Steady-state Temperature Increase Research Articles

On-chip heating effects in electronic measurements at cryogenic temperatures

Heating is a major concern for electrical measurements at cryogenic temperatures. In this study, the statics and dynamics of heating effects induced on a Si/SiO2 chip by the application of DC and AC power to an on-chip heating element are measured using on-chip cryogenic thermometers. It is found that large on chip temperatures, ∼ 100 mK above cryostat temperature, and large on-chip thermal gradients, 100s of mK over ∼ 10 μm distance, are generated at relatively small (∼ 0.1 μW) input powers. As expected, the heating effects are larger at lower cryostat temperatures. With applied AC voltages, it is found that the average temperatures generated are similar in magnitude to the DC voltages, but the temperature gradients are smaller. The average temperatures reached on the surface of the chip increase with increasing frequency. At 5 Hz frequency, the thermal dynamics are too slow to allow temperature oscillations following the input power, instead resulting in a steady state temperature increase.

Computational Optimization of the Size of Gold Nanorods for Single-Molecule Plasmonic Biosensors Operating in Scattering and Absorption Modes.

We present a comprehensive computational study on the optimization of the size of gold nanorods for single-molecule plasmonic sensing in terms of optical refractive index sensitivity. We construct an experimentally relevant model of single-molecule–single-nanoparticle sensor based on spherically capped gold nanorods, tip-specific functionalization and passivation layers, and biotin-streptavidin affinity system. We introduce a universal figure of merit for the sensitivity, termed contrast-to-noise ratio (CNR), which relates the change of measurable signal caused by the discrete molecule binding events to the inherent measurement noise. We investigate three distinct sensing modalities relying on direct spectral measurements, monitoring of scattering intensity at fixed wavelength and photothermal effect. By considering a shot-noise-limited performance of an experimental setup, we demonstrate the existence of an optimum nanorod size providing the highest sensitivity for each sensing technique. The optimization at constant illumination intensity (i.e., low-power applications) yields similar values of approximately 20 × 80 nm2 for each considered sensing technique. Second, we investigate the impact of geometrical and material parameters of the molecule and the functionalization layer on the sensitivity. Finally, we discuss the variable illumination intensity for each nanorod size with the steady-state temperature increase as its limiting factor (i.e., high-power applications).

Fatigue Life Assessment of Filled Rubber by Hysteresis Induced Self-Heating Temperature.

As a viscohyperelastic material, filled rubber is widely used as a damping element in mechanical engineering and vehicle engineering. Academic and industrial researchers commonly need to evaluate the fatigue life of these rubber components under cyclic load, quickly and efficiently. The currently used method for fatigue life evaluation is based on the S–N curve, which requires very long and costly fatigue tests. In this paper, fatigue-to-failure experiments were conducted using an hourglass rubber specimen; during testing, the surface temperature of the specimen was measured with a thermal imaging camera. Due to the hysteresis loss during cyclic deformation, the temperature of the material was found to first rise and then level off to a steady state temperature, and then it rose sharply again as failure approached. The S–N curve in the traditional sense was experimentally determined using the maximum principal strain as the fatigue parameter, and a relationship between the steady state temperature increase and the maximum principal strain was then established. Consequently, the steady state temperature increase was connected with the fatigue life. A couple of thousand cycles was sufficient for the temperature to reach its steady state value during fatigue testing, which was less than one tenth of the fatigue life, so the fatigue life of the rubber component could be efficiently assessed by the steady state temperature increase.

Improved Performance of Flexible Graphene Heater Based on Repeated Laser Writing

The electrothermal performance of laser-induced graphene (LIG)- based heater can be improved by increasing the laser power in a certain range. However, excessive laser power will cause the produced LIG film to detach from the substrate, resulting in the degradation of performance. In this letter, a novel strategy of repeated laser writing is proposed to further improve the electrothermal performance of LIG-based heater. The best optimized result appears after four- time repeated laser writing at a certain power. After the device is irradiated for four times, the best steady-state temperature increases by 103.8%, which is 43.8% higher than that of the heater fabricated by the method of only increasing the laser power while not repeated laser irradiation.

Analysis and control of heating of magnetic nanoparticles by adding a static magnetic field to an alternating magnetic field

In this paper, we analyzed the influence of a static magnetic field (SMF), generated by a pair of neodymium magnets, on the temperature rise and the specific absorption rate (SAR) of superparamagnetic nanoparticles when exposed to an alternating magnetic field (AMF). Furthermore, we attempted to verify the feasibility of controlling the SAR and steady state temperature increase of the magnetic nanoparticles (MNP) by controlling the magnitude of the applied SMF. Magnetic heating was produced by applying a high-frequency AMF generated by a solenoid coil and an SMF generated by using a pair of magnets; the SMF value could be controlled by varying the separation distance between the magnets. We performed two sets of experiments: in the first, heating was performed using different amplitudes of AMF and SMF. The results showed that while the increase in temperature and SAR was maximum in the absence of an SMF, these values decreased with increasing SMF value, tending to zero as the nanoparticles approached their magnetic saturation state. In the second set of experiments, we used the curve fitting of the experimental data to obtain the required SMF value to achieve two different specific temperatures and SAR values. The measured values of temperature rise and SAR were quite similar to those calculated using the curve-fitted data. Thus, the experimental results confirm the possibility of controlling the heating properties of MNP during magnetic hyperthermia through an applied SMF.

Improving Signal and Photobleaching Characteristics of Temporal Focusing Microscopy with the Increase in Pulse Repetition Rate

Wide-field temporal focused (WF-TeFo) two-photon microscopy allows for the simultaneous imaging of a large planar area, with a potential order of magnitude enhancement in the speed of volumetric imaging. To date, low repetition rate laser sources with over half a millijoule per pulse have been required in order to provide the high peak power densities for effective two-photon excitation over the large area. However, this configuration suffers from reduced signal intensity due to the low repetition rate, saturation effects due to increased excitation fluences, as well as faster photobleaching of the fluorescence probe. In contrast, with the recent advent of high repetition rate, high pulse energy laser systems could potentially provide the advantages of high repetition rate systems that are seen in traditional two-photon microscopes, while minimizing the negatives of high fluences in WF-TeFo setups to date. Here, we use a 100 microjoule/high repetition rate (50–100 kHz) laser system to investigate the performance of a WF-TeFo two-photon microscope. While using micro-beads as a sample, we demonstrate a proportionate increase in signal intensity with repetition rate, at no added cost in photobleaching. By decreasing pulse intensity, via a corresponding increase in repetition rate to maintain fluorescence signal intensity, we find that the photobleaching rate is reduced by ~98.4%. We then image live C. elegans at a high repetition rate for 25 min. as a proof-of-principle. Lastly, we identify the steady state temperature increase as the limiting process in further increasing the repetition rate, and we estimate that repetition rate in the range between 0.5 and 5 MHz is ideal for live imaging with a simple theoretical model. With new generation low-cost fiber laser systems offering high pulse energy/high repetition rates in what is essentially a turn-key solution, we anticipate increased adoption of this microscopy technique by the neuroscience community.

Comparison of temperature change and resulting ablation size induced by a 902–928 MHz and a 2450 MHz microwave ablation system in in-vivo porcine kidneys

Introduction: Microwave ablation (MWA) uses heat to ablate undesired tissue. Development of pre-planning algorithms for MWA of small renal masses requires understanding of microwave-tissue interactions at different operating parameters. The objective of this study was to compare the performance of two MWA systems in in-vivo porcine kidneys.Methods: Five ablations were performed using a 902–928 MHz system (24 W, 5 min) and a 2450 MHz system (180 W, 2 min). Nonlinear regression analysis of temperature changes measured 5 mm from the antenna axis was completed for the initial 10 s of ablation using the power equation and after the inflection point using an exponential equation. Thermal damage was calculated using the Arrhenius equation. Long and short axis ablation diameters were measured.Results: The average ‘a’ varied significantly between systems (902–928 MHz: 0.0299 ± 0.027, 2450 MHz: 0.1598 ± 0.158), indicating proportionality to the heat source, but ‘b’ did not (902–928 MHz: 1.910 ± 0.372, 2450 MHz: 2.039 ± 0.366), signifying tissue type dependence. Past the inflection point, average steady-state temperature increases were similar between systems but reached more quickly with the 2450 MHz system. Complete damage was reached at 5 mm for both systems. The 2450 MHz system produced significantly larger short axis ablations (902–928 MHz: 2.40 ± 0.54 cm, 2450 MHz: 3.32 ± 0.41cm).Conclusion: The 2450 MHz system achieved similar steady state temperature increases compared to the 902–928 MHz system, but more quickly due to higher output power. Further investigations using various treatment parameters and precise thermal sensor placement are warranted to refine equation parameters for the development of an ablation model.

A transient thermal analysis of microscale inorganic light-emitting diodes

Microscale inorganic light-emitting diodes ($\mu~$-ILEDs) that are integrated in flexible substrates are able to be deformed like a rubber band. Such flexible $\mu~$-ILEDs can be integrated with human tissue directly as a novel optical detector applied to human health monitoring and clinical diagnosis. However, large amounts of heat will be emitted when such devices are operating, and the heat-transfer characteristics of the device are critical for its applications. In this paper, an axial symmetry analytical model, validated by 3D finite-element analysis, is developed to describe the transient heat transfer process based on the separation variable method. Both the time and space variations of the temperature increase are obtained theoretically. According to the analytical model, the influence of the in-plane dimension ratio $R_{0}$/$R~$ on the steady-state temperature increase of the $\mu~$-ILED is discussed. These results are able to predict the heating process of flexible $\mu~$-ILEDs and also pave the basis for thermal management and relevant thermal design of flexible $\mu~$-ILEDs.

Thermal response of tissue to RF exposure from canonical dipoles at frequencies for future mobile communication systems

The level of protection against thermal hazard of the current RF EM field (EMF) exposure limits is estimated at the transition frequency where the basic restrictions change from specific absorption rate to power density. It is shown that the calculated steady-state temperature increase in the skin generated by a nearby dipole transmitting at maximum power to meet compliance with the EMF limits presents a significant discontinuity at this frequency. The results suggest that for exposure to limited areas of the body at frequencies where basic restrictions are provided in terms of power density, the currently existing exposure guidelines need to be revised. These findings might have large implications on the development of future radio access technologies operating at the millimetre wave.

Quantitative study of the photothermal properties of metallic nanowire networks.

In this article, we present a comprehensive investigation of the photothermal properties of plasmonic nanowire networks. We measure the local steady-state temperature increase, heat source density, and absorption in Ag, Au, and Ni metallic nanowire networks under optical illumination. This allows direct experimental confirmation of increased heat generation at the junction between two metallic nanowires and stacking-dependent absorption of polarized light. Due to thermal collective effects, the local temperature distribution in a network is shown to be completely delocalized on a micrometer scale, despite the nanoscale features in the heat source density. Comparison of the experimental temperature profile with numerical simulation allows an upper limit for the effective thermal conductivity of a Ag nanowire network to be established at 43 Wm(-1) K(-1) (0.1 κbulk).

Note: High turn density magnetic coils with improved low pressure water cooling for use in atom optics.

We describe a magnetic coil design utilizing concentrically wound electro-magnetic insulating (EMI) foil (25.4 μm Kapton backing and 127 μm thick layers). The magnetic coils are easily configurable for different coil sizes, while providing large surfaces for low-pressure (0.12 bar) water cooling. The coils have turn densities of ~5 mm(-1) and achieve a maximum of 377 G at 2.1 kW driving power, measured at a distance 37.9 mm from the axial center of the coil. The coils achieve a steady-state temperature increase of 36.7°C/kW.

Photoresponsive hydrogel networks using melanin nanoparticle photothermal sensitizers.

Photoreconfigurable and photodegradable polymeric networks have broad utility as functional biomaterials for many applications in medicine and biotechnology. The vast majority of these functional polymers are synthesized using chemical moieties that may be cytotoxic in vivo. Materials synthesized from these substituents also pose unknown risk upon implantation and thus will encounter significant regulatory challenges prior to use in vivo. This work describes a strategy to prepare photodegradable hydrogel networks that are composed of well-characterized synthetic polymers and natural melanin pigments found within the human body. Self-assembled networks of poly(l-lactide-co-glycolide)-poly(ethylene glycol) ABA triblock copolymers are doped with melanin nanoparticles to produce reconfigurable networks based on photothermal phase transitions. Self-assembled hydrogel networks with melanin nanoparticles exhibit a storage modulus ranging from 1.5 ± 0.6 kPa to 8.0 ± 7.5 kPa as measured by rheology. The rate of UV-induced photothermal heating was non-monotonic and varied as a function of melanin nanoparticle loading. A maximum steady state temperature increase of 20.5 ± 0.30 °C was measured. Experimental heating rates were in close agreement with predictions based on attenuation of light in melanins via photothermal absorption and Mie scattering. The implications of melanin nanoparticles on hydrogel network formation and light-induced disintegration were also characterized by rheology and dynamic light scattering. Taken together, this class of photoreconfigurable hydrogels represents a potential strategy for photodegradable polymers with increased likelihood for clinical translation.

A Green Solowian Model for Sustainable Economic Growth

This paper examines whether a non-instantaneous relationship between CO2 emission and temperature increase can affect asymptotic stability in a Golden rule context, when utility is not only determined by the level of per capita consumption but also allows for the negative influence of the temperature increase. In particular, by exploiting a difference-differential equation (DDE) framework, we determine an interval for the steady state temperature increase which characterizes asymptotic stability completely. We apply this framework to assess a Green Solowian Rule steady state, suggesting that any lacks or excesses in the saving rate of an economy with respect to the optimal growth path are affected by the propensity to prevent future climate catastrophes. It is important to emphasise that a lower return of physical capital, which generally affects developed countries, adversely influences asymptotic stability.

Estimation of specific absorption rate and temperature increases in the human head due to portable telephones

The bioheat equation is solved for an anatomically based model of the human head with a resolution of 2.5 × 2.5 × 2.5 mm to study the thermal implications of exposure to electromagnetic (EM) fields typical of cellular telephones at 900 MHz.Attention has first been posed on a particular phone model, and a comparison between the absorbed power distribution and steady-state temperature increases has been carried out. The antenna output power was set to be consistent with the portable telephones of 600 mW, maximum SAR values, averaged over 1 gm, from 2.1 to 3.6 W/kg depending on the considered phone. The maximum temperature increases are obtained in the ear and vary from 0.22°C to 0.39°C, while the maximum temperature increases in the brain lie from 0.07°C to 0.17°C. These steady-state temperature increases are obtained after about 48 min of exposure, with a time constant of approximately 6 min. Application of the ANSI/IEEE safety guidelines restricting the 1 gm averaged spatial peak SAR to 1.6 W/kg results in the maximum temperature rise in the brain from 0.07°C to 0.15°C at 900 MHz. Finally, considerations about the exposure limits in the considered studied frequency are made. Key words: Electromagnetic wave polarization, electromagnetic heating, finite-difference time-domain method, mobile phone, specific absorption rate, temperature increase.

Gold Nanoparticles Allow Optoplasmonic Evaporation from Open Silica Cells with a Logarithmic Approach to Steady-State Thermal Profiles

In this work, plasmonically heated solid-state gold nanoparticle (AuNP) arrays are investigated under novel conditions that include large (>35 °C) steady-state (SS) temperature increases (ΔT) dominated by conduction in open environments that allow vapor−liquid phase change. Evaporative cooling from the open system decreases SS ΔT of the system by as much as (11.6 ± 0.33) °C (45%), consistent with predictions from an energy balance model expanded in this work to account for evaporative cooling and associated decreasing thermal mass. Comparing dynamic and steady temperature profiles from water evaporating from a AuNP-coated Si cell at 50 mW laser irradiation with the model yielded an average accumulated residual sum of squares of 2.95 °C2 over 200 s. Temperature increases that distribute nonuniformly across sample cell surfaces due to high laser power (≤150 mW) and conductive heat transfer are accurately and uniformly (<0.7% difference) represented by an infinite fin model at laser powers from 50 to 150 mW,...

Modeling terahertz heating effects on water

We apply Kirchhoff's heat equation to model the influence of a CW terahertz beam on a sample of water, which is assumed to be static. We develop a generalized model, which easily can be applied to other liquids and solids by changing the material constants. If the terahertz light source is focused down to a spot with a diameter of 0.5 mm, we find that the steady-state temperature increase per milliwatt of transmitted power is 1.8?C/mW. A quantum cascade laser can produce a CW beam in the order of several milliwatts and this motivates the need to estimate the effect of beam power on the sample temperature. For THz time domain systems, we indicate how to use our model as a worst-case approximation based on the beam average power. It turns out that THz pulses created from photoconductive antennas give a negligible increase in temperature. As biotissue contains a high water content, this leads to a discussion of worst-case predictions for THz heating of the human body in order to motivate future detailed study. An open source Matlab implementation of our model is freely available for use at www.eleceng.adelaide.edu.au/thz.

Measuring local RF heating in MRI: Simulating perfusion in a perfusionless phantom

To overcome conflicting methods of local RF heating measurements by proposing a simple technique for predicting in vivo temperature rise by using a gel phantom experiment. In vivo temperature measurements are difficult to conduct reproducibly; fluid phantoms introduce convection, and gel phantom lacks perfusion. In the proposed method the local temperature rise is measured in a gel phantom at a timepoint that the phantom temperature would be equal to the perfused body steady-state temperature value. The idea comes from the fact that the steady-state temperature rise in a perfused body is smaller than the steady-state temperature increase in a perfusionless phantom. Therefore, when measuring the temperature on a phantom there will be the timepoint that corresponds to the perfusion time constant of the body part. The proposed method was tested with several phantom and in vivo experiments. Instead, an overall average of 30.8% error can be given as the amount of underestimation with the proposed method. This error is within the variability of in vivo experiments (45%). With the aid of this reliable temperature rise prediction the amount of power delivered by the scanner can be controlled, enabling safe MRI examinations of patients with implants.

A Bifunctional Molecularly Imprinted Polymer (MIP): Analysis of Binding and Catalysis by a Thermistor

Binding or catalysis? Both can be distinguished with a molecularly imprinted polymer (MIP) by the different patterns of heat generation. The catalytically active sites, like in the corresponding enzyme, generate a steady-state temperature increase. Thus, enzyme-like catalysis and antibody-analogue binding are analyzed simultaneously in a bifunctional MIP for the first time (see scheme).

Specific absorption rate and temperature increases in the head of a cellular-phone user

In this paper, a complete electromagnetic and thermal analysis has been performed considering the head of a subject exposed to various kinds of cellular phones available on the market, and focusing the attention on important organs like the eye lens and brain. Attention has first been posed on a particular phone model, and a comparison between the absorbed power distribution and steady-state temperature increases has been carried out. The influence of different antennas (dipole, monopole, whip, and planar inverted F antenna) on the power absorption and on the consequent tissue heating has then been analyzed. The obtained results show for a radiated power of 600 mW, maximum SAR values, averaged over 1 g, from 2.2 to 3.7 W/kg depending on the considered phone. The maximum temperature increases are obtained in the ear and vary from 0.22/spl deg/C to 0.43/spl deg/C, while the maximum temperature increases in the brain lie from 0.08/spl deg/C to 0.19/spl deg/C. These steady-state temperature increases are obtained after about 50 min of exposure, with a time constant of approximately 6 min. Finally, the results evidence a maximum temperature increase in the external part of the brain from 0.10/spl deg/C to 0.16/spl deg/C for every 1 W/kg of SAR, averaged over 1 g of brain tissue.

Thermal models for microwave hazards and their role in standards development.

We consider the thermal response of the body to radiofrequency (RF) energy, with emphasis on partial-body exposure, to assess potential thermal hazards. The thermal analysis is based on Pennes' bioheat equation. In this model, the thermal response is governed by two time constants. One (tau1) pertains to heat convection by blood flow and is (for physiologically normal perfusion rates) on the order of 3 min. The second (tau2) characterizes heat conduction, and varies as the square of a distance that characterizes the spatial extent of the heating. We examine three idealized cases. The first is a region of tissue with an insulated surface, subject to irradiation with an exponentially decreasing SAR, which models a large surface area of tissue exposed to microwaves. The second is a region of tissue in contact with a hemispherical electrode that passes current into it, which models exposure from contact with a conductor. The third is a region of tissue with an insulated surface, subject to heating from a dipole located close to it. In all three cases, we estimate the maximum steady-state temperature increase as a function of the relevant electrical and thermal parameters and the thresholds for thermal hazard. We conclude that thermal models are a potentially fruitful but underutilized means of analyzing thermal hazards from RF fields. A quantitative analysis of such hazards enables the development of data-based uncertainty factors, which can replace arbitrary "safety factors" in developing exposure limits. Finally, we comment on the need to marry quantitative modeling of data and risk assessment, and to incorporate contemporary approaches to risk assessment into RF standards development.