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Context Sudden cardiac deaths (SCDs) have accounted for nearly half of the line-of-duty deaths among US firefighters over the past 10 years. In 2018, 33% of all SCDs occurred after the end of a fire service call. Researchers have suggested that an imbalance in autonomic nervous system (ANS) regulation of heart rate postcall may interfere with recovery in firefighters. Objective To use heart-rate recovery (HRR) and heart-rate variability (HRV), 2 noninvasive markers of ANS function, to examine the ANS recovery profiles of firefighters. Design Cross-sectional study. Setting Firehouse and research laboratory. Patients or Other Participants Thirty-seven male career active-duty firefighters (age = 39 ± 9 years, height = 178.8 ± 5.4 cm, mass = 87.9 ± 11.2 kg). Main Outcome Measure(s) Percentage of maximal HR (%MHR) and HRV (natural log of the square root of the mean sum of the squared differences [lnRMSSD]) were collected after both submaximal and maximal exercise protocols during a 10-minute seated recovery. The HRR profiles were examined by calculating the asymptote, amplitude, and decay parameters of the monoexponential HRR curve for each participant. Results Differences in HRR parameters after 10 minutes of seated recovery were identified after submaximal versus maximal exercise (P < .001). In addition, although ANS was more suppressed after maximal exercise, HRV indicated incomplete recovery, and regardless of the test, recovery %MHR and lnRMSSD values did not return to pretest %MHR and lnRMSSD values. Conclusions Our results suggest that the ANS contributions to recovery in active-duty firefighters are exercise-intensity specific, and this is likely an important factor when establishing best-practice recovery guidelines.

We report the design, fabrication and experimental investigation of a spectrally wide-band terahertz spatial light modulator (THz-SLM) based on an array of 768 actuatable mirrors with each having a length of 220 μm and a width of 100 μm. A mirror length of several hundred micrometers is required to reduce diffraction from individual mirrors at terahertz frequencies and to increase the pixel-to-pixel modulation contrast of the THz-SLM. By means of spatially selective actuation, we used the mirror array as reconfigurable grating to spatially modulate terahertz waves in a frequency range from 0.97 THz to 2.28 THz. Over the entire frequency band, the modulation contrast was higher than 50% with a peak modulation contrast of 87% at 1.38 THz. For spatial light modulation, almost arbitrary spatial pixel sizes can be realized by grouping of mirrors that are collectively switched as a pixel. For fabrication of the actuatable mirrors, we exploited the intrinsic residual stress in chrome-copper-chrome multi-layers that forces the mirrors into an upstanding position at an inclination angle of 35°. By applying a bias voltage of 37 V, the mirrors were pulled down to the substrate. By hysteretic switching, we were able to spatially modulate terahertz radiation at arbitrary pixel modulation patterns.

In this work, a three-dimensional printable multilayer phased array system was designed to demonstrate the applicability of additive manufacturing for radio frequency (RF) systems. A hybrid process incorporating a thermal wire-mesh embedding method for conductors and thermoplastic material extrusion for dielectrics is employed. The designed phased array, operating at 3.5 GHz, consists of three functional layers: a 1-to-4 Wilkinson divider at the bottom, embedded voltage-controlled phase shifters at the center, and patch antennas on the top. Standalone parts of the proposed multilayer phased array were printed to verify the integrated dielectric-conductor printing process as well as the incorporation of active semiconductor devices at room temperature.

Interest in active metamaterial (MM) devices has recently increased due to their potential for tunable, switchable, and scalable optical responses. More specifically, a dynamic, on-chip MM polarizer has applications ranging from material characterization to sensing without the need for cumbersome external filters. This work demonstrates efforts to optimize MM devices for dynamic polarization filtering by combining elements from split-ring resonators, wire-pairs, and fishnet patterns. The polarization grid has been designed to operate under an applied voltage with simulated on/off ratios of 75% and dynamic polarization selectivity of 70%. Samples have been fabricated using epitaxial GaAs on sapphire with various n-type doping concentrations to approximate electrical tuning.

Polymer extrusion three dimensional (3D) printing, such as fused deposition modeling (FDM), has recently garnered attention due to its inherent process flexibility and rapid prototyping capability. Specifically, the addition of electrical components and interconnects into a 3D printing build sequence has received heavy interest for space applications. However, the addition of these components, along with the thermal load associated with space-based applications, may prove problematic for typical thermally insulating 3D printed polymer structures. The work presented here addresses thermally conductive polymer matrix composites (specifically, graphite, carbon fiber, and silver in an acrylonitrile butadiene styrene polymer matrix) to identify the effect of composite geometry and print direction on thermal anisotropic properties. The work also examines the effect of these composites on print quality, mechanical tensile properties, fracture plane analysis, micrograph imaging, and cube satellite thermal analysis. The thermal conductivity of 3D printed material systems in this work may enable the production of thermally stable 3D printed structures, supports, and devices. Key results of this work include anisotropic thermal conductivity for 3D printed structures related to print direction and filler morphology meaning that thermal conductivity can be controlled through a combination of print raster direction and material design. When the materials analyzed in this work are incorporated with other active cooling systems, space-based 3D printed applications can then be designed to incorporate increasing thermal loads, opening a new door to producing space-ready 3D printed structures.

We present a terahertz-SLM with a frequency working range from 1.0 THz to 2.3 THz. Over the complete frequency range, the spatial modulation contrast exceeds 50% with a peak modulation contrast of 87% at 1.38 THz. The pixels of the SLM consist of mirror arrays that can be selectively actuated by a bias voltage of 35 V. Each individual pixel can either work as a grating, that diffracts terahertz radiation away from the detector, or as a flat mirror, that reflects all terahertz radiation into the detector. The mirrors have a size of 220 μm x 100 μm. Due to the wide frequency working bandwidth of more than 1 THz, such modulators can be used as spatial light modulators in terahertz coded aperture imaging spectroscopes with single-pixel detectors.

We present a new type of terahertz spatial light modulator (THz-SLM) based on an array of large micromirrors. The electrostatically tunable micromirrors spatially modulate terahertz waves in a frequency range from 0.97 THz to 2.28 THz with a peak modulation contrast of 87% at 1.38 THz. The array consists of 768 micromirrors with each having a length of 220 µm and a width of 100 µm. The mirrors are grouped into 24 pixels. By application of a bias voltage, the pixels can be individually switched between a reflecting and scattering state, which allows spatial modulation of terahertz radiation.

Integrated circuits revolutionized electronics long time ago and paved the way towards minimized microprocessors today. In analogy, plasmonics aims at the creation of highly integrated optical networks on a small chip that enable the implementation of ultra-small sensors or optical processors. In the terahertz frequency regime, we investigate the propagation of tightly bound pure surface waves on specifically designed meta-surfaces. While most presented metasurfaces on a thin film in the literature support waveguide mode propagation in the thin film substrate, whose evanescent electromagnetic fields form the surface waves at the waveguide boundaries, we observed pure surface waves that are not coupled to a waveguide mode in the thin film. Such meta-surfaces are particularly advantageous for use as surface sensors, since the surface waves carry most of their energy in the space between the surface and air and almost no energy in the thin film substrate. This is in strict contrast to most of the presented meta-surfaces in literature so far, which guide a significant part of unusable energy in the inaccessible region of the substrate. Furthermore, we study structures of Goubau lines and meta-surfaces that combine excellent spectrally broadband terahertz surface wave guiding with frequency-selective meta-surface areas and meta-surface sub-wavelength resonators on a chip. In detail, we investigate the coupling efficiency between Goubau lines and meta-surfaces.

Izod impact test specimens were fabricated via a desktop grade material extrusion 3D printer process using ABS in four build orientations. The 3D printed impact test specimens were examined in order to compare the effect of stress concentrator fabrication on impact test data where two methods were used to fabricate the stress concentrating notch: (1) printing the stress concentrator; and (2) machining the stress concentrator where the dimensions of the notch matched those specified in the ASTM standard D256-10. In both test cases, sensitivity to build orientation was also observed. The sample sets with printed stress concentrators were found to be statistically similar to their counterparts with milled stress concentrators. The experiment was repeated again on a commercial grade material extrusion 3D printer using ABS, PC, PC-ABS, and Ultem 9085 and differences in impact test results were observed most notably when Ultem 9085 was tested. Scanning electron microscopy was utilized to perform fractograpy on impact test specimens to explore the effect of stress concentrator fabrication on the fracture surface morphology of the failed specimens. The work here demonstrates the need for materials testing standards that are specific to additive manufacturing technologies; as well as concluding that all-printed impact test specimens may offer the best representation of the impact characteristics of 3D printed structures.

Material extrusion 3D printing (ME3DP), based on fused deposition modeling (FDM) technology is currently the most widely available 3D printing platform. As is the case with other 3D printing methods, parts fabricated from ME3DP will exhibit physical property anisotropy where build direction has an effect on the mechanical properties of a given part. The work presented in this paper analyzes the effect of physical property-altering additives to acrylonitrile butadiene styrene (ABS) on mechanical property anisotropy. A total of six ABS-based polymer matrix composites and four polymer blends were created and evaluated. Tensile test specimens were printed in two build orientations and the differences in ultimate tensile strength and % elongation at break were compared between the two test sample versions. Fracture surface analysis was performed via scanning electron microscopy (SEM) which gave insight to the failure modes and rheology of the novel material systems as compared to specimens fabricated from the same ABS base resin. Here it was found that a ternary blend of ABS combined with styrene ethylene butadiene styrene (SEBS) and ultra high molecular weight polyethylene (UHMWPE) lowered the mechanical property anisotropy in terms of relative UTS to a difference of 22 ± 2.07% as compared to 47 ± 7.23% for samples printed from ABS. The work here demonstrates the mitigation of a problem associated with 3D printing as a whole through novel materials development and analyzes the effects of adding a wide variety of materials on the physical properties of a thermoplastic base resin.

Thermophotovoltaics (TPVs) are a potential technology for waste-heat recovery applications and utilize IR sensitive photovoltaic diodes to convert long wavelength photons (>800nm) into electrical energy. The most common conversion regions utilize Gallium Antimonide (GaSb) as the standard semiconductor system for TPV diodes due to its high internal quantum efficiencies (close to 90%) for infrared radiation (~1700nm). However, parasitic losses prevent high conversion efficiencies from being achieved in the final device. One possible avenue to improve the conversion efficiency of these devices is to incorporate metallic photonic crystals (MPhCs) onto the front surface of the diode. In this work, we study the effect of MPhCs on GaSb TPV diodes. Simulations are presented which characterize a specific MPhC design for use with GaSb. E-field intensity vs. wavelength and depth are investigated as well as the effect of the thickness of the PhC on the interaction time between the e-field and semiconductor. It is shown that the thickness of MPhC has little effect on width of the enhancement band, and the depth the ideal p-i-n junction is between 0.6μm and 2.1μm. Additionally, simulated results demonstrate an increase of E-field/semiconductor interaction time of approximately 40% and 46% for a MPhC thickness of 350nm and 450nm respectively.

Purpose – The purpose of this paper is to demonstrate the strategy for increasing the applicability of material extrusion additive manufacturing (AM) technologies, based on fused deposition modeling (FDM), through the development of materials with targeted physical properties. Here, the authors demonstrate materials specifically developed for the manufacture of electromechanical and electromagnetic applications, the use of FDM-type processes in austere environments and the application of material extrusion AM. Design/methodology/approach – Using a twin screw polymeric extrusion process, novel polymer matrix composites and blends were created where the base material was a material commonly used in FDM-type processes, namely, acrylonitrile butadiene styrene (ABS) or polycarbonate (PC). Findings – The work presented here demonstrates that, through targeted materials development, the applicability of AM platforms based on FDM technology can be increased. Here, the authors demonstrate that that the physical properties of ABS and PC can be manipulated to be used in several applications such as electromagnetic and X-ray shielding. Other instances of the development of new materials for FDM led to mitigation of problems associated with the process such as surface finish and mechanical property anisotropy based on build orientation. Originality/value – This paper is an overview of a research effort dedicated to increasing the amount of material systems available to material extrusion AM. Here materials development is shown to not only increase the number of suitable applications for FDM-type processes, but to be a pathway toward solving inherent problems associated with FDM such as surface finish and build orientation-caused mechanical property anisotropy.

Material-extrusion three-dimensional (3D) printing has recently attracted much interest because of its process flexibility, rapid response to design alterations, and ability to create structures ‘‘on-the-go’’. For this reason, 3D printing has possible applications in rapid creation of space-based devices, for example cube satellites (CubeSat). This work focused on fabrication and characterization of tungsten-doped polycarbonate polymer matrix composites specifically designed for x-ray radiation-shielding applications. The polycarbonate–tungsten polymer composite obtained intentionally utilizes low loading levels to provide x-ray shielding while limiting effects on other properties of the material, for example weight, electromagnetic functionality, and mechanical strength. The fabrication process, from tungsten functionalization to filament extrusion and material characterization, is described, including printability, determination of x-ray attenuation, tensile strength, impact resistance, and gigahertz permittivity, and failure analysis. The proposed materials are uniquely advantageous when implemented in 3D printed structures, because even a small volume fraction of tungsten has been shown to substantially alter the properties of the resulting composite.

Advances in the field of extrusion based 3-D printing have recently allowed the incorporation of embedded electronics and interconnects, in order to increase the functionality of these structures. This paper builds on previous work in the area of fine-pitch copper mesh and embedded copper wire capacitive sensors encapsulated within a 3-D printed structure. Three varieties of sensors were fabricated and tested, including a small area wire sensor (320-μm width), a large area mesh sensor (2 cm 2 ), and a fully embedded demonstration model. In order to test and characterize these sensors, three distinct tests were explored. Specifically, the capacitive sensors were able to distinguish between three metallic materials and distinguish salt water from distilled water. These capacitive sensors have many potential sensing applications, such as biomedical sensing, human interface devices, material sensing, electronics characterization, and environmental sensing. As such, this paper also characterizes the capacitive sensors for an active microfluidic mixer.

In this work, the design, fabrication and characterization of a 3-D printed microwave patch antenna is presented. The antenna is fabricated by combining fused filament fabrication method for the dielectric part and ultrasonic metal wire mesh embedding approach for the conductor part. Full wave finite-element simulations for different wire mesh structures and also the entire antenna have been done to make sure the embedded wire mesh has good performance at microwave frequency. A microstrip patch antenna working around 7.5 GHz is printed and characterized to demonstrate the efficiency and accuracy of this technique. The measured reflection coefficient shows a good resonance peak at 7.5 GHz. The measured gain of this antenna is 5.5 dB at the resonance frequency. Good agreement between simulation and measurement is obtained in both reflection coefficient and radiation pattern.

Material extrusion 3D printing (ME3DP) based on fused deposition modeling (FDM) technology is currently the most commonly used additive manufacturing method. However, ME3DP suffers from a limitation of compatible materials and typically relies upon amorphous thermoplastics, such as acrylonitrile butadiene styrene (ABS). The work presented here demonstrates the development and implementation of binary and ternary polymeric blends for ME3DP. Multiple blends of acrylonitrile butadiene styrene (ABS), styrene ethylene butadiene styrene (SEBS), and ultrahigh molecular weight polyethylene (UHMWPE) were created through a twin screw compounding process to produce novel polymer blends compatible with ME3DP platforms. Mechanical testing and fractography were used to characterize the different physical properties of these new blends. Though the new blends possessed different physical properties, compatibility with ME3DP platforms was maintained. Also, a decrease in surface roughness of a standard test piece was observed for some blends as compared with ABS.

We present gallium antimonide (GaSb) p–i–n photodiodes for use as thermophotovoltaic (TPV) cells grown on gallium arsenide (100) substrates using the interfacial misfit array method. Devices were grown using molecular beam epitaxy and fabricated using standard microfabrication processes. X-ray diffraction was used to measure the strain, and current–voltage (I–V) tests were performed to determine the photovoltaic properties of the TPV cells. Energy generation at low efficiencies was achieved, and device performance was critically analyzed.

Advanced technology, such as sensing and communication equipment, has recently begun to combine optically sensitive nano-scale structures with customizable semiconductor material systems. Included within this broad field of study is the aptly named frequency-selective surface; which is unique in that it can be artificially designed to produce a specific electromagnetic or optical response. With the inherent utility of a frequency-selective surface, there has been an increased interest in the area of dynamic frequency-selective surfaces, which can be altered through optical or electrical tuning. This area has had exciting break throughs as tuning methods have evolved; however, these methods are typically energy intensive (optical tuning) or have met with limited success (electrical tuning). As such, this work investigates multiple structures and processes which implement semiconductor electrical biasing and/or optical tuning. Within this study are surfaces ranging from transmission meta-structures to metamaterial surface-waves and the associated coupling schemes. This work shows the utility of each design, while highlighting potential methods for optimizing dynamic meta-surfaces. As an added constraint, the structures were also designed to operate in unison with a state-of-the-art Ti:Sapphire Spitfire Ace and Spitfire Ace PA dual system (12 Watt) with pulse front matching THz generation and an EOS detection system. Additionally, the Ti:Sapphire laser system would provide the means for optical tunablity, while electrical tuning can be obtained through external power supplies.

This work describes the design, fabrication, and evaluation of three sets of multi-plane patch antennas fabricated using an enhanced 3D printing system. The antennas were embedded using a unique combination of thermal and chemical embedding techniques enabling the fabrication of multiple conductive non-orthogonal antenna layers constructed in a single build sequence. Potential devices include waveguides and antennas in complex geometries with a wide range of electromagnetically tailored thermoplastics. Two sets of patch antennas were fabricated, both comprising two patches offset 5° from the build plane: one with embedded copper mesh and the second with copper foil. A third set of patch antennas was fabricated on a five-plane structure consisting of three patches with offset 60° from the build plane, and a fourth patch parallel to the build plane. The resulting 3D printed antennas were characterized in terms of return loss and radiation pattern and demonstrated agreement with simulations.

Small satellites, such as CubeSats, are increasingly being called upon to perform missions traditionally ascribed to larger satellite systems. However, the market of components and hardware for small satellites, particularly CubeSats, still falls short of providing the necessary capabilities required by ever increasing mission demands. One way to overcome this shortfall is to develop the ability to customize every build. By utilizing fabrication methods such as additive manufacturing, mission specific capabilities can be built into a system, or into the structure, that commercial off-the-shelf components may not be able to provide. A partnership between the University of Texas at El Paso, COSMIAC at the University of New Mexico, Northrop Grumman, and the NASA Glenn Research Center is looking into using additive manufacturing techniques to build a complete CubeSat, under the Small Spacecraft Technology Program. The W. M. Keck Center at the University of Texas at El Paso has previously demonstrated the ability to embed electronics and wires into the addtively manufactured structures. Using this technique, features such as antennas and propulsion systems can be included into the CubeSat structural body. Of interest to this paper, the team is investigating the ability to take a commercial micro pulsed plasma thruster and embed it into the printing process. Tests demonstrating the dielectric strength of the printed material and proof-of-concept demonstration of the printed thruster will be shown.

To achieve high gain and electronic beam steering, phased array systems are commonly used. The phased array technique plays an important role in sensing and communication systems. Practical phased array system usually consists of many components including RF power feeding networks, antennas, and active parts such as phase shifters and amplifiers. Single-layer implementation of phased arrays usually leads to large system size and limited applicability as the functionality becomes more complex. Multilayer structure is useful to make the system compact. In addition, it increases the flexibility to add more functions by increasing the total structure thickness without increasing the footprint size. However, it is more challenging to design and fabricate multilayer phased array. The vertical transitions between layers require careful design to have low loss and conventional fabrication technique may not be cost effective. Additive manufacturing (AM), which enables 3D objects of arbitrary shape to be printed automatically layer by layer, is a potentially promising technique to manufacture multilayer phased array that has reduced size and cost but still possesses good electromagnetic performance.

In this paper, a 3D printed multilayer microstrip line structure with vertical transition is designed, fabricated and characterized. The dielectric part of the structure is printed using the FDM method and the conductor part is printed using the ultrasonic wire embedding approach. The measured total insertion loss of the 3D printed multilayer microstrip (90 mm long) including the vertical transition is smaller than 2 dB below 6 GHz. The measured results agree well with the simulation. The performance of this structure demonstrates that 3D printing techniques may be able to realize functional multilayer RF components / systems. As an example, a 3D printed multilayer phased array is designed based on similiar microstrip and vertical transition structure in this work. The simulated results show good impedance matching around 3.5GHz and a high directive beam at expected direction.

In this paper, a polymer matrix composite method by mixing printable polymer with a number of possible nanoparticles is investigated to achieve artificially controlled effective electromagnetic properties for 3D additive manufacturing applications. The material properties are evaluated using effective medium theory. The results show that this method can achieve robust electromagnetic property control with various nanoparticle fillings.

Additive manufacturing (AM), often called 3D printing, has received much attention recently with impressive demonstrations ranging from musical instruments, to vehicles, to housing components or even entire buildings. Although it has been argued that 3D printing could be the future of manufacturing, the potential and applicability of these methods for creating functional electronics at RF / microwave frequency remain largely unexplored.

Through the application of a bias voltage, metamaterials can dynamically change their response, opening up new technological possibilities. Combining design elements from three common metamaterial patterns, we have created a metamaterial polarizing filter that will transmit all polarization orientations equally when in the static mode. When a bias voltage is applied, the filter will minimize the transmission of x-polarized light in the wavelength band of interest. Progress has been made on creating a sufficiently conductive metamaterial to enable the dynamic mode, as well as on incorporating several filters into a monolithic stack. Fabrication methods and transmission results for the required substrates will be discussed.

Recent advances in the field of 3D printing have utilized embedded electronic interconnects in order to construct advanced electronic devices. This work builds on these advances in order to construct and characterize arbitrarily formed capacitive sensors using fine-pitch copper mesh and embedded copper wires. Three varieties of sensors were fabricated and tested, including a small area wire sensor (320μm width), a large area mesh sensor (2cm2), and a fully embedded demonstration model. In order to test and characterize these sensors in FDM materials, three distinct tests were explored. Specifically, the sensors were able to distinguish between three metallic materials and distinguish salt water from distilled water. These capacitive sensors have many potential sensing applications, such as biomedical sensing, human interface devices, material sensing, electronics characterization, and environmental sensing. As such, this work specifically examines optimum mesh/wire capacitive parameters as well as potential applications such as 3D printed integrated material sensing.

A technology that can be used in place of, or in addition to, rectennas is thermophotovoltaics (TPVs). The ultimate function of TPVs, like that of the rectenna, is the conversion of electromagnetic radiation to DC current. Rectennas use a rectifying diode coupled with an antenna to achieve this conversion. TPVs achieve this conversion through a single diode which both receive the radiation and converts it to a current. While rectennas are superior at longer wavelengths (greater than 5 μm), TPVs are more efficient at shorter wavelengths (less than 5 μm). Although rectennas and TPVs have been investigated independently, a hybrid technology may be possible which incorporates components from both technologies.

The ability to efficiently transform heat into a usable energy using thermophotovoltaics (TPV) has been a topic of research for many years. Due to recent micro fabrication advances, TPV and photonic crystals (PhC) has been the subject of renewed interest. In particular, PhC have been shown to increase the efficiency of photon to current conversions for infrared photodetectors. Here, Photonic crystals and a back reflecting plane have been employed to increase the efficiency of TPV cells by creating a resonant cavity. The result is an increased interaction time between photons and excitons leading to an increase in electron/hole pair generation. A simulated 2D photonic crystal consisting of Si 3 N 4 rods in an ohmic contact material has demonstrated a possible 81% increase in absorption for a GaSb TPV cell.

Here, we report on the enhancement of photon conversion by integration of photonic crystal (PhC) and surface plasmon (SP) structures into thermophotovoltaic (TPV) cells. PhCs consisting of rods of air are incorporated into the base of semiconductor TPV cells to increase the duration of thermal photon absorption, thus significantly enhancing quantum and conversion efficiencies (QE and CE, respectively). The potential of PhCs to augment the CE of TPV cells for most IR wavelengths makes it a widely useful technology. The ability to turn waste heat into usable energy will improve efficiency in a variety of electrical and electromechanical systems.

We report a metamaterial design for a thermophotovoltaic (TPV) emitter. TPVs are similar to photovoltaic solar cells, but they convert heat to electricity instead of sunlight. The focus of this paper is on the emitter stage of the TPV system, which converts the heat into a spectral band which is easily absorbable by the TPV photodiode. The proposed structure consists of a platinum metallic element, an alumina dielectric spacer, and platinum grounding plane on a sapphire substrate. This perfect absorber based metamaterial emitter is shown to robustly operate at 600 °C. This temperature is high enough to enable TPV use for many industrial applications.

We report the development of a front-side contact design for thermophotovoltaics that utilizes metallic photonic crystals (PhCs). While this front-side grid replacement covers more surface area of the semiconductor, a higher percentage of photons is shown to be converted to usable power in the photodiode. This leads to a 30% increase in the short-circuit current of the gallium antimonide thermophotovoltaic cell.