Self-Powered Temperature-Mapping Sensors Based on Thermo

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Self-Powered Temperature Mapping Sensors Based on Thermo-Magneto-Electric Generator Jinsung Chun, Ravi Anant Kishore, Prashant Kumar, Min-Gyu Kang, Han Byul Kang, Mohan Sanghadasa, and Shashank Priya ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17686 • Publication Date (Web): 23 Feb 2018 Downloaded from http://pubs.acs.org on February 23, 2018

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ACS Applied Materials & Interfaces

Self-Powered Temperature Mapping Sensors Based on ThermoMagneto-Electric Generator

Jinsung Chun,† Ravi Anant Kishore,† Prashant Kumar,† Min-Gyu Kang,† Han Byul Kang,† Mohan Sanghadasa‡ and Shashank Priya*,†

†

Center for Energy Harvesting Materials and System (CEHMS), Bio-Inspired Materials

and Devices Laboratory (BMDL), Virginia Polytechnic Institute and State University, VA 24060, USA. ‡

U.S. Army Aviation and Missile Research, Development, and Engineering Center,

Redstone Arsenal, AL 35898, USA.

Corresponding Author Email: [email protected]

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: We demonstrate a thermo-magneto-electric generator (TMEG) based on second order phase transition of soft magnetic materials that provides promising pathway for scavenging low grade heat. It takes advantage of the cyclic magnetic forces of attraction and repulsion arising through ferromagnetic-to-paramagnetic phase transition to create mechanical vibrations which are converted into electricity through piezoelectric benders. In order to enhance the mechanical vibrations frequency and thereby output power of TMEG, we utilize the nonlinear behavior of piezoelectric cantilevers and enhanced thermal transport through silver (Ag) nanoparticles (NPs) applied on the surface of a soft magnet. This resulted in large enhancement of oscillation frequency reaching up to 9 Hz (300% higher compared to prior literature). Optimization of piezoelectric beam and Ag NP distribution resulted in realization of nonlinear TMEG that can generate high output power of 80 µW across the load resistance of 0.91 MΩ, which is 2200% higher compared to the linear TMEG. Using non-linear TMEG, we fabricated and evaluated self-powered temperature mapping sensors for monitoring the thermal variations across surface. Combined, our results demonstrate nonlinear TMEG that can provide additional functionality including temperature monitoring, thermal mapping and powering sensor nodes.

KEYWORDS: Thermomagnetics, Phase transition, Piezoelectrics, Nanogenerator, Selfpowered temperature mapping sensor


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INTRODUCTION Harvesting wasted energy, freely available in the environment in the form of heat, light, magnetic field and mechanical oscillations, for powering small electronic devices is attracting increasing attention.1-3 Recently, the concept of self-powered system has also been propagated, where a device can perform all its function by harvesting energy from the environment.4 Variety of exciting solutions have been proposed in literature for converting heat, light, magnetic field and mechanical oscillations into electricity including thermoelectrics, photovoltaics, multiferroics, and piezoelectrics based generators.5-23 Among all the choices for energy harvesting, low grade (< 230oC) wasted heat is a very important source as almost 40 ~ 60% of energy in all the processes is converted into wasted heat.24 Recently, we have proposed a promising methodology for high efficiency low grade thermal energy harvesting using thermo-magneto-electric generator (TMEG). The operating mechanism of TMEG is based on second order phase transition of soft ferromagnetic materials resulting in mechanical vibrations in the presence of a thermal gradient and a hard magnet.25 The driving force for soft magnet mechanical oscillation is related to balance of magnetic force of attraction (which moves the soft magnet towards hard magnet) and non-linear spring force (which moves the soft magnet away from hard magnet). Assuming that the soft magnet has phase transition around room temperature (RT) and the hard magnet is at least 20oC above RT, one can achieve continuous cycling of soft magnet between hot-side and cold-side,26-31 and this mechanical energy is converted into electrical energy through a piezoelectric material, as schematically depicted in Figure 1a. In past few years, theoretical approaches predicting the output



power density and the conversion efficiency for TMEG have been developed.25,32 Further, thermal interfaces for increasing the heat transfer from soft ferromagnetic material have also been the focus of development.33 A self-powered sensor node operates by harvesting energy from environment. Using TMEG, self-powered temperature sensor can be developed for feedback control in electronic cooling, motors, and vehicles. Here, we demonstrate a high power TMEG, composed of Gadolinium (Gd) soft magnet with piezoelectric polyvinylidene difluoride (PVDF) and macro fiber composite (MFC) nonlinear elastic cantilevers and thermal interface consisting of Ag nanoparticles (NPs). Based upon the prior literature, it can be clearly observed that bimorph cantilever based TMEG provides higher output voltage and vibration frequency up to 4.2 V and 3 Hz respectively. Our TMEG architecture resulted in a significant enhancement of the operating frequency, reaching 9 Hz (>300% increase compared to prior literature), and 8.8 V output voltage (>200% increase compared to prior literature) respectively.34 For optimum configuration corresponding to width ratio of 0.375 and Ag concentration of 1.12 wt%, the TMEG provided output power of 80 µW across resistance of 0.91 MΩ. This is more than 22-fold power enhancement (2200% increase), compared to the linear TMEG.34 Based on TMEG arrays, the fabricated selfpowered temperature mapping sensors were shown to provide precise temperature distribution on the heater surface.

EXPERIMENTAL SECTION Fabrication of nonlinear cantilever based TMEG. A 127 µm-thickness gadolinium (Gd, 99.9% purity) foil (Alfa Aesar Co., Inc., USA) of an area of 1 × 1 cm2


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was used to prepare soft ferromagnetic materials for the fabrication of TMEG, as described previously. Prior to locating Gd foil between the hard magnet and cooler of the generator, a 1.58 mm-thickness neodymium (Nd) of area 5 × 5 mm2 (MAGCRAFT, USA) with a higher Curie temperature than that of Gd was attached to Peltier heater (1.5 × 1.5 cm2) using polysynthetic silver thermal paste to enhance the heat transfer from heater to Nd. Thermal paste was dried for 1h on a hot plate at 50oC. The Peltier cooler was mounted onto an aluminum heat sink with a fan for fast cooling up to ~10oC. To control exact distance between hard magnet and cooler, a distance controller was made from an insulating plastic frame by using a CubePro 3D printer (3D Systems, Inc., USA). To induce nonlinearity, trapezoidal-shaped cantilevers of stainless steel with width ratio from 0.372 to 1 were fabricated and implemented in bimorph cantilever-type architecture with 200 µm thick PVDF consisting of vertical Pt electrodes (KUREHA, USA) and P2type MFC (Smart Material Corp., USA). A soft ferromagnetic material was suspended on the nonlinear cantilevers having an area of 1 × 5 cm2, and located adjacent to cooler for forming a restoring force like a spring. Preparation of Ag NPs coated soft magnet. An aqueous suspension of 60 nmdiameter Ag NPs (0.02 mg/mL, Sigma Aldrich, USA) was used to cover the surface of the soft magnet. Prior to spin-coating of the Ag NP suspension on Gd film, a drop of the suspension was pipetted onto the film. Spin-coating of the film was performed at 1300 rpm for 3 s, and then the sample was dried for 5 h in a dry box at room temperature. The spinning speed and slow drying were optimized because both were critical in obtaining uniform nanoparticle templates without nanoparticle-free regions or agglomeration.



Fabrication of self-powered temperature mapping sensors. Heater and cooler arrays (5 × 5) with an area of 1.5 × 1.5 cm2 were fabricated on a 2 mm-thickness aluminum plate and aluminum heat sink with a fan, respectively. Hard magnets (5 mm × 5 mm) and soft magnets suspended on the nonlinear cantilevers were attached to heaters and adjacent to coolers, respectively. The heater arrays were mounted onto cooler arrays with a specific spacer with thickness of 1.5 cm to maintain the distance of 3 mm between hard magnet and cooler. Finally, self-powered temperature mapping sensors based on TMEG could be obtained. Measurement of electrical output and characteristics. To detect voltages generated by TMEG, KEYSIGHT DSO 1014A oscilloscope was used for electrical measurements. The thermal conductivity of the Ag NP coated Gd was measured using the laser flash thermal conductivity measurement system TC-1200 (ULVAC-RIKO Inc., Japan). A digital force gauge (Mark-10 Corporation, USA) was used to measure the mechanical force of cantilever with displacement and magnetic force of soft magnet from hard magnet. The crystalline properties of Ag nanoparticle coated soft ferromagnetic material were measured using a high resolution X-ray diffractometer (Bruker, Germany). The resistance of the oscilloscope is 10 MΩ. The characterization of the soft ferromagnetic material for its magnetic properties was performed using an evercool physical property measurement system (PPMS, Quantum Design).

RESULTS AND DISCUSSION The schematic layout of TMEG structure is shown in Figure 1a and detailed information on assembly is described in Experimental Details section. The TMEG consists of a Peltier


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heater (to generate controlled hot-surface of 50oC) of an area of 1.5 × 1.5 cm2 with a rectangular hard magnet (Nd) attached to it, and a soft ferromagnetic material (Gd) of an area 1 × 1 cm2 suspended on the nonlinear cantilever with bimorph configuration (PVDF and MFC). A Peltier cooler was also mounted onto an aluminum heat sink with a fan for maintaining the cold-side at ~−10oC. Figure 1b shows the photograph of a fabricated TMEG having dimension of 5 × 1.5 cm2. To increase heat transport, 60 nm-diameter Ag NPs were coated on both top and bottom surfaces of 127 µm thick Gd, as shown in Figure 1c. It can be observed in Figure 1d that Ag NPs are uniformly coated on Gd surface. The morphology of Ag NP coated Gd films are shown in Figure S1. In the initial state, Gd is attracted to the hard magnet due to magnetic force of attraction. When heated above its Curie temperature (~294 K), Gd transitions into paramagnetic state, and is then pulled away from the hard magnet by restoring force of the cantilever. When Gd cools down below Curie temperature, it shifts back to ferromagnetic state and is attracted to the hard magnet. This continuous cycling between hard magnet and heat sink creates a mechanical drive that can be harvested using the piezoelectric material (see Figures S2 and S3, Movie M1). Gd films coated with Ag NP concentrations ranging between 0 to 1.12 wt% were characterized by X-ray diffraction (XRD). Figure 2a shows the XRD of Gd film without Ag NPs representing the baseline peaks for Gd. As Ag nanoparticles were coated on Gd surface, peak corresponding to (111) plane for Ag appeared at 38o. The intensity of Ag peak increased with the concentration of Ag NPs on Gd surface. To investigate the thermal effect of Ag NPs, out-of-plane thermal conductivity of Ag NP coated Gd films were measured as a function of temperature from 300 K to 400 K, as shown in Figure 2b.



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The Gd film without Ag NPs exhibited thermal conductivity of 11.2 W/mK at 400 K. However, as observed here, thermal conductivity of Ag NPs increased up to 12 W/mK at 400 K. This enhancement can be attributed to higher thermal conductivity of Ag NPs than that of Gd film.35 To verify the influence of Ag NPs, theoretical thermal conductivity values of Gd film with Ag NPs were calculated as a function NP concentration. (Figure S4) Since thickness of Ag film is less than 5 µm, the thermal contact resistance can be ignored, and equivalent conductivity (κeq) of Ag coated Gd can be theoretically expressed as:

=

( )

(1)

where LAg and LGd represent thickness of the silver film and Gd respectively along the direction of heat conduction, and κAg and κGd represent the thermal conductivity of Ag NPs and Gd respectively. Figure S5 depicts measured and calculated thermal conductivity as a function of Ag NP concentrations from 0 to 1.12 wt%. Calculated value for Ag concentration of 1.12 wt% is also compared with the experimental results. The maximum difference between the experimental and calculated results was found to be about ~5.6%. Temperature-dependent magnetization was measured to confirm the magnetic characteristics of Ag NP coated Gd films. Figure 2c shows magnetization (M) as a function of temperature (T) for the reference Gd film and Ag NP coated Gd films with concentrations ranging from 0 to 1.12 wt%, after field-cooling at 500 Oe. The results indicate that Curie temperature (294 K) of Gd films and the rate of transition (dM/dT) between ferromagnetic and paramagnetic state is not shifted by the addition of Ag NP coating, although magnetization is slightly reduced. Figure 2d shows the magnetic


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hysteresis for reference Gd film and Ag NP coated Gd films at 250 K. The magnetic moment of Ag NP coated Gd films is relatively reduced by ~14%, compared to Gd film without Ag NPs. Similar to the result in Figure 2c, this reduction can be attributed to nonmagnetic Ag NPs coated on Gd film. In order to quantify the elastic nonlinearity of cantilevers with width ratio from 0.375 to 1, deflection measurements were conducted. Figure 3a shows the forcedisplacement curve for cantilevers of dimension 1 × 9 cm2 as a function of width ratio. It is obvious that the cantilever with width ratio of 0.375 shows higher degree of elastic nonlinearity at larger deflections, compared to cantilever width ratio of 1. As the width ratio decreases, the resistance to deformation decreases as the cantilever has non-uniform strain distribution.36-39 To verify magnetic force relative to elastic nonlinearity of the cantilever, the magnetic force was measured by varying the distance of hard magnet from Gd film, as shown in Figure 3b. At the distance of 0.7 mm, the magnetic force shows the maximum value of 5.7 N. As the distance between hard magnet and Gd film decreases, the magnetic force decreases.25, 32 Given short gap distance, the magnetic field is mainly confined along the vertical axis of the device. Thus, the vibration frequency of cantilever is a resultant of three effects – vertically aligned magnetic field, vertically aligned spring force, and thermal transport from Nd to Gd and then from Gd to heat sink. To further investigate the elastic nonlinearity, we developed a numerical model that captured the behavior of cantilevers with different width ratios. (Figure S6) As shown in Figures 3c and 3d, the model was validated by calculating the force – displacement curve for a cantilever with uniform cross-section and comparing it with the known results.40



Figures 4a and 4b show the output voltage and the vibration frequency of the TMEG without Ag NPs, as a function of the width ratio from 0.375 to 1 at a fixed gap distance of 1 mm. In order to apply constant thermal gradient of 60oC, the surface temperature of hard magnet attached to heater and cooler was maintained and measured for 1 h. The thermal gradient (∆T) was saturated to 60oC (hard magnet at 50oC and cooler at −10oC) after 10 min. (see the Supporting Information Figure S7). Under a thermal gradient of 60oC, the TMEG with width ratio of 1, generated a peak voltage of 2.4 V at vibration frequency of 1.6 Hz. As the width ratio decreases to 0.375, the output voltage and the vibration frequency increase to 8.2 V and 7.3 Hz respectively, providing more than 3-fold power enhancement. To verify the effect of Ag NPs as a thermal interfacial material, the output voltage and the vibration frequency of TMEG as a function of Ag NP concentration from 0 wt% to 1.12 wt% were measured at a fixed gap distance of 1 mm, as shown in Figures 4c and 4d. As the Ag NP concentration increases to 1.12 wt%, the output voltage and the vibration frequency increased to 8.8 V and 9 Hz respectively, providing a 3.5-fold power enhancement, compared to the TMEG a cantilever with uniform cross-section (2.5 V and 1.8 Hz respectively). These results indicate that the enhanced heat transfer on Gd surface results in large output voltage and high vibration frequency. In addition, for the width ratio of 0.375, the output voltage of the TMEG measured as a function of Ag NP concentrations, resulted in further increase in output voltage and vibration frequency (see Figure S8). The above measurements were repeated with MFC cantilever and similar results were observed as shown in Figures 2a and 2c. At width ratio of 0.375 and Ag NP concentration of 1.12 wt%, the output voltage and the vibration frequency of MFC cantilever based TMEG was 5 V and 9 Hz respectively. The


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Stability test for 60 min is shown in Figure S9 in supporting information. To optimize the output power of TMEG, the output voltage was measured with external loads varying from 1 Ω to 10 MΩ, as shown in Figure 4e. The electrical output of the TMEGs at variable external loads were measured using an oscilloscope with internal impedance of 10 MΩ, as shown in the inset of Figure 4e, where Vout is the measured voltage drop across the resistance R. The current (Iout) through the resistance R could be calculated as Vout/R, (see the Supporting Information Figure S10) and the output power (Pout) of the TMEG could be defined as VoutIout with various resistances. It should be noted that R is required to be less than 10 MΩ due to the resistance of oscilloscope.41 The optimum resistance (Rext) at the maximum power output is ~1 MΩ, whereas the internal resistance (Ros) of oscilloscope is 10 MΩ. Therefore, the equivalent resistance is calculated to be ~0.91 MΩ. Consequently, the instantaneous power of 80 µW obtained across the resistance of 0.91 MΩ, as shown in Figure 4f, was 22 times higher than that of TMEG with uniform cross-section (3.6 µW). Figure 5a shows the average output voltage and the vibration frequency of the TMEG with a width ratio of 0.375 and Ag NP concentration of 1.12 wt%, as a function of temperature on hot side from 30oC to 50oC (thermal gradient from 40oC to 60oC) at a fixed gap distance of 1 mm and cold side temperature of −10oC. The generated vibration frequency decreases linearly from 9 Hz to 0.3 Hz as hot side temperature decreases to 30oC. (see the Supporting Information Figure S11) The normalized vibration frequency change is defined as ∆f/f0 = (f−f0)/f0, where f and f0 are the frequencies at hot side temperature of 30oC and 50oC, respectively. The frequency change exhibits linear dependence on temperature, and the frequency sensitivity (S) is defined as S = δ(∆f/f0)/δT,



indicating the slope of the curve, as shown in Figure 5a, where δT is the change in thermal gradient. The sensitivity of TMEG is calculated to be 2.2°C−1, which is better than that of the previously reported temperature sensors.42-46 The minimum detection limit here refers to the lowest temperature (30oC) where phase transition can be obtained in the soft magnetic material. However, this limit can be tailored as needed by selecting soft magnetic materials with suitable transition temperature. In our case, the temperature regimes were defined by keeping solar powered UAV (Unmanned aerial vehicle) and smart homes as desired applications. Based on the ability to sense temperature, we demonstrated self-powered temperature mapping sensor for monitoring the local heat distribution on aluminum plate. A heater (1.5 × 1.5 cm2) with a hard magnet was attached to aluminum plate of an area of 10 × 10 cm2. To detect the local heat distribution on the plate, 3 × 5 cooler arrays with Gd (1 × 1 cm2) suspended on piezoelectric cantilevers with non-uniform cross-section were fabricated. As heat on aluminum plate was distributed by the heater at the center, infrared (IR) image and two-dimensional color mapping of the peak value of the frequency responses show temperature distribution on the plate, as shown in Figure 5b. These plots elaborate the spatial resolution of the TMEG matrix for mapping the applied temperature. To further investigate the temperature sensing capability, the self-powered temperature sensors with 25 × 25 pixels were fabricated as shown in Figure 5c. We accomplished formation of letter-shaped temperature distributions, such as “V” and “T”. Figure 5d shows two-dimensional color mapping that reconstructs the applied temperature of each array and maps overall temperature obtained from the heater arrays. The self-powered temperature sensors reconstructed letters “V” and “T” using vibration frequency response (see the Supporting Information Figures S12


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and S13 for details). The resolution and sensitivity of the temperature mapping system depended on number of parameters, such as the number of TMEG devices in arrays, nonlinearity of cantilever, and Ag NP concentration. Since this sensor can quantify temperature distribution accurately and provide reliable temperature detection with vibration frequency, all forms of temperature variations from electronic devices can be detected. TMEG based temperature mapping sensor demonstrate the potential to be applied in temperature monitoring system to collect information from automobile engines, pipes carrying hot water, radiators, electrical heat sinks, CPUs of desktop computers, and solar powered UAVs.

CONCLUSIONS We report a thermo-magneto-electric generator (TMEG), composed of a soft magnet mounted on a piezoelectric polyvinylidene difluoride (PVDF) or macro fiber composite (MFC) cantilever and a hard magnet attached to the hot surface. Balancing the magnetic force of attraction with spring force of a cantilever resulted in sustained mechanical oscillations, while enhancing the thermal transport at soft magnet surface. In order to enhance the vibration frequency and output performance of TMEG, a non-uniform crosssection cantilever and Ag nanoparticles (NPs) based thermal interface was employed. It resulted in significant enhancement of the frequency and output performance up to 9 Hz and 8.8 V respectively. Under a width ratio of 0.375 and Ag concentration of 1.12 wt%, the TMEG could deliver output power of 80 µW across a load resistance of 0.91 MΩ. This output power is 22-fold higher compared to the TMEG with uniform cross-section cantilever. Self-powered temperature mapping sensors based on TMEG arrays were



developed that demonstrated high accuracy in mapping temperature distribution on heater surface. This work demonstrated the possibility of using TMEG for thermal energy harvesting, and for temperature monitoring and thermal management of electronics.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Fabrication of TMEG device and Ag NPs coating process; XRD, thermal conductivity, and magnetic characteristics for soft ferromagnetic films; Mechanical characteristics of nonlinear cantilevers via experiment and calculation; Output performance and optimization of output power of TMEG; Modeling of nonlinear cantilevers for calculation; Thermal gradient on TMEG; Sensing performance of TMEG arrays (PDF) A movie showing Operating of TMEG (AVI)

ACKNOWLEDGMENTS This work was supported by AMRDEC through Center for Energy Harvesting Materials and Systems (CEHMS). S.P. would like to acknowledge support from office of basic energy science, department of energy. P.K. would like to acknowledge the support from Office of Naval Research through grant number N00014-14-1-0158. M.G.K. would like to acknowledge the support from NSF-CREST Grant number HRD


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1547771. R.K. and H.B.K. would like to thank DARPA MATRIX program for financial support. We are thankful to Jue Wang and Prof. Scott T. Huxtable for their assistance with thermal conductivity measurement.



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Figure 1. (a) Schematic layout of TMEG structure that consists of a heater (50oC) and a cooler (−10oC) with rectangular hard magnet (Nd) and a soft ferromagnetic material (Gd) suspended on the nonlinear cantilever with bimorph configuration. (PVDF and MFC) (b) A photograph of the fabricated TMEG device mounted onto an aluminum heat sink with fan for maintaining the cold-side at ~−10oC. (c) Schematic diagrams for Ag NP coating process on Gd film as a thermal interface material. (d) Top-view and cross-sectional SEM images of Ag NP coated Gd film showing uniformly coated Ag NPs on Gd surface.



Figure 2. (a) XRD patterns with the peak corresponding to (111) plane for Ag appeared at 38o, (b) measured and calculated thermal conductivity, (c) Temperature-dependent magnetization measured after field-cooling at 500 Oe, and (d) the magnetizationmagnetic field hysteresis curves of Ag NP coated Gd film as a function of Ag NP concentration from 0 to 1.12 wt%. It can be observed here that Ag NP coating increases thermal conductivity of the soft ferromagnetic film up to 12 W/mK at 400 K due to higher thermal conductivity of Ag NPs than that of Gd film.


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Figure 3. (a) Force-displacement curve of nonlinear cantilevers as a function of width ratio from 0.375 to 1, showing higher-degree of elastic nonlinearity in the cantilever with width ratio of 0.375 at larger deflections. (b) Magnetic force of Gd film as a function of distance between hard magnet and Gd film. (c) Displacement distributions of rectangular (linear) and trapezoidal (nonlinear) shape cantilevers with an external load of 1 N. (d) Calculated force-displacement curve of nonlinear cantilevers as a function of width ratio from 0.375 to 1. These calculated results are in good agreement with the experiments.



Figure 4. (a, b) Generated output voltage and vibration frequency of nonlinear TMEG without Ag NPs as a function of width ratio from 0.375 to 1. (c, d) Generated output voltage and vibration frequency of nonlinear TMEG with Ag NP concentration of 1.12 wt% as a function of width ratio from 0.375 to 1. (e) The output voltage and current of nonlinear TMEG with a width ratio of 0.375 and a Ag NP concentration of 1.12 wt%, and (f) the output power of the nonlinear TMEG with an Ag NP concentration of 1.12 wt% with the resistance of external loads from 1 Ω to 10 MΩ as a function of width ratio from 0.375 to 1.


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Figure 5. (a) Generated voltage and vibration frequency of nonlinear TMEG with a width ratio of 0.375 and an Ag NP concentration of 1.12 wt% as a function of temperature on hot side from 30oC to 50oC. (b) IR image and two-dimensional color mapping of the peak value of the frequency responses showing temperature distribution on the plate, as heat on aluminum plate was distributed by the heater in center. (c) Pictures of the fabricated self-powered temperature sensors with 25 × 25 pixels and (d) Two-dimensional color mapping that reconstructs the applied temperature, such as “V” and “T”, of each array and maps overall temperature obtained from the heater arrays, resulting in reconstructing the letters “V” and “T” with vibration frequency response.



TOC figure


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Self-Powered Temperature-Mapping Sensors Based on Thermo

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