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M A T E R I A L S S C I E N C E

Long-term heat-storage ceramics absorbing thermal energy from hot waterYoshitaka Nakamura1*, Yuki Sakai2,3, Masaki Azuma2,3, Shin-ichi Ohkoshi4*

In thermal and nuclear power plants, 70% of the generated thermal energy is lost as waste heat. The temperature of the waste heat is below the boiling temperature of water. Here, we show a long-term heat-storage material that absorbs heat energy at warm temperatures from 38°C (311 K) to 67°C (340 K). This unique series of material is composed of scandium-substituted lambda-trititanium-pentoxide (-ScxTi3−xO5). -ScxTi3−xO5 not only accumulates heat energy from hot water but also could release the accumulated heat energy by the application of pressure. -ScxTi3−xO5 has the potential to accumulate heat energy of hot water generated in thermal and nuclear power plants and to recycle the accumulated heat energy on demand by applying external pressure. Furthermore, it may be used to recycle waste heat in industrial factories and automobiles.

INTRODUCTION Generated thermal energy cannot be efficiently converted to electric power at thermal and nuclear power plants. Seventy percent of the generated thermal energy is discarded as waste heat (1–4). The tem-perature of this waste heat is below the boiling temperature of water, i.e., 100°C (373 K) (5). The waste heat is currently released into the atmosphere through water or air, negatively affecting the environment (6–12). Storing and using this waste heat would pro-vide numerous benefits due to the improved energy efficiency and environmental compliance. In the present paper, we report a long-term heat-storage ceramic, scandium-substituted lambda-trititanium- pentoxide, absorbing thermal energy by a solid-solid phase transition below boiling temperature of water. The ceramic can repeatedly use thermal energy by pressure and heating. This heat-storage perfor-mance could provide a sophisticated energy reuse technology for thermal and nuclear power plants and mitigate negative environmen-tal impact of the waste heat.

RESULTSFirst-principles calculations of formation energyIn an effort to realize heat-storage materials (13, 14) capable of absorb-ing low-temperature waste heat, our research has focused on metal- substituted lambda-trititanium-pentoxide (-MxTi3O5). -Ti3O5 exhibits photo- and pressure-induced phase transitions (15–19). To date, several types of metal-substituted -Ti3O5 have been reported (20–22). We surveyed metal cations suitable for metal substitution of the Ti ion in -Ti3O5. Specifically, we conducted first-principles calculations and determined the formation energies of the various -MxTi3O5 using 54 different elements. Figure 1A and fig. S1 show the results, where blue denotes that metallic ion substitution stabilizes the formation energy, while orange destabilizes the formation energy.

Of these elements, only six have a stabilizing effect: Sc, Nb, Ta, Zr, Hf, and W (Fig. 1, B and C). Thus, we synthesized these -Mx-Ti3O5. Substituting with Nb, Ta, Zr, Hf, and W yields the phase. However, Sc-substituted Ti3O5 assumes the phase (fig. S2). Here, we report the synthesis, crystal structure, and heat-storage proper-ties of Sc-substituted -Ti3O5.

Crystal structureWe used an arc-melting technique to synthesize the Sc-substituted -Ti3O5 (23–27). Figure 2A overviews the synthetic procedure. Pre-cursors of Sc2O3, TiO2, and Ti powders are mixed, and an 8-mm pellet of the mixture is prepared. Arc melting was used to melt the pellet in an Ar atmosphere. Then, the sample is shaped into a spherical ball (Fig. 2A). The obtained sample is milled by hand. The formula of the sample is determined to be Sc0.09Ti2.91O5 by x-ray fluorescent (XRF) measurements (see Materials and Methods). We performed synchrotron x-ray diffraction (SXRD) measurements using beamline BL02B2 at SPring-8 to determine the crystal structure (28). Figure 2B shows the SXRD pattern of the as-prepared sample at room tem-perature. From the Rietveld analysis, the crystal structure is mono-clinic (space group C2/m) with lattice parameters of a = 9.84195 (4) Å, b = 3.79151 (1) Å, c = 9.98618 (4) Å,  = 91.1207 (3)°, and a unit cell volume of V = 372.572 (3) Å3 (fig. S3).

These characteristics correspond to the crystal structure of -Ti3O5. -Sc0.09Ti2.91O5 has a slightly larger unit cell volume than that of -Ti3O5 with a 0.4% expansion. In addition, the phase is present as the minor phase. The phase adopts a monoclinic crystal structure (space group C2/m) with lattice parameters of a = 9.7930 (4) Å, b = 3.8064 (14) Å, c = 9.4375 (4) Å,  = 91.5611 (3)°, and V = 351.66 (2) Å3. The scanning transmission electron microscopy (STEM) image shows stripe-like domains measuring approximately 100 nm × 200 nm (Fig. 2C).

Pressure-induced phase transitionNext, we measured the pressure-induced phase transition using SXRD (fig. S4). The as-prepared sample was compressed by pressures of 0.2 to 1.7 GPa with a hydraulic press. As the pressure increases, the -phase fraction decreases, while the -phase fraction increases (Fig. 3A). The crossover pressure is 670 MPa (Fig. 3B). The sample after the pressure-induced phase transition (Fig. 3A) was heated,

1Industrial Solutions Company, Panasonic Corporation, 1006 Kadoma, Kadoma City, Osaka 571-8501, Japan. 2Kanagawa Institute of Industrial Science and Technology 705-1 Shimoimaizumi, Ebina, Kanagawa 243-0435, Japan. 3Laboratory for Materials and Structures, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, Kanagawa 226-8503, Japan. 4Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.*Corresponding author. Email: nakamura.yoshi-taka@jp.panasonic.com (Y.N.); ohkoshi@chem.s.u-tokyo.ac.jp (S.O.)

Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution License 4.0 (CC BY).

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and the temperature evolution of the SXRD patterns was collected (fig. S5). Figure 3C shows the peaks of -(203), - and -(20-3), and -(023). The and peaks are constant until 50°C (323 K), and then the phase decreases and the phase increases at 75°C (348 K),

indicating reversibility due to pressure and heating. The phase transforms into the phase above 175°C (448 K) but, upon cool-ing, returns to the phase in the absence of a transition back to the phase (fig. S6).

Fig. 1. First-principles calculations of formation energies. (A) Periodic table colored by the total electronic energies of -Ti3O5 with an elemental substitution. Blue elements are those where substituted -Ti3O5 shows a lower formation energy than that of pure -Ti3O5. Orange elements are those where substituted -Ti3O5 shows a higher formation energy. (B) Calculated total electronic energies of -AxTi3−xO5 (A, trivalent elements) and (C) -BxTi3-xO5 (B, tetravalent elements) in order of atomic num-ber. One of three Ti sites in -Ti3O5 is substituted by a colored element for the first-principles calculations. Element A in -AxTi3−xO5 substitutes into the Ti1 site. Element B in -BxTi3-xO5 substitutes into the Ti2 site. Blue and orange squares represent that elemental substituted -Ti3O5 shows a lower formation and a higher formation energy, respectively. Black square denotes pure -Ti3O5.

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Heat-storage propertyWe measured the heat absorption mass of the sample after the pres-sure-induced phase transition by differential scanning calorimetry (DSC). We swept the sample compressed at 1.7 GPa with 22.7% of the phase and 77.3% of the phase from 0°C (273 K) to 300°C (573 K). Heat absorption is observed with an absorption peak at 67°C (340 K) (Fig. 3D). Considering the conversion of the and phases, the heat absorption mass is 75 kJ liter−1. The pressure- and heat-induced phase transitions were repeatedly observed (fig. S7).

Compared to the previous work (16), the heat-storage tempera-ture from the pressure-produced phase to phase in the present study is 67°C, which is a remarkable reduction from 197°C. This reduction is attributed to the decrease in the formation energy dif-ference between the two phases, which reduces the crossing tempera-ture of the two Gibbs energy curves (29). First-principles calculations

support these results. The Gibbs energy versus temperature is de-scribed in Fig. 4 and in Materials and Methods.

Thermodynamic mechanism of long-term heat storage and pressure-induced phase transitionAccording to previous reports on -Ti3O5 (15, 16), the reversible phase transition between the phase and phase by pressure and heat is considered to be attributed to the energy barrier between the two phases, which originates from the elastic interaction within the material. To under-stand the mechanisms of long-term heat storage and the low pressure– induced heat energy release, we show the Gibbs free energy of the system (Gsys) using a thermodynamic model based on the Slichter and Drickamer mean-field model (SD model) (30) (see Materials and Methods). The Gibbs free energy in the SD model (Gsys) is described as Gsys = xH + x(1 − x) + T{R[x ln x + (1 − x)ln(1 − x)] − xS}, with a

Fig. 2. Synthesis, crystal structure, and morphology of -Sc0.09Ti2.91O5. (A) -Sc0.09Ti2.91O5 sample synthesis. Pelletized mixture powder of Sc2O3, TiO2, and Ti metal with a diameter of 8 mm is prepared, melted, and rapidly cooled in an arc-melting process. After the melting process, the solidified (as-prepared) sample is milled by hand. Photo credit: Yoshitaka Nakamura, Panasonic Corporation. (B) Synchrotron x-ray diffraction (SXRD) pattern of the as-prepared Sc0.09Ti2.91O5 sample collected at room temperature with = 0.420111 Å. Upper blue and lower orange bars represent the calculated positions of the Bragg reflections of -Sc0.09Ti2.91O5 and -Sc0.09Ti2.91O5. (C) Scanning electron microscopy (SEM) image of the powdered sample shows a grain size below 100 m. Particle from the powdered sample is sliced by a focused ion beam. STEM image shows stripe-like domains with a size of about 100 nm × 200 nm. Scale bars show 100 m in the SEM image and 100 nm in the STEM image.

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cooperative interaction parameter () between the phase and phase due to the elastic interactions within the crystal. x is the ratio of phase, and R is the gas constant. From the result of the DSC measurement, the transition enthalpy (H) is 75 kJ liter−1 (4.0 kJ mol−1), and the transition entropy (S) is 0.22 kJ K−1 liter−1 (12 J K−1 mol−1). When the interaction parameters are set as a particular combina-tion of values, the SD model calculation well reproduces the mea-surement data (i.e., the phase transition of phase → phase occurs around 350 K). Then, the thermally produced phase is maintained even at low temperatures in the cooling process (Fig. 5, A and B). Thus, the reason why the phase is maintained for a long period is

that the presence of the energy barrier between the and phases prevents the transformation of the phase into the phase. The prepared -Sc0.09Ti2.91O5 shows good stability; i.e., -Sc0.09Ti2.91O5 is perfectly maintained after 248 days (about 8 months) and 367 days (1 year) from the XRD measurement.

Furthermore, we reproduced the pressure-induced phase transi-tion from the phase to phase. Applying pressure to the system causes the energy barrier to disappear and induces a phase transi-tion from the phase to phase (Fig. 5C). This pressure-induced phase transition is caused by the change in the value upon applying external pressure (see Materials and Methods). Therefore, the system

Fig. 3. Pressure-induced phase transition and heat-storage process. (A) SXRD patterns of Sc0.09Ti2.91O5 measured at room temperature and ambient pressure after compression between 0.2 and 1.7 GPa with a hydraulic press ( = 0.420111 Å). As the pressure increases, the -(20-3) and -(203) peaks (blue) decrease and the -(20-3) peak (orange) increases, indicating a pressure-induced phase transition. a.u., arbitrary units. (B) Pressure dependence of the phase fractions of Sc0.09Ti2.91O5 calculated from the SXRD patterns in (A). Crossover pressure (phase transition pressure) occurs at 670 MPa. (C) SXRD patterns of Sc0.09Ti2.91O5 measured between 27°C (300 K) and 300°C (573 K; = 0.999255 Å). The and peaks are constant until 50°C (323 K; orange), and then the phase decreases and the phase increases at 75°C (348 K; blue). The phase transforms into the phase above 175°C (448 K; black) but is restored upon cooling. (D) DSC chart of Sc0.09Ti2.91O5 shows an endothermic reaction at 67°C (340 K). Samples are compressed at 1.7 GPa before the variable temperature SXRD and DSC chart measurements.

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is trapped as the phase at room temperature, but applying pres-sure overcomes the energy barrier, resulting in a phase transition to the phase.

DISCUSSIONFigure 6 schematically illustrates the heat-storage system using Sc-substituted -Ti3O5. Cooling water for a turbine in a power plant is pumped from a river or sea. As the water passes through the turbine, the water temperature increases due to heat exchange. The energy of hot water is transferred to Sc-substituted -Ti3O5 in tanks. Sub-sequently, water with a reduced thermal energy returns to the river or the sea. This system can mitigate the rise of river or sea water temperature. Energy-stored Sc-substituted -Ti3O5 can release its stored thermal energy by application of pressure, allowing energy to be used on demand. For example, the stored thermal energy can be supplied to buildings or industrial plants that are close to power plants, without using electricity. Moreover, taking advantage of the charac-teristic of holding the latent heat energy until pressure application, if energy-stored Sc-substituted -Ti3O5 is transported by truck, the heat energy can be used at a distant location. As for the efficiency, the transformation energy efficiency (e) value is evaluated on the basis of the temperature dependence of the enthalpy for the phase and phase obtained by first-principles calculations and DSC mea-surements (fig. S8). For example, when the heat release temperature is 15°C (288 K) and the temperature increase is 1 K, the efficiency is

93%. When the temperature increase is 5 K, the efficiency is 77% (table S1).

In conclusion, we demonstrate heat-storage ceramics based on Sc-substituted -Ti3O5, which absorb heat from hot water. After conducting first-principles calculations, we synthesize Sc-substituted -Ti3O5 ceramics with a heat absorption below 100°C (373 K). This heat absorption material below 100°C can recover the thermal ener-gy from cooling water in power plant turbines, mitigating the rise in sea water temperatures. Moreover, the heat absorption temperature can be easily controlled by changing the Sc content in -Ti3O5 in ac-cordance to the target application.

These heat absorption temperature changes are attributed to the crossover temperature change of Gibbs energies. We successfully synthesize -Sc0.105Ti2.895O5 with a heat absorption temperature at 45°C (318 K) and -Sc0.108Ti2.892O5 with a heat absorption tem-perature at 38°C (311 K; see Materials and Methods and fig. S9). Sc-substituted -Ti3O5 will expand opportunities to use thermal energy as it can use thermal energy that is currently in the unused tempera-ture range. In addition to electric power plants, other applications of the present material such as heat-storage usage to collect waste heat from factories, transportation vehicles, mobile phones, and electronic devices should be possible.

MATERIALS AND METHODSFirst-principles calculationsIn consideration of the valences between six-coordinated Ti3+ and Ti4+ in -Ti3O5 (15, 16, 31), the total electronic energies of -Ti3O5 substituted by trivalent or tetravalent elements from one of three Ti sites were calculated by first-principles calculations using the Vienna ab initio simulation package (VASP) code. The crystal structure of -Ti3O5 shown in (16) was used as calculation models for the initial structure. The lattice parameters and atomic positions

Fig. 4. Mechanism for the decrease in transition temperature. Calculated ther-modynamic free energy of Ti3O5 and Sc0.09Ti2.91O5 with supercells (1 × 3 × 1). Sc substitution site for the calculation is set at Ti3, and 1 of 36 Ti atoms is substituted in - and -Ti3O5. Difference of the free energies of the and phases represents G (G = G − G). At G = 0 (black dotted line), the free energies of the and phases are equal, indicating the crossover temperature (calculated phase transi-tion temperature). Normalized temperature of 1.0 is set at Tp1 = 848 K (575°C), which is the calculated crossover temperature of - and -Ti3O5 (blue dotted line, GTi3O5 = 0). Red dotted line represents the crossover temperature of - and -Sc0.09Ti2.91O5 (GSc0.09Ti2.91O5 = 0) at Tp2 = 614 K (341°C). Crossover temperature of Sc0.09Ti2.91O5 decreases about 27.6% from the temperature of Ti3O5. This tempera-ture decrease ratio agreed well with the experimentally obtained decrease ratio of 27.7% calculated from the phase transition temperature of 470 K (197°C) reported in -Ti3O5 and 340 K (67°C) measured in -Sc0.09Ti2.91O5 (Fig. 3D).

Fig. 5. Mechanism of long-term heat storage and pressure-induced phase transition. (A) Gibbs free energy (Gsys) versus phase fraction (x) curves from 420 to 200 K with a 20 K interval, calculated by the SD model. Blue spheres indicate the thermal population of the phase. (B) Temperature dependence of the calculated phase (blue) and phase (red) fractions. (C) Gsys versus x under ambient pressures of 0.1, 400, and 700 MPa at 300 K.

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were optimized at standard pressure with a cutoff energy of 500 eV and a k-mesh of 7 × 7 × 2 until the electronic iterations converged below 10−5 eV. On the basis of first-principles calculations, we fo-cused on the synthesis of Sc-substituted Ti3O5 because Sc takes Sc3+ with a six- or eight-coordinated geometry, which hinders the higher valence states in Ti sites observed in -Ti3O5.

Material synthesis by arc meltingSc-substituted -Ti3O5 samples were synthesized by an arc-melting method with a pelletized mixture powder of Ti metal (99.9% purity), TiO2 (99.9% purity), and Sc2O3 (99.99% purity) in an Ar atmosphere at 0.05 MPa. Ti metal, TiO2, and Sc2O3 powders were mixed in a molar ratio of Ti:TiO2:Sc2O3 = 0.478:2.433:0.045. In this method, samples were melted and turned over three or four times on a copper cool-ing stage after solidification. These solidified samples were milled by hand before the measurements. The composition was confirmed by XRF, and the formula was determined to be Sc0.09Ti2.91O5: calcd: Ti, 62.4; O, 35.8; Sc, 1.8%; found: Ti, 62.0; O, 36.0; Sc, 2.0%, which is identical to the mixed ratio of the starting materials. Sc0.105Ti2.895O5 and Sc0.108Ti2.892O5 samples were also synthesized, and we measured the x-ray diffraction patterns (fig. S9). Moreover, Ti3O5 samples, sub-stituted by 3 atomic % (at %) of Zr, Nb, Hf, Ta, and W, were synthe-sized and their x-ray diffraction patterns were measured (fig. S2). They mainly showed -Ti3O5 patterns.

SXRD measurementCrystal structures of Sc-substituted Ti3O5 samples were determined by Rietveld analysis of the SXRD data collected in beamline BL02B2 at SPring-8 (28). The samples were sealed in glass capillaries for the SXRD measurements. The RIETAN-FP program was used to refine the structural parameters (32).

Thermal property measurementThe heat absorption properties of Sc-substituted Ti3O5 samples were measured by DSC (Seiko Instruments, DSC 220C) at a heating-cooling rate of 10 K/min and an air gas flow of 100 ml/min. Before DSC measurements, the samples containing both the phase and the phase were compressed at 1.7 GPa to transform them from the phase to the phase. In addition, the thermal properties of -Sc0.105Ti2.895O5 and -Sc0.108Ti2.892O5 samples were measured (fig. S9).

First-principles calculation of Gibbs free energyTo interpret the phase transition temperature, the Gibbs free energies of Sc-substituted -Ti3O5 and -Ti3O5 with supercells (1 × 3 × 1) and a k-mesh of 2 × 2 × 2 of the optimized structures were calculated using the Phonopy code in cooperation with the VASP code for the interatomic force constants calculations (33, 34). The Sc substitution ratio was set to about 3 at % (Sc0.09Ti2.91O5). That is, 1 of 36 Ti atoms was substituted by an Sc atom in the supercells. The differential en-ergy of and phase was calculated (G = G − G). The calculated G of Ti3O5 and Sc-substituted Ti3O5 are shown in Fig. 4. Ti3O5 shows G = 0 at 575°C (848 K), which is the crossover temperature of the calculated free energies corresponding to the phase transition temperature (29). Sc-substituted Ti3O5 showed G = 0 at 341°C (614 K). The normalized temperature in Fig. 4 was set at 575°C (848 K), which is the crossover temperature of the free energies of Ti3O5. The cross-over temperature of Sc-substituted Ti3O5 decreased about 27.6% from the temperature of Ti3O5. This temperature decrease ratio agreed well with the experimentally obtained decrease ratio of 27.7% calculated from the phase transition temperature of 470 K (197°C) reported in -Ti3O5 (16) and 340 K (67°C) measured in -Sc0.09Ti2.91O5. The calculated crossover temperature was overestimated compared with the phase transition temperature, which was likely because the magnetic interaction was not taken into account during the phonon calculations. The formation energies corresponding to the Gibbs free energies (G) at 0 K were −2362.47 eV (-Ti3O5), −2376.44 eV (Sc-substituted -Ti3O5), −2372.45 eV (-Ti3O5), and −2381.65 eV (Sc-substituted -Ti3O5).

Thermodynamic analysisIn the SD model calculations, the value depends on the tem-perature and pressure (i.e.,  = a + bT + cP). From the DSC mea-surement result, the H value was 4.0 kJ mol−1, and the S value was 11.7 J K−1 mol−1. When the parameters of were set as a = 7 kJ mol−1, b = −1.2 J K−1 mol−1, and c = −0.37 J MPa−1 mol−1, the SD model calculations reproduced the long-term heat storage and pressure-induced phase transition, as shown in Fig. 5.

SUPPLEMENTARY MATERIALSSupplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/27/eaaz5264/DC1

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Fig. 6. Application of Sc-substituted -Ti3O5 for power plants. Schematic illus-tration of a heat energy recycling system using Sc-substituted -Ti3O5 heat-storage ceramics. Cooling water for a turbine in a power plant is pumped from a river or sea. Water becomes hot after heat exchange through the turbine. This hot water energy is stored in tanks containing Sc-substituted -Ti3O5 heat-storage ceramics. Water with a reduced heat energy returns to the river or the sea, mitigating the rise of the sea temperature. Energy-stored Sc-substituted -Ti3O5 heat-storage ceramics can supply heat energy to buildings or industrial plants by applying pressure. Further-more, the energy-stored ceramics can be transported to distant locations by a truck.

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Acknowledgments Funding: This work was supported, in part, by JSPS Grant-in-Aid for Specially Promoted Research (grant 15H05697), Grant-in-Aid for Scientific Research(A) (grant 20H00369), and Collaborative Research Projects, Laboratory for Materials and Structures, Tokyo Institute of Technology. The synchrotron-radiation experiments were performed at SPring-8 with the approval of Japan Synchrotron Radiation Research Institute (JASRI; Proposal nos. 2018A1642 and 2018B1797). We are grateful to T. Takizawa (Panasonic Corporation) for setting up the first-principles calculation using VASP and Phonopy codes, H. Tamaki (Panasonic Corporation) for use of the arc-melting equipment, H. Kataoka (Panasonic Corporation) for use of a hydraulic press, and F. Shinsyu (Panasonic Corporation) for collecting the SEM and STEM images. We are grateful to F. Jia (The University of Tokyo) and H. Tokoro (University of Tsukuba) for the thermodynamic calculation, K. Imoto (The University of Tokyo) for the enthalpy calculation, K. Nakagawa (The University of Tokyo) for XRF measurement, and M. Yoshikiyo (The University of Tokyo) for the discussion of the manuscript. Author contributions: Y.N. designed and coordinated this study, contributed to all the measurements and calculations, and wrote the paper. Y.S. and M.A. conducted the SXRD measurements. S.O. designed this study and wrote the paper. All the authors discussed the results and commented on the manuscript. Competing interests: Y.N. is author on a patent filed by Panasonic Intellectual Property Management Co. Ltd. (no. PCT/JP2019/010402, published on 19 September 2019). Y.N. is author on a patent application filed by Panasonic Intellectual Property Management Co. Ltd. (no. JP 2019-124064, filed on |2 July 2019). The others declare no other competing interests. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. Additional data related to this paper may be requested from the authors.

Submitted 16 September 2019Accepted 19 May 2020Published 1 July 202010.1126/sciadv.aaz5264

Citation: Y. Nakamura, Y. Sakai, M. Azuma, S. Ohkoshi, Long-term heat-storage ceramics absorbing thermal energy from hot water. Sci. Adv. 6, eaaz5264 (2020).

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Long-term heat-storage ceramics absorbing thermal energy from hot waterYoshitaka Nakamura, Yuki Sakai, Masaki Azuma and Shin-ichi Ohkoshi

DOI: 10.1126/sciadv.aaz5264 (27), eaaz5264.6Sci Adv 

ARTICLE TOOLS http://advances.sciencemag.org/content/6/27/eaaz5264

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