Characterization of thermal diffusivity of micro/nanoscale wires by transient photo-electro-thermal technique

In this work, a transient photon-electro-thermal (TPET) technique based on step laser heating and electrical thermal sensing is developed to characterize the thermophysical properties of one-dimensional micro/nanoscale conductive and nonconductive wires. In this method, the to-be-measured thin wire/tube is suspended over two electrodes and is irradiated with a step cw laser beam. The laser beam induces a transient temperature rise in the wire/tube, which will lead to a transient change of its electrical resistance. A dc current is applied to the sample, and the resulting transient voltage variation over the wire is measured and used to extract the thermophysical properties of the sample. A 25.4-μm thick Pt wire is used as the reference sample to verify this technique. Sound agreement is obtained between the measured thermal diffusivity and the reference value. Applying the TPET technique, one can measure the thermal diffusivity of conductive single-wall carbon nanotube (SWCNT) bundles and nonconductive cloth fibers. For nonconductive wires, a thin (∼ nm) metallic film is coated on the outside of the wire for electrical thermal sensing. The measured thermal diffusivity for the SWCNT bundle is 2.53×10^-5 m²/s, much less than the thermal diffusivity of graphite in the layer direction. For microscale cloth fibers, our experiment shows its thermal diffusivity is at the level of 10^-7 m²/s. PACS 78.20.Nv; 42.62.-b; 65.80+n; 66.30.Xj     

Thermal transport in multiwall carbon nanotube buckypapers

A steady-state electro-Raman-thermal (SERT) technique is developed to characterize the thermal transport in multiwall carbon nanotube (MWCNT) buckypapers. This SERT technique involves steady-state joule heating of a suspended sample and measuring its middle point temperature based on Raman shift intensity. The thermal conductivity is determined from linear fitting of the temperature against heating power. Combined with the transient-electro-thermal technique, the thermophysical properties of two MWCNT buckypapers are characterized as 1.19 and 2.92 W/(m K) for thermal conductivity, 3.65×¹⁰−6 and 7.58×¹⁰−6 m²/s for thermal diffusivity, 459 and 543 kg/m³ for density. Detailed discussion and analysis are provided about the uncertainty of the SERT technique and its capacity for measuring wires down to sub-μm length. The low thermal conductivity of the buckypaper indicates its thermal transport is determined by the thermal contact resistance between MWCNTs. These contact points feature low thermal conductance. The mean distance between two adjacent contact points is estimated in the order 45-450 μm and 93-933 μm for the two samples, indicating low-density contacts within the buckypaper.     

Transient thermal characterization of micro/submicroscale polyacrylonitrile wires

In this work, the thermal diffusivity of single polyacrylonitrile (PAN) wires with diameters from 4.62 μm down to 324 nm is measured by using our recently developed transient electro-thermal technique. The wires span from 23 μm to 126.2 μm in our measurement. Since PAN wires are dielectric, a thin Au film is coated on the surface of the wires to make them conductive. In the experiment, a step current (with ∼2 μs rising time) is fed to the sample. The sample is heated and takes a certain time to reach its steady thermal state. The temperature rising response of the sample is sensed by measuring the resistance change of the thin Au coating. From the average temperature evolution of the sample, the thermal diffusivity can be extracted. Three PAN wires with different diameters are synthesized using the electro-spinning technique and are measured to obtain their thermal diffusivities (around 1.53×10^-7 m²/s), which are slightly smaller than the bulk value. PACS 65.80.+n; 66.30.Xj; 44.10.+i     

X-ray observations of tungsten wire array Z-pinch implosions on QiangGuang-1 facility

Z-pinch experiments with two arrays consisting, respectively, of 32 4-μm- and 6-μm-diameter tungsten wires have been carried out on QiangGuang-1 facility with a current rising up to 1.5MA in 80ns. At early time of implosion, x-ray framing images show that the initial emission comes from the central part of arrays, and double clear emission rings, drifting to the anode and the cathode at 5×10⁶cm/s and 2.4×10⁷cm/s respectively, are often produced near the electrodes. Later, in a 4-μm-diameter tungsten wire array, filamentation caused by ohmic heating is prominent, and more than ten filaments have been observed. A radial inward shift of arrays starts at about 30\,ns earlier than the occurrence of the x-ray peak power for both kinds of arrays, and the shrinkage rate of emission region is as high as 1.7×10⁷cm/s in a 4-μm-diameter tungsten wire array, which is two times higher than that in a 6-μm one. Emission from precursor plasmas is observed in implosion of 6-μm-diameter tungsten wire arrays, but not in implosion of a 4-μm-diameter tungsten wire array. Whereas, in a 4-μm-diameter tungsten wire array, the soft x-ray emission shows the growth of m=1 instability in the plasma column, which is caused by current. The reasons for the discrepancy between implosions of 4-μm- and 6-μm-diameter tungsten wire arrays are explained.     

Development of exploding wire ion source for intense pulsed heavy ion beam accelerator

A Novel exploding wire type ion source device is proposed as a metallic ion source of intense pulsed heavy ion beam (PHIB) accelerator. In the device, multiple shot operations are realized without breaking the vacuum. The basic characteristics of the device are evaluated experimentally with an aluminum wire of diameter 0.2 mm and length 25 mm. A capacitor bank of capacitance 3 μF and a charging voltage of 30 kV was used, and the wire was successfully exploded by a discharge current of 15 kA with a rise time of 5.3 μs. Plasma flux of ion current density around 70 A/cm² was obtained at 150 mm downstream from the device. The drift velocity of ions evaluated by a time-of-flight method was 2.7×10⁴ m/ s, which corresponds to the kinetic energy of 100 eV for aluminum ions. From the measurement of the ion current density distribution, the ion flow is found to be concentrated toward the direction where the ion acceleration gap is placed. From the experiment, the device is found to be acceptable for applying the PHIB accelerator.     

Radiation characteristics and implosion dynamics of tungsten wire array Z-pinches on the YANG accelerator

We investigated the radiation characteristics and implosion dynamics of low-wire-number cylindrical tungsten wire array Z-pinches on the YANG accelerator with a peak current 0.8-1.1 MA and a rising time ～ 90 ns.The arrays are made up of(8-32)×5 μm wires 6/10 mm in diameter and 15 mm in height.The highest X-ray power obtained in the experiments was about 0.37 TW with the total radiation energy ～ 13 kJ and the energy conversion efficiency ～ 9%(24×5 μm wires,6 mm in diameter).Most of the X-ray emissions from tungsten Z-pinch plasmas were distributed in the spectral band of 100-600 eV,peaked at 250 and 375 eV.The dominant wavelengths of the wire ablation and the magneto-Rayleigh-Taylor instability were found and analyzed through measuring the time-gated self-emission and laser interferometric images.Through analyzing the implosion trajectories obtained by an optical streak camera,the run-in velocities of the Z-pinch plasmas at the end of the implosion phase were determined to be about(1.3-2.1)×10 7 cm/s.     

Design and performance of a 2-D multi-wire position sensitive X-ray detector

A 2-D multi-wire position sensitive detector for X-ray diffraction and small angle X-ray scattering studies is described. The detector has an active area of 100 mm × 100 mm and consists of an anode plane with 10 μm SS wires at 3 mm spacing and a pair of orthogonal cathode readout planes with 25 μm SS wires placed at 1.5 mm spacing. The position information is obtained using charge division method and recorded using a laboratory built data acquisition system. The resolution and gas gain was measured for 5.9 keV X-rays (⁵⁵Fe-source) as a function of the anode wire voltage and gas pressure. It was observed that the proportional region of the PSD at 100 kPa pressure extended up to a high voltage value of around 1.5 kV and it shifted to high values up to 2 kV for gas pressure of 300 kPa. The energy resolution improved from 18% (FWHM) to 12% with increase in pressure. The spatial resolution of the PSD also showed improvement, with a value of 1.2 mm × 1.4 mm at 300 kPa gas pressure. A maximum gain of 5 × 10⁴ is obtained.     

Thermal and optical properties of femtosecond-laser-structured PMMA

Thermal diffusivity of laser micro- and nano-structured regions in polymethylmethacrylate (PMMA) was measured by the temperature wave method with a lateral resolution reduced to ～10 μm using an array of micro-sensors. The volume fraction of laser modified phase was maximized by implementing tightly focused femtosecond laser pulses inside PMMA and maintaining distance of few micrometers between the irradiation spots. The absolute value of thermal diffusivity of PMMA 1.066±0.08×10^?7 m²/s was reliably determined with the miniaturized sensors. Regions laser structured by single pulses had no trace of carbonization, almost the same thermal diffusivity as the host PMMA, and a stress-induced birefringence Δn～10^?4 modulated with period ～2 μm.     

Heat release in the cryogenic system of a superconducting integrated detector and the influence of heat on its operation

Heat release in the cryogenic system of a subterahertz-range superconducting integrated detector at ≈4.2 K is studied, and the influence of the released heat on its main characteristics is estimated. The detector chip mounted on a silicon lens is connected to a bias board by aluminum wires 25 μm in diameter, which are fixed by ultrasonic bonding. They are necessary for setting a bias current through the working components of the detector and represent an integral part of the system. The contact resistance between the wires and contact pads of the microchip is measured. The contact resistance is found to considerably exceed the resistance of the aluminum wire and, hence, makes a major contribution to heat release in the system. A “multipoint contact with one wire” technique is suggested. Tests show its efficiency: the contact resistance decreases considerably compared with the standard approach.     

Numerical simulation of low loss silicon photonic wire waveguide with multiple cladding layers

A different silicon photonic wire waveguide is proposed, which uses multiple thin cladding layers in order to reduce the index contrast between core and cladding interface. The reduced index contrast in the proposed waveguide has led to reduction in the scattering losses by 37% as compared to silicon wire waveguide for 400 nm × 220 nm waveguide dimension. The proposed waveguide has shown significant reduction in bending losses. It offers the bending loss of 0.0118 dB at the radius of 1 μm and 0.0063 dB for a radius of 2 μm at 1.55 μm wavelength as compared to 0.086 and 0.013 dB at the radius of 1 and 2 μm, respectively, offered by silicon photonic wire waveguide at 1.5 μm wavelength. The use of polymer material as top cladding layer resulted in decreasing the sensitivity of effective index against temperature for the designed waveguide by a factor of 2 as compared to silicon wire waveguide.     

Thermal Conductivity Measurement of Gases by the Transient Short-Hot-Wire Method

Measurements of the thermal conductivity of helium and hydrogen are performed using the transient short-hot-wire method. The short hot wire is made of platinum and has a diameter of about 10 μm and a length of about 15 mm. It is attached by spot welding to platinum terminals with a diameter of 1.5 mm. The probe is inserted into the sample vessel that has a volume of 35 cm³ and an inner diameter of 30 mm. The thermal conductivity is evaluated by comparing a numerical solution of the heat conduction in and around the short wire with the experimentally obtained temperature rise of the wire. The measured thermal conductivities show good reproducibility. Also, the measured thermal conductivities agree with the reference equations within a deviation of ± 1%.     

Cavity-enhanced optical feedback-assisted photo-acoustic spectroscopy with a 10.4 μm external cavity quantum cascade laser

An ultra-sensitive photo-acoustic spectrometer using a 10.4 μm broadly tunable mid-IR external cavity quantum cascade laser (EC-QCL) coupled with optical feedback to an optical power buildup cavity with high reflectivity mirrors was developed and tested. A laser optical power buildup factor of 181 was achieved, which corresponds to an intra-cavity power of 9.6 W at a wavelength of 10.4 μm. With a photo-acoustic resonance cell placed inside the cavity this resulted in the noise-equivalent absorption coefficient of 1.9 × 10^?10 cm^?1 Hz^?1/2, and a normalized noise-equivalent absorption of 1.1 × 10^?11 cm^?1 W Hz^?1/2. A novel photo-acoustic signal normalization technique makes the photo-acoustic spectrometer’s response immune to changes and drifts in the EC-QCL excitation power, EC-QCL to cavity coupling efficiency and cavity mirrors aging and contamination. An automatic lock of the EC-QCL to the cavity and optical feedback phase optimization permitted long wavelength scans within the entire EC-QCL spectral tuning range.     

Nonlinear refractive index of multicomponent glasses designed for fabrication of photonic crystal fibers

The second order nonlinear refractive index n ₂ of various multicomponent glasses was measured at the wavelength of 1240 nm close to the 1.3-μm fiber transmission window. With the refractive index covering the range from 1.45 to 2.3, a comparatively broad range of n ₂ with values from 1.1×10^?20 m²/W for boro-silicate based glass NC21 to 4.3×10^?19 m²/W for lead–bismuth-gallate based glass PBG08 was measured using the Z-scan method. Considering the broad infrared transmission range of multicomponent glasses, these materials pose a great potential for future applications as photonic crystal fiber sources of infrared supercontinuum.     

Investigation of black phosphorus field-effect transistors and its stability

In this paper, the electrical properties of black phosphorus(BP) are investigated. Back-gated field-effect transistors (FETs) array with different channel length are fabricated on the same BP nanoflake. The device exhibits high current on/off ratio (5 × 10³), high field-effect mobility (130 cm² V^?1 s^?1) and low contact resistance (917 Ω μm). In addition, the stability of BP device is also explored. Results show that the 10 nm Al₂O₃ dielectric layer can effectively depress the exposure of BP flakes with air and then protect the BP devices from ambient degradation. There is no noticeable degradation in device performance for the devices with 10 nm Al₂O₃ passivating layer even after being exposed in air for 2 weeks.     

Enhanced 2.0 μm emission in Ho3+/Yb3+ co-doped silica-germanate glass

A detailed investigation on thermal and spectroscopic properties of different Ho³⁺/Yb³⁺ concentration ratios in silica-germanate glasses is displayed. According to the measurement of thermal properties, the host glass possesses high transition temperature (585 °C) as well as the large ΔT(155 °C). The 2.0 μm fluorescence can be obtained from all the samples. Maximum stimulated emission cross-section of around 2.0 μm is 0.56 × 10⁻²⁰ cm² of Ho³⁺ as calculated by McCumber theory. Besides, the underlying mechanism is analyzed by means of fluorescence spectra. Thus, desirable thermal properties and spectroscopic characteristics of Ho³⁺/Yb³⁺ co-doped silica-germanate glass is a promising material in 2.0 μm emission.     

Effects of ambient pressure on Cu Kα X-ray radiation with millijoule and high-repetition-rate femtosecond laser

Hard Cu Kα X-ray radiation was generated with a millijoule and high-repetition-rate Ti: sapphire laser in air, helium or vacuum (2.7–1.3×10⁴ Pa) ambient. The characteristic X-ray was obtained by focusing the 0.06–1.46 mJ/pulse, 100 fs, 1 kHz repetition femtosecond laser onto a solid copper target to a spot 4.8 μm in diameter. The relationship between Kα X-ray conversion efficiency and atmospheric conditions was explained with a simple electron collision model that suggested that the electron mean free path is an important parameter in the generation of ultrafast pulsed X-rays in any ambient condition. We also demonstrated a high-intensity X-ray source working in helium at atmospheric pressure.     

Thermal resistance of CNTs-based thermal interface material for high power solid state device packages

Vertically aligned carbon nanotubes (VACNTs) were synthesized over copper substrate. The diameter and length of the CNTs were 100 nm and 2–3 μm, respectively. Synthesis of CNTs was confirmed by Raman spectrum and verified by TEM as multi walled CNTs. SEM images showed the vertically aligned CNTs over Cu substrate. Strengthening of CNTs was performed by filling with Cu and SU-8 epoxy sealant in gap between the CNTs. The observed density was high for epoxy sealed CNTs. The bending ability of CNTs was checked and observed as low for epoxy sealed CNTs. The thermal resistance of the samples was measured by JESD51-2 standard for various loads. The observed resistance was low (0.277 cm²?K/W) for epoxy sealed CNTs at 1100 kPa. The calculated resistance of CNTs alone was 0.097 cm²?K/W for epoxy sealed at 900 kPa.     

Preparation and comparative structural properties of porous SnO2 microrods and submicrorods

A facile two-step approach has been used for the synthesis of porous SnO₂ rods: the initial room-temperature precipitation of precursor SnC₂O₄ and its subsequent thermal decomposition at 550 °C. Both the as-obtained porous SnO₂ microrods (length ～10.0?±?3.5 μm, diameter ～1.1?±?0.4 μm) and submicrorods (length ～5.8?±?1.9 μm, diameter ～0.4?±?0.1 μm) are the crystalline mixtures of major tetragonal and minor orthorhombic crystal phases, showing a tetragonal fraction of 84.7 and 87.0 %, respectively. When applied as a lithium-ion battery anode, the porous submicrorods (specific surface area ～13.6 m² g^?1) can deliver an initial discharge capacity of 1,730.7 mAh g^?1 with a high coulombic efficiency of 61.6 % and show the 50th discharge capacity of 662.8 mAh g^?1 at 160 mA g^?1 within a narrow potential range of 10.0 mV to 2.0 V. Similarly, even the anode of porous microrods (specific surface area ～11.8 m² g^?1) can still exhibit an initial discharge capacity of 1,661.1 mAh g^?1 at 160 mA g^?1 with a coulombic efficiency of 60.9 %. Regardless of the polymorphic nature, the acquired porosity may only alleviate the huge volume change of anodes for the first cycle; thus, the structural parameters of average size and specific surface area can be feasibly associated with the enhanced lithium storage capability. Anyway, these indicate a facile oxalate precursor method for the controlling synthesis and high performance of rodlike SnO₂ for lithium-ion batteries.     

Ultrasonic preparation of tetraoxalyl ethylenediamine melamine resin-coated multiwalled carbon nanotubes for trace Ni(II) sensing

Under an aid of ultrasonic, tetraoxalyl ethylenediamine melamine resin-coated multiwalled carbon nanotubes were prepared for Ni(II) sensing in aqueous solution. The processes involved the fabrication of tetraoxalyl ethylenediamine melamine resin by one pot way, the coating of tetraoxalyl ethylenediamine melamine resin at multiwalled carbon nanotubes (MWCNTs), and the determination of Ni(II). The present materials were carefully examined by Fourier transform infrared spectroscopy, field emission scanning electron microscope, and electrochemistry techniques. A great deal of amorphous microsphere could be observed for tetraoxalyl ethylenediamine melamine resin with an average diameter of 1.2 μm, and MTE could evenly adhere at the surface of MWCNTs by the ultrasonic. Tetraoxalyl ethylenediamine melamine resin-coated multiwalled carbon nanotube-modified paraffin-impregnated graphite electrode was successfully used for the determination of Ni(II) by differential pulse adsorptive anodic stripping voltammetry. The current responses (?0.3 V) were linearly increased depending on the concentration from 1?×?10^?11 to 3?×?10^?10 M (i (μA)?=?11.1?+?7.9 c (1?×?10^?12 M); R?=?0.9901, 3σ?=?7?×?10^?12 M).