Thermochromic undoped and Mg-doped VO₂ thin films and nanoparticles: Optical properties and performance limits for energy efficient windows

Abstract

Undoped and Mg-doped thermochromic VO 2 films with atom ratios z ≡ Mg/(Mg + V) of 0 ≤ z < 0.21 were deposited by reactive DC magnetron sputtering onto heated glass and carbon substrates. Elemental compositions were found by Rutherford backscattering spectrometry. Optical constants were determined from transmittance and reflectance measurements and were used for modeling the optical properties of thin films and dilute nanoparticle composite layers below and above the critical temperature for thermochromic switching between a low-temperature infrared transparent state and a high-temperature infrared reflecting or absorbing state. Mgdoped films showed superior luminous transmittance T lum and solar transmittance modulation ΔT sol compared to undoped VO 2 films, and both of these parameters could be further enhanced by anti-reflection. VO 2 -containing nanocomposites had much larger values of T lum and ΔT sol than VO 2 -based films. Mg-doping was found to erode the properties of the nanocomposites. Approximate performance limits are given on T lum and ΔT sol for thermochromic VO 2 films, with and without Mg doping and antireflection coating, and also for VO 2 -containing dilute nanocomposites.--

Durability of thermochromic VO2 thin films under heating and humidity: Effect of Al oxide top coatings

“…It is likely that some or all of these oxide layers are able to impart oxidation protection of the underlying VO 2 . We also note that VO 2 -based nanoparticles may be of considerable interest for TC fenestration [11,[16][17][18]. Core-shell structures have been investigated recently with VO 2 cores surrounded by shells comprised of SiO 2 [44][45][46][47][48] or TiO 2 [49][50][51] and, again, the top layer may serve as oxidation protection.…”

“…It should be noted that such coatings may be desired not only for oxidation protection but for their optical functionality. Thus the top coating can provide anti-reflection and thereby enhance the luminous transmittance, which is advantageous for windows-related applications [11,16], and studies have been reported for anti-reflecting layers based on oxides of Si [32][33][34][35][36][37], Ti [36][37][38][39][40][41][42], Zr [43] and Al [37]. It is likely that some or all of these oxide layers are able to impart oxidation protection of the underlying VO 2 .…”

“…The deposition rate was ~7 nm/minute. The deposition conditions were carefully monitored and kept since they are of crucial importance for reproducible formation of thermochromic VO 2 films, as discussed elsewhere [16,23,24].…”

“…TC fenestration has been discussed for several decades [2][3][4][5][6][7][8][9][10], but practical implementation has been hampered by performance deficiencies such as a too high value of τ c , too large luminous absorptance, and too small modulation of the solar energy throughput [11]. However, there has been substantial progress towards the alleviation of all of these obstacles, and it is well known that tungsten doping can decrease τ c to room temperature [11,12], that luminous absorptance can be lowered by magnesium doping [13][14][15][16], and that solar energy modulation can be boosted by invoking VO 2 -based nanoparticles rather than thin films [16][17][18]. Another obvious requirement for practical implementation of TC VO 2 -based films and nanoparticles is that they must maintain their desirable properties during extended periods of time, which is far from obvious since V 2 O 5 , not VO 2 , is the thermodynamically stable oxide [19,20].…”

Thermochromic light scattering from particulate VO2 layers

“…10,11 Thin films and nanoparticle composites of VO 2 are of considerable interest for "smart" glazings in buildings as well as for numerous other technologies. 3,[12][13][14][15][16][17] Most optical applications of VO 2 require essentially non-scattering properties, and both films and nanoparticle composites have been subject to detailed studies recently. 13,15 Only a few prior investigations 9,[18][19][20][21][22][23][24][25] have considered light scattering, and we particularly note recent work by Jin et al 20,21 on diffuse transmittance through VO 2 -based functional fiber mats and by Moot et al 22 on composites of VO 2 and elastomers.…”