Development of heat-storage ceramics for automobiles - School of Science, the University of Tokyo
Dec 11, 2019

Development of heat-storage ceramics for automobiles

Low-pressure-responsive heat-storage ceramic

 

Key points of the work

  • A new heat storage material that can instantaneously release heat energy with a low pressure has been developed.
  • The developed material, block-type lambda trititanium pentoxide, is a heat-storage ceramic that can preserve the heat energy for a prolong period and release it only on demand by a very weak pressure.
  • Block-type lambda trititanium pentoxide is expected to be a heat-storage material to improve the fuel efficiency during the initial operation of automobiles by effectively using the thermal energy. Alternatively, block-type lambda trititanium pentoxide is a potential candidate for heat-storage systems in solar power plants.

 

Overview of the work

A research group lead by Professor Shin-ichi Ohkoshi (Department of Chemistry, School of Science, The University of Tokyo) has developed high-performance thermal storage ceramics that can store latent heat energy for a long period and release the heat energy upon demand by applying an extremely low pressure. The newly developed heat-storage ceramic has the crystal structure of lambda-type trititanium pentoxide (λ-Ti3O5), and its nanoparticles obtain block-type shape [1]. The block-type λ-phase accumulates a large heat energy of 237 kJ L−1, which is comparable to those of solid-liquid phase transition materials (about 70% of the fusion heat of water and 140% of the fusion heat of polyethylene glycol). This heat-storage ceramic exhibits a pressure-induced phase transition to beta trititanium pentoxide [2] by applying a weak pressure and releases the stored heat. The pressure-induced phase transition occurs by applying only several tens of bars, and half of the fraction transforms by 7 MPa (70 bar). 70 bar is approximately half of the pressure of a commercially available 7 m3 compressed gas cylinder and is the lowest value required for known pressure-induced phase transitions among solids. The long-term heat-storage property of block-type λ-Ti3O5 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.

Heat-storage materials with prolong storage period are capable of absorbing unnecessary exhaust heat and reusing them as thermal energy. Especially in automobiles, such a material can be mounted around components such as engines and mufflers in order to use the thermal energy released during driving and to improve fuel efficiency. Nevertheless, 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. Additionally, since the present material has both properties of long-term latent heat storage and sensible heat storage, it is expected to be useful for heat-storage systems in solar power plants.

 

Details of the work

Automobiles, such as cars, trucks, and buses gain power using heat energy from burning fuel in an engine. Upon initiating the engine, an automobile consumes energy to warm the internal system to the appropriate temperature in order to start driving. On the other hand, excessive heat energy is generated and released into the atmosphere while driving. Fuel consumption would be reduced if this excessive heat energy could be used when restarting a car. Materials capable of accumulating heat energy, which are known as heat-storage materials, are classified into two categories: sensible heat-storage materials and solid–liquid latent heat-storage materials. The former includes bricks and concrete, while the latter includes water, paraffin, and polyethylene glycol. Regardless of the category, these materials release their accumulated heat energy over time.

The research group focused on lambda trititanium pentoxide. Lambda trititanium pentoxide is a new phase of trititanium pentoxide discovered by Professor Ohkoshi. The accumulated energy in lambda trititanium pentoxide can be stored and released by an external pressure. Such a heat-storage behavior cannot be observed in typical pressure-induced phase transitions. Therefore, lambda trititanium pentoxide, which is a new type of a heat-storage material, suggests potential applications in the industry. More preferably, the pressure to extract the accumulated heat energy is desired to be less than 10 MPa (100 bar) so that the material can be used in a wider range.

In this research, a low-pressure responsive thermal storage ceramics was developed. The target material was prepared by sintering the precursor rutile-type TiO2 at 1300 °C for 2 hours under a hydrogen atmosphere. The sample is comprised of block-shaped crystals of sub-micrometer length on a side. According to the morphology of the primary particles, we call the present material block-type lambda trititanium pentoxide (block-type λ-Ti3O5) (Fig. 1).

Figure 1. Morphology and crystal structure of block-type λ-Ti3O5. (a) Crystal structure of block-type λ-Ti3O5 viewed along the b-axis (left) and c-axis (right). (b) TEM image of block-type λ-Ti3O5 (left) and enlarged figure of the TEM image with clear lattice fringes (right). Insets show the Fourier transform image (upper) and the atomic positions of the corresponding lattices (lower).

 

The block-type λ-phase exhibits a pressure-induced phase transition to beta trititanium pentoxide at low pressure. The pressure-induced phase transition occurs by applying only several tens of bars, and half of the fraction transforms by 7 MPa (70 bar) (Fig. 2a). 70 bar is approximately half of the pressure of a commercially available 7 m3 compressed gas cylinder and is the lowest value required for known pressure-induced phase transitions among solids. The temperature change of the sample during a pressure-induced phase transition was measured using thermography. Hitting the sample with a hammer instantly (in less than 67 ms) changes the temperature of sample from 26.8 °C to 85.5 °C (Fig. 2b, 3).

Figure 2. Pressure evolution of the phase fractions and pressure-induced heat release of block-type λ-Ti3O5. (a) Phase fractions of block-type λ-Ti3O5 (blue) and β-Ti3O5 (red) versus applied pressure. (b) Time dependence of the sample temperature on applying pressure as observed by thermography. Pressure is applied to the sample at t = 0.

 

Figure 3. Time evolution of the thermographic image on applying pressure to block-type λ-Ti3O5. Snapshots taken from the thermogram of the sample on applying pressure as observed by thermography. Pressure is applied to the sample at t = 0. The sample temperature reached a maximum of 85.5 °C.

 

Then the temperature exponentially decreases with a decay time of 1.7 s. The pressure-released energy was estimated to be 235 ± 7 kJ L−1. At the same time, the heat-storage temperature and accumulated heat energy were measured with a differential scanning calorimeter (DSC). 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. Conversely, in the cooling process from 600 K to room temperature, an exothermic peak (i.e., heat-release peak) is not observed. These data indicate that λ-Ti3O5 stores the latent heat energy of 237 kJ L−1.These observations indicate that the long-term heat-storage property of block-type λ-Ti3O5 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. Thermodynamic calculations show that applying weak pressure to the system causes the energy barrier to disappear and induces a phase transition from λ-Ti3O5 to β-Ti3O5, which release the heat being stored (Fig. 4).

Figure 4. Pressure-induced phase transition mechanism based on statistical thermodynamic calculations. (a) Calculated λ-phase fraction (x) versus temperature curves at P = 0.1 MPa (top) and 30 MPa (bottom). Calculations were performed for the cooling process and the heating process under the conditions of ΔH = 13.7 kJ mol−1, ΔS = 34.6 J K−1 mol−1, γb = −2.4 J K−1 mol−1, and γc = −0.12 kJ MPa−1 mol−1, while assuming the γa value has a normal distribution centered at 12.88 kJ mol−1 with a standard deviation of 0.3 kJ mol−1. (c) Calculated λ-phase and β-phase fractions versus pressure at 300 K.

 

Herein we report a newly developed heat-storage ceramic based on block-type λ-Ti3O5, which preserves the heat energy for a long period and shows a low pressure–induced heat energy release. Block-type λ-Ti3O5 accumulates a large latent heat energy of 237 kJ L−1, 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 accumulated heat energy can be extracted by applying an extremely weak pressure of only several MPa up to 7 MPa. The present heat-storage ceramic should be useful in automobile components near engines and mufflers, since the heat-storage ceramic can warm the cooled internal system when restarting the automobile (Fig. 5a).Additionally, an example of other possible applications is solar power plants (Fig. 5b). Since the present material has both properties of long-term latent heat storage and sensible heat storage, it is expected to be useful for heat-storage systems in solar power plants.

Figure 5. Possible applications of block-type λ-Ti3O5 for automobiles. Schematic image of where block-type λ-Ti3O5 could be applied as a heat-storage material in an automobile. Blue areas indicate the possible components to use the heat-storage material: combustion chamber, crankshaft, and muffler.

 

Publication journal

Journal

Scientific Reports

Title Low-pressure-responsive heat-storage ceramics for automobiles
Authors Shin-ichi Ohkoshi*, Hiroko Tokoro, Kosuke Nakagawa, Marie Yoshikiyo, Fangda Jia, and Asuka Namai
DOI No 10.1038/s41598-019-49690-0
Abstract URL https://www.nature.com/articles/s41598-019-49690-0

 

Glossary

[1] Block-type lambda-titanium pentoxide (block-type λ-Ti3O5)

lambda-titanium pentoxide is a titanium oxide material with a new crystal structure discovered by Professor Shinichi Ohkoshi in 2010 [Nature Chemistry, 2, 539 (2010)]. It has been recently proposed as a new concept named heat-storage ceramics [Nature Communications, 6, 7037 (2015)]. The material shows metallic properties. The material presented here is lambda trititanium pentoxide with a block-like shape.

[2] Beta-titanium pentoxide (β-Ti3O5)

A conventionally known brown crystal phase of trititanium pentoxide, which shows semiconducting properties.

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