Process for producing a metal oxide powder

Abstract

A process for producing a metal oxide powder comprising: providing a precursor solution or dispersion containing a metal complex; spraying the precursor solution on to a heated substrate in the presence of water, thereby depositing material on the substrate; drying the deposited material; and removing the deposited material from the substrate to produce the metal oxide powder.

Claims

1. A process for producing a metal oxide powder comprising of one of a metal (IV) oxide powder (MO.sub.2 where M is a metal) and a metal oxide powder comprising of a metal (III) oxide film (M.sub.2O.sub.3 where M is a metal), the process comprising: providing one of a precursor solution and a dispersion containing a metal (IV) complex or a metal (III) complex; spraying the precursor solution or dispersion on to a heated substrate in the presence of water, thereby depositing material on the substrate; drying the deposited material; cooling the deposited material on the substrate at a controlled cooling rate of at least 10 C./min; and removing the deposited material from the substrate to produce the metal oxide powder, wherein: the metal (M) is vanadium, molybdenum, tungsten, germanium or manganese; and the spraying, drying and cooling steps are carried out in at least one of a protective or inert atmosphere.

2. A process according to claim 1, wherein the precursor solution or the dispersion is selected from: an aqueous solution; the concentration of the precursor solution or dispersion is up to 0.1M; and the concentration of the metal (IV) complex or the metal (III) complex in the precursor solution is up to 0.1M.

3. A process according to claim 1, wherein the metal is selected from one of: (i) the metal is vanadium and the metal complex is vanadyl acetylacetonate; (ii) the metal is vanadium and the metal complex is vanadyl oxalate; (iii) the metal is molybdenum and the metal complex is molybdenyl acetylacetonate; and (iv) the metal is molybdenum and the metal complex is molybdenyl oxalate or a suitable molybdenum (IV) complex.

4. A process according to claim 1, wherein the oxidation state of the metal is the same in the metal complex as it is in the metal oxide powder.

5. A process according to claim 1, wherein the precursor solution or dispersion comprises one or more further metals as dopants.

6. A process according claim 1, wherein the precursor solution or dispersion is sprayed on to the heated substrate with the aid of a carrier gas optionally wherein the carrier gas is sprayed at a rate of up to 60 ml/min.

7. A process according to claim 1, wherein the precursor solution or dispersion is sprayed on to the heated substrate at a rate of up to 5 ml/min.

8. A process according to claim 1, wherein the substrate is heated to a temperature selected from one of: (i) at least 300 C.; (ii) at least 400 C.; (iii) at least 450 C.; and (iv) up to 550 C.

9. A process according to claim 1, wherein drying the deposited material comprises the steps of maintaining the substrate at a predetermined drying temperature for a predetermined drying time, wherein the predetermined drying temperature is selected from one of: (i) at least 300 C.; (ii) at least 400 C.; (iii) at least 450 C.; and (iv) up to 550 C.; and wherein the predetermined drying time is selected from: (a) at least 5 minutes; and (b) up to 60 minutes.

10. A process according to claim 1 comprising one of heat treating, and annealing, the deposited material, wherein the heat treating is carried out in one of the protective or inert atmosphere.

11. A process according to claim 1, wherein the steps of spraying the precursor solution or dispersion on to a heated substrate in the presence of water, thereby depositing material on the substrate and drying the deposited material are repeated one or more times.

12. A process according to claim 1, wherein the precursor solution has a pH which is selected from one of: at least 5 and up to 9.

13. A process according to claim 1, wherein the deposited material is removed from the substrate by any suitable non-chemical, non-thermal means selected from the group consisting of scraping the deposited material from the substrate, tipping the substrate, inverting the substrate, and shaking the substrate.

14. A process according to claim 1, wherein the process comprises a step of further processing the deposited material removed from the substrate.

15. A process according to claim 1, wherein the substrate comprises a glass.

16. A process according to claim 1, wherein the precursor solution containing the dopant(s) is one of: supplied to a nozzle and sprayed on to the heated substrate; the dopant(s) is/are supplied to a nozzle separately from the precursor solution or dispersion containing the metal complex and the dopant(s) is/are mixed with the precursor solution or dispersion containing the metal complex at the nozzle immediately before spraying; or one or more solutions or dispersions containing the dopant(s) is/are sprayed from one or more different nozzles from the precursor solution or dispersion containing the metal complex, such that the solutions or dispersions mix together as they are sprayed on to the substrate.

17. A process according to claim 1, wherein the method comprises a step of preparing the precursor solution.

Description

(1) In order that the invention may be well understood, it will now be described with reference to the accompanying drawings, in which:

(2) FIG. 1 shows the typical variation of electrical resistance with temperature for VO.sub.2 deposited as a film;

(3) FIG. 2 shows the typical variation of optical transmittance of VO.sub.2 deposited as a film at 2500 nm as a function of temperature;

(4) FIG. 3 is a scanning electron microscope (SEM) image of VO.sub.2 deposited as a film in accordance with the invention;

(5) FIG. 4 shows the variation of electrical resistance with temperature for VO.sub.2 deposited as a film shown in FIG. 3;

(6) FIG. 5 is an x-ray diffraction (XRD) spectrum for VO.sub.2 deposited as a film shown in FIG. 3;

(7) FIG. 6 is an XRD spectrum for another example embodiment of VO.sub.2 deposited as a film in accordance with the invention; and

(8) FIG. 7 is an XRD spectrum for another example embodiment of VO.sub.2 deposited as a film in accordance with the invention.

(9) FIG. 1 shows the typical variation of electrical resistance with temperature for VO.sub.2 deposited as a film. As can be seen in FIG. 1, there is a step-change in electrical resistance at around 60 C. to 70 C. The step-change in electrical resistance is approximately two orders of magnitude. This change in electrical resistance is a consequence of the semiconductor-to-metal transition. As can be seen in FIG. 1, the typical variation of electrical resistance with temperature for VO.sub.2 deposited as a film has a hysteresis of around 15 C. in width. In FIG. 1, one of a series of data points for heating is labelled 1 and one of a series of data points for cooling is labelled 2.

(10) FIG. 2 shows the typical variation of optical transmittance of VO.sub.2 deposited as a film at 2500 nm wavelength as a function of temperature. As can be seen in FIG. 2, there is a significant change in optical transmittance at around 60 C. to 70 C. This change in optical transmittance is a consequence of the semiconductor-to-metal transition. At temperatures above the transition, the optical transmittance of VO.sub.2 deposited as a film at 2500 nm is between 30% and 40%; at temperatures below the transition, the optical transmittance of the VO.sub.2 at 2500 nm is around 90%. As can be seen in FIG. 2, the typical variation of optical transmittance of VO.sub.2 deposited as a film at 2500 nm as a function of temperature has a hysteresis of around 15 C. in width. In FIG. 2, one of a series of data points for heating is labelled 3 and one of a series of data points for cooling is labelled 4.

(11) Thus, the optical transmittance of VO.sub.2 deposited as a film changes significantly, due to the semiconductor-to-metal transition. At temperatures below the transition, VO.sub.2 is substantially transparent. At temperatures above the transition, VO.sub.2 is significantly less transparent (more opaque). In VO.sub.2, the semiconductor-to-metal transition occurs at around 70 C. This is not that much higher than the temperatures that can be reached on hot days in some parts of the world. It is this change in optical properties and the temperature at which the change occurs, which makes the use of doped or undoped VO.sub.2 of particular interest in the manufacture of smart windows.

(12) In accordance with the invention, spray pyrolysis can be used to deposit VO.sub.2 as a film on a substrate. The film can then be removed from the substrate, in order to provide a VO.sub.2 powder.

(13) Several experiments were conducted, in order to determine suitable conditions for reliably producing VO.sub.2 films by spray pyrolysis.

(14) Generally, the process involves providing a precursor solution containing a vanadium (IV) complex. The precursor solution is then sprayed in the presence of water on to a heated substrate, thereby depositing material on the substrate. After spraying, the deposited material is dried by maintaining the substrate at a predetermined drying temperature for a predetermined drying time. After drying, the deposited material on the substrate is cooled to ambient temperature at a controlled, relatively quick, rate. The deposited material can then be removed from the substrate to produce a vanadium (IV) oxide powder.

(15) X-ray diffraction (XRD) data from VO.sub.2 deposited as films successfully produced by spray pyrolysis in accordance with the invention indicate two main phases: V.sub.3O.sub.5 (an oxygen-deficient semiconducting phase and VO.sub.2 in its low-temperature M1 monoclinic form. Typically, there is a peak in the XRD spectra at 9 and peaks at 30. The peak at 9 is associated with V.sub.3O.sub.5; the peaks at 30 are associated with V.sub.3O.sub.5 and VO.sub.2. In some instances, the peak at 9 was more intense than the peaks at 30; in other instances, the peaks at 30 were stronger.

(16) Table 1 below shows spray pyrolysis conditions that were found by the applicant to produce good quality VO.sub.2 (A-K), which was deposited as a film on a glass substrate. The VO.sub.2 was around 300 nm thick.

(17) TABLE-US-00001 TABLE 1 Carrier Precursor Substrate Cooling Drying Precursor gas flow solution temp. Carrier rate time Precursor solution rate spray rate Film ( C.) gas ( C./min) (mins) complex conc. (M) (ml/min) (ml/min) A 530 N.sub.2 15 60 [VO(acac).sub.2] 0.05 7 0.5 B 530 N.sub.2 15 60 [VO(acac).sub.2] 0.0228 15 1 C 530 N.sub.2 15 45 [VO(acac).sub.2] 0.0228 15 1 D 530 N.sub.2 15 60 [VO(acac).sub.2] 0.0228 15 1 E 530 N.sub.2 15 5 [VO(acac).sub.2] 0.0228 15 1 F 530 N.sub.2 15 30 [VO(acac).sub.2] 0.0228 15 1 G 530 N.sub.2 15 30 [VO(acac).sub.2] 0.0228 15 1 H 530 N.sub.2 40 30 [VO(acac).sub.2] 0.0228 15 1 I 465 N.sub.2 15 30 [VO(acac).sub.2] 0.0228 15 1 J 490 N.sub.2 15 30 [VO(acac).sub.2] 0.0228 15 1 K 530 N.sub.2 15 30 [VO(acac).sub.2], 0.0228 15 1 with 2 mol % [Zn(acac).sub.2 .Math. H.sub.2O]

(18) The spray pyrolysis conditions and parameters used to produce film A successfully produced a uniform film. A hysteresis typical of the semiconductor-to-metal transition in VO.sub.2 was seen in measurements of the variation of electrical resistance with temperature for film A.

(19) In producing film B, the effects of carrier gas flow rate, precursor solution spray rate and precursor solution concentration on the reliability of film production were tested. A hysteresis typical of the semiconductor-to-metal transition in VO.sub.2 was seen in measurements of the variation of electrical resistance with temperature for film B.

(20) Film C was produced using the same conditions and parameters as film B, except that the drying time was shorter (45 minutes instead of 60 minutes). A hysteresis typical of the semiconductor-to-metal transition in VO.sub.2 was seen in measurements of the variation of electrical resistance with temperature for film C.

(21) FIG. 3 is a scanning electron microscope (SEM) image of film D. The image was taken using an accelerating voltage of 5 kV and at a working distance of 10 mm. The magnification of the SEM image is 50000 times.

(22) FIG. 4 shows the variation of electrical resistance with temperature for film D. A hysteresis typical of the semiconductor-to-metal transition in VO.sub.2 can be seen. An arrow labelled 14 indicates heating; an arrow labelled 15 indicates cooling.

(23) FIG. 5 is an x-ray diffraction (XRD) spectrum of film D. The spectrum contains a prominent peak 16 at around 9 and smaller, significant peaks 17 at around 30.

(24) Film D was produced under the same spray pyrolysis conditions as film B. This was done to test the reproducibility of the process used to produce film B. The successful production of film D indicated that the process used to produce film B was reproducible.

(25) Film E appeared to be a normal-looking VO.sub.2 film produced in accordance with the invention. In producing the film E, a very short drying time was used (5 minutes). No significant hysteresis loop was seen in measurements of the variation of electrical resistance with temperature for film E. Without wishing to be bound by any theory, the absence of a significant hysteresis loop for film E could be a consequence of the very short drying time used in producing the film E. The other spray pyrolysis conditions used in the production of film E were the same as were used in the production of films B and D, both of which exhibited an observable, significant hysteresis loop in the variation of electrical resistance with temperature.

(26) A hysteresis typical of the semiconductor-to-metal transition in VO.sub.2 was seen in measurements of the variation of electrical resistance with temperature for film F and film G.

(27) Films F and G were produced using the same spray pyrolysis process conditions. This suggests that the practising the process using these process conditions produces reproducible results in terms of film production.

(28) The process for producing film H was successful in that it produced a good VO.sub.2 film. The process conditions included a relatively fast cooling rate of 40 C./min. A hysteresis typical of the semiconductor-to-metal transition in VO.sub.2 was seen in measurements of the variation of electrical resistance with temperature for film H.

(29) The processes by which films I and J were produced were both successful. In producing film I, the substrate temperature was 465 C.; in producing film J, the substrate temperature was 490 C. A hysteresis typical of the semiconductor-to-metal transition in VO.sub.2 was seen in measurements of the variation of electrical resistance with temperature for film I and film J.

(30) In the production of film K, zinc was introduced as a dopant. Consequently, film K was a zinc-doped VO.sub.2 film. The film exhibited a good change in resistance with temperature associated with the semiconductor-to-metal transition. The width of the observed hysteresis loop was relatively narrow.

Example of a Preferred Process

(31) The experimental data were analysed, in order to derive a preferred method and set of process conditions for reliably producing good VO.sub.2 powders by spray pyrolysis in accordance with the invention.

(32) A precursor solution is prepared by dissolving approximately 0.228 M vanadyl acetylacetonate ([VO(acac).sub.2]) in a 2:1 by volume mixture of ethanol to 7% by volume acetic acid in water. Preferably, the precursor solution may be prepared not more than 48 hours prior to use.

(33) The precursor solution is then deposited on to a substrate by spray pyrolysis in a humid atmosphere.

(34) In this preferred example process, the carrier gas is pure nitrogen, which is supplied to the nozzle of the spray pyrolysis system at a flow rate of 14.5 l/minute. Alternatively, the carrier gas may be water-saturated nitrogen. At the same time, the precursor solution is introduced into the nozzle at a flow rate of 1 ml/min. Droplets of the precursor solution are thereby produced at the nozzle, and carried to the substrate and deposited thereon.

(35) A glass substrate is used, which is held at a temperature of 490 C. during spray deposition. The duration of the spray deposition process is approximately 40 minutes.

(36) Following the spray deposition process, the deposited material and the substrate are held at 490 C. for a further 30 minutes. The deposited material forms a film on the substrate. Typically, the film has a thickness of approximately 300 nm.

(37) The person skilled in the art will appreciate that the process conditions used to produce the films A-K and of the preferred process may be varied without departing from the scope of the invention.

(38) After cooling, the material deposited on the substrate is removed from the substrate, thereby providing a metal oxide powder, e.g. VO.sub.2 powder. The material deposited on the substrate may be removed from the substrate by any suitable non-chemical, non-thermal means such as by scraping the deposited material from the substrate and/or by tipping (e.g. inverting) the substrate and/or shaking the substrate.

(39) The deposited material removed from the substrate may be further processed to provide the metal oxide powder, e.g. vanadium (IV) oxide powder, with one or more desired characteristics, e.g. a higher purity and/or a particular particle size distribution.

(40) In another example embodiment of the invention, vanadium (IV) oxide was deposited as a film by spray pyrolysis using vanadium (V) oxide (V.sub.2O.sub.5) as a precursor. 0.236 g V.sub.2O.sub.5 and 0.6 g oxalic acid solid precursors were dissolved in 25 ml of water and warmed until a blue vanadyl oxalate ([VO(ox)]) solution was formed. 15 ml of the [VO(ox)] solution was sprayed, with nitrogen as a carrier gas, at a rate of 0.5 ml/min on to a glass substrate held at 500 C. The deposited film was then annealed at 525 C. under flowing nitrogen for three hours. Optionally, acetone may be added to the [VO(ox)] solution.

(41) The VO.sub.2 film produced was uneven and powdery. FIG. 6 is an XRD spectrum of the VO.sub.2 film. Peaks corresponding to the (011), (211), (020), (212) and (021) planes are labelled in FIG. 6.

(42) After cooling, the material deposited on the substrate was removed from the substrate by a suitable non-chemical, non-thermal method, thereby producing a VO.sub.2 powder.

(43) In another example embodiment of the invention, vanadium (IV) oxide was deposited as a film by spray pyrolysis using vanadium (V) oxide (V.sub.2O.sub.5) as a precursor.

(44) 0.103 g V.sub.2O.sub.5 and 0.2 g oxalic acid solid precursors were dissolved in 10 ml of water and warmed until a blue vanadyl oxalate ([VO(ox)]) solution was formed. The [VO(ox)] solution was sprayed, with nitrogen as a carrier gas, at a rate of 1.0 ml/min on to a glass substrate held at 450 C. The deposited film was then annealed at 525 C. under flowing nitrogen for three hours. Optionally, acetone may be added to the [VO(ox)] solution.

(45) The VO.sub.2 film produced was relatively uniform and thin. FIG. 7 is an XRD spectrum of the VO.sub.2 film. Peaks corresponding to the (100) and (011) are labelled in FIG. 7.

(46) After cooling, the material deposited on the substrate was removed from the substrate by a suitable non-chemical, non-thermal method, thereby producing a VO.sub.2 powder.

(47) Methods using an oxalate precursor, e.g. oxalic acid, may be preferred, since oxalate precursor materials generally may be relatively cheap. In addition, the powder products formed by methods using an oxalate precursor may be relatively clean.

(48) There are two vanadyl oxalates, [VO(ox)(H.sub.2O).sub.2] and [VO(ox).sub.2].sup.2. When the vanadium (IV) oxalate is produced by dissolving V.sub.2O.sub.5 in an excess of an oxalate solution, more of the dianionic complex ([VO(ox).sub.2].sup.2) than the neutral complex ([VO(ox)(H.sub.2O).sub.2]) typically may be present.

(49) In embodiments of the invention, dopants may be added to the solution, in order to modify the powder produced. Suitable dopant precursor materials may include water-soluble metal salts, e.g. a water-soluble tungsten salt.

(50) Metal oxide powders, e.g. vanadium (IV) oxide powders, obtained or obtainable by the production process of the invention may have utility in a wide range of applications. For instance, powders obtained or obtainable by the process of the invention may be used in thermochromic coatings, inks or paints. Such powders may, for example, be used in the coating of glass to produce thermochromic windows (smart windows) for buildings or vehicles. Such coatings may also be applied to objects, in order to reduce the thermal (infrared) images of the objects.

(51) The production process of the present invention has several advantages over known methods of producing metal oxide powders, e.g. vanadium (IV) oxide powder.

(52) Importantly, in some embodiments, the metal, e.g. vanadium, generally may not change oxidation state during the spray pyrolysis process, i.e. the metal exists in the same oxidation state in the precursor complex as in the final powder product. Without wishing to be bound by any theory, it is thought that this results in the metal oxide powder, e.g. vanadium (IV) oxide powder, produced in accordance with the invention being of very good purity (i.e. at least 90% pure).

(53) The coordination and decomposition chemistry of precursor vanadyl complexes means that the spray pyrolytic process of the invention is successful. The complexes decompose, i.e. the ligands become separated from the vanadyl ions, due to the temperature of the substrate. The water provides the oxygen that is required to react with the vanadyl ion to produce vanadium (IV) oxide (VO.sub.2). Preferably, the water may be the principal, e.g. only, source of oxygen available for the reaction. The water may be provided by an aqueous solution (e.g. an aqueous precursor solution) and/or an aqueous solvent mixture and/or a water-containing (e.g. water-saturated) carrier gas.

(54) While knowledge of the coordination and decomposition chemistry of targeted precursors (e.g. the coordination and decomposition chemistry of targeted vanadyl precursors such as [VO(acac).sub.2] and [VO(ox)]) underpins this invention, in further developing the invention the spray pyrolytic process has been tested, controlled, developed and optimised.

(55) For example, the process allows for the production of metal oxide, e.g. vanadium (IV) oxide, at lower temperatures than flame spray pyrolysis. Also, the process may be more acceptable from an environmental and/or health and safety perspective than processes such as APCVD. One environmental benefit is that the process may provide the ability to produce metal oxide powders, e.g. vanadium (IV) oxide powders, from an aqueous precursor solution.

(56) Furthermore, careful selection of the precursor complex can reduce or minimise any harmful emissions (e.g. nitrous oxide) when the precursor complex decomposes. For example, the acetylacetonate (acac) ligand has reasonable water solubility and does not produce very harmful emissions on decomposition of the precursor complex, making it a suitable choice for use in the present invention. The oxalate (ox) ligand has good water solubility and also does not produce very harmful emissions on decomposition of the precursor complex, making it a suitable choice for use in the present invention.

(57) Advantageously, the process can be scaled up to produce relatively large quantities of very pure doped or undoped powder.

(58) As a result of the use of aqueous solutions, relatively low temperatures and/or low-harmful emission ligands, the equipment cost and complexity for practising the invention may be relatively low. Thus, it may be relatively economical to scale-up the invention. Furthermore, since the ligand(s) may remain intact after decomposition of the precursor complex, in some embodiments, it may be possible to recover the ligand(s) and subsequently re-use the recovered ligand(s) in the preparation of the precursor complex. Thus, the ligand(s) may be recyclable, thereby further reducing or minimising the cost of practising the invention.

(59) While the invention has been described specifically in relation to the production of vanadium (IV) oxide powders, it will be appreciated that other metal (IV) oxide powders may be produced by the process of the invention. For instance, the spray pyrolytic process of the invention may be used to produce doped or undoped molybdenum (IV) oxide powder, tungsten (IV) oxide powder, germanium (IV) oxide powder or manganese (IV) oxide powder.

(60) In addition, the process may be utilised to produce metal oxide powders, in which the metal has other oxidation states. For instance, the process may be utilised to produce metal (II) oxide powders or metal (III) oxide powders such as vanadium (III) oxide powder.

(61) It will be appreciated that the methods of the invention are typically performed in an inert atmosphere and generate intermediate oxidation states. By performing the methods in an inert atmosphere, advantageously the formation of higher, e.g. highest, oxidation states is avoided.