Introduction

Vanadium dioxide (VO2) undergoes a reversible phase transition from a low-temperature monoclinic VO2(M1) semiconductive phase to a high-temperature tetragonal VO2(R) metallic phase at a transition temperature (Ttr) of approximately 68 °C for the bulk material1,2. The abrupt decrease of infrared transmittance without attenuation of luminous transmittance in the metallic state makes VO2-based coatings a promising candidate for thermochromic smart windows reducing the energy consumption of buildings. In spite of recent significant progress in fabrication and performance of thermochromic VO2-based materials (see, for example, reviews3,4,5,6,7,8 and the works cited therein), there are still serious drawbacks hindering their application in smart windows. These are: a high temperature needed for fabrication, a high transition temperature, a low luminous transmittance (Tlum), a low modulation of the solar transmittance (ΔTsol) and low environmental stability. To meet the requirement for large-scale implementation on building glass, VO2-based coatings should satisfy the following criteria simultaneously: deposition temperature close to 300 °C or lower9,10,11,12,13, Ttr close to 20 °C14, Tlum > 60%3,15,16, ΔTsol > 10%17,18,19, and long-time environmental stability8,20,21,22.

Decrease of the deposition temperature of thermochromic VO2-based coatings to 300 °C is of key importance: (1) to facilitate their large-scale production by reducing the energy consumption, simplifying substrate heating and cooling procedures and minimizing problems with a temperature non-uniformity over large substrate surfaces, and (2) to allow deposition of these coatings onto temperature-sensitive flexible substrates.

Magnetron sputter deposition with its versatility and the ease of scaling up to large substrate sizes is probably the most important preparation technique of thermochromic VO2-based coatings8,12,13. In our recent works23,24, reactive high-power impulse magnetron sputtering (HiPIMS) with an effective pulsed oxygen flow control (applicable to large-area coaters25) was used for low-temperature (300 °C) deposition of thermochromic VO2 films onto conventional soda-lime glass without any substrate bias voltage and without any interlayer. Except for the work11 with the same substrate surface temperature Ts = 300 °C, there is no work in the literature reporting a magnetron sputter deposition of thermochromic VO2 films onto unbiased amorphous substrates at Ts < 400 °C23,24. The possibility to prepare thermochromic VO2-based coatings without any substrate bias voltage is of key importance for their deposition on large area non-conductive (glass) substrates (no RF-induced bias needed). Here, it should be mentioned that HiPIMS techniques are compatible with existing magnetron sputtering systems utilized in industrial deposition devices26,27.

In this work, we report on high-performance three-layer thermochromic ZrO2/V0.982W0.018O2/ZrO2 coatings with a low transition temperature (20–21 °C) prepared on soda-lime glass using a low-temperature (330 °C) magnetron sputter deposition without any substrate bias voltage. We present basic principles of this new solution for a low-temperature scalable deposition of high-performance durable thermochromic VO2-based coatings for smart-window applications.

Methods

Coating preparation and elemental composition

The V0.982W0.018O2 layers were deposited by controlled HiPIMS of V target, combined with a simultaneous pulsed DC magnetron sputtering of W target, at a low substrate surface temperature Ts = 330 °C and without any substrate bias voltage in an argon–oxygen gas mixture. The argon flow rate was 60 sccm corresponding to an argon partial pressure of 1 Pa, while the oxygen flow rate (Φox) was not fixed but pulsing between 0.6 and 1.5 sccm (see Fig. 1), and the duration of the Φox pulses (injecting oxygen in front of the sputtered V target toward the substrate24) was determined during the deposition by a programmable logic controller29 using a pre-selected critical value of the average discharge current on V target in a period of the power supply (\(\stackrel{-}{I}\)d)cr = 0.43 A. The basic principle of the pulsed oxygen flow control is illustrated in Fig. 1, which shows the time evolution of the magnetron voltage (Ud(t)) and the target current density (Jt(t)), averaged over the total target area, for both targets at the minimum and maximum value of the oxygen partial pressure in the vacuum chamber corresponding to the minimum and maximum \(\stackrel{-}{I}\)d, respectively, during the deposition. Here, it should be mentioned that the effective pulsed oxygen flow control makes it possible to utilize two benefits of the reactive HiPIMS deposition23,24. The first benefit is highly ionized fluxes of particles with many V+ and V2+ ions onto the substrate and enhanced energies (up to 50 eV relative to ground potential) of the ions bombarding the growing films, allowing us to achieve the VO2 crystallinity at a low Ts and without any substrate bias voltage. The second benefit is a very high degree of dissociation of the O2 molecules injected into the high-density plasma in front of the V target, allowing us to achieve the required VO2 stoichiometry at a low compound fraction in the target surface layer. This is of key importance for reduced arcing, increased sputtering of V atoms, and low production of O− ions at the target29.

Figure 1
figure 1

Waveforms of the magnetron voltage (Ud) and the target current density (Jt) for preset deposition-averaged target power densities of 12.9 W cm−2 and 33 mW cm−2 for V and W target, respectively, during a deposition of the V0.982W0.018O2 films (the Jt values for W target are magnified 1,000 times). Time evolution of the average discharge current on V target in a period of the power supply (\(\stackrel{-}{I}\)d) during the deposition is shown in the inset. A pre-selected critical value (\(\stackrel{-}{I}\)d)cr = 0.43 A determining the switch between the oxygen flow rates Φox = 0.6 sccm and Φox = 1.5 sccm is marked by dots. Reprinted from the work28.

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The depositions were performed in an ultra-high vacuum multi-magnetron sputter device (ATC 2200-V AJA International Inc.) using two unbalanced magnetrons with planar V and W targets (99.9% purity, diameter of 50 mm and thickness of 6 mm in both cases). The magnetron with a V target was driven by a high-power pulsed DC power supply (TruPlasma Highpulse 4002 TRUMPF Huettinger)24. The voltage pulse duration was 50 µs at a repetition frequency of 200 Hz (duty cycle of 1%) and the deposition-averaged target power density was 12.9 W cm−2. The magnetron with a W target was driven by a pulsed DC power supply (IAP-1010 EN Technologies Inc.). The voltage pulse duration was 16 µs at a repetition frequency of 5 kHz (duty cycle of 8%) and the deposition-averaged target power density was 33 mW cm−2. Under these conditions, the W content in the metal sublattice of V1−xWxO2, as measured on a dedicated 285 nm thick film in a scanning electron microscope (SU-70, Hitachi) using wave-dispersive spectroscopy (Magnaray, Thermo Scientific), was 1.8 ± 0.6 at.% (i.e., x = 0.018).

Both bottom and top ZrO2 antireflection layers were deposited by reactive mid-frequency AC magnetron sputtering without ohmic heating (Ts < 100 °C) and without any substrate bias voltage in an argon–oxygen gas mixture. The argon partial pressure was 1 Pa and the oxygen partial pressure was 0.35 Pa (oxide mode). The depositions were performed using two strongly unbalanced magnetrons with planar Zr targets (99.9% purity, diameter of 100 mm and thickness of 6 mm) driven by a mid-frequency AC power supply (TruPlasma MF 3010, TRUMPF Huettinger)30. The oscillation frequency was close to 85 kHz and the deposition-averaged target power density was 15.5 W cm−2.

The thickness of individual layers was measured by spectroscopic ellipsometry using the J. A. Woollam Co. Inc. VASE instrument31.

The presented deposition technique for preparation of the thermochromic ZrO2/V0.982W0.018O2/ZrO2 coatings is, just like the deposition of low-emissivity coatings, compatible with the existing magnetron sputtering systems in glass production lines.

Coating structure and properties

For structural investigation of the films, X-ray diffraction (XRD) measurements were carried out using a PANalytical X´Pert PRO diffractometer working with a CuKα (40 kV, 40 mA) radiation at a glancing incidence of 1°. The average size of coherently diffracting regions of the VO2(R)/VO2(M1) phase was estimated from the full width at half maximum of the main VO2(R)/VO2(M1) diffraction peak, corrected for instrumental broadening, using the Scherrer’s equation.

The surface morphology of the films was determined by atomic force microscopy (AFM) using a SmartSPM Microscope (AIST-NT) with a diamond tip (nominal radius below 10 nm) in a semicontact mode. The root-mean-square roughness of the surface, Rrms, was computed from a randomly selected square area of 1 × 1 μm2. The AFM images were processed by Gwyddion 2.41 software32, and an implemented “watershed” method was used for grain analysis. The grains identified were approximated by an equivalent disc diameter with the same projected area as the grain.

The hardness of VO2 (without the ZrO2 overlayer) was measured using a Hysitron TI 950 triboindenter with a cube corner tip at a maximum load of 100 μN.

The normal-incidence coating transmittance was measured by spectrophotometry using the Agilent CARY 7000 instrument equipped with an in-house made heat/cool cell. Spectroscopic measurements were performed in the wavelength range λ = 300 to 2,500 nm at the temperatures Tms = − 5 °C (semiconducting state below Ttr) and Tmm = 60 °C (metallic state above Ttr). Hysteresis curves were measured at λ = 2,500 nm in the temperature range Tm = − 10 to 60 °C. The luminous transmittance (Tlum) and the solar transmittance (Tsol) are defined as follows

$${T}_{\mathrm{l}\mathrm{u}\mathrm{m}}\left({T}_{\mathrm{m}}\right)=\frac{\underset{380}{\overset{780}{\int }}{\varphi }_{\mathrm{l}\mathrm{u}\mathrm{m}}{\left(\lambda \right)\varphi }_{\mathrm{s}\mathrm{o}\mathrm{l}}\left(\lambda \right)T\left(\lambda , {T}_{\mathrm{m}}\right)d\lambda }{\underset{380}{\overset{780}{\int }}{\varphi }_{\mathrm{l}\mathrm{u}\mathrm{m}}{\left(\lambda \right)\varphi }_{\mathrm{s}\mathrm{o}\mathrm{l}}\left(\lambda \right)d\lambda },$$
(1)
$${T}_{\mathrm{s}\mathrm{o}\mathrm{l}}\left({T}_{\mathrm{m}}\right)=\frac{\underset{300}{\overset{2500}{\int }}{\varphi }_{\mathrm{s}\mathrm{o}\mathrm{l}}\left(\lambda \right)T\left(\lambda , {T}_{\mathrm{m}}\right)d\lambda }{\underset{300}{\overset{2500}{\int }}{\varphi }_{\mathrm{s}\mathrm{o}\mathrm{l}}\left(\lambda \right)d\lambda },$$
(2)

where φlum is the luminous sensitivity of the human eye and φsol is the sea-level solar irradiance spectrum33 at an air mass of 1.5. The modulation of the luminous transmittance (ΔTlum) and of the solar transmittance (ΔTsol) are defined as

$$\Delta {T}_{\mathrm{l}\mathrm{u}\mathrm{m}}= {T}_{\mathrm{l}\mathrm{u}\mathrm{m}}\left({T}_{\mathrm{m}\mathrm{s}}\right)- {T}_{\mathrm{l}\mathrm{u}\mathrm{m}}\left({T}_{\mathrm{m}\mathrm{m}}\right),$$
(3)
$$\Delta {T}_{\mathrm{s}\mathrm{o}\mathrm{l}}= {T}_{\mathrm{s}\mathrm{o}\mathrm{l}}\left({T}_{\mathrm{m}\mathrm{s}}\right)- {T}_{\mathrm{s}\mathrm{o}\mathrm{l}}\left({T}_{\mathrm{m}\mathrm{m}}\right).$$
(4)

Using relation (2) it can be written

$${\Delta T}_{\mathrm{s}\mathrm{o}\mathrm{l}}=\frac{\underset{300}{\overset{2500}{\int }}{\varphi }_{\mathrm{s}\mathrm{o}\mathrm{l}}\left(\lambda \right)\Delta T\left(\lambda \right)d\lambda }{\underset{300}{\overset{2500}{\int }}{\varphi }_{\mathrm{s}\mathrm{o}\mathrm{l}}\left(\lambda \right)d\lambda },$$
(5)

where ΔT(λ) = T(λ, Tms) − T(λ, Tmm) is the modulation of the transmittance at the wavelength λ. The average luminous transmittance (Tlum) is defined as Tlum = [Tlum(Tms) + Tlum(Tmm)]/2.

Results and discussion

Design and transition temperature of ZrO2/V0.982W0.018O2/ZrO2 coatings

The three-layer structure of ZrO2/V0.982W0.018O2/ZrO2 coatings, formed by an active layer in the middle and two antireflection (AR) layers, is shown in Fig. 2. Let us emphasize the combination of properties which makes ZrO2 a proper candidate for the AR-layers. First, ZrO2 has a refractive index (n) close to the required geometric mean of refractive indices of V0.982W0.018O2 and glass (bottom AR-layer) or V0.982W0.018O2 and air (top AR-layer). Second, ZrO2 has almost zero extinction coefficient (k) for visible and infrared wavelengths (λ), allowing one to utilize higher-order AR-layers without concessions in terms of absorption. Third, crystalline structure of the bottom ZrO2 layer can be achieved even at a low deposition temperature, which in turn improves the V0.982W0.018O2 crystallinity and the process reproducibility. Fourth, ZrO2 is a hard (for an oxide) and stable material, which allows the top AR-layer to provide a mechanical protection and environmental stability for the active V0.982W0.018O2 layer. The hardness of ZrO2 prepared by the present technique is 15–17 GPa30, compared to the hardness of VO2 of only 12 GPa. Note that ZrO2 layers are being increasingly applied in architectural glass as a protective overcoat for advanced low-emissivity stacks34. These properties cannot be matched by many other potential or occasionally used AR-layer materials due to their, e.g., non-zero k (Cr2O3), lower hardness (SiO2, Ta2O5), high deposition temperature of the hardest phase (α-Al2O3), too low n for the bottom AR-layer (SiO2) or usable but too high n (rutile TiO2).

Figure 2
figure 2

The three-layer thermochromic VO2-based coating on a soda-lime glass substrate investigated in this paper. Here, hb, h, and ht represent the thickness of the bottom ZrO2 layer, the thickness of the active V0.982W0.018O2 layer, and the thickness of the top ZrO2 layer, respectively. Below, individual coatings are referred to as (hb, h, ht). The refractive index (n550) at the wavelength of 550 nm of all layers is also given. Tms and Tmm denote the temperatures when the V0.982W0.018O2 layer is in the semiconductive (below Ttr) and metallic (above Ttr) state, respectively. Reprinted from the work28.

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We examined the effect of smoothly varied hb and ht in our recent work31 and thereby identified the optimum value hb = ht = 180 nm leading to a second-order interference maximum of Tlum (consistently with the optimization of ht in the work35). This choice constitutes a crucial part of the efforts to maximize Tlum and ΔTsol (at a given h) in parallel: while the frequently used first-order AR-layers (λ/4-layers; see e.g. the work36 for a first-order ZrO2 AR-layer) lead to a high transmittance modulation only in the far infrared (where it is weighted by weak solar irradiance when calculating ΔTsol), second-order AR-layers (3λ/4-layers) lead to a high transmittance modulation mainly in the near infrared (where it is weighted by much higher solar irradiance; see below for a graphical example). Furthermore, we use two different h values of 45 nm or 48 nm (leading to higher Tlum) and 69 nm (leading to higher ΔTsol) in order to demonstrate the corresponding tradeoff. Here, it should be mentioned that the thickness of the V0.982W0.018O2 layer deposited onto amorphous soda-lime glass was 48 nm while it was 45 nm for the same layer deposited using the same discharge conditions (Fig. 1) and deposition time onto the crystalline ZrO2 layer. To avoid any changes in the composition of the V0.982W0.018O2 layer, which would be caused mainly by a larger erosion of the V target (see the much higher power compared with the W target) in additional (much later) depositions, we used the original configurations of the thermochromic VO2-based coatings with the slightly different h = 45 nm and 48 nm in this work. The aforementioned effect of the thicker V0.982W0.018O2 layer with h = 69 nm is presented for two important configurations, denoted as (180, 69, 0) and (180, 69, 180).

Figure 3 shows the dependence of the transmittance at λ = 2,500 nm (T2500) on the measurement temperature (Tm) for two of the coatings prepared. The evaluated transition temperature (center of the hysteresis curves) was reduced by the aforementioned W doping to Ttr = 20–21 °C (ZrO2/V0.982W0.018O2/ZrO2 coatings in Fig. 2) or 23 °C (V0.982W0.018O2 without AR-layers in Table 1). The Ttr value is reproducible (almost the same for two different h values) and in agreement with the requirement for smart-window applications14. It is very important that using the present deposition technique, we did not experience any tradeoff (indicated in the literature19,37,38) between lowering Ttr by W doping and optimizing the other optical properties: the differences in the V(W)O2 optical constants at λ = 550 nm were within the measurement error and reproducibility noise and did not exhibit any systematic dependence on the W content. The present W content of 1.8 ± 0.6 at.% and the transition temperature Ttr = 57 °C achieved for undoped VO2 prepared by the same technique23 collectively lead to a gradient of approximately (23–57)/1.8 = − 19 K/at.% of W in the metal sublattice, consistent with those (− 13 to − 22 K/at.%) reported in the literature18,31,39.

Figure 3
figure 3

Adapted from the work.28

Temperature (Tm) dependence of the transmittance (T2500) at 2,500 nm for the ZrO2/V0.982W0.018O2/ZrO2 coatings with hb = 180 nm, h = 45 nm or 69 nm, and ht = 180 nm deposited onto 1 mm thick glass substrates (Fig. 2). The transition temperatures (Ttr) are also given.

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Table 1 Thermochromic properties of different configurations of the VO2-based coatings on 1 mm thick glass substrates.
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Structure and morphology of V0.982W0.018O2 layers

Figure 4 shows the coating structure at the temperature Tm = 25 °C, i.e., essentially during the thermochromic transition. The bottom ZrO2 AR-layer [denoted as (180,0,0)] consists of a mixture of m-ZrO2 (PDF#04-013-687540) and t-ZrO2 (PDF#01-081-1544 valid for ZrO1.95). The strongest peaks around 2Θ = 28.0° and 29.5° correspond well to the peaks of m-ZrO2 [(− 111) planes diffracting at 2Θ = 27.95°] and t-ZrO2 [(101) planes diffracting at 2Θ = 29.81°]. The V0.982W0.018O2 layer with Ttr = 23 °C (Table 1) deposited onto glass [denoted as (0,48,0)] and the V0.982W0.018O2 layer with Ttr = 20 °C deposited onto the crystalline ZrO2 layer [denoted as (180,45,0)] consist of a mixture of the high- and low-temperature thermochromic phase, VO2(R) (PDF#01-073-2362) and VO2(M1) (PDF#04-003-2035), respectively, which are hard to distinguish. The strongest and sharp peak around 2Θ = 27.8° corresponds well to the peaks of the VO2(R), (110) planes diffracting at 2Θ = 27.91°, and the VO2(M1), (011) planes diffracting at 2Θ = 27.80°. In spite of a small content of the non-thermochromic VO2(B) phase (PDF#01-084-3056), the V0.982W0.018O2 layer on the crystalline ZrO2 layer exhibits a better crystallinity of the thermochromic VO2(R)/VO2(M1) phase. This is quantified in terms of larger size of the coherently diffracting regions, obtained using the peak around 27.8°. The size is 47 nm along the scattering vector (that is, for the 1° glancing incidence used, about 47·cos[1° + 27.8°/2] = 45 nm vertically, which is equal to the layer thickness) for the V0.982W0.018O2 layer on ZrO2, compared to 23 nm for the V0.982W0.018O2 layer on glass.

Figure 4
figure 4

X-ray diffraction patterns taken at Tm = 25 °C from the V0.982W0.018O2 layers with the thickness h = 48 nm deposited onto glass [denoted as (0,48,0)] and with the thickness h = 45 nm deposited under the same discharge conditions (Fig. 1) onto the ZrO2 layer with the thickness hb = 180 nm on glass [denoted as (180,45,0)]. For comparison, the X-ray diffraction pattern from the ZrO2 layer with the thickness hb = 180 nm on glass [denoted as (180,0,0)] is given. The main diffraction peaks of VO2(R), VO2(M1), VO2(B), ZrO2(m) and ZrO2(t) are marked. Reprinted from the work28.

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The surface morphology of V0.982W0.018O2 layers (without the top AR-layer) is shown in Fig. 5. The figure constitutes an independent confirmation of the fact that while the present deposition technique allows a low-temperature crystallization of VO2-based layers on amorphous glass, their crystallinity on crystalline ZrO2 is even better. Most importantly, the grains identified by the “watershed” method make up 80% of the projected surface area for the V0.982W0.018O2 layer (Rrms = 1.1 nm) deposited onto the bare soda-lime glass (Fig. 5a), while they make up 94% of the projected surface area for the V0.982W0.018O2 layer (Rrms = 1.2 nm) deposited onto the crystalline ZrO2 AR-layer (Fig. 5b). Furthermore, the latter V0.982W0.018O2 layer exhibits also a narrower distribution of the horizontal grain sizes (Fig. 5c).

Figure 5
figure 5

Surface morphology of the V0.982W0.018O2 layers with the thickness h = 48 nm deposited onto glass [denoted as (0,48,0); panel a] and with the thickness h = 45 nm deposited onto the ZrO2 layer with the thickness hb = 180 nm on glass [denoted as (180,45,0); panel b], together with the corresponding grain-size (approximated by an equivalent disc diameter) distributions on the area of 1 × 1 μm2; panel c. Reprinted from the work28.

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Thermochromic properties of ZrO2/V0.982W0.018O2/ZrO2 coatings

Figure 6 shows in detail the aforementioned role of second-order AR-layers in optimizing Tlum and ΔTsol, given by Eqs. (1) and (5), respectively, in parallel. On the one hand, Fig. 6a,b show that Tlum depends only on a narrow range of wavelengths: the transmittance T(λ, Tm) is weighted by a narrow function φlum(λ). There is an easily explainable increase of T(λ, Tm) in the corresponding narrow visible λ range, resulting from using only the bottom AR-layer, only the top AR-layer (stronger increase than the previous one) and both AR-layers (the strongest increase). This phenomenon is almost independent of Tm, which means that the low ΔTlum (Table 1) is almost independent of the coating design. Furthermore, Fig. 6a,b confirm that owing to the absorption in V0.982W0.018O2, T(λ, Tm) is generally higher at h = 45 nm and 48 nm than at h = 69 nm.

Figure 6
figure 6

Adapted from the work.28

Spectral transmittance T(λ, Tm) of different configurations (Table 1) of the thermochromic VO2-based coatings on 1 mm thick glass substrates measured at Tms = − 5 °C (panel a) and Tmm = 60 °C (panel b), together with the corresponding modulation of the transmittance ΔT(λ); panel c. The coatings are denoted as (hb, h, ht), where hb and ht are the thicknesses of the bottom and top ZrO2 layers, respectively. The thickness of the V0.982W0.018O2 layer is h = 45 nm or 48 nm (solid lines) and h = 69 nm (dashed lines). The luminous sensitivity of the human eye (φlum) normalized to a maximum of 100%, and the sea-level solar irradiance spectrum (φsol) at an air mass of 1.5 are also given.

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On the other hand, Fig. 6c shows that the dependence of ΔTsol on the coating configuration is much more difficult to explain, because the transmittance modulation ΔT(λ) is weighted by a wide and complicated function φsol(λ) and there is no coating configuration leading to the highest ΔT(λ) in the whole λ range shown. Indeed, while ΔT(λ) in the far infrared above ≈1,600 nm (weighted by relatively low φsol) is actually the highest without any AR-layer, ΔT(λ) in the near infrared below ≈1,600 nm (weighted by relatively high φsol) is the highest when using both second-order AR-layers or at least the top one. The reason is that the second-order interference maxima on both AR-layers in the visible are followed by lower-order interference minima and maxima in the infrared, and that the overall improvement of the near infrared transmittance by this interference is more significant below than above Ttr. The fact that this kind of effect cannot be achieved by thinner first-order AR-layers is discussed in more detail in our recent work31. Furthermore, Fig. 6c confirms that ΔT(λ) is generally higher at h = 69 nm than at h = 45 nm or 48 nm.

The transmittance-based integral quantities, given by Eqs. (1)–(5), are summarized in Fig. 7 and Table 1. In agreement with the discussion of the transmittance in itself (Fig. 6), it can be seen that the transition from (1) bare V0.982W0.018O2 through coatings with (2) only the bottom AR-layer and (3) only the top AR-layer to the coating with (4) both AR-layers, at almost the same thickness (45 nm and 48 nm) of the V0.982W0.018O2 layer, leads to a gradual improvement of the optical performance (average Tlum and ΔTsol). The performance of the best coating configuration (hb = ht = 180 nm) is characterized by Tlum = 59.4% and ΔTsol = 5.5% (at h = 45 nm) and by Tlum = 48.0% and ΔTsol = 10.4% (at h = 69 nm), in both cases accompanied by low ΔTlum and the aforementioned Ttr = 20–21 °C. It is possible to state that our results are close to the requirements (see the introductory part and the gray area in Fig. 7) for smart-window applications.

Figure 7
figure 7

Adapted from the work.28

The average luminous transmittance (Tlum) and the modulation of the solar transmittance (ΔTsol) for different configurations (Table 1) of the thermochromic VO2-based coatings on 1 mm thick glass substrates. The coatings are denoted as (hb, h, ht), where hb and ht are thicknesses of the bottom and top ZrO2 layers, respectively. The thickness of the V0.982W0.018O2 layer is h = 45 nm or 48 nm (full symbols) and h = 69 nm (empty symbols). The gray region represents the required values for smart-window applications.

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In addition to comparing the thermochromic properties of the present coatings with the industrial requirements, it is worth comparing them with the properties of coatings reported in the literature (Table 2). We focus on coatings on glass substrates and on plastic foils42,43, which can be pasted on the glass, with an at least somewhat lowered Ttr ≤ 40 °C.

Table 2 Comparison between this work and previously reported studies on Tlum and ΔTsol of VO2-based coatings with a transition temperature Ttr ≤ 40 °C prepared on glass substrates or polyethylene terephthalate (PET) foils.
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The ZrO2/V0.988W0.012O2/ZrO2 coating31 was deposited using the same method as in the present work. The V0.958Tb0.031W0.011O2 coating17 was fabricated on a glass substrate from Tb- and W-codoped VO2 nanopowders prepared using hydrothermal synthesis. An additional annealing at 400 °C for 60 min in argon atmosphere was ultimately applied to increase the adhesion and the coating crystallinity. The V0.872Sr0.119W0.009O2 layer, forming a basis for the V0.872Sr0.119W0.009O2/SnO2 coating18, was deposited by RF magnetron co-sputtering of V, Sr and W from a single composed V-Sr-W target in an argon–oxygen gas mixture at a substrate temperature of 450 °C. The V0.931Fe0.069O2 coating19 and V0.878Fe0.092Mg0.030O2 coating37 were prepared by DC magnetron co-sputtering of V and Fe, and V, Fe and Mg, respectively, from a single composed V–Fe, and V–Fe–Mg target in an argon–oxygen gas mixture at a substrate temperature of 60 °C, with an additional in-situ annealing at 450 °C for 30 min in oxygen. The V0.98W0.02O2 coating15 was prepared on silica substrate by spin coating via a sol–gel process and annealing at 500 °C for 30 min in ammonia atmosphere. The V0.99W0.01O2 coating41 was prepared on fused quartz using a sol–gel method and annealing at 600 °C for 30 min in argon gas.

The thermochromic coatings, which were prepared on a polyethylene terephthalate (PET) substrate, are derived from V0.971F0.029O242 or V0.99W0.01O243 nanoparticles dispersed in polyurethane. These nanoparticles were produced by complex hydrothermal reactions. Here, it should be mentioned that the F-doping and W-doping of these nanoparticles resulted in the required reduction in the transition temperature, but it led also to a decrease in the modulation of the solar transmittance. The ΔTsol value decreased from 13.1% for the coating with pure VO2 nanoparticles to 10.7% (see Table 2) for the coating with 2.93 at.% F-doped VO2 nanoparticles42. In case of the coating with 1 at.% W-doped VO2 nanoparticles43, the ΔTsol value decreased to 12.7% (see Table 2) from 22.3% for the coating with pure VO2 nanoparticles. Note that the transition temperature Ttr = 36 °C of the high-performance thermochromic coating with the V0.99W0.01O2 nanoparticles (see Table 2) was determined as a mean value from the temperature of 46 °C, related to an endothermic peak, and 26 °C, related to an exothermic peak, detected using differential scanning calorimetry during the heating-up and cooling-down period, respectively.

As can be seen in Table 2, we achieved an excellent combination of the required characteristics: the lowest maximum glass temperature during the preparation of the coatings (Ts)max = 330 °C, an appropriate transition temperature Ttr = 20–21 °C, and Tlum up to 60% at ΔTsol close to 6% or Tlum up to 50% at ΔTsol above 10%. These optical properties are comparable with those achieved for the thermochromic VO2-based coatings which were prepared using long and too complicated chemical processes on flexible PET foils42,43 at a very low (Ts)max ≤ 100 °C, but with too high transition temperatures Ttr = 35 °C and 36 °C, respectively.

Conclusion

High-performance thermochromic ZrO2/V0.982W0.018O2/ZrO2 coatings with a low transition temperature were prepared on soda-lime glass by a low-temperature scalable deposition technique which is compatible with the existing magnetron sputtering systems in glass production lines. The V0.982W0.018O2 layers were deposited by controlled HiPIMS of V target, combined with a simultaneous pulsed DC magnetron sputtering of W target (doping of VO2 by W to reduce the transition temperature to Ttr = 20–21 °C without any degradation of thermochromic properties), at a low substrate surface temperature Ts = 330 °C in an argon–oxygen gas mixture. The effective pulsed oxygen flow control of the reactive HiPIMS deposition makes it possible to utilize the enhanced energies of the ions bombarding the growing V0.982W0.018O2 layers for the support of the crystallization of the thermochromic VO2 phase in them at the low Ts = 330 °C and without any substrate bias voltage. Our design of the three-layer VO2-based coatings utilizes the second-order interference in two antireflection ZrO2 layers to increase both the luminous transmittance and the modulation of the solar transmittance. The ZrO2/V0.982W0.018O2/ZrO2 coatings exhibit the optical properties which are relatively close to the requirements (Tlum > 60% and ΔTsol > 10%) for smart-window applications. For applications in large-scale systems, it is important that the presented controlled deposition of the active VO2-based layers can be performed also at prolonged duty cycles (up to 5%). This results in up to 5 times lower target power density in a pulse at the same deposition-averaged target power density (approximately 13 W cm−2 in this work). Moreover, the deposition rate of these layers is higher.