Technical field

Chemistry; chemical engineering; synthesis o† sjubmicron powder material.

Technical problem

Technical problem is current lack of flame (spr_ky) pyrolysis of water based precursor to A ₂ .x.yB _x C _y Zr ₂ 0 material where A can be Eit er Gd or La, while B and C are optional and may be selected from Ce, Pr, Nd,| Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ga, Sc, whereby moiety of B and C are preferably between 0 at.% in 10 at.%.

State of the Art

There are several ways how to achieve powder preparation, briefly described below by referencing state of the art patents or patient applications as well as technical articles in the field.

C 102502816 describes a method for preparing Gd2Zr ₂ 0 ₇ nano-powder through coprecipitation and relates to a method for preparing nuclear radiation resistant nano- powder. The method includes steps: firstly, preparing solution, namely weighing rare earth oxides Gd ₂ 0 ₃ and ZrOCI ₂ 8(H ₂ 0) according to a Gd ₂ Zr ₂ 0 ₇ component, dissolving in concentrated nitric acid and addipg distilled water to obtain rare earth solution; secondly, preparing precipitating agen ; ammonia water into diluted ammonia water, preparing precursor, dropwise adding he prepared rare earth solution into precipitating agent, and subjecting the obtained precursor precipitate to aging, cleaning, filtering, drying and sieving to obtain precursor powder; and thirdly, subjecting the precursor: powder to roasting and ball-mitling to obtain a powder sample.

CN103396119 describes a .preparation method of single-phase pyrochlore-type La ₂ Zr ₂ 0 ₇ nano-powder. The preparation method is characterized in that the La _z Zr ₂ C>7 nano-powder is obtained by ageing] drying and high-temperature roasting by using Zr<4+> and La<3+> hydration inorganic salt as a precursor, citric acid as a complexing agent, an amide organic matter as a gel accelerant and polyethylene glycol as a dispersing agent.

Method of combustion synthesis is commonly used in laboratory scale preparation of materials, and is based on water solution of metal salts - nitrates which together with fuel in form of urea, glycine, alanine, citric acid at elevated temperatures form gel where exo-thermal reaction is expected, said reaction causing fine porous oxide powder, An example : is described in work "Characterization and luminescent properties of Eu 3+ doped: Gd ₂ Zr ₂ 07 nanopowders" (S. S. S. M. S. Rabasovic, D. Sevic , J. Krizan , M. Terzic, J. Mozina, B. Marinkovic and and N. R. M. Mitric, M. D. Rabasovic, "Characterization and luminescent properties of Eu 3+ doped Gd2Zr207 nanopowders," J. Alloys Compd., 2014).

Method of flame pyrolysiis jis based on burning of precursor which is alcohol based, resulting in oxide powder. The elements of precursor are organo-metal compounds.

Further example of state of the art synthesis can be found in work "Synthesis of Single Phase La ₂ Zr ₂ G ₇ by Wet Mechanochemical Treatment" (M. O. D. Jarligo, Y. Kang, A. Kawasaki, and R. Watanabe, "Synthesis of Single Phase La 2 Zr 2 O 7 by Wet Mechanochemical Treatment" vol. 45. no. 8, pp. 2634-2637, 2004).

Further example of ;state of the art hydrothermal method can be found in workj "Synthesis, Characterization and Luminescent Properties of La ₂ Zr ₂ Or:Eu3+Nanorods" (H. Sc-hg, L. Zhou. Y. Huang, L. Li, T. Wang, and L. Yang _j "Synthesis, Characterization and Luminescent Properties of La[sub 2]Zr[sub 2]0(sub' 7]:Eu[sup 3+] Nanorods,"; Chinese J. Chern. Phys., vol. 26, no. 1, p. 83, 2013). Last but not least various synthesis methods can be found in work "Synthesis of fine- dispersed oxides La ₂ Zr ₂ 0 ₇ , La ₂ Hf ₂ 0 ₇ , Gd ₂ Zr ₂ 0 ₇ , Gd ₂ Hf ₂ 0 ₇ " (N. T. K. V.G. Sevastyanovl , E.P. Simonenkol , 2, N.P. Simonenkol , K.A. Sakharov2, "Synthesis of fine-dispersed oxides La2Zr207, La2Hf207, Gd2Zr207, Gd2Hf207," vol. 7, no. June, pp. 24-28, 2012).

None of these methods solve above referenced technical problem. The solution to presented technical problem can be attempted by use of reactor for synthesis of oxidized or reduced or carbonised submicron powder compounds using thermoacoustic burner.

SI24270 describes a reactor for synthesis of oxidized or reduced or carbonised submicron powder compounds usjing thermoacoustic burner solving a problem of synthesis of submicron, powder materials in a single pass from a liquid precursor in a reducing atmosphere. _; Thermoacoustic burner allows the heating of the reaction space in the form of a tubular reactor to a temperature required for the synthesis, in the range of 500 to 12,00 °C. In trie case of a precursor such as a mixture of nitrate and fuel exothermal reactiojn occurs, said reaction producing a compound of the size of submicron particles. Thermoacoustic burner produces acoustic waves resulting in a homogeneous temperature field jand transport of the material towards the exit of the resonance tube. Due; to the atmosphere with presence of soot, and because of presence of an excess of reducinjg gas oxidation of the particles is not possible, and at the same time said particles jobtain thin carbon coating. After exiting from the reaction space the flue gases with particles within a narrow passage which allows access to the cooling air cool ijapidly to the level of 200 to 300 °C, wherein all combustible residues of flue gases receive sufficient oxygen to burn. Since said particles already have said carbon coating said particles are not oxidized while flue gases are allowed to finish combustion, , Description of new invention

Process for continuous synthesis of submicron oxide powder ceramic materials for thermal coatings solves abovei referenced technical problem by combining method of flame pyrolysis of water soluble precursor with combustion synthesis in continuous process using special reactor.

Process for continuous synthesis according to this patent application is performed in reactor for the synthesis of reduced submicron powder compounds comprising thermoacoustic burner described in SI24270, and is incorporated herewith by reference.

Within this reactor the oxidants and! reductants react (both are present in precursor), and a result are powder (final product) and flue gases. Powder is formed as any ash would be formed during; combustion. In this case said powder happens to be of utility as final product which is more or less crystalline or amorphous, however with desired chemical composition. All other products of reaction are in form of flue gases such as N2, C02, and H20, and arei released.

Said reactor for the synthesis of reduced submicron powder compounds comprising thermoacoustic burner is suitable for flammable and water precursors. Said thermoacoustic burner comprises of thermoacoustic burner section, and of resonance pipe. Said thermoacoustic burner is a Helmholtz resonator with adjustable frequency of operation. The thermoacoustic burner section works at 50 to 250 Hz frequency. By adjusting its natural frequency, it is possible to create a resonance mode that is tied to the resonance pipe; in this apparatus the reaction space and part for the cooling and capturing of particles. When working with flammable precursors, they can be injected into the adjustable volume of the burning chamber; when working with water-basedi precursors they can be injected into the resonance pipe where right temperature conditions for the reaction were formed beforehand.

In the case of reduction atmosphere, the reactor is divided into two parts. The first part is the resonator with the icombustion chamber where we need to create a I suitable ratio of fuel and air for the stable function of the thermoacoustic burner; it can' be either in the oxidation or reduction field but so the thermoacoustic burner's function is stable. Creating an additional reduction atmosphere is possible in the resonance pipe; an additional iinjection of reduction gas creates higher concentrations of the reduction atmosphere. Gases that enable reduction may be H2, C ₂ H ₂ , C ₃ H ₈ , GH ₄ , C2H4, C3H6. CO; Because they are injected into a heated and working resonance reactor, the combustion of gases is certain. In this case, the rest of the oxygen from the reaction chamber is used up first, if still present. In the continuation, we have a: presence of unburned reduction gases at temperatures from 600 to 1500 (preferably around 1200 °C).

Injection of the precursor into the resonance pipe is behind the reduction gas supply. Spraying is done with a two-component nozzle, there can be several spray gases used for propelling, if : either nitrogen or argon is used this does not affect the reaction. Spraying forms tilny droplets of the precursor which disintegrate rapidly because of environment temperatures from 500 to 1200 °C. Precursor components are metal salts (nitrates) which react by the principle of combustion synthesis in the combination of fuel in the form of urea, carbohydrazide, glycine. The nitrate reacts with the fuel: in the precursor, which assures a reduction reaction giving off additional exothermic reaction heat. Flue gasses with the surplus of reduction gasses ensure the reaction environment at a desired temperature an prohibit the oxidation of the combustion synthesis product. ! During reaction between components injected particles of final product are crystallized in form of powder, in addition to hot flue gas formation. By extending the distance and time when the particles are at the reaction

i

temperature, it is possible o influence the crystalline structure of the product. This is connected with the natural frecjuency of the thermoacoustic burner that can be changed by the adjustable volume. The nature of the thermoacoustic burner guarantees high turbulence in the resonance pipe that additionally sprays the precursor droplets, maintains aj homogeneous temperature field along the entire cross-section of the [ reaction pipe, and provides transport of the powder product) towards the pipe outlet. in Said resonance pipe comprises the reduction environment until the exit of flue gasses and synthesized particles from the resonance pipe. At the exit of the resonance pipe, hot flue igasses mix with air, ignite and combust the leftover gasses in a prolonged flame. At the same tirne, the mixed in amount of air strongly cools the flue gasses and powder-like product that entered the extended pipe of the reactor. The extended pipe is where cooling jof the product and final oxidation of the gasses takes place, but the product does not oxidize because of the rapid lowering of the temperature below the oxidation line (200X... 300 °C). The cooling gas is air, producing both cooling effect, serving in the combustion of the left-over reduction gasses, and being the cheapest; way of finishing the work in the reduction atmosphere but other forms of coolants may be used as well. Due to safety precautions the final oxidation of flue gasses must occur before a filter, but because of the rapid temperature fall the particles don't oxidize.

The precursor in this invention is comprised of a raw material, which in the correct stoichiometric ratio contains necessary metal ions of the crystalline matrix, such as metal ions, Gd, La and Zr in the form of water-soluble salts, such as nitrates, acetates, or in the form of organic metals. Fuel in the precursor is based on organic compounds such as urea, glycine, alanine, citric acid, glycol, glycerol, hexamethylenetetramine. or similar organic compounds, and mixtures of two or more thereof. Other metal elements which are acting as dopants may be added as any water-solublle compounds such as metal salts of Mg, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Qa , Sc, mainly as nitrates, up to 10 atomic percent (hereinafter at. %), preferably at around 5 at. %. The addition of these elements or mixture of several elements has vital influence on the material properties, in particular) fluorescence.

Described precursor is propelled in a reactor by spray gas such as oxygen, air, or! nitrogen, in the device described in SI24270, wherein an atmosphere in said reacto may be oxidative or reductive, ajt a temperature ranging from 500 to 1.500 ⁰ C and a frequency of thermoacoustic burner between 50 and 300 Hz. In this way, a continuous process of the reaction of a precursor for the combustion synthesis is achieved. At the exit of the reaction tube material is rapidly cooled, preferably with air or other suitable cooling ;gas. The recovery of particles is carried out by conventional methods of filtration as _; known in the art (hereinafter also referred to as filtratiJn system or filter). The purpose of obtained material, preferably Gd2Zr20 ₇ and/or La2Zr ₂ 0 ₇ is a coating against thermal wear. This material may be doped using rare earths resulting in fluorescent properties which allow for the monitoring of temperature and wear of the coating. Monitoring of the temperature is possible in two ways, with the method of the relationship between the intensity of fluorescent peaks, or duration of fluorescence.

Process for continuous synthesis according to this application is comprised of the following steps:

(i) Precursor preparation

(ii) Reactor preparation

(iii) Pyrolysis

(iv) Cooling with further optional step of annealing wherein said continuous synthesis is performed in reactor for the synthesis of reduced submicron powder compounds comprising a thermoacoustic burner, said thermoacoustic burner comprising a thermoacoustic burner comprised of thermoacoustic burner section and a resonance pipe, and further wherein material obtained is A2-x-yB _x CyZr207 material;

wherein A is selected from Gd, La;

wherein B is optional, and; is selected from Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm,

Yb, Lu, Hf, Ga, Sc; i

wherein C is optional; and is selected from Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm,

Yb, Lu, Hf, Ga, Sc;

wherein x is moiety of B and is preferably between 0 at.% and 10 at.%;

wherein y is moiety of C aind is preferably between 0 at. % in 10 at %;

wherein 2-x-y is a result of mathematical operation subtracting moieties x, and y froml 2, respectively.

In a way of example ^; , if material A is Gd, material B is Nd, and material C is Eu. and further, if moiety of B is 5 at. %, and moiety of C is 2% the result of mathematical operation 2-x-y is 1 ,93, x is 0,05, and y is 0,02 so the material in this example is

Gdi 93Ndo.05EUo,02Zr207.

Precursor preparation according to this invention is further comprised of following steps:

(i) Dissolving precursor in the first solvent;

(ii) Dissolving appropriate additives, preferably oxidants and reducers in the second solvent;

(iii) Mixing of said dissolved precursor with said dissolved fuel to prepare a solution.

Said precursor can be selected from range of materials, including gadolinium oxide (Gd ₂ 0 ₃ ), lanthanum nitrate La(N0 ₃ )3, or mixtures or combinations thereof.

Said additives can be selected from range of materials, including zirconium oxynitrate (ZrO(N0 ₃ ) ₂ ), urea ((NH ₂ )2GO), europium nitrate (Eu(N0 ₃ )3). erbium nitrate Er(N0 ₃ )3. ytterbium nitrate Y (N0 ₃ )3,i or mixtures or combinations thereof.

Said first solvent can be selected from range of materials, including nitric acid (HNOg), water, preferably demineralized.

Said second solvent can be selected from range of materials, including water.

Reactor preparation according to this invention is comprised of following steps: said solution is put into a liquid feed system means, preferably syringe, pump| or similar, for feeding said solution into said reactor;

said thermoacoustic burner is started preferably by turning on a blower, a spark plug and opening a; fuel valve.

said thermoacoustic burner then operates at its natural frequency, : saicl frequency risijng wjith increased temperature within said reactor;

said thermoacoustic burner and said resonance pipe are heated up ; to la working temperature between 700°C and 1.500°C, preferably between 000°C and 1.200 ^e C; Reactor preparation may be performed in a reactor for the synthesis of reduced submicron powder compounds further comprising a blower for blowing mixture, a spark plug for igniting volatile mixture within said reactor, and fuel valve for allowing volatile mixture to enter; said reactor through.

Reactor preparation may further include the following step: by heating the resonance pipe a filter forming part of said reactor for the synthesis of reduced submicron powder compounds comprising thermoacoustic burner is also heated up; said filter is cooled with a secondary air input so that its [temperature remains above dew point, preferably above 100 ^e C in order to prevent condensation inside the filter.

Pyrolysis according to this invention is comprised of following steps:

opening spray gas opening valve; flow of said spray gas is preferably measured;

said liquid feed system means is started and said precursor is sprayed into said reactor propelled by said spray gas forming spray droplets;

said , solvent evaporates from the said spray droplets, followed by I an exothermic reaction between additives, preferably between oxidizers, preferably nitrates and reducers, preferably urea, forming hot flue gases iand particles;

Said spray gas can be air or oxygen in the case of oxidizing atmosphere and/orj nitrogen or any of inert gases for; reductive atmosphere.

During this phase the pulsations of burning have an effect on heat transfer phenomena; the higher turbulence enhances the homogeneity of the temperature field over the whole jcross section of the pipe and at the same time a good transport of formed particles towards the end of resonance pipe is ensured.

Batch processes using precursor is most often used in laboratory processes, here the combustion is not continuous. The nature of combustion synthesis leaves! veiy porous products; in the case of continuous operation and using spray methods, the product is more homogeneous and it is not batch dependent.

Cooling according to this invention is comprised of following steps:

at the end of said resonance pipe said hot flue gases are mixed with air and then resulting mixture enters said filter; particles and flue gases are cooiejd down, preferably fast, to a jtemperature below 200 "C because of relatively good mixing at the exit of the resonance pipe; at temperatures below 200 °jC there should be no more ongoing reactions or crystallization,

in said filter said particles are separated from said flue gases, which can be achieved most known filtration techniques.

Annealing according to this; invention is comprised of following steps,

should desired crystallinity inot be reached in said reactor, a product can ,be further annealed in separate furnace, said furnace known in state of the art at a temperature up to 1200 ^* C for 2-6 hours.

The main difference : between this synthesis and flame spray synthesis is in t e solvent - whereas flame spray synthesis only uses flammable solvents and organic precursors, the pulse combustiqn synthesis can also use water based precursors with inorganic metal: salts and, ensures good transport of particles through ithe resonance pipe. The energy [required for evaporation and reaching reaction conditions is achieved through pulse burning of gaseous or liquid fuel. Furthermore, it is also possible to operate in fuel) rich and fuel lean conditions.

Resonance frequency:

Helmholtz resonator

The frequency of fuel burning can be approximated with the use of the equation fcjr frequency of Helmholtz resonator A is the cross section of the neck (outlet pipe), L is the length of the neck (outlet pipe), V is the volume of the combustion chamber and c the speed of sound in the resonator. The frequency is temperature dependent, because the speed of sound changes with density of the medium.

Resonance pipe frequency

The resonance pipe has a frequency similar to an open-end air column, which base frequency can be approximated with this equation:

_ c

f ^o ~ 2 L

L is the length of the pipe and c is the speed of sound in the oscillating medium. T|he resonance pipe can operate at the base frequency or any other higher harmonips. The natural frequency of the therfmoacoustic burner and the operating frequency of the resonance pipe have to be harmonized.

The process is further described by means of examples, below

Example 1 - precursors pnd process for synthesis of Gdi,9sEu _0> os r207

In concentrated nitric acid (65% w/w HN0 ₃ ) 84 g Gd ₂ 03 is dissolved, further 50 g ZrO(N0 ₃ )2 _> 50 g urea, and 18 g Eu(NO;j) ₃ is dissolved. Said reactor for the synthesis of reduced submicron powder compounds is operated at around 1200 °C in oxidizig atmoshpere, and said thermoacpustic burner is operated at frequency 220 Hz. Said spray gas is oxygen.

Example 2 - precursors and process for synthesis of Lai,92Eu _0< 08Zr2O ₇

In demoralized water 80 g of La(N0 ₃ ) ₃ , 50 g of ZrO(N0 ₃ ) ₂ , 50 g of urea and! 36 g of Eu(N0 ₃ ) ₃ is dissolved. Said reactor for the synthesis of reduced submicron powder compounds is operated at around 1200 °C in oxidizig atmoshpere, and sa d thermoacoustic burner is operated at frequency 220 Hz. Said spray gas is oxygen. | Example 3 - precursors and process for synthesis of

In concentrated nitric acid (65% w/w HN0 ₃ ) 84 g Gd ₂ 0 ₃ is dissolved, further 50 g ZrO(N0 ₃ ) _2> 50 g urea, 17,7 g of Er{N0 ₃ ) ₃ and 4,5 g of Yb(N0 ₃ ) ₃ is dissolved. Said reactor for the synthesis of reduced submicron powder compounds is operated at around 1200 ^Q C in oxidizig atmoshjpere, and said thermoacoustic burner is operated at frequency 220 Hz. Said spray gas is oxygen

Example 4 - precursors and process for synthesis of Lai.gsEro.w bo.oiZrjO?

In demineralized water 80 g of La(N03)3, 50 g of ZrO(N0 ₃ ) ₂ , 50 g of urea, 17,7 giof Er(N0 ₃ ) ₃ and 4,5 g of Yb(N0 ₃ ) _3l is dissolved. Said reactor for the synthesis Of reduced submicron powder compounds is operated at around 1200 "C in oxidizig atmoshpere, and said thermoacoustic burner is operated at frequency 220 Hz. Said spray gas is oxygen.

Said synthesis results in crystalline materials with specific fluorescence.

Fluorescence specters are presented in figures accompanying this patent application, and forming its integral part.

Fig. 1 shows fluorescent response of Gdi g ₅ Euo _, o5 r ₂ 0 ₇ illuminated by light of wavelength of 365 nm. There are three specific peaks seen in spectre, at 590 hm, 61 nm, and 629 nm.

si. 2 shows

wavelength

548 nm.

si. 3 shows

wavelength

548 nm.