| Abstract | Vanadijev dioksid (VO2) je termokromni materijal u kojem pri 68 °C dolazi do reverzibilne fazne transformacije iz niskotemperaturne, monoklinske, poluvodičke faze koja je propusna za infra-crveno (IC) zračenje u visokotemperaturnu, rutilnu, metalnu fazu koja IC zračenje reflektira. Ova transformacija čini VO2 materijalom prikladnim za izradu pametnih prozora, čija je funkcija pri visokim temperaturama okoliša spriječiti prolaz dijela IC zračenja u objekt, pri čemu nije onemogućen prolaz vidljivog zračenja, odnosno svjetla. Iako VO2 ima brojne prednosti u odnosu na druge anorganske i organske termokromne materijale, ima i ozbiljne nedostatke. Osnovni nedostaci VO2 su visoka temperatura fazne transformacije, Tc, mala propusnost za vidljivo svjetlo (mali omjer propuštenog i upadnog svjetla), Tlum, zbog čega sloj VO2 na staklu mora biti vrlo tanak, ograničena sposobnost modulacije (mala razlika u propusnosti ukupnog sunčeva zračenja metalne i poluvodičke faze), ΔTsol, te neprivlačna žućkasta boja. Stoga se VO2, modificira dopiranjem s Zr kako bi se smanjili navedeni nedostatci. Dopiranje sa Zr smatra se najkorisnijim za unaprjeđenje svih navedenih svojstava budući da razlika u radijusu V4+ i Zr4+ destabilizira monoklinski VO2, što dovodi do sniženja Tc. Dopiranje sa Zr proširuje zabranjenu zonu VO2 što može dovesti do porasta Tlum i ΔTsol, a Zr modificira i boju VO2. Dopirani VO2 pripravlja se nizom tehnika poput kemijskog nanošenja iz plinovite faze, magnetronskog rasprašivanja ili sol-gel sinteze. Poznata metoda za pripravu VO2, sinteza preko vanadil glikolata, VO(OCH2CH2O), nije do sada korištena za pripravu Zr dopiranog VO2. Stoga je u ovom radu prvo iz amonijeva metavanadata, NH4VO3, etilenglikola, C2H6O2, i cirkonijeva butoksida, Zr(OC4H9)4, prvo pripravljen prekursor iz kojeg se termičkom obradom može dobiti VO2. Pripravljeni su uzorci ciljane stehiometrije V1-xZrxO2, gdje je x=0; 0,02; 0,04 i 0,06 te je promatran utjecaj Zr na tijek termičke evolucije vanadijevih oksida. Korištene su metode diferencijalne termičke i termo-gravimetrijske analize (DTA/TGA), rendgenske difrakcije (XRD), infracrvene spektroskopije s Fourierovom transformacijom (FTIR) i pretražne elektronske mikroskopije (SEM). Utvrđeno je da udio Zr značajno utječe na fazni sastav i morfologiju prekursora, u kojem s porastom udjela Zr ima sve više amorfne faze. Nadalje, utvrđeno je da zagrijavanjem prekursora dolazi do kristalizacije različitih vanadijevih okisda poput VO2, V3O7, V2O5 te cirkonijeva vanadata, Zr(V2O7). Zapaženo je da udio Zr utječe na tijek kristalizacije i veličinu kristalita VO2. Također je utvrđen termički režim pri kojem se u uzorcima bez ili s malim udjelom Zr može u potpunosti ili gotovo u potpunosti izbjeći kristalizacija svih faza osim VO2. Međutim, analize ukazuju da opisanim načinom priprave nije došlo do ugradnje Zr u kristalnu rešetku VO2. |
| Abstract (english) | Vanadium dioxide (VO2) is a thermochromic material in which at 68 °C a reversible phase transformation occurs from a low temperature, monoclinic, semiconductor phase permeable to infrared (IC) radiation to a high-temperature, rutile, metal phase that reflects IC radiation. This transformation makes VO2 a suitable material for making smart windows, whose function is to prevent the passage of a part of IC radiation to an object at high ambient temperatures, without preventing the passage of visible light. Although VO2 has many advantages over other inorganic and organic thermochromic materials, there are serious drawbacks. The basic flaws of VO2 are high phase transformation temperature, Tc, low bandwidth for visible light (low ratio of transmitted and incident light), Tlum, which is why the layer of VO2 on the glass has to be very thin, the limited modulation ability (small difference in permeability of total solar radiation of metal and semiconductor phases), ΔTsol, and unattractive yellowish color. Therefore, VO2 is modified by doping with Zr to reduce these defects. Doping with Zr is considered to be most useful for improving all of the above mentioned properties since the difference in the radius of V4+ and Zr4+ destabilizes the monoclinic VO2, leading to a decrease in Tc. Doping with Zr widens the VO2 band-gap, which can lead to a rise in Tlum and ΔTsol, and Zr also modifies the VO2 color. The doped VO2 could be prepared by a series of techniques such as chemical deposition from a gas phase, magnetron sputtering or sol-gel synthesis. Although it is a known method for the preparation of VO2, the synthesis through vanadyl glycolate, VO(OCH2CH2O), has not been used up to date for the preparation of Zr doped VO2. Therefore, in this work, first from the ammonia metavanadate, NH4VO3, ethylene glycol C2H6O2, and zirconium butoxide, Zr(OC4H9)4, a precursor was prepared from which VO2 can be obtained by thermal treatment. Samples of targeted stoichiometry V1-xZrxO2 were prepared, where x = 0; 0.02; 0.04 and 0.06 and the influence of Zr on the course of thermal evolution of vanadium oxide was observed. The methods of differential thermal and thermogravimetric analysis (DTA / TGA), X-ray diffraction (XRD), Fourier transform (FTIR) infrared spectroscopy and scanning electron microscopy (SEM) were used. It was found that Zr significantly influences the phase composition and morphology of the precursor, and that with the increase of the Zr share the quantity of the amorphous phase increases. Furthermore, it has been found that by thermal treatment of the precursor, crystallization of various vanadium phases such as VO2, V3O7, V2O5 and zirconium vanadate, Zr(V2O7) occurs. It has been noted that the Zr share influences the course of crystallization and the size of the crystalline VO2. A thermal regime was also established by which in the samples without or with a small Zr content, the crystallization of all phases except VO2 can completely or almost completely be avoided. However, the analyzes indicate that the described method of preparation did not result in the incorporation of Zr into the crystal lattice of VO2. |
