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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 10796−10803
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, Blacksburg, Virginia 24060, United States ‡ U.S. Army Aviation and Missile Research, Development, and Engineering Center, Redstone Arsenal, Huntsville, Alabama 35898, United States ACS Appl. Mater. Interfaces 2018.10:10796-10803. Downloaded from pubs.acs.org by TULANE UNIV on 01/09/19. For personal use only.
S Supporting Information *
ABSTRACT: We demonstrate a thermo-magneto-electric generator (TMEG) based on second-order phase transition of soft magnetic materials that provides a 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 that are converted into electricity through piezoelectric benders. To enhance the mechanical vibration frequency and thereby the output power of the 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 results in large enhancement of the oscillation frequency reaching up to 9 Hz (300% higher compared with that of the prior literature). Optimization of the piezoelectric beam and Ag NP distribution resulted in the realization of nonlinear TMEGs that can generate a high output power of 80 μW across the load resistance of 0.91 MΩ, which is 2200% higher compared with that of the linear TMEG. Using a nonlinear TMEG, we fabricated and evaluated self-powered temperature-mapping sensors for monitoring the thermal variations across the surface. Combined, our results demonstrate that nonlinear TMEGs can provide additional functionality including temperature monitoring, thermal mapping, and powering sensor nodes. KEYWORDS: thermomagnetics, phase transition, piezoelectrics, nanogenerator, self-powered 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 functions by harvesting energy from the environment.4 A variety of exciting solutions have been proposed in the literature for converting heat, light, magnetic field, and mechanical oscillations into electricity including thermoelectrics, photovoltaics, multiferroics, and piezoelectrics-based generators.5−23 Among all choices for energy harvesting, lowgrade (300% increase compared with that of the prior literature), and 8.8 V output voltage (>200% increase compared with that of the prior literature), respectively.34 For the optimum configuration corresponding to a width ratio of 0.375 and Ag concentration of 1.12 wt %, the TMEG provided an output power of 80 μW across a resistance of 0.91 MΩ. This is more than 22-fold power enhancement (2200% increase), compared with that of the linear TMEG.34 On the basis of TMEG arrays, the fabricated self-powered temperaturemapping sensors were shown to provide precise temperature distribution on the heater surface.
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RESULTS AND DISCUSSION The schematic layout of the TMEG structure is shown in Figure 1a, and detailed information on the assembly is described in the Experimental Section. The TMEG consists of a Peltier heater (to generate a controlled hot surface of 50 °C) 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 ∼−10 °C. Figure 1b shows the photograph of a fabricated TMEG having a dimension of 5 × 1.5 cm2. To increase the 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 the Gd surface. The morphology of Ag NP-coated Gd films is shown in Figure S1. In the initial state, Gd is attracted to the hard magnet because of the magnetic force of attraction. When heated above its
EXPERIMENTAL SECTION
Fabrication of Nonlinear Cantilever-Based TMEG. A 127 μm thick gadolinium (Gd, 99.9% purity) foil (Alfa Aesar Co., Inc., USA) of an area of 1 × 1 cm2 was used to prepare soft ferromagnetic materials for the fabrication of TMEGs, as described previously. Prior to locating the Gd foil between the hard magnet and cooler of the generator, a 1.58 mm thick neodymium (Nd) of area 5 × 5 mm2 (MAGCRAFT, USA) with a higher Curie temperature than that of Gd was attached to 10797
DOI: 10.1021/acsami.7b17686 ACS Appl. Mater. Interfaces 2018, 10, 10796−10803
Research Article
ACS Applied Materials & Interfaces
Figure 2. (a) XRD patterns with the peak corresponding to the (111) plane for Ag appeared at 38°, (b) measured and calculated thermal conductivity, (c) temperature-dependent magnetization measured after field cooling at 500 Oe, and (d) the magnetization−magnetic field hysteresis curves of the Ag NP-coated Gd film as a function of the Ag NP concentration from 0 to 1.12 wt %. It can be observed here that the Ag NP coating increases the thermal conductivity of the soft ferromagnetic film up to 12 W/m K at 400 K because of higher thermal conductivity of Ag NPs than that of the Gd film.
Figure 3. (a) Force−displacement curve of nonlinear cantilevers as a function of the width ratio from 0.375 to 1, showing higher degree of elastic nonlinearity in the cantilever with a width ratio of 0.375 at larger deflections. (b) Magnetic force of the Gd film as a function of distance between the hard magnet and Gd film. (c) Displacement distributions of rectangular (linear) and trapezoidal (nonlinear) cantilevers with an external load of 1 N. (d) Calculated force−displacement curve of nonlinear cantilevers as a function of the width ratio from 0.375 to 1. These calculated results are in good agreement with the experiments.
Curie temperature (∼294 K), Gd transitions into the paramagnetic state and is then pulled away from the hard
magnet by restoring force of the cantilever. When Gd cools down below the Curie temperature, it shifts back to the 10798
DOI: 10.1021/acsami.7b17686 ACS Appl. Mater. Interfaces 2018, 10, 10796−10803
Research Article
ACS Applied Materials & Interfaces
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) Output voltage and current of nonlinear TMEG with a width ratio of 0.375 and an Ag NP concentration of 1.12 wt %, and (f) 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 the width ratio from 0.375 to 1.
ferromagnetic state and is attracted to the hard magnet. This continuous cycling between the 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 and 1.12 wt % were characterized by X-ray diffraction (XRD). Figure 2a shows the XRD of the Gd film without Ag NPs, representing the baseline peaks for Gd. As Ag NPs were coated on the Gd surface, the peak corresponding to the (111) plane for Ag appeared at 38°. The intensity of the Ag peak increased with the concentration of Ag NPs on the 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 to 400 K, as shown in Figure 2b. The Gd film without Ag NPs exhibited a thermal conductivity of 11.2 W/m K at 400 K. However, as observed here, the thermal conductivity of Ag NPs increased up to 12 W/m K at 400 K. This enhancement can be attributed to higher thermal conductivity of Ag NPs than that of the Gd film.35 To verify the influence of Ag NPs, theoretical thermal conductivity values of the Gd film with Ag NPs were calculated as a function of the NP concentration (Figure S4). Because the thickness of the 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
κeq =
κAgκGd(LGd + 2LAg ) 2LAg κGd + LGdκAg
(1)
where LAg and LGd represent the 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 the measured and calculated thermal conductivity as a function of Ag NP concentrations from 0 to 1.12 wt %. The 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 the Curie temperature (294 K) of Gd films and the rate of transition (dM/dT) between the ferromagnetic and paramagnetic state is not shifted by the addition of Ag NP coating, although magnetization is slightly reduced. Figure 2d shows the magnetic hysteresis for the 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 with that of the Gd film without 10799
DOI: 10.1021/acsami.7b17686 ACS Appl. Mater. Interfaces 2018, 10, 10796−10803
Research Article
ACS Applied Materials & Interfaces
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 the hot side from 30 to 50 °C. (b) IR image and two-dimensional color mapping of the peak value of the frequency responses, showing the temperature distribution on the plate, as heat on the 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 the overall temperature obtained from the heater arrays, resulting in reconstructing the letters “V” and “T” with a vibration frequency response.
apply a constant thermal gradient of 60 °C, the surface temperature of the hard magnet attached to the heater and cooler was maintained and measured for 1 h. The thermal gradient (ΔT) was saturated to 60 °C (hard magnet at 50 °C and cooler at −10 °C) after 10 min (see the Supporting Information Figure S7). Under a thermal gradient of 60 °C, the TMEG, with a width ratio of 1, generated a peak voltage of 2.4 V at the vibration frequency of 1.6 Hz. As the width ratio decreases to 0.375, the output voltage and the vibration frequency increased 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 the TMEG as a function of Ag NP concentration from 0 to 1.12 wt % were measured at a fixed gap distance of 1 mm, as shown in Figure 4c,d. 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 with those of the TMEG with a cantilever with a uniform cross section (2.5 V and 1.8 Hz, respectively). These results indicate that the enhanced heat transfer on the Gd surface results in a 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 exhibited further increase in the output voltage and vibration frequency (see Figure S8). The above measurements were repeated with a MFC cantilever, and similar results were observed as shown in Figure 2a,c. At a width ratio of 0.375 and an Ag NP concentration of 1.12 wt %, the output voltage and the vibration frequency of the MFC cantilever-based TMEG were 5 V and 9 Hz, respectively. The stability test for 60 min is shown in Figure S10 in the Supporting Information. To
Ag NPs. Similar to the result in Figure 2c, this reduction can be attributed to nonmagnetic Ag NPs coated on the Gd film. To quantify the elastic nonlinearity of cantilevers with a width ratio from 0.375 to 1, deflection measurements were conducted. Figure 3a shows the force−displacement curve for cantilevers of dimension 1 × 9 cm2 as a function of the width ratio. It is obvious that the cantilever with a width ratio of 0.375 shows a higher degree of elastic nonlinearity at larger deflections, compared with the cantilever width ratio of 1. As the width ratio decreases, the resistance to deformation decreases as the cantilever has nonuniform strain distribution.36−39 To verify the magnetic force relative to elastic nonlinearity of the cantilever, the magnetic force was measured by varying the distance of the hard magnet from the 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 the hard magnet and Gd film decreases, the magnetic force decreases.25,32 Given the 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 effectsvertically aligned magnetic field, vertically aligned spring force, and thermal transport from Nd to Gd and then from Gd to the 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 Figure 3c,d, the model was validated by calculating the force−displacement curve for a cantilever with a uniform cross section and comparing it with the known results.40 Figure 4a,b shows 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. To 10800
DOI: 10.1021/acsami.7b17686 ACS Appl. Mater. Interfaces 2018, 10, 10796−10803
Research Article
ACS Applied Materials & Interfaces
obtained from the heater arrays. The self-powered temperature sensors reconstructed letters “V” and “T” using the vibration frequency response (see the Supporting Information Figures S12 and S13 for details). The resolution and sensitivity of the temperature-mapping system depended on the number of parameters, such as the number of TMEG devices in arrays, nonlinearity of the cantilever, and Ag NP concentration. Because 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. The TMEG-based temperature-mapping sensor demonstrates the potential to be applied in the temperature monitoring system to collect information from automobile engines, pipes carrying hot water, radiators, electrical heat sinks, CPUs of desktop computers, and solarpowered UAVs.
optimize the output power of TMEGs, 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 an 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 S9), 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 because of the resistance of the oscilloscope.41 The optimum resistance (Rext) at the maximum power output is ∼1 MΩ, whereas the internal resistance (Ros) of the 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 the TMEG with a 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 the Ag NPs with a concentration of 1.12 wt %, as a function of the temperature ranging on the hot side from 30 to 50 °C (thermal gradient from 40 to 60 °C) at a fixed gap distance of 1 mm and the cold side temperature of −10 °C. The generated vibration frequency decreases linearly from 9 to 0.3 Hz as the hot side temperature decreases to 30 °C (see the Supporting Information Figure S11). The normalized vibration frequency change is defined as Δf/f 0 = (f − f 0)/f 0, where f and f 0 are the frequencies at the hot side temperature of 30 and 50 °C, respectively. The frequency change exhibits linear dependence on temperature, and the frequency sensitivity (S) is defined as S = δ(Δf/f 0)/δT, indicating the slope of the curve, as shown in Figure 5a, where δT is the change in the thermal gradient. The sensitivity of the 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 (30 °C) where the phase transition can be obtained in the soft magnetic material. However, this limit can be tailored as needed by selecting soft magnetic materials with a suitable transition temperature. In our case, the temperature regimes were defined by keeping solar-powered unmanned aerial vehicles (UAVs) and smart homes as desired applications. On the basis of the ability to sense temperature, we demonstrated a self-powered temperature-mapping sensor for monitoring the local heat distribution on an aluminum plate. A heater (1.5 × 1.5 cm2) with a hard magnet was attached to the aluminum plate with 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 a nonuniform cross section were fabricated. As the heat on the aluminum plate was distributed by the heater at the center, the infrared (IR) image and the 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 the formation of letter-shaped temperature distributions, such as “V” and “T”. Figure 5d shows twodimensional color mapping that reconstructs the applied temperature of each array and maps the overall temperature
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CONCLUSIONS We report a TMEG, composed of a soft magnet mounted on a piezoelectric PVDF or MFC cantilever and a hard magnet attached to the hot surface. Balancing the magnetic force of attraction with the spring force of a cantilever resulted in sustained mechanical oscillations, while enhancing the thermal transport at the soft magnet surface. To enhance the vibration frequency and output performance of TMEGs, a nonuniform cross section cantilever and Ag NP-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 an output power of 80 μW across a load resistance of 0.91 MΩ. This output power is 22-fold higher compared with the TMEG with the uniform cross section cantilever. Self-powered temperature-mapping sensors based on TMEG arrays were developed that demonstrated high accuracy in the mapping temperature distribution on the heater surface. This work demonstrated the possibility of using TMEGs for thermal energy harvesting and for temperature monitoring and thermal management of electronics.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b17686. Fabrication of the TMEG device and Ag NP 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 the output power of TMEGs; modeling of nonlinear cantilevers for calculation; thermal gradient on TMEG; and sensing the performance of TMEG arrays (PDF) A movie showing operating of TMEG (AVI)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jinsung Chun: 0000-0003-1646-8620 Min-Gyu Kang: 0000-0001-9247-6476 10801
DOI: 10.1021/acsami.7b17686 ACS Appl. Mater. Interfaces 2018, 10, 10796−10803
Research Article
ACS Applied Materials & Interfaces Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by AMRDEC through Center for Energy Harvesting Materials and Systems (CEHMS). S.P. would like to acknowledge the support from the 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 1547771. R.A.K. and H.B.K. would like to thank DARPA MATRIX program for the financial support. We are thankful to Jue Wang and Prof. Scott T. Huxtable for their assistance with thermal conductivity measurement.
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DOI: 10.1021/acsami.7b17686 ACS Appl. Mater. Interfaces 2018, 10, 10796−10803
Research Article
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DOI: 10.1021/acsami.7b17686 ACS Appl. Mater. Interfaces 2018, 10, 10796−10803
