Noninvasive monitoring of brain temperature during mild hypothermia

The main purpose of this study was to verify the feasibility of brain temperature mapping with high-spatial- and reduced-spectral-resolution magnetic resonance spectroscopic imaging (MRSI). A secondary goal was to determine the temperature coefficient of water chemical shift in the brain with and without internal spectral reference. The accuracy of the proposed MRSI method was verified using a water and vegetable oil phantom. Selective decrease of the brain temperature of pigs was induced by intranasal cooling. Temperature reductions between 2°C and 4°C were achieved within 20 min. The relative changes in temperature during the cooling process were monitored using MRSI. The reference temperature was measured with MR-compatible fiber-optic probes. Single-voxel ¹H MRS was used for measurement of absolute brain temperature at baseline and at the end of cooling. The temperature coefficient of the water chemical shift of brain tissue measured by MRSI without internal reference was −0.0192±0.0019 ppm/°C. The temperature coefficients of the water chemical shift relative to N-acetylaspartate, choline-containing compounds and creatine were −0.0096±0.0009, −0.0083±0.0007 and −0.0091±0.0011 ppm/°C, respectively. The results of this study indicate that MRSI with high spatial and reduced spectral resolutions is a reliable tool for monitoring long-term temperature changes in the brain.     

The spectral sensitivity of long period gratings fabricated in elliptical core D-shaped optical fibre

Long period gratings (LPGs) were written into a D-shaped optical fibre that has an elliptical core with a W-shaped refractive index profile and the first detailed investigation of such LPGs is presented. The LPGs’ attenuation bands were found to be sensitive to the polarisation of the interrogating light with a spectral separation of about 15 nm between the two orthogonal polarisation states. A finite element method was successfully used to model many of the behavioural features of the LPGs. In addition, two spectrally overlapping attenuation bands corresponding to orthogonal polarisation states were observed; modelling successfully reproduced this spectral feature. The spectral sensitivity of both orthogonal states was experimentally measured with respect to temperature and bending. These LPG devices produced blue and red wavelength shifts depending upon the orientation of the bend with measured maximum sensitivities of −3.56 and +6.51 nm m, suggesting that this type of fibre LPG may be useful as a shape/bend orientation sensor with reduced errors associated with polarisation dependence. The use of neighbouring bands to discriminate between temperature and bending was also demonstrated, leading to an overall curvature error of ±0.14 m⁻¹ and an overall temperature error of ±0.3 °C with a maximum polarisation dependence error of ±8 × 10⁻² m⁻¹ for curvature and ±5 × 10⁻² °C for temperature.     

Optical decoherence times and spectral diffusion in an Er-doped optical fiber measured by two-pulse echoes, stimulated photon echoes, and spectral hole burning

Two-pulse and stimulated photon echoes and spectral hole burning were measured on the transition from the lowest component of the ⁴I_15/2 manifold to the lowest component of ⁴I_13/2 of Er³⁺ in a silicate optical fiber at 1.6 K. The two-pulse echo decays gave decoherence times as long as 230 ns for magnetic fields above 2 T. A large field dependent contribution to the homogeneous line width of >2 MHz was found and interpreted in terms of coupling to magnetic tunneling modes (TLS) in the glass. The stimulated echoes measured at 2 T showed spectral diffusion of 0.8 MHz/decade of time between 0.4 and 500 μs. Spectral diffusion in this high field region is attributed to coupling to elastic TLS modes which have a distribution of flip rates in glasses. Time-resolved spectral hole burning at very low field showed stronger spectral diffusion of 5.7 MHz/decade of time, attributed to coupling to magnetic spin-elastic TLS modes.     

Fiber optic Fabry-Perot pressure sensor with low sensitivity to temperature changes for downhole application

Pressure and temperature are two important parameters in reservoir engineering. The fiber optic sensors can be used for permanent downhole monitoring. In this paper, we propose an extrinsic fiber Fabry-Perot interferometer (EFPI) sensor for pressure measurement with low sensitivity variation. The pressure sensitivity of EFPI sensor and of the fiber Bragg grating (FBG) sensors have been measured. The experimental pressure sensitivity for EFPI and FBG sensors are measured to be 2.75 × 10⁻⁸ 1/kPa and 1.52 × 10⁻⁸ 1/kPa, respectively. The temperature cross-sensitivity problem of the EFPI sensor has been solved by a new technique. The temperature sensitivity of EFPI sensor has been decreased to 1.2 × 10⁻⁶/°C, while the temperature sensitivity of non-compensated EFPI sensor has been measured to be 16.4 × 10⁻⁶/°C. The results show that the EFPI sensor has a higher pressure sensitivity and good capability to decrease temperature sensitivity in comparison to FBG sensor.     

Characterization of phosphorus-doped homoepitaxial (1 0 0) diamond films grown using high-power-density MWPCVD method with a conventional quartz-tube chamber

Phosphorus-doped n-type homoepitaxial diamond films have been successfully grown at high substrate temperatures (>1000 °C) on high-pressure/high-temperature-synthesized type-Ib single-crystalline diamond (1 0 0) substrates, by using a conventional microwave plasma chemical-vapor-deposition (CVD) system with high power densities. The deposition system employed in this work had an easily exchangeable 36 mm inner-diameter quartz-tube growth chamber. The homoepitaxial diamond films thus grown were characterized by means of Hall-effect measurements with an AC magnetic field, atomic force microscope observations and secondary ion mass spectrometry techniques. The dependences of the substrate temperature (≤1300 °C) and the P/C ratio in the source gas (≤9900 ppm) on the specimen features were investigated. The optimum substrate temperature deduced was ≈1160 °C, which was also applicable to the CVD growth of undoped homoepitaxial diamond layers. The n-type conductions with an activation energy ≈0.6 eV were observed for the specimens with amounts of the P atoms incorporated to ≈1.5 × 10¹⁸ cm⁻³ whereas the doping efficiencies changed from ≈0.06% to ≈0.92% with the growth condition. Possible origins for these results are discussed in relation to the growth mechanism.     

Anisotropic nonlinear optical absorption of gold nanorods in a silica matrix

Nanocomposite films consisting of gold nanospheres or gold nanorods embedded in a silica matrix have been prepared using a hybrid deposition technique consisting of plasma-enhanced chemical vapor deposition of SiO₂ and co-sputtering of gold, followed by annealing at 900 °C. Subsequent irradiation with 30 MeV heavy ions (Cu⁵⁺) was used to form gold nanorods. Linear and nonlinear optical properties of this material are closely related with the surface plasmon resonance in the visible. The nonlinear absorption coefficient (α₂@532 nm) for the films containing gold nanospheres was measured by Z-scan and P-scan techniques, and it was found to be isotropic and equal to −4.8 × 10⁻² cm/W. On the contrary, gold nanorods films exhibited two distinct surface plasmon resonance absorption bands giving rise to a strong anisotropic behavior, namely a polarization-dependent linear absorption and saturable absorption. Z-scan and P-scan measurements using various light polarization directions yielded nonlinear absorption coefficient (α₂@532 nm) values varying from −0.9 × 10⁻² cm/W up to −3.0 × 10⁻² cm/W. Linearity of the P-scan method in the context of nanocomposite saturable absorption is also discussed.     

Observation of stable mixed alkanethiolate-chloride adlayer on Ag(1 1 1): structural correlation with pure alkanethiolate monolayers

The self-assembly of 1-alkanethiols, CH₃(CH₂)_n−1SH (n=2-16), on Ag(1 1 1) film initially covered with a native oxide monolayer and that on a chloride-covered Ag(1 1 1) were comparatively studied by X-ray photoelectron spectroscopy and scanning tunneling microscopy. The native oxide monolayer was readily substituted completely by thiolates irrespective of chain length, leading to a dense (√7×√7)R19.1° monolayer for n<3, or a distorted (√7×√7)R19.1° structure for n>3 accompanied by distinct island and fine domain structures previously reported by other groups. In contrast, the chloride-to-thiolate conversion was far from complete for long alkanethiols (n?8), and at sufficiently high conversion temperature (>50 °C), we found a highly stable mixed thiolate-chloride monolayer with a well-defined 2:1 S/Cl atomic ratio, suggesting the occurrence of a mixed (√7×√7)R19.1° adlayer ordering. The combined effects of substrate-molecule and intermolecular interactions behind these contrastive phenomena are discussed.     

A novel polyol method to synthesize colloidal silver nanoparticles by ultrasonic irradiation

A polyol synthesis of silver nanoparticles in the presence of ultrasonic irradiation was compared with other configurations (at ambient temperature, 120 °C, and 120 °C with injected solutions) in the absence of ultrasonic irradiation in order to obtain systematic results for morphology and size distribution. For applying ultrasonic irradiation, rather fine and uniform spherical silver particles (21 ± 3.7 nm) were obtained in a simple (at ambient temperature without mechanical stirring) and fast (within 4 min, 3.61 × 10⁻³ mol min⁻¹) manner than other cases (at ambient temperature (for 8 h, 0.03 × 10⁻³ mol min⁻¹): 86 ± 16.8 nm, 120 °C (for 12 min, 1.16 × 10⁻³ mol min⁻¹): 64 ± 14.9 nm, and 120 °C with injected solutions (during 12 min): 35 ± 6.8 nm; all other cases contained anisotropic shaped particles). Even though the temperature of polyol reaction reached only at 80 °C (<120 °C) in the presence of ultrasonic irradiation, a uniform mixing (i.e. enhanced collision between silver particle and surrounding components) by ultrasonic irradiation might induce a better formation kinetics and morphological uniformity.     

Magneto-optical imaging using a garnet indicator film prepared on glass substrates

We observed the magnetic domains of a magnetic card by using the bismuth-substituted yttrium iron garnet (Bi-YIG; Bi₁Y₂Fe₅O₁₂) thin films as the indicator films. The magnetic domains’ dependence on the preparation conditions of the garnet thin film crystals was visualized by using a magneto-optical microscope. Garnet thin films were fabricated on glass substrates using a metal-organic decomposition method (MOD). We found that bigger Faraday rotation was measured in the better crystallized indicator films. Polycrystalline Bi-YIG thin films were successfully obtained for the annealing temperature of 700 °C and an annealing time of 1 h. The thickness of the film was about 47 nm for a single coating during the MOD process. The Faraday-rotation angle of the films was estimated as −2.47°/μm and the angle was comparable to other synthesis methods, such as the sol-gel and the RF-magnetron sputtering. Using these indicator films, we could image the magnetic domains of magnetic materials.     

Densification, microstructural evolution, and dielectric properties of CaTiO3-LaGaO3 microwave ceramics

The relations among the densification, microstructural evolution, and microwave dielectric properties of the (1−x)CaTiO₃-xLaGaO₃ ceramics with x=0.34 and 0.36 were investigated in this study. The results indicated that (1−x)CaTiO₃−xLaGaO₃ ceramics can be densified at 1300 °C with at least 97% of the theoretical value. The ceramics reported an orthorhombic perovskite structure, and no other detectable phases were found. Both ε_r and Q×f values can be improved by slowing the cooling rate during sintering. The ε_r and Q×f values of the 0.64CaTiO₃-0.36LaGaO₃ ceramics at cooling rates of >10 °C/min and 0.1 °C/min are 48.1 and 27,500 and 48.7 and 38,000, respectively. The higher densification obtained at a slower cooling rate plays an important role in improving the microwave dielectric properties.     

Evolution of magnetic phase at low aging temperature in a heavily cold-drawn stainless steel fiber

The evolution of the magnetic phase upon aging at 300–520 °C in a heavily cold-drawn AISI 316L austenitic stainless steel fiber was studied using thermomagnetic analysis (TMA) and magnetic force microscopy with a heating stage. An increasing trend of magnetization from 50 °C to around 470 °C in the heating curves of TMA in austenitic stainless steels after a cold-drawing process was observed. No significant M_s temperature signal in the TMA curve at cooling indicated an increase in magnetization upon cooling period without significant phase transformation. A series of in situ magnetic force microscopy observations reveal a growth of the magnetic domain structure after aging at 300 °C for 2.5 h. Results show that the ferromagnetic increase during aging at lower annealing temperature resulted from the growth of martensite.     

Using ultrasound CBE imaging without echo shift compensation for temperature estimation

Clinical trials have demonstrated that hyperthermia improves cancer treatments. Previous studies developed ultrasound temperature imaging methods, based on the changes in backscattered energy (CBE), to monitor temperature variations during hyperthermia. Echo shift, induced by increasing temperature, contaminates the CBE image, and its tracking and compensation should normally ensure that estimations of CBE at each pixel are correct. To obtain a simplified algorithm that would allow real-time computation of CBE images, this study evaluated the usefulness of CBE imaging without echo shift compensation in detecting distributions in temperature. Experiments on phantoms, using different scatterer concentrations, and porcine livers were conducted to acquire raw backscattered data at temperatures ranging from 37 °C to 45 °C. Tissue samples of pork tenderloin were ablated in vitro by microwave irradiation to evaluate the feasibility of using the CBE image without compensation to monitor tissue ablation. CBE image construction was based on a ratio map obtained from the envelope image divided by the reference envelope image at 37 °C. The experimental results demonstrated that the CBE image obtained without echo shift compensation has the ability to estimate temperature variations induced during uniform heating or tissue ablation. The magnitude of the CBE as a function of temperature obtained without compensation is stronger than that with compensation, implying that the CBE image without compensation has a better sensitivity to detect temperature. These findings suggest that echo shift tracking and compensation may be unnecessary in practice, thus simplifying the algorithm required to implement real-time CBE imaging.     

A 3D trajectory for undersampling k-space in MRSI applications

Magnetic resonance spectroscopic imaging (MRSI) is a noninvasive technique for producing spatially localized spectra. MRSI presents the important challenge of reducing the scan time while maintaining the spatial resolution. The preferred approach for this is to use time-varying readout gradients to collect the spatial and chemical-shift information. Fast, three-dimensional (3D) spatial encoded methods also reduce the scan time. Despite the existence of several new and faster 3D encoded methods, or k-space trajectories, for magnetic resonance imaging (MRI), only stack of spirals and echo planar have been studied in 3D MRSI. A novel formulation for designing fast, 3D k-space trajectory applicable to 3D MRSI is presented. This approach is simple and consists of rays expanding from the origin of k-space into a revolving sphere, collecting spectral data of all 3D spatial k-space at different times in the same scan. This article describes this new method and presents some results of its application to 3D MRSI. This technique allows some degree of undersampling; hence, it is possible to reconstruct high-quality undersampled spectroscopic imaging in order to recognize different compounds in short scan times. Additionally, the method is tested in regular 3D MRI. This proposed method can also be used for dynamic undersampled imaging.     

ZrB2-based Ohmic contacts to p-GaN

The annealing temperature dependence of contact resistance and layer stability of ZrB₂/Ti/Au and Ni/Au/ZrB₂/Ti/Au Ohmic contacts on p-GaN is reported. The as-deposited contacts are rectifying and transition to Ohmic behavior for annealing at ≥750 °C, a significant improvement in thermal stability compared to the conventional Ni/Au Ohmic contact on p-GaN, which is stable only to <600 °C. A minimum specific contact resistance of ∼2 × 10⁻³ Ω cm⁻² was obtained for the ZrB₂/Ti/Au after annealing at 800 °C while for Ni/Au/ZrB₂/Ti/Au the minimum value was 10⁻⁴ Ω cm⁻² at 900 °C. Auger Electron Spectroscopy profiling showed significant Ti, Ni and Zr out diffusion at 750 °C in the Ni/Au/ZrB₂/Ti/Au while the Ti and Zr intermix at 900 °C in the ZrB₂/Ti/Au. These boride-based contacts show promise for contacts to p-GaN in high temperature applications.     

Sintering temperature dependence of room temperature magnetic and dielectric properties of Co0.5Zn0.5Fe2O4 prepared using mechanically alloyed nanoparticles

Co_0.5Zn_0.5Fe₂O₄ nanoparticles were prepared using mechanical alloying (MA) and sintering. The crystallite size, coercivity, retentivity and saturation magnetization were also measured. The frequency dependence of dielectric and the magnetic parameters, namely, real permittivity ε′, loss tanget tan δ, real permeability μ′ and loss factor μ″ were measured at room temperature for samples sintered from 600 to 1000 °C, in the frequency range 10 MHz to 1.0 GHz. The results show that the crystallite size of the resulting products ranges between 16 and 67 nm for as-milled sample and the sample sintered at 1000 °C, respectively. The sample sintered at 1000 °C, measured at room temperature exhibited a saturation magnetization of 37 emu g⁻¹. The values of permittivity remain constant within the measured frequency, but vary with sintering temperature. The permeability values, on the other hand however vary with both the sintering temperature and the frequency, thus, the absolute value of the permeability decreased after the natural resonance frequency.     

A temperature sensor based on optical microfiber knot resonator

A type of compact temperature sensor based on microfiber knot resonator is proposed and demonstrated experimentally. The microfiber knot, which is assembled by two fiber probes, is placed on a plate glass substrate and coated with low-index polymer to keep the system robust. Sensitivities of this kind of temperature sensor as 0.27 nm/°C in heating process (when temperature ranges from 28 to 140 °C) and −0.28 nm/°C in cooling process (when temperature ranges from 135 to 25 °C) are obtained. Temperature resolution of 0.5 °C is demonstrated and higher resolution is predicted with a high-resolution spectrometer.     

EPR study of the diluted magnetic semiconductor CuGa1−xMnxTe2

Electron paramagnetic resonance (EPR) experiments were made in the diluted magnetic semiconductor CuGa_1−xMn_xTe₂, in the temperature range 70<T<300 K. The samples were synthesized by direct fusion of stoichiometric mixtures of the elements, with Mn composition from x=0.0 to 0.25. The EPR spectra were measured as function of temperature, Mn composition, and field orientation. The temperature variation of the resonance field shows a critical point at about 235 K, and is associated with a transition from the ferromagnetic to the superparamagnetic state. The resonance field was also measured as a function of the field angle, and displays a well-defined uniaxial symmetry. This uniaxial field depends on the Mn concentration and is due to tetragonal distortions induced by Mn²⁺ at Ga sites, and the demagnetizing effects due to formation of ferromagnetism (FM) Mn-clusters.     

The influence of substrate temperature on the morphology, optical and electrical properties of thermal-evaporated ZnTe Thin Films

The structural, morphological, optical and electrical properties of ZnTe films deposited by evaporation were investigated as a function of substrate temperature (at −123 and 27 °C) and post-deposition annealing temperature (at 200, 300 and 400 °C). It was determined that films deposited at both substrate temperatures were polycrystalline in nature with zinc-blende structure and a strong (1 1 1) texture. A small Te peak was detected in XRD spectra for both substrate temperatures, indicating that as-deposited ZnTe films were slightly rich in Te. Larger grains and a tighter grain size distribution were obtained with increased substrate temperature. Scanning electron microscopy (SEM) studies showed that the microstructures of the as-deposited films agreed well with the expectations from structure zone model. Post-deposition annealing induced further grain growth and tightened the grain size distribution. Annealing at 400 °C resulted in randomization in the texture of films deposited at both substrate temperatures. Optical spectroscopy results of the films indicated that the optical band gap value increased from 2.13 to 2.16 eV with increased substrate temperature. Increasing the annealing temperature sharpened the band-edge. Resistivity measurements showed that the resistivity of films deposited at substrate temperatures of −123 and 27 °C were 32 Ω cm, and 1.0 × 10⁴ Ω cm, respectively with corresponding carrier concentrations of 8.9 × 10¹⁵ cm⁻³ and 1.5 × 10¹⁴ cm⁻³. Annealing caused opposite changes in the film resistivity between the samples prepared at substrate temperatures of −123 and 27 °C.     

MgB2 fabrication by diffusion-controlled three layered (B-Mg-B) technique

MgB₂ was successfully fabricated through diffusion-controlled three-layered (B-Mg-B) technique under high pressure. Due to melting temperature of Mg, the material was pre-heat treated at 600 °C between 1 and 48 h. Optimum pre-heat treatment condition was found to be 600 °C for 48 h. Then, the compacted material was grinded and pelletized under pressure of 2 ton. The pellets were heat treated at 600-900 °C for 1-48 h. Optimum heat treatment condition was determined to be 800 °C for 1 h for formation of almost pure MgB₂. Diffusion coefficient was determined with Fick's law and EDX data. Diffusion coefficient value for B in Mg matrix and Mg in B matrix was determined to be 1.66×10⁻⁷ and 3.14×10⁻⁸ cm²/sn, respectively. Best T_c value (39.4 K) was obtained for material heat treated at 800 °C for 1 h. A symmetric hysteresis was obtained for the best MgB₂ material and magnetization decreased with increase in the temperature and the applied magnetic field.     

Formation of functional groups on graphite during oxygen plasma treatment

Improved sample wettability was obtained by oxygen plasma functionalization of pyrolytic graphite. The samples were exposed to highly dissociated oxygen plasma with the density of 1 × 10¹⁶ m⁻³, the electron temperature of about 5.5 eV and the density of neutral oxygen atoms of 8 × 10²¹ m⁻³ for 20 s. The surface wettability was measured by a contact angle of water drop. The contact angle dropped from original 112° down to about 1°. The functional groups were detected by XPS analyses. The survey spectrum showed a substantial increase of oxygen concentration on the surface, while high-resolution analyses showed additional oxygen was bonded onto the graphite surface in the form of C-O polar functional group responsible for the increase of the surface energy.