Introduction

Heat is known to be a considerable form of energy available in the surrounding environment, e.g., solar-thermal energy and exhaust heat, which are generated by natural and artificial actions, respectively. Thus, heat is a primitive but a crucial form of energy in daily life. Hence, heat storage technology is indispensable for efficient energy utilization and sustainable development. This technology can appropriately balance the supply and demand amounts of thermal energy for different times and/or spaces, i.e., heat compensation by the shift in the time or space1. Thus, heat storage materials are crucial for realizing the time-/space shift in motor vehicles and solar-thermal electric generation. In particular, as the time-/space shift permits the storage of heat during the day and radiates the heat during the night, its use is also expected for the retention of temperature in a severe area experiencing a large temperature difference, e.g., an extra-terrestrial space or a planet. Particularly, in the candidate places for migration, which are accessible from the earth, i.e., the moon and Mars, the temperature-difference between the day and night is quite large, and the nighttime is extremely cold. This leads us to consider that a huge electricity is needed to keep the temperature adequate for our existence in the dwelling area and space colony. If the stored solar thermal energy is released in the nighttime, a sharp reduction of the electricity is expected. Thus, heat storage material, which has a good productivity and is expandable, are quite attractive.

Latent heat storage is based on the capture or release of energy when a material undergoes a phase change from, for example, solid to liquid or vice versa. Note that heat storage materials based on this phase transition are referred to as phase change materials (PCMs)2,3,4,5,6. PCMs can accumulate thermal energy, which exhibits intermittent characteristics, and subsequently generate heat at a constant (phase-transition) temperature related to the latent heat. For example, ice, paraffin, fatty acids, and inorganic hydrates are well-known PCMs that can store thermal energy at low temperatures (<150 °C)7. The heat storage mechanism of these PCMs involves a solid–liquid phase transition; hence, it is imperative to ensure that the PCM inside the container is maintained in the liquid state. Moreover, some hazards are inevitable, i.e., damage to the container because of the large volume change occurring during the phase transition and leaking out of the liquid. In these circumstances, it is of particular interest in developing new candidates for novel heat storage material, e.g., λ-trititanium pentoxide (Ti3O5) based solid–solid phase transition, microencapsulated metal-based PCM, and so on7,8: Vanadium dioxide (VO2) exhibits a strongly correlated electron system, which exhibits a structural phase transition at 68 °C. A reversible change is observed between the monoclinic (low-temperature phase) and tetragonal (high-temperature phase) crystal structures accompanied with an exothermic or endothermic reaction (Fig. 1)9,10,11,12. Hence, the above-mentioned hazards can be avoided. In addition, the latent heat of VO2 (~237 J/cm3) is comparable to that of the PCMs reported thus far13, making it adequate for practical applications. Recently, VO2 powder for PCM has been developed and is commercially-available from the companies; Kojundo chemical laboratory Co., Ltd. (Smartec® HS) and NIPPON DENKO CO., LTD. However, as VO2 is available as a powder, it is mandatory to use a container if the VO2 powder is used as the PCM. In addition, vanadium is a rare metal; hence, VO2 reagents are expensive. Thus, VO2 exhibits some disadvantages that need to be improved so as to enable large-scale PCM applications.

Figure 1
figure 1

Endothermic or exothermic reaction of the VO2 phase caused by phase transition. DTA measurement was carried out using reagent-grade VO2 powder. In the DTA curves, endothermic and exothermic peaks were observed in the heating and cooling processes, respectively, corresponding to the phase transition between the monoclinic and tetragonal (rutile-type) structures at a Tc of ~68 °C.

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In this study, a new PCM is proposed, i.e., VO2-dispersed glass, where the VO2 powders embedded in the glass matrix, serves as a durable container-free PCM and prevents the degradation of the VO2 phase because of oxidation and moisture. Glass exhibits immense advantages from the viewpoint of materials science, e.g., large-scale or mass production, flexibility, and formability. Hence, recently, glass has been extensively examined not only for photonics but also as energy-related materials14,15,16,17. Based on this background, VO2-dispersed glass exhibits immense potential for PCM on the basis of the solid–solid phase transition. An incorporation method18 is utilised for fabricating the dispersed glass (Fig. 2), aiming to realise the all-solid PCM.

Figure 2
figure 2

Incorporation method for the preparation of the VO2-dispersed glass. (a) The powdered glass matrix and VO2 powder were mixed, and the mixture was subsequently added into a crucible with a lid. (b) The mixture was melted in an electric furnace. (c) The melts were poured onto a steel plate and were quenched by another steel plate. (d) The VO2-dispersed glass was obtained.

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Results

Selection of the glass matrix composition

First, several glasses were prepared to investigate the composition of the glass matrix suitable for the VO2 dispersion. In this study, three compositions as candidates for the glass matrix, i.e., 35BaO–65B2O3, 15B2O3–10P2O5–75V2O5, and 30BaO–10TeO2–60V2O5, were examined. A borate system was selected because of a previously reported study in which vanadate compounds have been embedded in a borate glass matrix19. Vanadate systems have been selected because a low-processing temperature is expected for synthesis20,21,22.

Differential thermal analysis (DTA) (Fig. 3(a)) revealed that the BaO–B2O3 and BaO–TeO2–V2O5 systems exhibit high thermal stability against crystallisation because the exothermic peak corresponding to the amorphous–crystal phase transformation was absent, whereas a strong, sharp crystallisation peak was observed for the B2O3–P2O5–V2O5 system, corresponding to formation of the V2O5 phase (inset). On the other hand, vanadate systems exhibited a considerably lower glass-transition temperature (Tg) compared with that of the borate system. A low Tg is preferable for the VO2 dispersion to proceed at a low temperature with respect to energy savings as well as the prevention of degradation of VO2 because of high-temperature exposure.

Figure 3
figure 3

Preliminary assessment of the glass matrix. (a) DTA results of 35BaO–65B2O3, 15B2O3–10P2O5–75V2O5, and 30BaO–10TeO2–60V2O5 glasses in the bulk form, which were selected as candidate matrices for the VO2 dispersion. Inverted triangles indicate the glass-transition temperatures, namely, Tg = 580 °C, 225 °C, and 276 °C for the BaO–B2O3, B2O3–P2O5–V2O5, and BaO–TeO2–V2O5 systems, respectively. In addition, the inset shows the powder XRD pattern of the B2O3–P2O5–V2O5 sample heat-treated at a crystallisation peak temperature, i.e., Tp = 279 °C, in addition to the pattern based on the ICDD of V2O5 (#41-1426). (b) Powder XRD patterns of the candidate glasses dispersed with VO2 powder on the basis of the incorporation method (see Method). The values at the end of glass composition represent the additive amount of dispersed VO2 (in mol%). In addition, the ICDD patterns of VO2 (#43-1051) and V3O5 (#72-0977) are included. (c) The immersion test for water resistivity in the V2O5-based matrix glass.

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Next, the incorporation method was carried out as a trial to fabricate VO2-dispersed glasses, which were subsequently analysed by powder X-ray diffraction (XRD) (Fig. 3(b)). The VO2 phase in the borate system was transformed into V3O5 (or the so-called Magneli phase)23, and the size of the Magneli phase (~10 μm, Supplementary Fig. S2) is less than that of the VO2 reagent (~20 μm, Supplementary Fig. S1), suggesting the elution or diffusion of vanadium into the BaO–B2O3 glass matrix. On the other hand, the VO2 phase was stably retained in case of the vanadate systems. This result led us to expect that vanadate systems are suitable for fabricating VO2-dispersed samples. In addition, the water stability test was carried out using the candidate vanadate system glasses. After the immersion of the B2O3–P2O5–V2O5 system glass into water, a pale-yellow colour was immediately observed, and then the water became black after 1 week (Fig. 3(c)). Meanwhile, the BaO–TeO2–V2O5 system remained transparent, indicative of its high resistively against water or moisture. Thus, the BaO–TeO2–V2O5 system demonstrates immense potential as glass matrices, which can be used for the VO2-dispersed PCM. Hence, 30BaO–10TeO2–60V2O5 is selected as the glass matrix.

Characterisation of the VO2-dispersed glass

VO2-dispered glasses with different additive amounts (x; mol%) of VO2 powder, i.e., 30BaO–10TeO2–60V2O5:xVO2 composites, were fabricated according to the protocol described in the Method section. Scanning electron microscopy (SEM) analyses revealed that the incorporated VO2 powders are stably embedded in the matrix, with no significant aggregation of the powders (x = 80; Fig. 4(a)). Elemental mapping results revealed a clear boundary between the VO2 particles and glass phases. In addition, the migration of barium (Ba) and tellurium (Te) surrounding the VO2 particles and the diffusion of vanadium (V) into the glass matrix were barely observed. To primarily examine the heat storage property, thermal cycling test was also carried out by means of DTA in the VO2-dispersed sample. Clear endothermic and exothermic peaks were observed in the temperature range of ~60–80 °C, which are attributed to the phase-transition of VO2, during the heating and cooling process, respectively. (Fig. 4(b)). The peaks of 10-cycle could be almost superimposed on that of 1-cycle. In addition, the SEM observation revealed no significant change in the microstructure for the sample after the cycling test (Supplementary Fig. S3). Since any collapses were hardly observed in the sample subjected to the cycling test, it is suggested that the dispersed glass possesses a high thermal repeatability.

Figure 4
figure 4

Characteristics of the VO2-dispersed sample in 30BaO–10TeO2–60V2O5: 80VO2. (a) SEM (secondary electron) and the elemental mapping results of the sample with x = 80. Scale bar corresponds to 20 um. (b) DTA results in cooling-/heating-cyclic condition at 1-, 5- and 10-cycles (heating and cooling rate: 10 K/min; sample weight: ~40 mg).

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Thermal properties

To examine the latent heat storage function in detail, DSC measurements were carried out. A steep change in the specific heat (peak of the Cp–T curve), related to the phase transition of the dispersed VO2 powder, was observed around the phase-transition temperature of VO2 (Tc~68 °C), and then the transition enthalpy (ΔH), which corresponded to the latent heat storage capacity, was estimated (Fig. 5a,b, and c) for x = 60, 80, and 120, respectively). The ΔH values increased with x, and finally the sample with x = 120 exhibited a ΔH value of ~14.3 J/g (Fig. 5(d)), corresponding to ca. 30% of that of the VO2 reagent (~45 J/g, Supplementary Fig. S1). Thus, the latent heat storage function is imparted to the glass-based material via the VO2 dispersion. In addition, the temperature retention property of the VO2-dispersed glass was assessed. Small pieces of matrix glass (free from VO2, bulk state) and dispersed glass (x = 120, bulk) and VO2 reagent (powder), with similar weight (~2.2–2.4 g), were heated at around ~100 °C, and then were left at room temperature (RT). Variations in the surface temperature were monitored by thermography. In the matrix glass, the temperature monotonically decreased with time, eventually returning to RT (Fig. 6a)). On the other hand, the VO2 reagent exhibited a temperature plateau for ~2.5 min, corresponding to the latent heat related to the phase transition, with subsequent cooling to RT. In addition, the dispersed glass exhibited a plateau for ~1.5 min, indicative of the temperature retention properties. Thermographic images also aided in the better understanding of the temperature variation. The dispersed glass and VO2 reagent exhibited similar temperature distribution after ~1 min (start of the plateau) (Fig. 6(b)). Notably, the VO2 reagent exhibited a fluctuation in the image colour, indicative of a non-homogeneous temperature gradient. This fluctuation was related to the difference in the packing density of the VO2 powder. However, the dispersed glass barely exhibited any fluctuation.

Figure 5
figure 5

Thermal properties of the VO2-dispersed samples. (a) Heat capacity and transition enthalpy for the VO2-dispersed glasses in the BaO–TeO2–V2O5 system with x = 60, (b) 80, and (c) 120. The transition enthalpy, ΔH, was obtained by thermodynamic treatment based on \(\triangle H\,=\int {C}_{p}{\rm{d}}T\) because the enthalpy corresponded to the latent heat storage capacity. (d) The latent heat storage capacity as a function of x.

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Figure 6
figure 6

Temperature retention of the examined samples. (a) Temperature change of the matrix glass in BaO–TeO2–V2O5 (bulk; ~2.4 g), VO2 (powder; ~2.3 g), and VO2-dipersed glass (x = 120, bulk; ~2.2 g) vs. time. Dispersed samples exhibited a plateau around the phase transition temperature, related to the latent heat of the incorporated VO2 phase. (b) Thermal images of the dispersed sample and VO2. In the both experiments of (a) and (b), temperature of the samples were risen to ~100 °C by a resistance heater, and then their surface-temperatures were monitored by use of the thermography. Black bars in (a) and (b) correspond to the scale of ~1 cm.

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Discussion

V2O5 is a glass-forming oxide according to Sun’s classification24, and is easily vitrified by addition of network-modifiers25. V2O5 is the major component in the examined B2O3–P2O5–V2O5 and BaO–TeO2–V2O5 glasses. Although both of these glasses exhibited low Tg, their thermal stabilities against crystallisation and water resistivity were considerably different. A previous study on the glass structure in the P2O5–V2O5 system have reported that the glass network comprises layers of pyramidal VO5 units, which are interlinked together by PO4 units26. Because of the open network structure of glass, water molecules easily attack the PO4 units27,28,29,30, eventually destroying the glass network. Hence, the low stability and resistivity in B2O3–P2O5–V2O5 are thought to be related to a similar structural scenario. Meanwhile, the glasses in the TeO2–V2O5 system comprise of V–O and Te–O polyhedral units, leading to a three-dimensional network structure in binary and multicomponent systems20,21,22. Hence, it is hypothesised that a 3D structure provides a stable network and considerably contributes to the high thermal stability against crystallisation and water resistivity in the BaO–TeO2–V2O5 system.

The homogeneity in the temperature retention (no fluctuation of temperature distribution, Fig. 5) was observed in the dispersed glass, and the presence of the glassy phase probably contributed to the minimisation of the pores occurring between the VO2 particles, which necessarily occurred in the powder state. Temperature fluctuations should be enlarged via the expansion of the material in case the PCM powder is used; hence, dispersion or incorporation plays a crucial role in maintaining the homogeneity of the spatial temperature distribution. Moreover, generally, thermal conductivity (Λ) of multicomponent glass possesses Λ ~ 1 Wm−1K−1, regardless of its composition/system, and such a low Λ originates in prevention of phonon-propagation due to the random-network (disordered) structure without transition symmetry, e.g., 40P2O5–60V2O5 glass; Λ ~ 0.8 Wm−1K−1 (at 350 K or 77 °C)31. Although the Λ-value of matrix glass in this study (30BaO–10TeO2–60V2O5) is considered to be lower than that of VO2 crystal (Λ ~ 4 Wm−1K−1 and Λ ~ 5 Wm−1K−1 in monoclinic and tetragonal systems, respectively)32, the low Λ-value of glass is possibly preferable from the viewpoint of heat-retaining property because a gradual cooling is expected in the VO2-dispersed glass before/after temperature retention (plateau), and we can also see this trend in Fig. 6(a). The acquisition of the temperature retention properties in glass materials with the dispersion or immobilisation of the VO2 phase is expected to be valuable to exploit new functions and applications.

There are some reports about heat storage material consisting of polymers, e.g., polymer blend PCM. However, taking that oxide (or ceramic) material basically possess a high mechanical/thermal properties compared to polymer (organic) material into account, glass-based PCM is expected to have a long-term reliability. Furthermore, glass-based PCMs are considered to demonstrate potential for massive thermal storage applications, for example, in space development and terraforming, on the basis of the advantages of glass materials, e.g., large-scale and mass production. In particular, the presence of abundant glass and related minerals (e.g., pyroxene and olivine) on Mars33 has also vigorously stimulated the study of glass-based PCMs.

In conclusion, a new PCM based on glass materials, i.e., VO2-dispered glass in multicomponent systems, is reported, and its latent heat storage and temperature retention properties are demonstrated. On the other hand, because of the dispersed glass still being a prototype, some issues should be overcome, e.g., improvement of the heat storage amount. Nevertheless, as the technology for glass–crystal composites and their industrialisation has been previously reported34, the results obtained herein demonstrate significance as the first step in the development of all-solid PCMs.

Methods

Preparation of the matrix glass

The glass matrix compositions were 35BaO–65B2O3, 15B2O3–10P2O5–75V2O5, and 30BaO–10TeO2–60V2O5 (mol%). Commercial reagent-grade powders of BaCO3, B2O3, (NH4)2HPO4, V2O5, and TeO2 were used as raw materials. Glasses were prepared by a conventional melt-quenching technique using an alumina crucible with a lid. Melting conditions were as follows: 1200 °C for 30 min (BaO–B2O3 system), 800 °C for 60 min (B2O3–P2O5–V2O5), and 800 °C for 60 min (BaO–TeO2–V2O5) under air. The melts were poured onto a steel plate heated at ~200 °C, followed by pressing using another steel plate to obtain the as-quenched samples (Quenching rate: ~101–102 K/sec). Their samples were confirmed to be in the glassy state as evidenced by X-ray diffraction (XRD) analysis.

Dispersion of VO2 in the glass matrix

To fabricate the VO2-dispersed glass, the incorporation method reported in the study by Komatsu et al.18 was utilised. Figure 2 shows the schematics of the procedure. A powdered matrix glass and VO2 powder (purity: 99.9%; Kojundo Chemical Laboratory Co., Ltd.) were thoroughly mixed using an alumina mortar. Second, the mixture was added into an alumina crucible and melted under the following conditions: 1200 °C for 10 min (BaO–B2O3 system), 1200 °C for 15 min (B2O3–P2O5–V2O5), and 900 °C for 10 min (BaO–TeO2–V2O5) under air. The quenching process was similar to that utilised during the preparation of the matrix glass, finally furnishing VO2-dispered glasses with different matrices.

Characterisation of the matrix glass and VO2-dipersed glass

In the studied glasses and VO2-dispersed samples, the state of the material and crystals were observed by XRD (Cu-Kα radiation). Microscopic observation was carried out by scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectroscopy. Water stability was examined by the immersion of the samples (~0.2 g; bulk form) into water.

The thermal properties of the matrix and VO2-dispersed glasses were examined by differential thermal analysis (DTA; heating rate of 10 K/min, Rigaku Corporation, Thermoplus TG8120). The transition enthalpy (ΔH), corresponding to the amount of the stored latent heat, of the VO2-dispersed samples was evaluated by differential scanning calorimetry (DSC; heating rate of 1 K/min, Seiko Instruments Inc., DSC6220). The specific heat Cp [\(={(\partial H/\partial T)}_{p}\)] was measured as a function of temperature (T), and ΔH was estimated on the basis of the thermodynamic relation: \(\triangle H\,=\int {C}_{p}{\rm{d}}T\). The sample state used in the measurement was the bulk form with a weight of ca. 10 mg. The time dependence of temperature in the examined samples was evaluated by a thermography test (Nippon Avionics Co., Ltd.; R300SR-S).