Low-pressure-responsive heat-storage ceramics for automobiles

The accumulated heat energy of a heat-storage material is typically released over time. If a heat-storage material could preserve its accumulated heat energy for a prolonged period, the applicability of such materials would be expanded greatly. Herein we report a newly fabricated heat-storage material that can store latent heat energy for a long period and release the heat energy upon demand by applying an extremely low pressure. This material is a block-type lambda trititanium pentoxide (block-type λ-Ti3O5). The block-type λ-phase accumulates a large heat energy of 237 kJ L−1 and exhibits a pressure-induced phase transition to beta trititanium pentoxide. The pressure-induced phase transition occurs by applying only several tens of bars, and half of the fraction transforms by 7 MPa (70 bar). Such a low-pressure-responsive heat-storage ceramic is effective to reuse excessive heat in automobiles or waste heat at industrial factories.


Results and Discussion
Material, crystal structure, and morphology. The target material was prepared by sintering the precursor rutile-type TiO 2 at 1300 °C for 2 hours under a hydrogen atmosphere. The detail synthesis is described in the Methods section (Fig. S1). Elemental analysis by X-ray fluorescence (XRF) suggests that the formula of the obtained sample is Ti 3.00 O 5.00 (Calc.: Ti 64.2%, O 35.8%; Found: Ti 64.1%, O 35.9%). The powder X-ray diffraction (PXRD) pattern with Rietveld analysis indicates a monoclinic crystal structure in the C2/m space group with lattice parameters of a = 9.8256(2) Å, b = 3.78889(4) Å, c = 9.9723(2) Å, and β = 91.2751 (14)°. These features correspond to a crystal structure of λ-Ti 3 O 5 (Fig. 1a,b, Table S1) 5 . In addition, the β-phase is included as a minor phase (monoclinic, C2/m, a = 9.7659(3) Å, b = 3.79907(7) Å, c = 9.4445(3) Å, β = 91.533(3)°). The morphology of the sample was investigated by transmission electron microscopy (TEM). The sample is comprised of block-shaped crystals of sub-micrometer length on a side (Figs 1c and S2). This crystal size is remarkably large compared to previously reported samples. According to the morphology of the primary particles, we call the present material block-type lambda trititanium pentoxide (block-type λ-Ti 3 O 5 ).
www.nature.com/scientificreports www.nature.com/scientificreports/ Release of heat energy by applying pressure to the λ-phase. The pressure effect on block-type λ-Ti 3 O 5 was investigated by PXRD measurements after applying an external pressure (P) of 2.5, 5, 7.5, 10, 12.5, 15, 30, 45, 60, 230, or 600 MPa (Figs 2a and S3). As the pressure increases, the phase fraction of λ-Ti 3 O 5 decreases while that of β-Ti 3 O 5 increases (Fig. 2b). Above 30 MPa, the phase fractions show constant values. The transition pressure (P 1/2 ), which is where the phase fractions of λ-Ti 3 O 5 and β-Ti 3 O 5 are equal, is 7 MPa. www.nature.com/scientificreports www.nature.com/scientificreports/ thermograms before and after a pressure application. We visually measured the temperature change of the sample during a pressure-induced phase transition using thermography (Fig. 2c, Supplementary Movie S1). Pressure was applied by hitting the sample using a hammer. Initially, the temperature is 26.8 °C, and the thermal image is blue. Hitting the sample with a hammer instantly changes the thermal image color to white, which successively turns red, orange, yellow, green, and then back to blue (Fig. 3). The maximum temperature of the white area is 85.5 °C, indicating a temperature increase of 60 °C. The sample temperature reaches a maximum value in less than 67 ms after applying pressure, indicating that the heat energy is immediately released upon applying pressure. Then the temperature exponentially decreases with a decay time of 1.7 s.
We carried out the estimation of the pressure-released heat energy using thermography. Based on the heat capacity versus temperature curve of β-phase 24 , and by considering the temperature increase and the conversion ratio after hitting the sample with a hammer, the pressure-released energy was estimated to be 235 ± 7 kJ L −1 . The details of the estimation process are described in Supplementary Section 6.
Heat-storage process from the βto λ-phase. The heat-storage temperature and accumulated heat energy were measured with a differential scanning calorimeter (DSC). For the measurement, the pressure-produced β-phase was used. In the initial heating process from room temperature to 600 K, an endothermic peak (i.e., heat-storage peak) is observed at 471 K (198 °C). Analyses of the DSC curve shows that the accumulated heat energy is 237 kJ L −1 (Fig. 4a). Conversely, in the cooling process from 600 K to 300 K, an exothermic peak (i.e., heat-release peak) is not observed. These data indicate that λ-Ti 3 O 5 stores the latent heat energy of 237 kJ L −1 . www.nature.com/scientificreports www.nature.com/scientificreports/ thermal hysteresis loop of the phase transition between the βand λ-phases. To investigate the origin of such a low pressure-induced phase transition, we measured the temperature dependence of the magnetic susceptibility (χ) of block-type λ-Ti 3 O 5 using a superconducting quantum interference device (SQUID) magnetometer (Figs 4b and S4). In the cooling process from 600 K, the χ value remains nearly constant around 0.0003 emu per Ti atom, gradually decreases below 150 K, but rapidly increases below 30 K. In the heating process from 2 K to 190 K, the χ values are the same as those in the cooling process. In the heating process around 190 K, the χ value begins to diverge; it takes lower values but abruptly increases at 455 K until it returns to the original values. A thermal hysteresis loop is observed with a branch point in the low temperature region (T L ) of 190 K and a closing point in the high temperature region (T H ) of 455 K. The temperature width of the thermal hysteresis (ΔT ≡ T H −T L ) is 265 K. Such a thermal hysteresis loop has not been observed in the previous λ-Ti 3 O 5 . It should be noted that the χ value of ~0.0003 emu per Ti atom in the cooling process indicates Pauli paramagnetism. The gradual decrease below 150 K is due to the spin-orbital interaction of the Ti 3+ ions, while the increase below 30 K is attributed to the Curie paramagnetic component due to lattice defects.
Mechanism of the appearance of thermal hysteresis loop and low pressure-induced heat energy release. Next we considered the origin of the thermal hysteresis between the λand β-phases and the phase transition with an extremely weak pressure using thermodynamic analysis based on the Slichter and Drickamer mean-field model (SD model) (see Methods) 39 . In the SD model, the Gibbs free energy (G) of the system is described as x)] − xΔS} + G β with the Gibbs free energy of β-phase (G β ) as the standard, and the interaction parameter (γ) between the λand β-phases related to the elastic interactions inside the crystal is defined by γ = γ a + γ b (T) + γ c (P). From the result of the DSC measurement, the transition enthalpy (ΔH) is 13.7 kJ mol −1 . When the transition entropy (ΔS) and interaction parameters are set as a particular combination of values, the SD model calculation well reproduces the observed thermal hysteresis; i.e., the phase transitions of β-phase → λ-phase and λ-phase → β-phase occur at T L of 194 K and T H of 458 K, respectively, at a pressure of 0.1 MPa (1 bar) (Fig. S5a, black line). Hence, the thermal hysteresis loop appears due to the existence of the energy barrier between two bistable phases with close energy states. Furthermore, by assuming the γ value has a distribution due to the inhomogeneity of the primary crystal size, the SD model calculation well reproduces the observed hysteresis loop as shown in Fig. 5a, where the transition in the cooling process is gradual and that in the heating process is abrupt.
Next, we calculated the pressure dependence of the λ-Ti 3 O 5 phase fraction. Applying pressure to the system causes the energy barrier to disappear and induces a phase transition from λ-Ti 3 O 5 to β-Ti 3 O 5 (Fig. 5b). Figure S5b shows the x vs. T plots at P = 30 MPa. The pressure dependence of x shows a threshold in the pressure-induced phase transition. By considering the distribution of the γ value (Fig. S5c), a gradual pressure-induced phase transition as in Fig. 2b is well reproduced (Fig. 5c).

conclusion
Herein we report a newly developed heat-storage ceramic based on block-type λ-Ti 3 O 5 , which preserves the heat energy for a long period and shows a low pressure-induced heat energy release. Block-type λ-Ti 3 O 5 accumulates a large latent heat energy of 237 kJ L −1 , and the accumulated heat energy can be extracted by applying www.nature.com/scientificreports www.nature.com/scientificreports/ an extremely weak pressure of only several MPa with a P 1/2 value of 7 MPa. For example, the pressure of a compressed gas cylinder is in the range from 12 to 30 MPa 40 , suggesting that the present material could be triggered using a gas cylinder. The long-term heat-storage property of block-type λ-Ti 3 O 5 and its release of accumulated heat energy by low pressure originate from the bistability (λ-phase and β-phase) of the present material and the existence of an energy barrier between the two phases. Due to the energy barrier, block-type λ-Ti 3 O 5 exhibits one of the largest thermal hysteresis loops among condensed matter with a ΔT value of 265 K. Because the energy barrier disappears under weak pressure, the λ-phase transforms into the β-phase and releases the accumulated latent heat energy, which is comparable to the latent heat energies of solid-liquid phase-transition materials, e.g., water (320 kJ L −1 ), paraffin (140 kJ L −1 ), and polyethylene glycol (165 kJ L −1 ). The energy barrier is attributed to the elastic interaction within the material. The behaviors of the temperature dependence and pressure dependence of block-type λ-Ti 3 O 5 are well reproduced by thermodynamic calculations. From the viewpoint of automobile applications, transition pressures below 10 MPa are preferable. Therefore, the present heat-storage ceramic should be useful in automobile components near engines and mufflers (Fig. 6) 37,38 , since the heat-storage ceramic can warm the cooled internal system when restarting the automobile. Additionally, an example of other possible applications is solar power plants. In solar power plants, nitrates have been used in the heat storage tanks. Since the present material has both properties of long-term latent heat storage and sensible heat storage, it is expected to be useful for the heat storage system at solar power plants (Fig. S6).

Methods physical measurements.
Elemental analysis of the prepared sample was performed using X-ray fluorescence spectroscopy (Rigaku, ZSX PrimusII). TEM measurements were conducted using a JEOL JEM-2000EXII and JEM-4000FXII. The XRD measurements were conducted by a Rigaku Ultima IV with Cu Kα radiation (λ = 1.5418 Å). Rietveld analyses were performed by the RIETAN-FP program. The magnetic properties were