OXIDE

BACKGROUND

[0001] This application is directed to methods for preparing nickel/aluminum spinel- coated aluminum oxide, the materials obtained by the methods, and uses thereof.

[0002] A spinel-type composite metal oxide is a complex oxide formed by combining two or more metal oxides, and has a chemical formula which can be written as AB2O4 or ABCO4. AB204-type spinels have cubic crystal systems, with a face-centered cubic lattice, wherein O ² is face-centered cubic close packed (CPP).

[0003] Nickel aluminate ( AI2O4) is a spinel material containing a transition metal ion (Ni). It is a narrow-bandgap semiconductor, and has advantages such as good visible light absorption properties, catalytic properties, and resistance to acids and alkalis.

Moreover, AI2O4 ceramic-metal composite materials have very high conductivity, and may be used in the aluminum electrolysis industry as inert anode materials.

[0004] A known method for synthesizing NiAl204-coated alumina (AI2O3) is ball milling NiO powder and AI2O3 powder, followed by oven-drying and heat treatment.

However, this process has a long cycle, generally requiring several hundred hours, and and high energy consumption. During preparation, the chemical reaction in which AI2O4 is synthesized can only be realized slowly at a temperature of as high as 1600°C. LIU Tao et al. have described a high-temperature oxidation method to prepare an AI2O3 composite powder coated with a nanosized AI2O4 layer. The surface of the AI2O3 was chemically plated with Ni first, then subjected to high-temperature oxidizing treatment at 1350°C or higher to obtain the corresponding powder. The method is complex and multi-step, with a high heat- treatment temperature. Another method for preparing NiAl204-coated AI2O3 is a liquid-phase combustion synthesis method. The powder obtained by that method is a mixed powder of N1AI2O4 and NiO in a non-stoichiometric ratio, the components being (Nii _-x Al204-x'xNiO). The value of x is gradually reduced by raising the preheating temperature, so as to obtain the corresponding N1AI2O4 powder. The control of preheating temperature in the method is not precise, and the rate of temperature increase has a significant influence on the product, so the purity of the AI2O4 obtained is low.

[0005] In summary, the preparation of the nano structured NiAl204-coated AI2O3 has high energy consumption, and relies on complex steps. Moreover, in the case of the liquid- phase combustion synthesis method, component control is difficult, and purity is low. There accordingly remains a need in the art for new methods for the preparation of nanostructured NiAl ₂ 04-coated AI2O3.

SUMMARY

[0006] In an embodiment, a method for preparing a NiAhC^-coated AI2O3 powder, the method comprising: contacting an uncoated AI2O3 powder with N1SO4 and H4HCO3 in an aqueous medium to provide an AI2O3 powder coated with a composition comprising nickel; and sintering the AI2O3 powder coated with a composition comprising nickel at a temperature and for a time effective to provide the Ni AhC^-coated AI2O3 powder.

[0007] In a preferred embodiment, a method for preparing a nano-NiAhC^-coated AI2O3 powder comprises:

first, AI2O3 powder is dispersed in deionized water, and using nickel sulfate and ammonium bicarbonate as starting materials, the surface of the AI2O3 powder is uniformly coated with NiC03'2Ni(OH)2'2H20 by a liquid-phase reaction method, with the nano-layer thickness being regulated by controlling the nickel sulfate content, and after centrifugal separation, the powder is oven-dried and sieved, wherein the reaction equation is as follows:

3 SO4+6 H4HCO3→NiC0 ₃ 2Ni(OH) ₂ 2H ₂ 0 I +( H ₄ )2S04+5C0 ₂ †;

secondly, the mixed powder is sintered, wherein during sintering,

N1CO3 2Ni(OH)2 2H2O is decomposed into nano nickel oxide, then nickel oxide reacts with AI2O3 to produce nano AI2O4, the inverse equations being as follows:

N1CO3 2Ni(OH) ₂ 2H ₂ 0→ 3 O +4H ₂ 0†+C0 ₂ †

AI2O3 +N1O →NiAl ₂ 0 ₄ ; and

thirdly, the sintered powder is sieved to obtain nano-Ni AhC^-coated AI2O3 powder.

[0008] Another embodiment is a NiAhC^-coated AI2O3 made by the above-described methods.

[0009] The embodiments are further described with reference to the following Figures, Detailed Description, and Examples.

BRIEF DESCRIPTION OF THE FIGURES

[0010] The embodiments are further illustrated by the following Figures, in which:

[001 1] FIG. 1 is an X-ray diffraction (XRD) spectrum of Example 1 wherein (a) is the original AI2O3 powder and (b) is NiAb04-coated powder. [0012] FIG. 2 is a scanning electron microscope (SEM) image showing the overall morphology of the NiAhC^-coated AI2O3 powder obtained in Example 1.

[0013] FIG. 3 is an SEM image showing the local morphology of the NiAhC^-coated AI2O3 powder obtained in Example 1, and shows that AI2O4 is uniformly distributed on the surface of the AI2O3.

DETAILED DESCRIPTION

[0014] Described herein is a liquid-phase reaction method for preparing a

nickel/aluminum spinel-coated aluminum oxide. To solve the abovementioned technical drawbacks, the technical solution includes dispersing an AI2O3 powder in an aqueous medium; and contacting the dispersed AI2O3 powder with nickel sulfate and ammonium bicarbonate to provide a surface coating comprising nickel carbonate. The thickness of the surface coating can be controlled by adjusting the nickel sulfate content. The coated AI2O3 powder is then separated from the liquid phase, dried, and optionally sized. The dried, optionally sized powder is then sintered at a temperature effective to provide to provide a nanosized N1AI2O4 coating on the AI2O3 powder. The NiAkC^-coated AI2O3 powder can then be optionally sized again.

[0015] In particular, AI2O3 powder, nickel sulfate (N1SO4), and ammonium bicarbonate (NH4HCO3) are contacted in an aqueous medium. The AI2O3 powder comprises particles that can be of any shape, for example spherical, plate-shaped, irregular, or a combination comprising at least one of the foregoing. In an embodiment the powder is approximately spherical. The particle size can be monomodal, bimodal, or higher, or a distribution of sizes. The average largest particle size can vary widely, for example from 10 nanometers (nm) to 1 millimeter (mm), or from 50 nm to 500 micrometers (μιη), or from 100 nm to 100 μιη. In an embodiment the average largest particle size is from 500 nm to 50 μιτι, or from 750 nm to 1.25 μιη. For example, the AI2O3 powder can have an average largest particle size of 0.8 to 1.2 μιτι, or 0.9 to 1.1 μιτι, or 1 μιη.

[0016] The aqueous medium comprises water, preferably deionized water. The aqueous medium can contain only water, or can further comprise a water-miscible organic solvent, for example ethanol, isopropanol, dimethylsulfoxide, dimethyl formamide, and the like. When present, the water-miscible organic solvent can comprise 1 to 80 volume percent (vol. %) of the total volume of the aqueous solution, for example 2 to 70 vol.%, or 5 to 50 vol.%, or 5 to 40 vol.%, or 10 to 30 vol.% of the total volume of the aqueous medium. [0017] The AI2O3 powder and N1SO4, and H4HCO3 starting materials can be added to the aqueous medium in any order, or even simultaneously to provide a liquid-phase reaction system. For example, the AI2O3 powder can be dispersed in aqueous medium, followed by the addition of H4HCO3 and then and N1SO4. Alternatively, the H4HCO3 can be dissolved in the aqueous medium, followed by the addition of the AI2O3 powder and then the N1SO4 to provide the liquid-phase reaction system. The additions can be with agitation, for example stirring. Preferably, the the pH of the liquid-phase reaction system is maintained at 6 to 9, preferably at 7 to 8. The pH of the reaction can be adjusted by adjusting the rate of addition of the N1SO4.

[0018] The liquid-phase reaction system is maintained for a period of time and at a temperature effective to provide a coating on the AI2O3 powder. For example, the liquid- phase reaction system can be stirred for 30 minutes to 10 hours, or 1 hour to 5 hours. The temperature of the liquid-phase reaction system can be maintained at 10 to 50°C, for example at 20 to 25°C. In an embodiment, the temperature is ambient temperature to minimize energy costs.

[0019] Reaction of the starting materials provides a product where the the surface of the AI2O3 powder is uniformly coated with a composition comprising nickel. Without being bound by theory, it is believed that the coating comprises nickel carbonate (N1CO3). The composition can further comprise various oxides or hydroxides of nickel, for example nickel hydroxide (2Ni(OH)2). For example, the coating composition can comprise

NiC03'2Ni(OH)2'2H20 formed in a liquid-phase reaction. The reaction equation can be as shown in Equation 1.

3 SO4+6 H4HCO3 → N1CO3 2Ni(OH) ₂ 2H ₂ 0 I +( H ₄ )2S04+5C0 ₂ † (Eq. 1)

[0020] The thickness of the layer formed can be regulated by controlling the nickel sulfate content of the liquid-phase reaction system. In an embodiment, the layer can be a nano-layer having a thickness of 1 to 500 nm, or 2 to 250 nm, or 3 to 100 nm. The thickness can be uniform, for example varying by less than 20% on a given particle, or varying by less than 10%.

[0021] The nickel-coated AI2O3 powder can be isolated from the aqueous medium, for example by filtration or or centrifugal separation. The nickel-coated AI2O3 powder can be dried, for example in an oven or under a flow of hot air. The size ranges of powders after drying can correspond approximately to the size of the starting aluminum powders. In a preferred embodiment, the nickel-coated AI2O3 powder can have can have an average particle size of less than 300 micrometers, most preferably less than 60 micrometers, or less than 40 micrometers, or less than 30 micrometers, for example 1 to 40 micrometers, or 1 to 30 micrometers.

[0022] The dried nickel-coated AI2O3 powder can then be sieved to isolate particles of more uniform size. A variety of sizes or ranges of sizes can be used. In an embodiment, the particles are sieved to a size of less than 50 micrometers, or less than 35 micrometers, or less than 20 micrometers, for example 1 to 25 micrometers, or 1 to 20 micrometers.

[0023] The nickel-coated AI2O3 powder can then be sintered to provide the NiAl ₂ 0 ₄ - coated AI2O3 powder. In a preferred embodiment, the NiAl ₂ 0 ₄ coating thus obtained is nano structured. Without be being bound by theory, it is believed that the

N1CO3 2Ni(OH)2 2H2O coating is decomposed into nickel oxide as shown in Equation (2); then the nickel oxide reacts with AI2O3 to produce AI2O4, as shown in Equation (3).

N1CO3 2Ni(OH) ₂ 2H ₂ 0 → 3 O +4H ₂ 0†+C0 ₂ † (Eq. 2)

AI2O3 +NiO → AI2O4 (Eq. 3)

[0024] Sintering can be by methods known in the art. For example the nickel-coated AI2O3 powder can be loaded into a corundum crucible and heated at a rate of temperature increase of 5 to 10°C/minute, followed by heating at 1150 to 1350°C for 1 to 3 hours.

Sintering can be conducted in air or under an inert atmosphere.

[0025] After sintering the NiAhC^-coated AI2O3 powder can be sieved to provide a powder of more uniform particle size. For example, the powder can have an average particle size of less than less than 25 micrometers, or less than 15 micrometers, or less than 10 micrometers, for example 1 to 15 micrometers, or 1 to 10 micrometers.

[0026] Compared with the prior art, the present invention provides a method for preparing a nickel-aluminum-spinel-coated aluminum oxide powder material having simple operations and high reproducibility. The methods provide a dense and uniform coating of nickel-aluminum- spinel on the surface of the aluminum oxide, with uniform grain sizes. Moreover, the method is safe and reliable, requires less energy, and saves space. During preparation by sintering, the rate of temperature increase is high, the sintering temperature is low, the surface is coated uniformly, the reaction time is shortened, and the preparation cost is low.

[0027] The nickel/aluminum-spinel-coated aluminum oxide powder prepared as described herein can be used in a variety of applications, for example as catalytic materials and as high-temperature thermosensitive materials.

[0028] The various embodiments are further illustrated by the following particular examples.

EXAMPLES

Example 1

[0029] First, 4.5 g of AI2O3 powder of average particle size 1 μιη is weighed out, and dispersed into 300 ml of deionized water and 1.5 mol/L H4HCO3 solution; an N1SO4 solution is added dropwise while stirring magnetically, wherein the total duration of the dropwise addition and reaction is 2 hours (h), to give a solution with a pH of 8. After the reaction, centrifugation and water-washing are performed, then oven-drying and sieving are performed to obtain an AI2O3 powder coated with N1CO3 2Ni(OH)2 2H2O. The powder obtained is loaded into a corundum crucible and sintered, causing the N1CO3 2Ni(OH)2 2H2O to decompose to give an NiO-coated AI2O3 powder; NiO then reacts with AI2O3 to obtain nickel-aluminum-spinel; the rate of temperature increase is 5°C/min, the sintering

temperature is 1200°C, and the temperature is maintained for 2 h. The sintered powder is sieved, to obtain a nickel-aluminum-spinel-coated aluminum oxide composite material.

[0030] The XRD spectrum of alumina is shown in FIG. 1 at (a), and the XRD spectrum of the alumina after coating is shown in FIG. 1 at (b).

[0031] The XRD spectrum of FIG. 1 shows that NiAl ₂ 0 ₄ material is prepared under the conditions of Example 1.

[0032] FIG. 2 is an SEM image of the powder of Example 1 showing the overall morphology of the NiAkC^-coated AI2O3 powder obtained in Example 1. FIG. 3 is an SEM image showing the local morphology of the NiAkC^-coated AI2O3 powder obtained in Example 1. FIGS. 2 and 3; these show that the NiAl ₂ 0 ₄ prepared in Example 1 is uniformly distributed on the surface of the AI2O3.

Example 2

[0033] 4.5 g of AI2O3 powder of average particle size 50 μιη is weighed out, and put into 300 ml of deionized water and 1.5 mol/L H4HCO3 solution; an N1SO4 solution is added dropwise while stirring magnetically, wherein the total duration of the dropwise addition and reaction is 2 h, to give a solution with a pH of 7. After the reaction, centrifugation and water- washing are performed, then oven-drying and sieving are performed to obtain an AI2O3 powder coated with N1CO3 2Ni(OH) ₂ 2H ₂ 0. The powder obtained is loaded into a corundum crucible and sintered, causing the N1CO3 2Ni(OH) ₂ 2H ₂ 0 to decompose to give an NiO-coated Al ₂ 03 powder; NiO then reacts with Al ₂ 03 to obtain nickel-aluminum-spinel; the rate of temperature increase is 8°C/min, the sintering temperature is 1300°C, and the temperature is maintained for 2 h. The sintered powder is sieved, to obtain a nickel- aluminum-spinel-coated aluminum oxide composite material.

Example 3

[0034] 4.5 g of Al ₂ 03 powder of average particle size 1 μιη is weighed out, and put into 300 ml of ethanol solution and 1.5 mol/L NH4HCO3 solution; an S04 solution is added dropwise while stirring magnetically, wherein the total duration of the dropwise addition and reaction is 2 h, to give a solution with a pH of 8. After the reaction, centrifugation and water- washing are performed, then oven-drying and sieving are performed to obtain an Al ₂ 03 powder coated with N1CO3 2Ni(OH) ₂ 2H ₂ 0. The powder obtained is loaded into a corundum crucible and sintered, causing the N1CO3 2Ni(OH) ₂ 2H ₂ 0 to decompose to give an NiO-coated Al ₂ 03 powder; NiO then reacts with Al ₂ 03 to obtain nickel-aluminum-spinel; the rate of temperature increase is 5°C/min, the sintering temperature is 1200°C, and the temperature is maintained for 2 h. The sintered powder is sieved, to obtain a nickel- aluminum-spinel-coated aluminum oxide composite material.

Example 4

[0035] 4.5 g of Al ₂ 03 powder of average particle size 100 μιη is weighed out, and put into 300 ml of ethanol solution and 1.5 mol/L NH4HCO3 solution; an S04 solution is added dropwise while stirring magnetically, wherein the total duration of the dropwise addition and reaction is 2 h, to give a solution with a pH of 8. After the reaction, centrifugation and water- washing are performed, then oven-drying and sieving are performed to obtain an Al ₂ 03 powder coated with N1CO3 2Ni(OH) ₂ 2H ₂ 0. The powder obtained is loaded into a corundum crucible and sintered, causing the N1CO3 2Ni(OH) ₂ 2H ₂ 0 to decompose to give an NiO-coated Al ₂ 03 powder; NiO then reacts with Al ₂ 03 to obtain nickel-aluminum-spinel; the rate of temperature increase is 5°C/min, the sintering temperature is 1300°C, and the temperature is maintained for 2 h. The sintered powder is sieved, to obtain a nickel- aluminum-spinel-coated aluminum oxide composite material.

Example 5

[0036] 4.5 g of AI2O3 powder of average particle size 100 μιη is weighed out, and put into 300 ml of deionized water solution and 1.5 mol/L H4HCO3 solution; an N1SO4 solution is added dropwise while stirring magnetically, wherein the total duration of the dropwise addition and reaction is 2 h, to give a solution with a pH of 7. After the reaction,

centrifugation and water-washing are performed, then oven-drying and sieving are performed to obtain an AI2O3 powder coated with N1CO3 2Ni(OH)2 2H2O. The powder obtained is loaded into a corundum crucible and sintered, causing the N1CO3 2Ni(OH)2 2H2O to decompose to give an NiO-coated AI2O3 powder; NiO then reacts with AI2O3 to obtain nickel-aluminum-spinel; the rate of temperature increase is 8°C/min, the sintering

temperature is 1250°C, and the temperature is maintained for 2 h. The sintered powder is sieved, to obtain a nickel-aluminum-spinel-coated aluminum oxide composite material.

[0037] The compositions and methods disclosed herein are further illustrated by the following aspects, which are non-limiting:

[0038] Aspect 1 : A method for preparing a NiAkC^-coated AI2O3 powder, the method comprising: contacting an uncoated AI2O3 powder with N1SO4 and H4HCO3 in an aqueous medium to provide an AI2O3 powder coated with a composition comprising nickel; and sintering the AI2O3 powder coated with a composition comprising nickel at a temperature and for a time effective to provide the NiAkC^-coated AI2O3 powder.

[0039] Aspect 2: The method according to aspect 1, further comprising isolating the AI2O3 powder coated with a composition comprising nickel before sintering.

[0040] Aspect 3 : The method according to aspect 2, further comprising sizing the the AI2O3 powder coated with a composition comprising nickel after isolation and before sintering.

[0041] Aspect 4: The method according to any one or more of aspects 1 to 3, further comprising sizing the NiAkC^-coated AI2O3 powder.

[0042] Aspect 5: The method according to aspect 1, comprising: dispersing the uncoated AI2O3 powder in deionized water; adding NH4HCO3 and N1SO4 to the uncoated AI2O3 powder to provide a coating on the surface of the AI2O3 powder, wherein the coating comprises nickel; separating the nickel-coated AI2O3 powder from the deionized water; oven- drying the separated AI2O3 powder; sieving the dried AI2O3 powder; sintering the sieved AI2O3 powder to provide the NiAhC^-coated AI2O3 powder; and optionally, sieving the sintered Ni AhC^-coated AI2O3 powder.

[0043] Aspect 6: The method according to any one or more of aspects 1 to 5, wherein the coating comprises nickel comprises NiC03'2Ni(OH)2'2H20.

[0044] Aspect 7: The method according to any one or more of aspects 1 to 6, wherein the N1AI2O4 coating is nano structured.

[0045] Aspect 8: The method according to any one or more of aspects 1 to 7, wherein the uncoated AI2O3 powder has a particle size of 10 nanometers to 1 millimeter, preferably 1 to 100 μιη.

[0046] Aspect 9: The method according to any one or more of aspects 1 to 8, wherein the aqueous medium medium is deionized water or a water/ethanol solution.

[0047] Aspect 10: The method according to any one or more of aspects 1 to 9, comprising combining the uncoated AI2O3 powder and the NH4HCO3; then adding the N1SO4, preferably adding the H4HCO3 dropwise.

[0048] Aspect 11 : The method according to any one or more of aspects 1 to 10, wherein the pH of the aqueous medium is 7 to 8.

[0049] Aspect 12: The method according to any one or more of aspects 1 to 11, wherein a rate of temperature increase of sintering is 5 to 10°C/min, the sintering temperature is 1150 to 1350°C, and the temperature is maintained for 1 to 3 hours.

[0050] Aspect 13. A NiAhC^-coated AI2O3 powder prepared by any one or more of the methods of aspects 1 to 12.

[0051] While particular embodiments have been described, alternatives,

modifications, variations, improvements, and substantial equivalents that are or may be presently unforeseen may arise to applicants or others skilled in the art. Accordingly, the appended claims as filed and as they may be amended are intended to embrace all such alternatives, modifications variations, improvements, and substantial equivalents.