30results about "Mangesium aluminates" patented technology

The invention relates to the technical field of preparation methods of nano precursor powder, in particular to a synthesis method of Cr < 3 + > doped magnesium aluminum oxide nano powder, which comprises the following steps: weighing magnesium nitrate, aluminum nitrate and chromic nitrate according to the stoichiometric ratio of MgAl2-xO4: xCr < 3 + >; stirring and mixing the prepared magnesium nitrate solution, aluminum nitrate solution and chromic nitrate solution together to prepare a mixed solution I; ammonia water is dripped into the first mixed solution, then the first mixed solution dripped with the ammonia water is stirred and mixed, and a second mixed solution is prepared; pouring the mixed solution II into a high-pressure reaction kettle, and putting the high-pressure reaction kettle into a constant-temperature box for reaction to prepare a white precipitate; adding deionized water into the white precipitate for washing and precipitating, and centrifuging to obtain white precipitate powder; drying the white precipitate powder at a constant temperature; calcining the dried white precipitate powder to prepare nano precursor powder; the prepared nanometer precursor powder has the advantages of being high in purity and small in particle size.

The invention relates to a fast ion conductor-stabilizer composite coated lithium manganese iron phosphate material as well as a preparation method and application thereof. The lithium manganese iron phosphate material comprises a lithium manganese iron phosphate inner core and a coating layer on the surface of the lithium manganese iron phosphate inner core, the coating layer is obtained by mixing sodium aluminosilicate, magnesium metaaluminate, a coating carbon source and a lithium supplement agent, coating the surface of the lithium manganese iron phosphate inner core with the mixture, and performing sintering treatment. According to the invention, sodium aluminosilicate and magnesium metaaluminate are combined with conductive carbon to carry out co-coating treatment on lithium manganese iron phosphate, and in a high-temperature sintering process, part of Na < + > / Al < 3 + > / Mg < 2 + > multi-ions are co-embedded into a near surface layer of a lithium manganese iron phosphate crystal, so that a unit cell structure which is more stable in structure and easier in Li < + > diffusion is formed. In the high-temperature sintering process, part of magnesium metaaluminate reacts with the lithium supplement agent to form an Al-based fast ion conductor, and sodium aluminosilicate has a wider Li < + > diffusion channel, so that the material has excellent rate capability.

The invention provides nano magnesium aluminate spinel with high sintering activity and high dispersity and a preparation method thereof, and belongs to the field of nano material preparation. The method comprises the following steps: mixing aluminum chloride, magnesium chloride and deionized water to obtain a mixed solution; adding aluminum bicarbonate into the mixed solution, and carrying out hydrothermal reaction, so that aluminum hydroxide and magnesium hydroxide are precipitated at the same time to obtain a magnesium aluminate spinel precursor; the magnesium aluminate spinel precursor is subjected to first-stage roasting, so that the magnesium aluminate spinel precursor is subjected to flash pyrolysis to form a fine crystal nucleus, and a pyrolysis product is obtained; and performing second-stage roasting on the pyrolysis product to control the crystal nucleus of the pyrolysis product to grow along a specific crystal face, thereby obtaining the nano magnesium aluminate spinel powder. A nanoscale precursor is precisely regulated and controlled to be controllably and orderly separated out through a wet-process system micro-area reaction; the crystal form development, the microstructure and the size uniformity of the powder are controlled through a flash pyrolysis nucleation-crystal face oriented growth two-stage roasting technology.

A manufacturing apparatus (1A) includes a first gas introduction portion (20) provided on one end portion (10a) side of a calcination furnace (10) to introduce a gas into the calcination furnace (10), a gas discharge portion (30) provided on the other end portion (10b) side of the calcination furnace (10) to discharge the gas in the calcination furnace (10) to the outside, and a conveying apparatus (40) to convey a metal compound and a flux from the gas discharge portion (30) side to the first gas introduction portion (20) side. The calcination furnace (10) has a temperature increasing region (12A), a cooling region (14A), and a reaction region (13A) provided between the temperature increasing region (12A) and the cooling region (14A) and causing the metal compound and the flux to react. In the manufacturing apparatus (1A), a gas flow (AF1) generated by the gas introduced from the first gas introduction portion (20) causes the flux gasified in the reaction region (13A) to be powdered in the cooling region (14A), and the gas containing the powdered flux is sent to the gas discharge portion (30).

The invention belongs to the technical field of inorganic luminescent materials, and particularly relates to a preparation method of a Mn < 4 + >-doped magnesium aluminum salt red luminescent material containing Lewis acid sites. The method comprises the following steps: preparing a precursor solution according to a chemical formula MgAl (2-x) MnxO4, and carrying out a co-precipitation reaction to generate a precipitate; sequentially carrying out preheating and high-temperature calcination to form a matrix; then activating through Na < + > exchange and H < + > exchange, directionally introducing oxygen vacancies and exposing Al3 < + > octahedral sites, and cooperatively regulating 2E-4A2 red light transition of Mn < 4 + >. The obtained material has excellent thermal stability, is suitable for blue light excited warm white light LED with high color rendering index and low color temperature, and solves the technical defects of insufficient oxygen vacancy and uncontrollable coordination environment of the traditional red light material. The ion exchange method disclosed by the invention solves the problem that the luminescence application is limited due to the fact that Mn < 2 + > is doped through a high-temperature solid-phase method in the prior art, the oxygen vacancy concentration is low, Lewis acid sites are insufficient, and the local coordination environment of Mn < 4 + > cannot be accurately regulated and controlled. The preparation method is simple and easy to operate, and the product is stable in luminescence.

The invention belongs to the technical field of asbestos tailing utilization, and particularly discloses an asbestos tailing full-resource utilization process. The method provided by the invention comprises the following steps: crushing and sieving asbestos tailings, adding a sulfating roasting reagent and ammonium bisulfate to obtain a mixed material, and roasting the mixed material in a fluidized bed; after heat preservation treatment, water is added for a leaching reaction, and leaching residues and filtrate are obtained through solid-liquid separation; adjusting the pH value of the filtrate to obtain a mixed precipitate of ferric hydroxide and nickel hydroxide and a sulfate solution, performing evaporative crystallization on the sulfate solution, and performing roasting treatment to obtain a mixed oxide; and adding aluminum powder into the mixed oxide, and carrying out aluminothermic reduction reaction to obtain the magnesium ingot. According to the method, high-valued utilization of various elements in the asbestos tailings can be achieved, circular economy and closed-loop utilization can also be achieved, and the economic effect of enterprises is improved.

This invention provides a method for calcining dolomite, a carbonization method, a calcium-magnesium separation method, and a comprehensive utilization method. The comprehensive utilization method of dolomite includes: calcining dolomite powder to obtain calcined white powder; subjecting the calcined white powder to digestion and aging treatments sequentially to obtain an aging liquid; carbonizing the aging liquid to obtain a carbonization reaction liquid; performing solid-liquid separation on the carbonization reaction liquid to obtain heavy magnesium water and a first solid separate; pyrolyzing the heavy magnesium water to obtain a pyrolysis reaction liquid; performing solid-liquid separation on the pyrolysis reaction liquid to obtain a decomposition liquid and a second solid separate; calcining the second solid separate with magnesium carbonate to obtain magnesium oxide; and reducing the magnesium oxide to obtain metallic magnesium and magnesium aluminum spinel. This invention achieves comprehensive utilization of dolomite resources, with no waste residue or exhaust gas emissions. The main product is metallic magnesium, and the by-products are light calcium carbonate, magnesium aluminum spinel, and solid carbon dioxide.

The present invention provides an optimized method for recovering high-purity lithium sulfate from a lithium-containing composite oxide deposited on the eroded surface of a waste refractory crucible to be discarded. Therefore, by using the method for producing high-purity lithium sulfate from the waste refractory crucible of the present invention, not only can the discarded waste refractory crucible be reused to produce high-purity lithium sulfate that can be used in the production of lithium secondary batteries, but it is also expected that the positive electrode active material, iron oxide, alumina, silicate, and calcium carbonate obtained incidentally in the production process can be reused.

The present invention provides an optimized method for recovering high-purity lithium sulfate from a lithium-containing composite oxide deposited on an eroded surface of a waste sagger discarded. Therefore, when the method for producing high-purity lithium sulfate from a waste sagger of the present invention is used, it is expected not only to be able to produce high-purity lithium sulfate that can be used for manufacturing lithium secondary batteries by recycling a discarded waste sagger, but also to be able to recycle a positive electrode active material, iron oxide, alumina, silicate, and calcium carbonate obtained as by-products during the production process of the lithium sulfate.

The invention relates to a preparation method of high-purity magnesium aluminate spinel powder and the high-purity magnesium aluminate spinel powder. The preparation method comprises the following steps: S1, weighing raw materials in percentage by mass, wherein the raw materials comprise 65-80% of magnesium oxide powder and 20-35% of aluminum powder; s2, adding fluoride accounting for 1-10% of the mass of the raw materials into the raw materials, mixing, stirring and grinding uniformly to obtain a mixture; s3, pressurizing and pressing the mixture into a solid blank; s4, the solid blank is subjected to a calcination reaction for 1-4 h at the temperature of 900-1500 DEG C in a vacuum environment, and a sintered blank is obtained; and S5, cooling the sintered blank to room temperature, and crushing to obtain the high-purity magnesium aluminate spinel powder. The preparation method provided by the invention is based on a low-temperature solid-phase reaction between several raw materials and the fluoride, and by controlling specific reaction parameters, the preparation method is simple, the roasting temperature is low, the dosage of the fluoride is small, and the obtained magnesium aluminate spinel powder is relatively high in phase purity and small and uniform in particle size.

This invention relates to a doped yttrium oxide flake powder, its preparation method, and its applications. The method uses 0.2-0.4 M Y(NO₃)₂. 3 ) 3 ·6H 2 O solution as Y 3+ Source, then add MgCl to the solution 2 and Al(NO 3 ) 3 ·9H 2 O was used as a dopant. After adjusting the pH of the mixed solution to 8-10, a hydrothermal reaction was carried out at 170-200°C for 9-15 hours to achieve uniform distribution of the dopant in the yttrium oxide lattice. Finally, the powder was calcined at 600-700°C for 3-5 hours to improve the structural stability and properties of the material. The synthesized doped yttrium oxide powder had a plate-like morphology with a particle size between 5-25 micrometers, and the final product composition included Y. 2 O 3 MgAl 2 O 4 And Y-Al-O compounds. Compared with traditional physical doping methods, the preparation method of this invention not only achieves a more uniform doping effect, but also shows significant advantages in terms of energy consumption, cost, and particle size control. The doped yttrium oxide obtained by this method has performance superior to or equivalent to high-purity yttrium oxide, providing a new avenue for the further application and development of yttrium oxide.

The present invention relates to the technical field of fire-resistant wall materials and discloses a method for preparing a fire-resistant wall material that can automatically decompose formaldehyde, comprising the following steps: (1) mixing MgO and Al2O3, subjecting the mixture to ball milling, ultrasonic treatment, and heating reaction to obtain a porous precursor; (2) mixing the porous precursor with nano-LiF and calcining the mixture under nitrogen protection to obtain a calcined product; (3) loading the calcined product with titanium dioxide nanotubes through a coupling reaction to obtain a fire-resistant wall material. The present invention utilizes the excellent fire-resistant properties of spinel materials and, through the loading of titanium dioxide nanotubes, enables the wall material to catalytically decompose toxic and hazardous substances such as formaldehyde and benzene; by first synthesizing the porous precursor and then adding nano-LiF, the calcination time is greatly shortened and energy loss is reduced while meeting the material properties.

This invention provides a method for surface modification of lithium-rich manganese-based cathode materials, comprising: (1) dissolving a polymer with multiple carboxyl functional groups in deionized water to obtain a coating agent aqueous solution of 0.1–10 mg / mL; (2) dissolving a soluble inorganic salt containing a modified metal element in deionized water to obtain a metal precursor solution; (3) uniformly dispersing the lithium-rich manganese-based cathode material in the coating agent aqueous solution, and then adding the metal precursor solution, or mixing the coating agent aqueous solution and the metal precursor solution before uniformly dispersing the lithium-rich manganese-based cathode material therein; (4) washing and filtering the reactants obtained in step (3), and then heat-treating them under an oxygen-free atmosphere to form a metal ion gradient doping in the lithium-rich manganese-based cathode material, and forming a metal oxide layer, a spinel phase, and a carbon layer sequentially from the inside to the outside on the surface of the lithium-rich manganese-based cathode material. This invention improves the comprehensive electrochemical performance of the modified lithium-rich manganese-based cathode material.

A method for producing spinel-type aluminum oxide using a water-in-oil Pickering emulsion is provided. [Solution] The method includes the steps of: (1) mixing an aqueous dispersion of boehmite alumina sol with an aqueous solution containing one of the metal species Ni, Co, or Mg; (2) mixing a water-insoluble organic solvent with the aqueous solution from step (1) to form a phase-separated state of aqueous phase and organic solvent phase; (3) forming a water-in-oil (W / O) Pickering emulsion by applying shear to the entire liquid in the phase-separated state from step (2) to produce a spinel precursor in the organic solvent phase; (4) allowing the sheared liquid to stand to separate into an aqueous phase and an organic solvent phase; (5) removing the organic solvent phase from the phase-separated liquid from step (4); (6) drying the organic solvent phase from step (5) to obtain a cake; (7) baking the cake from step (6) at 700 to 1200°C for 2 to 8 hours; and (8) pulverizing the baked product obtained from step (7).

Disclosed is a ceramic sintered body comprising magnesium aluminate spinel of composition MgAl2O4 having from 90 to 100% by volume of a cubic crystallographic structure and a density of from 3.47 to 3.58 g / cc, wherein the ceramic sintered body is free of sintering aids. A method of making the ceramic sintered body comprising spinel is also disclosed.

This invention relates to a fast-ion conductor-stabilizer composite-coated lithium manganese iron phosphate material, its preparation method, and its applications. The lithium manganese iron phosphate material of this invention comprises a lithium manganese iron phosphate core and a coating layer on its surface. The coating layer is obtained by coating the surface of the lithium manganese iron phosphate core with a mixture of sodium aluminosilicate, magnesium aluminate, a carbon source, and a lithium supplement, followed by sintering. This invention utilizes sodium aluminosilicate and magnesium aluminate combined with conductive carbon for co-coating lithium manganese iron phosphate, achieving partial Na+ ionization during high-temperature sintering. + / Al 3+ / Mg 2+ Multiple ions are co-intercalated into the near-surface layer of lithium manganese iron phosphate crystals, thereby forming a more stable structure and Li + A cellular structure that facilitates diffusion. During high-temperature sintering, some magnesium aluminate reacts with the lithium supplement to form an Al-based fast ion conductor, coupled with the wider Li-ion diffusion of sodium aluminosilicate. + The diffusion channels give the material excellent rate performance.

A method of manufacturing a nanocomposite may include combining a magnesium salt, an aluminum salt, and a manganese salt in stoichiometric proportions within 5 mol. % in an aqueous solvent including menthol or dextrose, to obtain a first mixture. The method further may include heating the first mixture to remove at least 99.5 percent by weight (wt. %) of the aqueous solvent to obtain a first solid, grinding the first solid into a first powder, calcining the first powder at a temperature of about 600 to 800° C. for a time of about 2 to 4 hours to obtain a second solid, grinding the second solid and urea, into a second powder, heating the second powder at a temperature of about 550 to 650° C. for a time of about 15 minutes to 1.5 hours to obtain the nanocomposite.

To provide a positive electrode active material for a secondary battery having high capacity and excellent charge / discharge cycle characteristics.SOLUTION: A positive electrode active material includes an aggregate of particles including a first particle group and a second particle group, the aggregate of particles contains lithium, cobalt, nickel, aluminum, magnesium, oxygen, and fluorine, when the number of atoms of cobalt contained in the aggregate of particles is 100, the number of atoms of nickel is 0.05 or more and 2 or less, the number of atoms of aluminum is 0.05 or more and 2 or less, and the number of atoms of magnesium is 0.1 or more and 6 or less, and when the aggregate of particles is subjected to particle size distribution measurement by a laser diffraction / scattering method, the first particle group has a first peak. The second particle group has a second peak, the first peak has a maximum value in a range of 2 μm or more and 4 μm or less, and the second peak has a maximum value in a range of 9 μm or more and 25 μm or less.SELECTED DRAWING: Figure 26

The invention provides a method for synthesizing magnesium aluminate spinel from secondary aluminum ash through an in-situ wet method, belongs to the technical field of solid waste treatment, and aims to solve the technical problems of complexity and high cost in the prior art. The invention discloses a method for synthesizing magnesium aluminate spinel from secondary aluminum ash through an in-situ wet method. The method comprises the following steps: (1) mixing, ball-milling and screening secondary aluminum ash and a magnesium-containing material to prepare fine powder; (2) mixing and stirring the fine powder and water, and performing solid-liquid separation and drying to prepare desalted fine powder; (3) mixing and stirring the desalted fine powder and acid liquor, and performing solid-liquid separation to prepare magnesium-rich aluminum liquid; (4) sequentially carrying out evaporation concentration, cooling crystallization and solid-liquid separation on the magnesium-rich aluminum liquid to prepare crystallized magnesium-aluminum salt; and (5) mixing and stirring the crystallized magnesium-aluminum salt and alkali liquor, and carrying out solid-liquid separation and roasting to prepare the magnesium aluminate spinel. The method is simple in process and low in cost, aluminum and magnesium resources in the secondary aluminum ash are efficiently recycled, and meanwhile the problem that the secondary aluminum ash pollutes the environment is solved.

Disclosed is a ceramic sintered body comprising magnesium aluminate spinel of composition MgAl2O4 having from 90 to 100% by volume of a cubic crystallographic structure and a density of from 3.47 to 3.58 g / cc, wherein the ceramic sintered body is free of sintering aids. A method of making the ceramic sintered body comprising spinel is also disclosed.

The present invention relates to modified layered double hydroxides (LDHs), particles comprising such modified LDHs, resin compositions (comprising the particles and a resin), dispersions (comprising the particles and a liquid), and methods for producing such modified LDHs. The modified layered double hydroxides according to the present invention comprise a modified layered double hydroxide of formula (I) below, which comprises a modified outer layer: [[[M1 2+ ] y (M2 2+ ) z ] 1-x [M3 3+ ] x (OH2)](A n- ) x / n mH2O (I) During the ceremony, M1 and M2 are each independently a divalent metal selected from the group Mg, Zn, Ca, Sr, Cu, Fe, Mn, Co, Ni, Sn, Pb, Cd and Ba, in particular from the group Mg, Zn, Cu, Fe, Mn, Co, Ni and Cd; M3 is a trivalent metal such as Al and / or Fe, especially Al; A n- is one or more intercalating n-valent anions, m, x, y, and z are values ​​within the ranges indicated below, 0≦m<2 0 <x≦0.5 0.5≦y+z≦1 The outer layer is modified with a derivative of an organic acid or its salt.

The present invention provides an optimized method for recovering high-purity lithium sulfate from a lithium-containing composite oxide deposited on the eroded surface of a waste refractory crucible to be discarded. Therefore, by using the method for producing high-purity lithium sulfate from the waste refractory crucible of the present invention, not only can the discarded waste refractory crucible be reused to produce high-purity lithium sulfate that can be used in the production of lithium secondary batteries, but it is also expected that the positive electrode active material, iron oxide, alumina, silicate, and calcium carbonate obtained incidentally in the production process can be reused.

The invention provides a surface modification method of a lithium-rich manganese-based positive electrode material, which comprises the following steps: (1) dissolving a high-molecular polymer with multiple carboxyl functional groups in deionized water to obtain a 0.1-10mg / mL coating agent aqueous solution; (2) soluble inorganic salt containing modified metal elements is dissolved in deionized water, and a metal precursor solution is obtained; (3) uniformly dispersing the lithium-rich manganese-based positive electrode material in the coating agent aqueous solution, and then adding the metal precursor solution, or mixing the coating agent aqueous solution and the metal precursor solution, and then uniformly dispersing the lithium-rich manganese-based positive electrode material in the mixture; and (4) washing and filtering the reactant obtained in the step (3), and then carrying out heat treatment in an oxygen-free atmosphere, so that metal ion gradient doping is formed in the lithium-rich manganese-based positive electrode material, and a metal oxide layer, a spinel phase and a carbon layer are sequentially formed on the surface of the lithium-rich manganese-based positive electrode material from inside to outside. The comprehensive electrochemical performance of the lithium-rich manganese-based positive electrode modified material is improved.

The invention belongs to the field of sodium ion batteries, and particularly relates to a NaxMn1-yMyO2-coated MgAl2O4 composite material, which has a core-shell structure, the core is NaxMn1-yMyO2 with a P2 / T composite phase or an O3 / P3 composite phase, x is 0.5-0.7, y is more than 0 and less than or equal to 0.6, M is at least one of Ca, Fe, Mg, Sr, Ta, Ti, Cu, Zn, V, Cr, Sn, Zr, Y, La, Ce, Nb, Mo, Ru and Ir, and the core is NaxMn1-yMyO2-coated MgAl2O4 composite material. And the shell is made of MgAl2O4. The invention also provides a preparation method and application of the material. According to the invention, the MgAl2O4 material is innovatively adopted as a coating material, and combined control of a special P2 / T composite phase or O3 / P3 composite phase of a nickel-free manganese-rich matrix is matched, so that synergy can be realized, the problems of dissolution of manganese and the like of the composite material can be remarkably improved, the structural stability and ion transmission performance of the composite material are improved, and the service life of the composite material is prolonged. Therefore, the long cycle performance of the material in high magnification and wide temperature range is improved.

A heat-resistant member (1) according to the present disclosure contains alumina as a main component, and magnesium aluminate and boron. The content percentage of the magnesium aluminate at the surface is higher than the content percentage of the magnesium aluminate in a surface layer section located directly below the surface.

A system and method for continuous production of Mg from metallothermic reduction of magnesium bearing ore from both the reactor side and condenser side of the system, using a separate collection vessel. The furnace is a heated tube through which a moving bed of tableted feed flows. The condenser is a common heat exchanger design (shell / tube, plate / plate, etc.) and uses a heat transfer liquid to cool and condense magnesium gas under vacuum or pressure conditions. The cooling medium can be molten salts or metals which are not in direct contact with the magnesium metal. Liquid magnesium flows from the condenser into a collection vessel for further processing. Continuous operation is achieved by supplying a constant feed of tablets into the furnace, producing a constant stream of Mg gas to the condenser. Magnesium liquid product is tapped periodically from the collection vessel.

The present invention provides a composite metal oxide that has exceptional anion adsorptivity and is capable of removing multiple types of anions, each at a high adsorption rate.　The present disclosure is a composite metal oxide represented by formula (1). The composite metal oxide has a solid base amount of 3.70 mmol / g.　Formula (1): (Mg1−xAlx)O1+x / 2 [In formula (1), x is a number that satisfies 0.18≤x≤0.45.]

The invention relates to a modified layered double hydroxide (LDH), particles comprising said modified LDH, a resin composition comprising a resin and said particles, a dispersion comprising a liquid and said particles, and a method for producing said modified LDH. The modified layered double hydroxide according to the invention comprises a modified layered double hydroxide according to formula (I), wherein: M1 and M2 are each independently a divalent metal selected from the group of Mg, Zn, Ca, Sr, Cu, Fe, Mn, Co, Ni, Sn, Pb, Cd, and Ba; in particular Mg, Zn, Cu, Fe, Mn, Co, Ni, and Cd; M3 is trivalent metal such as Al and / or Fe; in particular Al; and An− is one or more of an intercalating n-valent anion, wherein m, x, y, and z are values in the ranges represented by: 0≤m<2 0<x≤0.5 0.5≤y+z≤1 further comprising a modified outer layer wherein the outer layer is modified with an organic acid derivative or salts thereof.

The invention relates to a modified layered double hydroxide (LDH), particles comprising said modified LDH, a resin composition comprising a resin and said particles, a dispersion comprising a liquid and said particles, and a method for producing said modified LDH. The modified layered double hydroxide according to the invention comprises a modified layered double hydroxide according to formula (I), wherein: M1 and M2 are each independently a divalent metal selected from the group of Mg, Zn, Ca, Sr, Cu, Fe, Mn, Co, Ni, Sn, Pb, Cd, and Ba; in particular Mg, Zn, Cu, Fe, Mn, Co, Ni, and Cd; M3 is trivalent metal such as Al and / or Fe; in particular Al; and An- is one or more of an intercalating n-valent anion, wherein m, x, y, and z are values in the ranges represented by: 0 ≤ m < 2 0 < x ≤0.5 0.5 ≤ y + z ≤ 1 further comprising a modified outer layer wherein the outer layer is modified with an organic acid derivative or salts thereof.

Heat-resistant element, comprising: Aluminum oxide as a main component, Magnesium aluminate and Boron, where In a case where a surface layer area of ​​the heat-resistant element is a region from a surface of the heat-resistant element to a depth of 0.5 mm and a region deeper than the surface layer area is an inner section, both the surface layer area and the inner section contain the aluminum oxide and the magnesium aluminate, a boron content in the form of B2O3 in the surface layer area is lower than the boron content in the inner section and The percentage of magnesium aluminate at the surface is greater than the percentage of magnesium aluminate in the surface layer area located directly below the surface.