共查询到20条相似文献,搜索用时 15 毫秒
1.
Beop-Min Kim Jacques S.L. Rastegar S. Thomsen S. Motamedi M. 《IEEE journal of selected topics in quantum electronics》1996,2(4):922-933
A nonlinear finite-element program was developed to simulate the dynamic evolution of coagulation in tissue considering temperature and damage dependence of both the optical properties and blood perfusion rate. These dynamic parameters were derived based on the Arrhenius rate process formulation of thermal damage and kinetics of vasodilation. Using this nonlinear model, we found that the region of increased blood flow that formed at the periphery of the coagulation region significantly reduces the heat penetration. Moreover, increased scattering in the near-surface region prevents light penetration into the deeper region. Therefore, if the dynamic parameters are ignored, a relatively significant overestimation of the temperature rise occurs in a deeper area resulting in an overestimation in predicted depth of coagulation. Mathematical modeling techniques that simulate laser coagulation may not provide reliable information unless they take into account these dynamic parameters 相似文献
2.
Kavec M Gröhn OH Kettunen MI Silvennoinen MJ Penttonen M Kauppinen RA 《Magma (New York, N.Y.)》2001,12(1):32-39
Inadequate blood supply relative to metabolic demand, a haemodynamic condition termed as misery perfusion, often occurs in
conjunction with acute ischaemic stroke. Misery perfusion results in adaptive changes in cerebral physiology including increased
cerebral blood volume (CBV) and oxygen extraction ratio (OER) to secure substrate supply for the brain. It has been suggested
that the presence of misery perfusion may be an indication of reversible ischaemia, thus detection of this condition may have
clinical impact in acute stroke imaging. The ability of single spin echo T2 to detect misery perfusion in the rat brain at 1.5 T owing to its sensitivity to blood oxygenation level dependent (BOLD)
contrast was studied both theoretically and experimentally. Based on the known physiology of misery perfusion, tissue morphometry
and blood relaxation data, T2 behaviour in misery perfusion was simulated. The interpretation of these computations was experimentally assessed by quantifying
T2 in a rat model for cerebral misery perfusion. CBF was quantified with the H2 clearance method. A drop of CBF from 58 ± 8 to 17 ± 3 ml/100 g min in the parieto-frontal cortex caused shortening of T2. from 66.9 ± 0.4 to 64.6 ± 0.5 ms. Under these conditions, no change in diffusion MRI was detected. In contrast, the cortex
with CBF of 42 ± 7 ml/100 g min showed no change in T2. Computer simulations accurately predicted these T2, responses. The present study shows that the acute drop of CBF by 70% causes a negative BOLD that is readily detectable by
T2 MRI at 1.5 T. Thus BOLD may serve as an index of misery perfusion thus revealing viable tissue with increased OER. 相似文献
3.
Aluminum nitride (AlN) films were prepared by using reactive magnetron (RMS). The substrate temperature (Ts) was varied from room temperature to 700°C, and RF power (P w) from 100 to 250 W, and three different substrate materials (e.g. Si(100), Pt(111)/Si(100) and Al2O3(00·1)) were used. The mean free path of the species in the vacuum was controlled to be much shorter than the target-to-substrate distance, so that Ts and P w have the equivalent effect in affecting the effective thermal energy of the species after landing on the substrate. With these conditions, the film structure was found to cover a broad range, varying from nearly amorphous (na-), polycrystalline (p-), (00·1) texture (t-) and epitaxial (e-) structure. The structural change is supposed to be governed the successive increase in the thermal energy of the species on the substrate. The use of substrate material having lattice matching with the AlN structure further assists the (00·1) preferential growth when the species are mobile enough at the settings of high Ts and P w. 相似文献
4.
《Integrated ferroelectrics》2013,141(1):607-618
Vertical ferroelectric Pb(Zr,Ti)O3 (PZT) 1 μm thick film capacitor was fabricated by pulsed laser deposition technique (PLD) onto conducting La0.5Sr0.5CoO3(LSCO) 100 nm thick bottom electrode on both side polished YAlO3 + 1% Nd2O3 (Nd:YAlO3) single crystal substrate to operate as a Pockels cell optical modulator. On top of the PZT film, semitransparent 30 nm thick Au electrode was deposited by thermal evaporation. Intensity of the chopped 670 nm polarized laser radiation transmitted through the Au/PZT/LSCO/Nd:YAlO3 cell was measured at various temperatures and bias voltage applied. Applying 20 V (200 kV/cm) across the capacitive cell, modulation of the transmitted light as high as 3% was achieved while the voltage tunability measured at 1 kHz from C-V characteristics was about 70%. Thermo-optical measurements performed for PZT/Nd:YAlO3 sample in the range up to 400°C showed the phenomenon of critical opalescence in the vicinity of Curie temperature at 208°C. Optical transmission through the PZT film biased with electric field was studied in the range 400 to 1000 nm. Film thickness, refraction index and absorption coefficient have been determined from the interference pattern observed in the PZT transmission spectrum. A simple model yields the dispersion relation for the electro-optic coefficient. 相似文献
5.
ABSTRACTThin films of magnesium ferrite (MgFe2O4) are synthesized on the glass substrate by using thermal evaporation technique and studied their anti-bacterial biofilm activity against the gram-negative and gram-positive bacterium. The MgFe2O4 thin films (100 nm) were prepared at a deposition rate of 10 ± 2 Å/S, followed by annealing at 600°C. The morphological and structural study using scanning electron microscope, X-ray diffraction, and transmission electron microscope reveals crystalline structure of the magnesium ferrite. The model organisms for anti-bacterial biofilm activity were tested with E. coli (gram negative) and S. aureus (gram positive). The E. coli and S. aureus biofilms grown on the glass slides and MgFe2O4 coated glass slide were imaged using fluorescent microscope. It is observed that the MgFe2O4 coated glass slides exhibit minimum bacterial growth (both negative and positive bacteria) compared to the uncoated glass slide. 相似文献
6.
William J. O'Connor 《International Journal of Numerical Modelling》2002,15(2):195-203
In a companion paper, a transmission line matrix (TLM) scheme was presented for the solution of the 1-D equation of waves in moving media, ytt+αyxt+βyxx=0, describing the superposition of two waves with direction-dependent speeds. The TLM model achieved controlled, direction-dependent wave-speed bias by means of supplementary link lines with notional ‘diodes’. Expressions for wave speed, presented without proof, were verified. Here a proof is presented using a new, time-domain approach. The apparent (direction-dependent) inductance and capacitance as seen by a single wave are first established in a generalized way from the TLM algorithm, from which two wave speeds and impedances are then derived. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
7.
A. L. Koay A. J. Wilkinson S. H. Pulko 《International Journal of Numerical Modelling》2008,21(5):351-368
The transmission line matrix (TLM) method is an established technique for modelling thermal transients in heat transfer systems. However, initial and boundary conditions have always been slightly problematic, particularly when the boundary condition is specified as a temperature (Transmission Line Matrix (TLM) Techniques for Diffusion Applications. Gordon & Breach: London, 1998), for example, when the body of interest is suddenly exposed to a different temperature on its surface. In such a case the modelled solution contains additional dynamics that are associated with the two sub‐meshes in the TLM network and the two timesteps necessary for the temperature change to be fully communicated. These initialization problems are related to the fact that the boundary temperature in merely imposed as a fixed value on the network; the fundamental information‐carrying quantity, on the other hand, is the pulse, the thermal state of the body being represented by the distribution of pulses. Here, we aim to provide an alternative initialization approach, using nodal state estimation to derive pulse distributions from boundary and initial conditions specified by temperature. Consideration is given to the accuracy of the estimator by comparison with the first timestep solution proposed by Enders (Int. J. Numer. Model. 2002; 15 :251–259). Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
8.
S. Aliouat Bellia A. Saidane M. Benzohra J. M. Saiter A. Hamou 《International Journal of Numerical Modelling》2008,21(6):531-549
The skin is sensitive to temperature change and the effect may not be significant while the temperature at the surface is below 44°C. However, higher surface temperatures (above 44°C) will further incur time burning and carbonization so that irreversible damage may happen. An investigation of the heating intensity and the duration of the exposure to the heating source suggested that when the surface temperature is greater than 51°C, the exposure time required to destroy the epidermis is so short that trans‐epidermal necrosis may occur. In this paper, we present one‐ and two‐dimensional numerical models based on transmission line matrix (TLM) method for a quantitative prediction of skin burn injury resulting from the exposure of the skin surface to a high temperature heat source. Transient temperatures are numerically estimated by solving the Pennes' bioheat equation, and the damage function denoting the extent of burn injury is calculated using the Arrhenius assumptions for protein damage rate. The TLM model is used to analyse the effects of exposure time and geometrical dimensions of mutlilayered skin, on the transient temperature distribution and damage extension. TLM results showed good agreement with other numerical sources, suggesting that TLM modelling can be used as a tool for an effective thermal diagnostic of burn injuries. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
9.
This study is an experimental investigation on thermal energy harvesting using pyroelectric ceramics. Hot and cold air sources were used for generating temperature gradient in PbZrxTi1?xO3 (PZT-5H) ceramics. Four different frequencies of (heating and cooling process) 0.025, 0.05, 0.1 and 0.2 Hz were investigated for optimizing power/voltage output. Maximum output voltage of 2.51 V (across 4.7 μF and 5 MΩ) was achieved at 0.05 Hz frequency with temperature variation between 100°C and 65°C. Furthermore, maximum power density of 2.35 μW/cm3 was obtained at 0.05 Hz across 4.7 μF load capacitor and 5 MΩ load resistance. In addition to this a battery of 7 mAh capacity was charged up to 1.21 V in 55 minutes using continuous heating and cooling processes with frequency of 0.4 Hz. 相似文献
10.
Peter Enders Susan H. Pulko David M. Stubbs 《International Journal of Numerical Modelling》2002,15(3):251-259
In initializing a transmission line matrix (TLM) diffusion model it is necessary to consider both initial concentration (temperature) and initial flow. As usual, only one of them is given; an auxiliary formula is necessary to calculate the distribution for the first time step. It has been shown that the standard formula may introduce additional numerical errors (International Journal of Numerical Modelling: Electronic Networks, Devices and Fields 1993; 6 :135; International Journal of Numerical Modelling: Electronic Networks, Devices and Fields 1993; 6 :161) and that these errors can persist over many time steps. In this paper, we show how an initial modification to the normal TLM algorithm can remove such errors, and we demonstrate the method by applying it to a two‐dimensional TLM heat diffusion model for a copper plate. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
11.
The balanced‐unbalanced transformer (balun) is extensively used in radio communication and measuring instruments. In recent years, with the development of television and portable telephones, wideband transformers tend to be miniaturized. In this paper, the wideband transformer is analyzed in terms of distributed coupled‐line theory. A main result of the analysis is that the balance and unbalance transmission characteristics of the balun are markedly improved in the high‐frequency range by using a delay line element for compensation. If the connection point Sw between the coil winding and delay line is off the ground, the balance transmission band is about doubled in the low‐frequency range. If the point Swis grounded and if the impedance ratio m2 is small, a higher degree of unbalance attenuation can be obtained. Better balance transmission characteristics can be obtained when the matching factor K approaches half the optimum. In addition, we show that an in‐phase or anti‐phase transformer can be made on the basis of our isolation wideband transformer. The equivalent circuits of in‐phase and anti‐phase wideband n:1 transformers are obtained from our theoretical analysis and these can be formed by the same transformer. The transmission characteristics of in‐phase and anti‐phase wideband transformers are analyzed and improved characteristics are obtained in the high‐frequency range. The theoretical and experimental values agree well over the wide frequency range of 100 KHz to 1000 MHz. ©1998 Scripta Technica, Electr Eng Jpn, 126(2): 1–14, 1999 相似文献
12.
In what follows, AT-cut quartz crystal microbalance (QCM) effective properties for two commercial QCM substrates, are calculated as a function of temperature, while varying the degree of implementation regarding the temperature dependence of input quartz parameters in the transmission line (TL) model. Results for the effective quartz viscosity, oscillation area and external capacitance, are in qualitative agreement per property, irrespective of the implemented, input quartz parameter set for the calculations. However, the effective quartz thickness accounts for the quartz thermal expansion effect, only when quartz density is temperature adjusted, whereas quantitative deviations in the effective oscillation area, are qualitatively explained in the light of literature correlations, which relate the implemented input quartz parameter set, with inherent QCM energy dissipation metrics, and the magnitude of energy trapping. Moreover, the varying degree of input quartz parameter adjustment with temperature, yields qualitative and quantitative deviations from the expected response, in terms of a QCM quality metric, the C0/C1 ratio. Results suggest that the precision in the determination of the static capacitance C0 and the QCM effective properties incorporated therein, can be improved, with potential improvements in relevant, liquid or gas phase QCM applications. 相似文献
13.
S. J. Woods S. P. Wilson Alison B. Walker 《International Journal of Numerical Modelling》2000,13(1):37-57
We report non‐unique solutions for the potential in a Drift Diffusion (DD) model of a two terminal phototransistor. These solutions are present under bias without illumination, and persist until high illumination levels. It is well known that the DD equations can yield non‐unique solutions for pn structures which contain three or more junctions and two terminals with applied biases greater than kBT log 2 where kBT is the thermal energy at a temperature T, but DD models of phototransistors under illumination have been less well studied. The implicit belief is that one needs to artificially impose a potential in the base of the phototransistor in order to obtain a unique solution. We show here that this is only necessary because of a weakness in the numerical methods used to solve the equations, and describe two methods which circumvent this for which we show that this problem does not occur. These methods are used to investigate the operation of GaAs and In0·53Ga0·47As homojunction phototransistors, including the influence of the position of the illumination region and base doping. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献
14.
Mehdi Salmani-Jelodar Abhijeet Paul Timothy Boykin Gerhard Klimeck 《Journal of Computational Electronics》2012,11(1):22-28
The phonon spectra in zinc blende InAs, GaAs and their ternary alloy nanowires (NWs) are computed using an enhanced valence
force field (EVFF) model. The physical and thermal properties of these nanowires such as sound velocity, elastic constants,
specific heat (C
v
), phonon density of states, phonon modes, and the ballistic thermal conductance are explored. The calculated transverse and
longitudinal sound velocities in these NWs are ∼25% and 20% smaller compared to the bulk velocities, respectively. The C
v
for NWs are about twice as large as the bulk values due to higher surface to volume ratio (SVR) and strong phonon confinement
in the nanostructures. The temperature dependent C
v
for InAs and GaAs nanowires show a cross-over at 180°K due to higher phonon density in InAs nanowires at lower temperatures.
With the phonon spectra and Landauer’s model the ballistic thermal conductance is reported for these III–V NWs. The results
in this work demonstrate the potential to engineer the thermal behavior of III–V NWs. 相似文献
15.
Theera Anukunprasert Chintana Saiwan Elisabetta Di Bartolomeo Enrico Traversa 《Journal of Electroceramics》2007,18(3-4):295-303
Nanostructured powders of Nb-doped TiO2 (TN) and SnO2 mixed with Nb-doped TiO2 in two different atomic ratios—10 to 1 (TSN 101) and 1 to 1 (TSN 11)—were synthesized using the reverse micelle microemulsion
of a nonionic surfactant (brine solution/1-hexanol/Triton X-100/cyclohexane). The powders were characterized by transmission
electron microscopy (TEM) and X-ray diffraction (XRD). Thick films were fabricated for gas sensors and characterized by XRD
analysis and field emission scanning electron microscopy (FE-SEM). The effects of the film morphology and firing temperature
in the range 650–850 °C on CO sensitivity were studied. The best gas response, expressed as the ratio between the resistance
in air and the resistance under gas exposure (R
air/R
gas), was measured for TSN 11 at 11 for 1,000 ppm CO exposure. All types of sensors showed good thermal stability. The electrochemical
impedance spectroscopy (EIS) measurements were performed in different gas atmospheres (air, O2, CO and NO2) to better understand the electrical properties of the nanostructured mixed metal oxides. 相似文献
16.
Objective: Integration of transformers into transmission lines suppresses radiofrequency (RF)-induced heating. New figure-of-eight-shaped transformer coils are compared to conventional loop transformer coils to assess their signal transmission properties and safety profile.
Materials and methods: The transmission properties of figure-of-eight-shaped transformers were measured and compared to transformers with loop coils. Experiments to quantify the effect of decoupling from the B1 field of the MR system were conducted. Temperature measurements were performed to demonstrate the effective reduction of RF-induced heating. The transformers were investigated during active tracking experiments.
Results: Coupling to the B1 field was reduced by 18 dB over conventional loop-shaped transformer coils. MR images showed a significantly reduced artifact for the figure-of-eight- shaped coils generated by local flip-angle amplification. Comparable transmission properties were seen for both transformer types. Temperature measurements showed a maximal temperature increase of 30K/3.5 K for an unsegmented/ segmented cable. With a segmented transmission line a robotic assistance system could be successfully localized using active tracking.
Conclusion: The figure-of-eight-shaped transformer design reduces both RF field coupling with the MR system and artifact sizes. Anatomical structure close to the figure-of-eight-shaped transformer may be less obscured as with loop-shaped transformers if these transformers are integrated into e.g. intravascular catheters. 相似文献
17.
Xiang Gui 《International Journal of Numerical Modelling》2003,16(5):465-478
In diffusion modelling by means of the transmission‐line matrix (TLM) method, a nodal arrangement of using lossless transmission‐line segments and series resistors is almost exclusively adopted and is currently considered as a standard approach. In this paper, the use of shunt resistors instead of series resistors is shown to represent an equally valid configuration. As a starting point, we have derived the telegrapher's equation in its most general form for TLM modelling of diffusion processes. A general algorithm based on the shunt‐resistor TLM model for implementing a numerical solution of the diffusion equation in multiple dimensions is given. Fundamental analysis and calculated examples confirm that the alternative shunt‐resistor configuration does not exhibit the unwanted absorption effects suggested by a recent paper (Internat. J. Numerical Model.: Electronic Networks, Devices and Fields 2002; 15 :261). Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
18.
Tatjana Asenov Neboja Don
ov Bratislav Milovanovi John Paul 《International Journal of Numerical Modelling》2013,26(5):457-463
In this paper, a numerical model of electromagnetic left‐handed metamaterials is proposed. The dispersive properties of these materials are accounted for in the time domain by using the transmission‐line matrix method based on Z‐transforms. The close agreements obtained between the analytic and numerical results verify the validity, accuracy and stability of the approach. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
19.
Hiroshi Naganuma Kayoko Yamada Hiromi Shima Kensuke Akiyama Takashi Iijima Hiroshi Funakubo Soichiro Okamura 《Journal of Electroceramics》2009,22(1-3):78-81
BaPbO3 films were fabricated by a chemical solution deposition on the SiO2/Si(100) and MgO(100) substrates followed by a post-deposition annealing at the temperatures between 673 and 1073 K under oxygen flow. Polycrystalline BaPbO3 films were formed together with secondary phases such as PbO and Pb3O4 onto MgO(100) substrates at around 750 K, and the films were crystallized into single phase of BaPbO3 above 823 K. Endothermic peak in differential thermal analysis due to crystallization of BaPbO3 was observed at 750 K, which is consistent with crystallization temperature of BaPbO3 estimated from X-ray diffraction. The electrical resistivity depended on the annealing temperature even in the single phase BaPbO3 films, the lowest resistivity of 3?×?10?6 μΩ·m which was comparable to that of bulk BaPbO3 was achieved at the annealing temperature of 873 K. 相似文献
20.
Taku Noda Naoko Nose Naoto Nagaoka Akihiro Ametani 《Electrical Engineering in Japan》1999,129(1):29-38
For transmission‐line surge studies, the inclusion of corona discharge due to high voltage surges is important as well as the inclusion of frequency‐dependent effects. Because the charge‐voltage (q‐v) curve of a lightning surge is different from that of a switching surge, a corona model should reproduce different q‐v curves for different wave‐front times. The present paper proposes a wave‐front time dependent corona model which can express the dependence by a simple calculation procedure as accurately as a rigorous finite‐difference method which requires an enormous calculation time. The simplicity enhances the incorporation of the corona model into a line model, because a large number of models are to be inserted into the line model by discretization. The q‐v curves calculated by the proposed method agrees well with field tests. This paper also proposes an efficient method to deal with nonlinear corona branches in distributed‐parameter line model using the trapezoidal rule of integration and the predictor‐corrector method. © 1999 Scripta Technica, Electr Eng Jpn, 129(1): 29–38, 1999 相似文献