Cyclic thermal conductivity changes of Pd-catalyzed Ni–Mg alloy films by gasochromic hydro- and dehydrogenations

To investigate the thermal-switching properties of Pd-catalyzed Ni–Mg alloy films, we conducted in situ analyses of the films’ electrical, optical, and thermal properties through hydrogen gasochromic reactions. These reactions allow the films to reversibly switch between metallic (dehydride) and semiconductor (hydride) phases. The thermal conductivities of the metallic and semiconductor states were found to be 14 and 1.0 W m−1 K−1, respectively. By applying the Wiedemann–Franz law, we attributed the significant decrease in thermal conductivity during hydrogenation to the reduction in free electrons.


S
][3] Due to the significant changes in transmittance and reflectance of SMM films in the visible to near-infrared region during these hydro/dehydrogenation processes, they have been explored as oxidative-coloring films, serving as counter electrodes to reductive-coloring amorphous WO 3 films in electrochromic smart windows. 4)n this study, our aim is to fabricate high-performance thermal-switching devices [5][6][7] that utilize the metallic and semiconductor states of SMMs, as this technology holds promise for enhancing the efficient utilization of thermal energy. Tese thermal-switching devices, comprising SMM films, enable the control of thermal energy flow by leveraging their low and high thermal conductivities, respectively.Such a switch is a simple conceptual device with ON and OFF states of the heat flow.However, it would be difficult if realized by thermal conductivity contrast caused by any physical phenomena.Based on reviews of solid-state thermal conductivity switch materials near room temperature, 6,8) only a few materials have switch ratios greater than 3 between two different states such as phase transformation, crystal structures, and spin arrangement caused by temperature, electric field, magnetic field, and light irradiation.To gain a higher contrast of the thermal conductivity, the thermal conductivity switches would be to switch the heat carrier itself rather than changing the atomic arrangement or bond.Herein, we propose SMMs as the new concept of thermal conductivity switch driven by the chemical reaction, namely the hydro/ dehydrogenation of SMMs.
Since free electrons are the primary heat carriers in metals, it is expected that the hydro/dehydrogenation of SMMs will result in not only optical and electrical variations but also significant changes in thermal conductivity.The metal state after dehydrogenation is expected to act as a mirror state with a high thermal conductivity owing to high free electron density.Two methods are considered to realize the switching process of hydro/dehydrogenation.The first is the gasochromic method, where the films are directly hydrogenated using a dilute H 2 gas (several %) in Ar or N 2 gases. 2,3)The second method is the electrochromic method, in which the films are hydrogenated through reductive electrochemical proton-intercalations. 9)We have previously investigated the alteration of thermal conductivity in Y-Mg alloy SMM films controlled by electrochemical reactions in a 1 M KOH solution. 9)It was found that the thermal conductivity of Y-Mg alloy films in the electrochemical dehydrogenated (metal) state (−0.5 V versus Ag/AgCl) was approximately 3.4 times higher than that in the electrochemical hydrogenated (semiconductor) state (−1.1 V versus Ag/AgCl). 10)onsidering resource availability and cost, our focus in this study is on Ni-Mg alloy films 11,12) as SMMs for thermal switches, which consist of commonly abundant elements.The crystal structures of Ni-Mg alloys comprise a combination of hexagonal closest packing (Mg) and hexagonal (Mg 2 Ni) structures.Upon hydrogenation, they transform into semiconductor phases, namely MgH 2 and Mg 2 NiH 4 , 9) with rutile and monoclinic crystal structures, respectively.Previous reports have indicated that hydrogenation leads to an approximate 25% increase in the thickness of Ni-Mg alloy films, 13) a factor that should be considered when estimating thermal conductivities.
The deposition process involved the use of DC magnetron sputtering to deposit nominal 200 nm thick Ni-Mg films, with a 5 nm thick Pd layer acting as the catalyst, on unheated synthetic quartz substrates or 100 nm thick Mo-coated synthetic quartz glass substrates.The thicknesses of the Ni-Mg and Pd films were the optimized values for the durability and the efficiency of the hydro/dehydrogenation reactions, respectively.A Ni (14 at%)-Mg (86 at%) alloy target was employed for the Ni-Mg deposition, and the sputter depositions were carried out with Ar gas (99.999% purity) maintained at 5.0 Pa.Two-layered films of Pd/Ni-Mg were used for in situ measurements of electrical and optical properties, while three-layered films of Pd/Ni-Mg/Mo were utilized for in situ thermophysical measurements, where the Mo layer served as a reflective film.The thin Pd layer served the dual purpose of protecting the Ni-Mg layer against oxidation and catalyzing the hydro/dehydrogenation reactions.With the Pd catalyst at the surface, the hydrogenated Ni-Mg in the air easily releases hydrogens and returns to the dehydrogenated state, which could be attributed to the relaxation of internal stress (compressive stress) caused by the volume expansion of the hydrogenation.Table I provides the sputter deposition conditions for the films.Electron probe microanalysis (EPMA; JXA-8200, JEOL) was employed to analyze the element compositions of all the Ni-Mg films, confirming that the chemical compositions of Mg in the films were nearly identical to those in the target.X-ray diffraction (XRD) analyzes were conducted using Cu K α1 radiation at 40 kV and 20 mA (XRD-6000, Shimadzu), and for all the two-layered films, only a broad peak corresponding to Mg 6 Ni (8 2 2) was observed, indicating the nanocrystalline structure of the Ni-Mg alloy films.Figures 1(a In situ measurements of optical transmittance were performed using a spectrophotometer (UV-3600iPlus, Shimadzu) in the spectral range of 185-3300 nm for the Pd/Ni-Mg sample before and after two gasochromic hydro/dehydrogenation cycles using an Ar-H 2 (H 2 : 3%) mixture gas at atmospheric pressure (below the explosive limit of hydrogen).A gas chamber customized for the spectrophotometer was utilized for the in situ optical analyzes under the Ar-H 2 gas atmosphere.The flow rate of the Ar-H 2 (H 2 : 3%) mixture gas was controlled at 50 sccm.The gasochromic properties, as shown in Fig. 2, exhibited a distinct increase in transmittance from the visible to infrared region due to hydrogenation (semiconductor state), 9) followed by a decrease to zero upon dehydrogenation (metallic state).
[12][13][14][15][16][17][18][19][20][21] The measurements were conducted in situ using a rear-heating/ rear-detection (RR) type picosecond pulsed light heating thermoreflectance apparatus.The 3-layered films of Pd/Ni-Mg/Mo were placed in a specially designed sealed chamber, where the measurements were performed under open air and N 2 -H 2 (H 2 : 3%) mixture gas atmospheres (see Fig. 3).Initially, measurements were conducted on the as-deposited sample in open air.Subsequently, the chamber was evacuated, and the N 2 -H 2 (H 2 : 3%) mixture gas was introduced to achieve hydrogenation of the Ni-Mg layer.After waiting for at least 40 min, thermal conductivity measurements for the first hydrogenation were carried out.To ensure sufficient dehydrogenation, the sample was left in air for over 16 h.Next, thermal conductivity measurements for the first dehydrogenation were conducted in air.To verify reproducibility, the same procedures for thermal conductivity measurements were repeated for the second hydro/dehydrogenation states.Figure 4 illustrates the RR-thermoreflectance signals obtained during the two hydro/ dehydrogenation cycles.Clear variations in heat diffusion are observed between the hydrogenation and dehydrogenation reactions.The temperature decays during the hydro/dehydrogenation states are indicative of the semiconductor and metallic states, respectively.Table II presents the estimated thermal conductivities derived from the temperature decays of the TDTR signals.The analyzed thermal conductivities for the metallic and semiconductor states were approximately 14 and 1.0 W m −1 K −1 , respectively.This suggests the potential for reversible high-performance thermal switches with a dynamic range control of more than single digits.In comparison, for thermal switches utilizing VO 2 thermochromic films, 7,15) the changes in thermal conductivities during the phase transition (Mott-Hubbard transition) between metal and insulator were reported to be approximately 1.5 times higher for the metallic phase.Therefore, the hydro/dehydrogenation states of Ni-Mg films appear to be promising candidates for thermal-switching films.
To estimate the thermal conductivities carried by free electrons using the Wiedemann-Franz law (WFL), electrical conductivity measurements were conducted using the fourpoint probe method (HL-5500PC, Bio-Rad) under air or N 2 -H 2 gas atmosphere.These measurements were performed during the hydro/dehydrogenation cycles, which were repeated twice and demonstrated reversibility (Figs. 5 and 6). Figure 5 displays the thermal conductivities (k) of the Ni-Mg films as a function of electrical conductivity (σ).The straight line represents the WFL, with a Lorenz number of 2.45 × 10 −8 W Ω K −2 used.Similar approaches have been reported for various transparent conductive oxide films, such as Sn:In 2 O 3 , 16,17) Al:ZnO, 18) In-Ga-Zn-O, 19) Nb:TiO 2 , 20) and Sb:SnO 2 . 21)The thermal conductivities carried by free electrons, estimated through the WFL, are presented in Table II.The higher thermal conductivity in the metallic state is attributed to heat transfer facilitated by free electrons, while the significant decrease in thermal conductivity during hydrogenation is attributed to the absence of free electrons.
In summary, the Ni-Mg films, which are one of the SMM films composed of common elements, were investigated for their potential use in thermal-switching devices.In situ analyzes of the optical, thermal, and electrical properties of       095503-4 © 2023 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd ) and 1(c) present crosssectional transmission electron microscopy (TEM) images, while Figs.1(c) and 1(d) display selected area electron diffractions (SAEDs) of the three-layered films of Pd/Ni-Mg/Mo.Images (a) and (b) represent the as-deposited sample, while images (c) and (d) represent the sample after undergoing two cycles of hydro/dehydration.The crosssectional images reveal complete coverage of the Ni-Mg layer by the thin Pd layer.The SAEDs display diffraction rings corresponding to Mg 6 Ni (6 6 4), Mg 6 Ni (8 2 2), Mg 2 Ni (2 1 5), and other phases, suggesting the presence of nanocrystalline structures in the Ni-Mg films.
Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd the Ni-Mg films were conducted during gasochromic hydro/ dehydrogenation.We fabricated three-layered films consisting of Pd (5 nm thick), Ni-Mg (200 nm thick, Ni: 14 at%), and Mo (100 nm thick) using DC magnetron sputtering.The in situ TDTR analyzes (the RR type picosecond pulsed light heating thermoreflectance measurements), employing a gas mixture of Ar and H 2 (H 2 : 3%) at 1 atm, revealed that the

Fig. 5 .
Fig. 5. Thermal conductivity, k, of the Ni-Mg film as a function of the electrical conductivity, σ.The solid line is drawn based on Wiedemann-Franz law (WFL) using the Lorenz number L = 2.45 × 10 −8 W Ω K −2 and temperature T = 300 K.

Fig. 6 .
Fig. 6.Thermal conductivity changes of the Ni-Mg film during the two hydro/dehydrogenation cycles.

Table I .
Deposition conditions for the Ni-Mg, Pd and Mo films.