembled in aqueous solution

In-doped zinc oxide (ZnO:In) nanorods were grown onto SiO₂/n-Si substrate without catalyst in aqueous solution. The ZnO:In nanorods/SiO₂/n-Si heterostructure photovoltaic device was prepared. The structural and photoelectric properties of the as-grown ZnO:In nanorods were analyzed. ZnO:In nanorods had a strong and broad UV surface photovoltage response in the range of 300–400 nm, and the bands were identified. The photoelectric conversion properties of ZnO:In nanorods/SiO₂/n-Si heterostructure were investigated. ZnO:In/SiO₂/n-Si heterostructure showed a wide range photocurrent spectral response with high intensity in the UV and visible region. The rectifying behavior of this heterostructure was observed. Moreover, the device had a low turn-on voltage and a high breakdown voltage. Current–voltage characteristic was studied for the heterostructure, and the open-circuit voltage and short-circuit current were obtained. PACS 73.40.Lq; 85.35.Be; 81.16.Dn     

Fast and selective Cu2O nanorod growth into anodic alumina templates via electrodeposition

The fast and selective growth of cuprous oxide (Cu₂O) nanorods into anodic aluminum oxide (AAO) templates is achieved under optimized alkaline conditions via electrochemical deposition. The growth rate of Cu₂O nanorods at room temperature reached 360 nm/min, the fastest rate reported to date. The synthesis of Cu₂O nanorods by applying a constant current by using Cu₂O nanotubes as a transition state is extensively discussed; a Pt pottery-shaped layer played a key role as a seed layer for the fast Cu₂O growth. We report here the existence of regions of nanostructured Cu₂O based on our studies and previous relevant works, which include potential-pH curves for Cu²⁺-lactate solutions.     

Elastic,impedance spectroscopic and dielectric properties of TiO2 doped nanocrystalline NiCuZn spinel ferrites

ABSTRACT

Nanocrystalline Ni_0.4Cu_0.3Zn_0.3Fe₂O₄ ferrites doped with TiO₂ (0–10?wt %) were prepared by the sol-gel method. Elastic properties of synthesized samples were studied with the help of ultrasonic pulse transmission method. The elastic constants initially increase with an increase in TiO₂ up to 1?wt % and then decline. LCR-Q meter was used to study the dielectric properties within 50?Hz to 5?MHz range of the frequency. The dielectric constant (?′) and dielectric loss tangents were decreased continuously with increased frequency for all the selected samples at room temperature revealing normal dielectric behavior of ferrites. Also, the AC conductivity was increased with an increase in the frequency for all the selected samples. Cole-Cole plots were obtained for all investigated samples and showed single semicircle which indicates that the electrical conduction process appears only due to grain boundaries.     

Ultrasound assisted synthesis of {[Cu2(BDC)2(dabco)].2DMF.2H2O} nanostructures in the presence of modulator; new precursor to prepare nano copper oxides

As a new precursor to prepare nano copper oxide, nanostructures of porous metal organic framework (MOF) {[Cu₂(BDC)₂(dabco)].2DMF.2H₂O} (1) have been synthesized in the presence of acetic acid as a modulator via sonochemical method. Different concentrations of metal ion, organic linkers, modulator reagent and also different sonication times were held to improve the quality of nanostructures. Ultrasound irradiation helps nucleation step of the oriented attachment of modulation method and nanoparticles with a few nanorods has been prepared. As prepared MOF was calcinated at 500 °C to prepare nano CuO and Cu₂O. Compound 1, CuO and Cu₂O nanostructures were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray powder diffraction (XRD).     

Synthesis of magnetite nanorods and porous hematite nanorods

Fe₃O₄ nanorods with average diameters of 40-50 nm and lengths of up to 1 μm were synthesized through hydrolysis of FeCl₃ and FeSO₄ solutions containing urea in the temperature range from 90 to 95 °C in reflux condition for 12 h, following an aging time of 12 h. The porous hematite nanorods were prepared by calcination of the precursor which was obtained from hydrolysis of FeCl₃ and FeSO₄ solutions containing urea at a temperature of 90 °C for 10 h in hydrothermal condition. The formation of the porosity of hematite was due to the decomposition of FeCO₃ and FeOOH. Urea played a key role in the formation of the iron oxide nanorods. Transmission electron microscopy (TEM) images showed that the morphology of magnetite particles is homogeneous in the shape of rods and hematite rods are full of porosity. The values of saturation magnetization (M) and coercivity (H) of magnetite nanorods are 67.55 emu/g and 114 Oe, respectively. The samples were also characterized by X-ray powder diffraction (XRD) and electron diffraction (ED). At last, the forming mechanism of both the magnetite and porous hematite nanorods was discussed.     

Mössbauer study of EuBa2Cu3O6+x material

Mössbauer spectra of a mixed EuBa₂Cu₃O_6+x oxide where x>0.5 were recorded in temperature range of 76–300 K. Between 85–300 K temperature the parameters of the appearing line in the Mössbauer spectrum are consistent with an Eu(III) state. At 76 K, however, the spectrum of the freshly prepared material exhibits two separated lines.     

Room temperature ferromagnetism in Fe doped CuO nanorods

One dimensional CuO and Fe doped CuO nanorods have been synthesized by template free solution phase hydrothermal methods. The typical diameter and the length of the Cu_1−xFe_xO nanorods (x=0, 0.02, 0.05, 0.10) are 20-25 and 300-400 nm. Pure CuO nanorods show weak ferromagnetism and the introduction of Fe within CuO lattice improves significantly the ferromagnetic property with the Curie temperature far above room temperature. The shape anisotropy is the key point to understand ferromagnetism in Fe doped CuO nanorods.     

Synthesis and physical properties of mixed Co3O4/CoO nanorods by microwave hydrothermal technique

A mixture of crystalline Co₃O₄/CoO nanorods with non-uniform dense distribution has been successfully synthesized by microwave hydrothermal technique. The synthesized nanorods have been characterized by several techniques such as X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDX), and Fourier transforms infrared spectroscopy (FT-IR). The results showed that the as synthesized specimens contained mixed crystalline Co₃O₄/CoO nanorods with an average length of around 80 nm and an average diameter of 42 nm. UV–Vis spectrum of the nanorods exhibited a strong UV emission. The band energy gap of the product was 1.79 eV which lies between the energy gap of CoO and that for Co₃O₄. The obtained carrier concentration is of the order 4.32 × 10²⁷ m⁻³ and the dielectric constant is found to be 4.89. The electrical conductivity increases with increasing temperature and behaves as a semiconducting material with an activation energy of a bout 0.26 eV. This makes the as synthesized mixed Co₃O₄/CoO nanorods very useful for supercapacitor devices application. Magnetic hysteresis loops at room temperature of the as synthesized mixed oxides (Co₃O₄/CoO) nanorods exhibit typical soft magnetic behavior.