Absorbed Power Density Research Articles

Microstructural evolution and thermal fatigue damage mechanism of second-phase dispersion strengthened tungsten composites under repetitive thermal loads

• After repetitive thermal loads, the ODS/CDS-W composites experience obvious grain growth and particles shedding, leading to a decrease in mechanical properties. • The interface debonding and progressive shedding of second-phase particles from the W matrix is the main reason for the cracks along the grain boundaries. • The W-ZrC composite exhibits a better thermal shock resistance than W-Y 2 O 3 due to the intrinsic properties of second particles. In a fusion reactor, plasma-facing tungsten (W) materials inevitably suffer severe thermal shock, and the performance of W materials under repetitive high heat loads is one of the key concerns for long-term stable operation of the reactor. In this work, the microstructural evolution and thermal fatigue resistance of two representative W-0.5 wt.% ZrC (WZC) and W-1.0 wt.% Y 2 O 3 (WYO) composites were investigated under cyclic heat loads. Due to the intrinsic properties of ZrC and Y 2 O 3 particles such as coefficients of thermal expansion, particle size and distributions in W grains, the WZC composite exhibited a better thermal shock resistance than WYO. After thermal loads with the absorbed power density (APD) ≥ 22 MW/m 2 , WYO showed obvious grain growth, Y 2 O 3 particles shedding and degradation of mechanical properties. While, in the case of WZC, these damage behaviors only occurred when APD ≥ 25 MW/m 2 . Furthermore, an interesting crack mechanism in W composites was revealed due to interface debonding and progressive shedding of second-phase particles from the W matrix. The microstructures and tensile properties of the thermally loaded WZC and WYO specimens were also investigated and the correlations between the microstructure evolution and performance degradation are demonstrated. The results are useful for evaluating the thermal fatigue resistance of oxide/carbide dispersion strengthened W composites and their application in future fusion reactors.

Area-Averaged Transmitted and Absorbed Power Density on a Realistic Ear Model

At millimeter waves (MMW), the current state of research in computational dosimetry is mainly relying on flat-surface tissue-equivalent models to simplify the exposure assessment by disregarding geometrical irregularities characteristic of conformal surfaces on realistic models. However, this can lead to errors in estimation of dosimetric quantities on non-planar body parts with local curvature radii comparable to the wavelength of the incident field. In this study, we address this problem by developing an averaging technique for the assessment of the absorbed power density ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$S_{\text{ab}}$</tex-math></inline-formula> ) on the anatomically-accurate electromagnetic (EM) model of the human ear. The dosimetric analysis is performed for the plane-wave exposure at 26 and 60 GHz, and the accuracy of the proposed method is verified by using two commercial EM software. Furthermore, we compare the two definitions of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$S_{\text{ab}}$</tex-math></inline-formula> provided in the international guidelines and standards for limiting exposure to EM fields above 6 GHz. Results show marginal relative differences between the obtained values from the two different definitions (within about 6 %) in all considered scenarios. On the other hand, in comparison to flat models, the spatial maximum <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$S_{\text{ab}}$</tex-math></inline-formula> on the ear is up to about 20 % larger regardless of definition. These findings demonstrate a promising potential of the proposed method for the assessment of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$S_{\text{ab}}$</tex-math></inline-formula> on surfaces of anatomical models at frequencies upcoming for the 5th generation (5G) wireless networks and beyond

Estimation of Absorbed Power Density Based on Spherical Wave Expansion to Plane Wave Expansion for Exposure Assessment at 30 GHz

The international guidelines and standard specified the area-averaged absorbed/epithelial power density (APD/EPD) as the basic restriction at frequencies above 6 GHz. Assessment methods for APD are current under the consideration of standardization organizations. In this study, we first showed the enhancement of the APD due to the antenna/body interaction using full-wave simulations. We then proposed a hybrid method combining the spherical near-field measurement and numerical simulation of the exposure dose in human body, using an equivalent source reconstructed on the total field/scattered field boundary using the spherical wave expansion to plane wave expansion. By comparing with the full-wave simulated APD at 30 GHz for various antenna arrays, the proposed method gives a rather accurate estimation of the upper bound of the area-averaged APD (with relative differences < 28%). It can be utilized as an efficient method for electromagnetic safety assessment as the need for specific measurement equipment is eliminated.

Bubble Formation in ITER-Grade Tungsten after Exposure to Stationary D/He Plasma and ELM-like Thermal Shocks

Plasma-facing materials (PFMs) in the ITER divertor will be exposed to severe conditions, including exposure to transient heat loads from edge-localized modes (ELMs) and to plasma particles and neutrons. Tungsten is the material chosen as PFM for the ITER divertor. In previous tests, bubble formation in ITER-grade tungsten was detected when exposed to fusion relevant conditions. For this study, ITER-grade tungsten was exposed to simultaneous ELM-like transient heat loads and D/He (6%) plasma in the linear plasma device PSI-2. Bubble formation was then investigated via SEM micrographs and FIB cuts. It was found that for exposure to 100.000 laser pulses of 0.6 GWm−2 absorbed power density (Pabs), only small bubbles in the nanometer range were formed close to the surface. After increasing Pabs to 0.8 and 1.0 GWm−2, the size of the bubbles went up to about 1 µm in size and were deeper below the surface. Increasing the plasma fluence had an even larger effect, more than doubling bubble density and increasing bubble size to up to 2 µm in diameter. When using deuterium-only plasma, the samples showed no bubble formation and reduced cracking, showing such bubble formation is caused by exposure to helium plasma.

Intensity-dependent Temperature Rise Induced by Local Exposure to 26.5 GHz Quasi-Millimeter-Wave in Rat.

The widespread use of fifth-generation 5G millimeter-waves (MMW) generates concern about potential adverse health effects. The latest international guidelines for MMW exposure adopt an absorbed power density (APD) of 200 W/m2 to avoid a local temperature rise of 5°C in human tissues as an operational adverse health effect threshold. However, because APD is estimated by simulations using human tissue models, it is unknown whether a similar value can be confirmed for living tissues. The aim of this study was to investigate the relationship between APD and skin temperature rise in vivo, and to validate the estimated values. The rat dorsal skin was locally exposed to a 26.5 GHz quasi-MMW (qMMW) for 18 min using a patch antenna. The qMMW exposure intensities estimated by dosimetry were set to 0-500 W/m2 of APD. The temperatures in the dorsal skin and rectum were simultaneously measured during exposure. The qMMW-induced local temperature increase at different sites. The dorsal skin temperature increased by approximately 11.3°C at a maximum intensity of 500 W/m2, but the rectal temperature increased by only 0.6°C, indicating highly localized effects of exposure to rats. A significant correlation was observed between APD and skin temperature rise. The relationship provided a linear regression model, and a temperature rise of less than 5°C was estimated in the skin exposed to 200 W/m2 of APD. These results suggest that the operational threshold for the MMW exposure guidelines is valid under the present experimental conditions using rats.

Assessment of Transmitted Power Density in the Planar Multilayer Tissue Model due to Radiation from Dipole Antenna

Recent relevant safety guidelines IEEE-Std C95.1- 2019 and ICNIRP-RF Guidelines 2020 have converged towards 6 GHz as a transition frequency from specific absorption rate (SAR), as basic restriction quantity, to absorbed power density (APD). Namely, the penetration of electromagnetic waves into the human tissue rapidly decreases as frequency increases, therefore, tissue heating can be considered as superficial above 6 GHz. However, besides the APD, an alternative internal dosimetric quantity transmitted power density or TPD is sometimes computed since its relation to SAR is more obvious and is easier to obtain. This paper deals with an analytical/numerical approach to determine TPD in planar multi-layered model of the human tissue exposed to the dipole antenna radiation. Analytical approach deals with assumed sinusoidal current distribution, while numerical approach pertains to the determination of current by solving the corresponding Pocklington integro-differential equation via Galerkin-Bubnov Indirect Boundary Element Method. The novelty presented in this paper with respect to previous work is a multilayer geometry whose effects are considered via the corresponding Fresnel plane wave reflection/transmission approximation. Some illustrative results for current distribution, transmitted field, volume power density (VPD) and TPD at various frequencies and distances of the antenna from the interface are given.

Generation and delivery of free hydroxyl radicals using a remote plasma

We demonstrate a new gas-based generation source using a low power radio frequency driven atmospheric pressure plasma configured to deliver the radical flux into the far effluent region, well away from interference from other plasma factors such as electric fields, currents, and ultraviolet radiation. Using He–H2O gas chemistry isolated from the laboratory air, the plasma generated flux contains and other radicals including, O and HO2 as well as H2O2 which, along with, was found to vary with H2O vapour content and absorbed power density. Peak flux values were 2.3 nmol s−1 and 0.23 nmol s1 for H2O2 and respectively at a distance of 50 mm from the plasma, with 790 ppmv H2O and a power density of ∼108 W m−3. The maximum flux density was 4.5 × 1019 m−2s−1 falling to 1.7 × 1019 m2 s1 at 110 mm, equivalent to generation rates of 74 µM s1 and 28 µM s−1. Despite high recombination rates at the plasma exit, the escaping flux is still significant, indicating a viable delivery capability to downstream targets. Its performance with regard to generation rates compares well with traditional generation techniques such as radiolysis, advanced oxidation processes and enhanced Fenton-chemistry approaches where production rates are sub-µM s−1. Delivering precisely quantifiable fluxes provides new opportunities for scientific studies and technological opportunities in cell biology, atmospheric chemistry, protein unfolding and systematic dose studies for plasma-based and other related potential medical treatments.

Assessment of EMF Human Exposure Levels Due to Wearable Antennas at 5G Frequency Band.

(1) Background: This work aims to assess human exposure to EMF due to two different wearable antennas tuned to two 5G bands. (2) Methods: The first one was centered in the lower 5G band, around f = 3.5 GHz, whereas the second one was tuned to the upper 5G band, at 26.5 GHz. Both antennas were positioned on the trunk of four simulated human models. The exposure assessment was performed by electromagnetic numerical simulations. Exposure levels were assessed by quantifying the specific absorption rate averaged on 10 g of tissue (SAR10g) and the absorbed power density (Sab), depending on the frequency of the wearable antenna. (3) Results: the higher exposure values that resulted were always mainly concentrated in a superficial area just below the antenna itself. In addition, these resulting distributions were narrowed around their peak values and tended to flatten toward lower values in farther anatomical body regions. All the exposure levels complied with ICNIRP guidelines when considering realistic input power. (4) Conclusions: This work highlights the importance of performing an exposure assessment when the antenna is placed on the human wearer, considering the growth of wearable technology and its wide variety of application, particularly regarding future 5G networks.

Computation of Absorbed Power Densities in High-Resolution Head Models by Considering Skin Thickness in Quasi-Millimeter and Millimeter Wave Bands

The fifth-generation wireless communications system has been increasingly deployed by telecommunication companies worldwide. This system uses a high-frequency band ranging from 24 GHz to 28 GHz. However, the number of studies assessing the human protection from the electromagnetic fields is limited in this frequency band. In addition, two main international bodies—the International Commission on Non-Ionizing Radiation Protection and IEEE—have revised their guidelines and standard in 2020 and 2019, respectively. One primary change is the exposure metric and corresponding exposure limit at frequencies >6 GHz because the penetration depth of the electromagnetic waves in biological tissues is 1 cm or less for these frequencies. Therefore, modeling of the skin and subcutaneous tissues is important. This study evaluated the total absorbed power (TAP), the absorbed power density (APD), and variations in APD owing to skin thickness in seven types of realistic human head models with different skin thicknesses. Our results showed that the model without surface smoothing resulted in higher TAP than the model with smoothing. The amount of the TAP change in frequency domain in the model with realistically varied skin thickness was less than that in the model with uniform skin thickness because of the suppression of strong standing waves. The maximum variations in the APD, averaging over 4 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> or 1 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> with respect to skin thickness, were approximately 20% and 10%, respectively. For the limit of incident power density, APDs were below the guideline. These results will be useful for future revisions of international guidelines.

Effect of potassium bubbles on the thermal shock fatigue behavior of large-volume potassium-doped tungsten alloy

A newly developed large-volume potassium-doped tungsten (W–K) plate with a thickness of 15 mm and a weight of 25 kg by powder metallurgy plus hot rolling was prepared to meet the requirements of the International Thermonuclear Experimental Reactor (ITER) in engineering application. In order to clarify the effect of K doping on the thermal shock performance of W–K alloy, transient thermal shock tests with a single-pulse duration of 1 ms for 100 shots at room temperature were performed. The absorbed power density is set to 0.33, 0.44, 0.55 and 0.66 GW m−2, respectively. Furthermore, the microstructure, Vickers micro-hardness before and after the transient thermal shock, thermal conductivity and relative density were also characterized. The results indicate that the cracking threshold of rolled W–K is 0.44–0.55 GW m−2, which possesses a better transient thermal shock resistance compared with the most of advanced W-based materials. This is mainly because K doping can significantly improve the high-temperature stability and mechanical properties of W material without reducing its thermal conductivity. In particular, K bubbles can also effectively inhibit the formation and propagation of cracks during thermal shock. Moreover, the cracking mechanism of rolled W–K alloy is also discussed in detail. This study is helpful for building a trusted ITER database on advanced W-based materials that provides useful references for the selection of future plasma-facing materials.

Design and characterization of "flow-cell" integrated-flow active cooling for high-average-power ceramic optics.

We used COMSOL Multiphysics to design a prototype actively cooled "flow-cell" substrate targeted at high-average-power applications, fabricated the prototype from cordierite ceramic, and tested the substrate under load in our thermal loading test stand. Sub-aperture testing revealed average-power handling up to 3.88-W/cm2 absorbed power density, in excellent agreement with model predictions. Gratings fabricated on 2-in.-diam cordierite coupons were subjected to laser-damage testing and showed a damage threshold of 250 mJ/cm2.

Modeling the mechanism of micro / nanostructured surface formation in Al-Co-Cr-Fe-Ni and Co-Cr-Fe-Mn-Ni high-entropy alloys treated with a high current pulsed electron beam

In this work, we investigated, using transmission and scanning electron microscopy, the microstructure of Al-Co-Cr-Fe-Ni and Co-Cr-Fe-Mn-Ni non-equimolar high-entropy alloys treated by high current pulsed electron beams with an energy density of 30 J / cm2. Both alloys revealed a cellular crystallization structure with an average cell size of 192 ± 5 nm, and 453 ± 6 nm, correspondingly. The study aims to improve the model of the mechanism of formation of micro- and nanostructured surface layers taking into account combined thermocapillary, concentration-capillary, evaporative-capillary and thermoelectric instabilities at the melt-plasma interface. The results of the linear stability analysis showed that the absorbed power density is lost by evaporation that leads to a decrease in the values of the gradient of the undisturbed temperature and, an increase in the wavelength at which the maximum rate of growth of disturbances is observed. An analysis of the dispersion equation showed that the values of the wavelengths of disturbances on the surface of alloys in the liquid phase are 454 nm for Co-Cr-Fe-Mn-Ni and 189 nm for Al-Co-Cr-Fe-Ni, which deviate from the experimental data by no more than 2 %.

A Performance-Enhanced Absorbing Boundary Condition for FDTD Methods on Face-Centered Cubic Grids

Based on the auxiliary differential equation (ADE), a performance-enhanced convolution perfectly matched layer (CPML) for the face-centered cubic finite-difference time-domain (FCC-FDTD) method is developed in this letter. Unlike the traditional FDTD methods on Yee’s grids, for which the CPML technique is relatively straightforward to be implemented, poor performance, and even instability may occur in the FCC-FDTD method. To avoid these problems, the ADE technique is incorporated into the derivation, and the impact of node locations on absorbing boundary constitutive parameters is carefully considered. In addition, the derivation of the time-marching formulations is substantially simplified. The optimal ranges of constitutive parameters are found through a numerical sweep. Two numerical examples, the electromagnetic pulse propagation and absorbed power density of a brain model during mobile communication, are examined to validate the performance. The results suggest that the proposed ADE-CPML for the FCC-FDTD method shows good stability and accuracy.

Rapid sintering of barium titanate ceramics using direct and susceptor-assisted microwave heating

24 GHz microwave sintering of BaTiO3 ceramics has been studied under rapid heating at a rate of up to 300 °C/min to a maximum temperature of 1150–1300 °C with no isothermal hold. Under direct volumetric microwave heating the temperature difference measured between the center and periphery of the samples was 200–300 °C, and the estimated volumetrically absorbed power density was about 40 W/cm3. The use of a SiC susceptor made it possible to reduce the required microwave power and equalize the temperature distribution, but in this case densification started at a higher temperature. Evidence of recrystallization-induced microstructure transformation propagating outward from the core of the sample is presented. The possiblity of implementing control over the microstructure and functional properties by choosing the parameters of the microwave sintering regime is discussed.

Absorbed power density approach for optimal design of heaving point absorber wave energy converter: A case study of Durban sea characteristics

This work proposes an approach for the optimal sizing of a cylindrical heaving wave energy converter (WEC). The approach is based on maximising the absorbed power density (APD) of the buoy, with the diameter being the decision variable. Furthermore, two types of buoy shapes were compared to get the best option. The two buoy shapes are the cone cylinder buoy (CCB) and the hemisphere cylinder buoy (HCB). The aim was therefore to determine the best shape and as well as the optimal size of the cylindrical point absorber. To validate the approach, the simulation was performed under Durban (South Africa) sea characteristics of 3.6 m wave significant height and 8.5 s peak period, using the openWEC simulator. The buoy diameter range considered was from 0.5 m to 10 m for both shapes. Simulation results revealed that a diameter of 1 m was the optimal solution for both buoy shapes. Furthermore, the APD method revealed that the HCB was more efficient than the CCB. The power density of the HCB was 1070 W/m2, which was almost double the power density of the CCB, while the two shapes present almost the same absorbed power.

Monte Carlo Simulation of Clothed Skin Exposure to Electromagnetic Field With Oblique Incidence Angles at 60 GHz

This study presents an investigation of clothed human skin exposure to obliquely incident electromagnetic waves at 60 GHz. We clarified the combined impacts of the cloth material, incidence angle, and polarization on the assessment of transmittance and absorbed power density (APD) at the skin surface. A Monte Carlo simulation was conducted considering the thickness variation of the cloth material and skin tissue. For the case of transverse magnetic™ wave exposure, the transmittance increases with increasing incident angle up to the maximum transmittance angle in the range from 60 to 80°, which is known as the Brewster effects, regardless of textile materials and air gap between cloth and skin. The air gap results in a periodic fluctuation of the APD, where the variation is almost within 1 dB when the incident power density is constant and the incident angle is smaller than 40°. Our results also show that as the air gap increases to 2.5 mm, i.e., half-wavelength at 60 GHz in the air, the APD within the skin surface covered by typical cloth materials increases up to 40% compared with that of bare skin. Although the use of several cloth materials may increase the transmittance and APD in oblique incidence scenarios, all the results of the APD do not exceed the basic restriction for local exposure, demonstrating that the current guidelines for human exposure to electromagnetic fields are appropriate for preventing the excessive exposure at 60 GHz considering the impacts of oblique incidence angles and cloth materials.

Detection of sub-micrometer thermomechanical and thermochemical failure mechanisms in titanium with a laser-based thermoreflectance technique

This work reports on a pump–probe laser-based heating and sensing metrology to study the failure mechanisms of materials during extreme heat fluxes localized near surfaces, the localization of which is controlled by the focus of the laser beam and sensed by the reflection of a secondary probe laser. We focus the demonstration of these power density at failure tests on the damage mechanisms of commercially pure titanium metal during and after high heat fluxes induced from the absorbed laser energy. Using this steady-state thermoreflectance pump–probe metrology, a localized region of the material was irradiated at a low modulated frequency, while the average change in the thermoreflectance signal was monitored. We observe surface and cross-sectional oxidation of the titanium, revealing correlations between microstructural evolution events and shifts in thermoreflectance trends as a function of absorbed power density. Furthermore, the damage morphology was shown to be heavily influenced by the size of the heater (dictated by the radius of the pump laser beam), which controlled the relative degree of thermomechanical, melting, and oxidative decohesion failure mechanisms in the samples. The analysis of the temperature distribution coupled with the observed microstructural damage gives rise to a high-throughput experimental technique to induce desired deformation modes through cyclic thermal testing.

Damage behaviors in microstructures and mechanical properties of pure tungsten induced by repetitive thermal loads

Damage behaviors of ITER grade tungsten (W) induced by repetitive thermal loads have been investigated using an electron beam device EBMP-30. The metallographic observation, backscattered electron scanning electron microscopy, electron backscattered diffraction, atomic force microscopy analysis and tensile tests were carried out after the repetitive heat loads. Results indicate that there are negligible effects on the microstructures, surface morphologies and mechanical properties with the absorbed power densities (APD) lower than 10 MW/m 2 . While serious damages including the severe surface roughness with protruding structures along grain boundaries, and significant grains coarsening with the evolution of the grain shape occur undesirably due to the full recrystallization during the repetitive 30 MW/m 2 heat loads. Furthermore, after exposed to an APD of 30 MW/m 2 with 50 heat loads, the tensile properties deteriorate dramatically, resulting in a limited ultimate tensile strength of only 328 MPa and even a zero total elongation at 300 °C. The relationships between the thermal loads, microstructures evolution and degradation of mechanical properties have been analyzed.

Low-temperature internal quantum efficiency of GaInN/GaN quantum wells under steady-state conditions

We compare the low-temperature photoluminescence (PL) intensities of a range of GaInN/GaN quantum well (QW) structures under identical excitation conditions, mounting the samples side by side. Normalizing the measured intensity to the absorbed power density in the QWs, we find that low-temperature PL efficiencies of several samples, which show close to 100% internal quantum efficiency (IQE) in time-resolved PL, saturate at nearly an identical value. Of course, this is strong indicative of being 100% IQE at low temperature for those efficient samples. Using the low-temperature PL efficiency as a ‘Reference’, on the other hand, we observe not only the effects of temperature-independent non-radiative losses on the low-temperature IQE, but also are able to determine the IQE of arbitrary samples on an absolute scale. Furthermore, we prove the experimental results by comparing the low-temperature efficiencies of a sample with an initial 100% IQE after intentionally introducing structural defects with argon-implantation.

Assessment of Human Exposure Levels Due to Mobile Phone Antennas in 5G Networks.

The recent deployment of 5G networks is bringing benefits to the population but it is also raising public concern about human RF-EMF exposure levels. This is particularly relevant considering the next 5G mobile devices, which are placed in close proximity to the subjects. Therefore, the aim of the following paper is focused on expanding the knowledge of the exposure levels in 5G exposure scenarios, specifically for mobile applications, using computational methods. The mobile antenna was designed considering the 5G technology innovations (i.e., mm-wave spectrum, beamforming capability, high gain and wide coverage), resulting in a phased-array antenna with eight elements at the working frequency of 27 GHz. To assess the exposure levels, different types of skin models with different grades of details and layers were considered. Furthermore, not only was the presence of a mobile phone user simulated, but also that of a person in their proximity, who could be hit by the main beam of the phased-array antenna. All the simulations were conducted in Sim4Life platform, where the exposure levels were assessed in terms of absorbed power density averaged over 4 cm2 and 1 cm2, following the ICNIRP guidelines. The results highlighted that the use of the homogeneous skin model led to the absorbed power density peaks being greatly underestimated, with respect to those obtained in multilayer skin models. Furthermore, interestingly, we found that the exposure levels obtained for the person passing nearby were slightly higher than those experienced by the mobile phone user himself. Finally, using the allowed input power for real mobile applications, all the values remained below the limits indicated by the ICNIRP guidelines.