CN112830798B

CN112830798B - Preparation method of boron carbide granulation powder for pressureless sintering

Info

Publication number: CN112830798B
Application number: CN202110069549.6A
Authority: CN
Inventors: 王志江
Original assignee: Harbin Institute of Technology
Current assignee: Harbin Institute of Technology
Priority date: 2021-01-19
Filing date: 2021-01-19
Publication date: 2022-05-06
Anticipated expiration: 2041-01-19
Also published as: CN112830798A

Abstract

The invention discloses a preparation method of boron carbide granulation powder for pressureless sintering, and relates to a preparation method of boron carbide granulation powder for pressureless sintering. The invention aims to solve the problems that the density of the existing pressureless sintering boron carbide is difficult to be higher than 90% and the mechanical property is poor. In the process of spray granulation, the two are instantly mixed in an atomizing disc rotating at high speed, and then water is rapidly evaporated in a high-temperature spray granulation tower to obtain dry granulated powder, so that the agglomeration phenomenon is prevented. The method is applied to the field of preparation of boron carbide ceramics.

Description

Preparation method of boron carbide granulation powder for pressureless sintering

Technical Field

The invention relates to a preparation method of boron carbide granulation powder for pressureless sintering.

Background

Boron carbide was first discovered in 1858 and had a stoichiometric formula of B₄Compounds of C were not recognized until 1934. The boron carbide has the advantages of higher melting point, higher hardness, good wear resistance, acid and alkali corrosion resistance, small density and very high thermal neutron absorption capacity due to the property of covalent bonds. The hardness of the material is second to that of diamond and cubic boron nitride, and the material belongs to a non-metallic material and has important physicochemical properties. Boron carbide has the performance of high-temperature extraordinary hardness and is widely applied to the fields of body armor, grinding tools and the like. Boron carbide is the lightest ceramic material, and can be used as a jet blade due to low density, and is widely applied in the field of aerospace. The material can be used as a control rod of a nuclear reactor and a material for preventing radioactive substances from leaking due to the strong neutron absorption capacity of the material.

The boron carbide ceramic contains a covalent bond proportion of about 94 percent, has very low self-diffusion coefficient, and requires extremely high temperature for eliminating air holes and a mass transfer mechanism of grain boundary and volume diffusion. At present, the preparation method of the industrialized high-performance boron carbide ceramic mainly comprises two processes of hot-pressing sintering and pressureless sintering. The hot pressing process is to apply certain pressure to the boron carbide sintered body while sintering to promote the densification of the boron carbide ceramic, but the single furnace yield of the hot pressing process is limited, and only structural products with smaller size and simpler shape can be prepared, so the comprehensive cost of hot pressing boron carbide is very high. The pressureless sintering of the pure boron carbide ceramic is extremely difficult, and the pressureless sintering process has the advantages of being suitable for large-scale production and being capable of preparing structural components with large sizes and complex shapes. At present, the main precondition of pressureless sintering densification of pure boron carbide is to adopt superfine powder with low oxygen content less than or equal to 3 μm and a sintering temperature range of 2250-2350 ℃, but the densification is difficult to be higher than 90% and the mechanical property is poor.

Disclosure of Invention

The invention aims to solve the problems that the density of the existing pressureless sintering boron carbide is difficult to be higher than 90% and the mechanical property is poor, and provides a preparation method of boron carbide granulation powder for pressureless sintering.

The preparation method of the boron carbide granulation powder for pressureless sintering of the invention is completed according to the following steps:

firstly, raw material dispersion: placing the nano boron carbide powder in deionized water containing a dispersant A to be uniformly dispersed to obtain a nano boron carbide mixed solution; placing the micron boron carbide powder in deionized water containing a dispersant B to be uniformly dispersed to obtain a micron boron carbide mixed solution;

secondly, mixing the raw materials: respectively adding a sintering aid and a binder into the nano boron carbide mixed solution and the micron boron carbide mixed solution, and performing ball milling and mixing for 10-24 hours to respectively obtain nano boron carbide slurry and micron boron carbide slurry;

thirdly, drying the raw materials: and pumping the micron boron carbide slurry and the nanometer boron carbide slurry into an atomizing disc through two feed inlets of a spray granulation tower respectively, mixing in the atomizing disc, and directly atomizing and drying to obtain micron and nanometer boron carbide mixed granulation powder.

Wherein the mass ratio of the micron boron carbide powder to the nanometer boron carbide powder is 1: (0.01 to 0.5); the mass ratio of the nano boron carbide powder to the micron boron carbide powder to the sintering aid is 1: (0.01-0.1), wherein the mass ratio of the nano boron carbide powder to the micron boron carbide powder to the binder is 1: (0.01-0.15).

According to the invention, the nanometer boron carbide powder is added into the micron boron carbide powder which is difficult to sinter without pressure to improve the sintering activity, so that the boron carbide block with high densification degree, small grain size and excellent mechanical property is obtained. The strengthening and toughening effects of the nanometer boron carbide particle pairs are mainly realized by the functions of pinning and strengthening the grain boundary of the matrix by the nanometer particles, inhibiting the growth of the crystal grains of the matrix and improving the bonding strength of the grain boundary.

Because the performance of the ceramic is partially determined by the maximum defect size, in order to achieve the above effect, the key is to obtain the granulation powder in which the micron boron carbide and the nanometer boron carbide are uniformly mixed with each other, so as to prevent the particle agglomeration phenomenon from generating and leave the defect with larger size in the sintering process. However, commercially available micron boron carbide powders tend to contain boron oxide, the surface properties of which are determined primarily by boron oxide. Residual B₂O₃Formation of B (OH) in aqueous solution₃Since B is an electron-deficient atom, it will bind to H₂OH in O molecule^-(coordination of the lone pair of electrons on the O atom to the hollow P orbital of the B atom) to release H⁺Ions. Generated H⁺The ions are adsorbed on the interface of the micron boron carbide and the water solution, so that the surface of the micron boron carbide particles is positively charged. The used nano boron carbide powder has high purity, extremely low oxygen content and residual carbon content, and produces H⁺The probability of the nano powder is greatly reduced, the capability of the nano powder for absorbing negative charges is enhanced, and the surface property of the nano powder is mainly determined by the boron carbide body. Therefore, it is difficult to uniformly disperse the micro boron carbide powder and the nano boron carbide powder under the same condition. The invention uses two dispersing agents respectively aiming at nano boron carbide and micron boron carbide to prepare the nano boron carbide slurry and the micron boron carbide slurry which are uniformly and stably dispersed. In the process of spray granulation, the two are instantly mixed in an atomizing disc rotating at high speed, and then water is rapidly evaporated in a high-temperature spray granulation tower to obtain dry granulated powder, so that the agglomeration phenomenon is prevented. The method solves the problem that micron and nanometer boron carbide can not be well dispersed under the same condition, and can obtain the boron carbide granulation powder with uniform micro-nano mixing.

The invention has the following specific effects:

first, by adding a water-soluble dispersant, deionized water can be used as a dispersion medium. Avoids using flammable, explosive or toxic and harmful substances such as ether, methanol, ethanol and the like to disperse the powder.

And additives (metal oxides, metal simple substances, transition metal carbides, borides and the like) of the sintering aid can accelerate the densification process, reduce the sintering temperature and prolong the heat preservation time.

And thirdly, the contained nano boron carbide greatly improves the integral specific surface energy, increases the sintering driving force and reduces the temperature and the heat preservation time required by sintering.

Fourthly, the contained nano boron carbide can promote densification in the subsequent sintering process, inhibit the growth of crystal grains through the pinning and strengthening of crystal boundaries, improve the bonding strength of the crystal boundaries and form fine crystal grains, thereby simultaneously improving the strength and the toughness of the ceramic.

And fifthly, the invention adopts a mode of dispersing the nano boron carbide powder and the micron boron carbide powder and instantly mixing and drying in the granulation process, thereby greatly improving the uniformity of powder mixing.

The advantages of the present invention can be explained by several mechanisms as follows. 1. The nano boron carbide has the characteristics of small particle size, large specific surface area, high surface energy, large proportion of surface atoms and the like, and after the nano boron carbide is added into the micron boron carbide, the sintering driving force of the powder is greatly improved, and the densification process is accelerated. 2. The added nanometer boron carbide particles can fill in the pores formed among the micron boron carbide particles, so that the overall porosity is reduced, the density of the green body is improved, and the densification of sintering is facilitated. 3. The method comprises the steps of dispersing nano boron carbide and micron boron carbide into slurry respectively, and then mixing and drying instantly in the spray granulation process, so that the problem of agglomeration of micro-nano powder in the same slurry is solved, the uniformity of mixing of the micro-nano boron carbide powder is greatly enhanced, and the ceramic material with uniform microstructure and excellent mechanical property can be obtained by sintering. 4. The refinement of crystal grains is a known method which can improve the strength and enhance the toughness, and the added nano boron carbide particles form fine crystal grains after sintering, so that the toughness and the strength of the boron carbide ceramic can be improved.

Drawings

Fig. 1 is a schematic structural diagram of a spray granulation tower provided by the invention, wherein 1 is a charging barrel, 2 is a peristaltic pump, 3 and 4 are feed inlets, 5 is an atomizing disc, and 6 is a spray granulation tower body;

FIG. 2 is a scanning electron microscope image of the product obtained in example one, at 400 times magnification;

FIG. 3 is a scanning electron microscope image of the product obtained in example one, at a magnification of 7000 times.

Detailed Description

The first embodiment is as follows: the preparation method of the boron carbide granulation powder for pressureless sintering in the embodiment is completed according to the following steps:

thirdly, drying the raw materials: and pumping the micron boron carbide slurry and the nanometer boron carbide slurry into an atomizing disc through two feed inlets of a spray granulation tower respectively, mixing in the atomizing disc, and directly atomizing and drying to obtain micron and nanometer boron carbide mixed granulation powder. Wherein the mass ratio of the micron boron carbide powder to the nanometer boron carbide powder is 1: (0.01 to 0.5); the mass ratio of the nano boron carbide powder to the micron boron carbide powder to the sintering aid is 1: (0.01-0.1), wherein the mass ratio of the nano boron carbide powder to the micron boron carbide powder to the binder is 1: (0.01-0.15).

In this embodiment, the spray granulation tower is an improved double-feed-port spray granulation tower, and is respectively connected with the nano boron carbide slurry and the micron boron carbide slurry, and the feed rate ratio of the peristaltic pump is 1: (5-100). The inlet temperature in the tower is 150-250 ℃, and the outlet temperature is 65-100 ℃. The spraying mode is centrifugal spraying, and the rotating speed of an atomizing disc is 5000-20000 rpm.

The present embodiment has the following specific effects:

The second embodiment is as follows: the first difference between the present embodiment and the specific embodiment is: the particle size of the nanometer boron carbide powder in the step one is 20 nm-600 nm, and the particle size distribution of the micrometer boron carbide powder is 0.6 μm-50 μm. The rest is the same as the first embodiment.

The third concrete implementation mode: the present embodiment differs from the first or second embodiment in that: in the first step, the mass ratio of the nano boron carbide powder to the micro boron carbide powder to the deionized water is 1: 1. The others are the same as in the first or second embodiment.

The fourth concrete implementation mode: the difference between this embodiment mode and one of the first to third embodiment modes is: the mass ratio of the nano boron carbide powder to the dispersant A is 1: 0.001 to 0.03; the mass ratio of the micron boron carbide powder to the dispersant B is 1: 0.001 to 0.03. The rest is the same as one of the first to third embodiments.

The fifth concrete implementation mode: the difference between this embodiment and one of the first to fourth embodiments is: the dispersant A in the step one is polyethyleneimine, ammonium lignosulfonate or polyvinyl butyral. The rest is the same as one of the first to fourth embodiments.

The sixth specific implementation mode: the difference between this embodiment and one of the first to fifth embodiments is: the dispersant B in the first step is tetramethyl ammonium hydroxide, polyvinyl pyrrolidone, sodium hydroxymethyl cellulose or polyethylene glycol. The rest is the same as one of the first to fifth embodiments.

The seventh embodiment: the difference between this embodiment and one of the first to sixth embodiments is: the dispersion mode in the step one is ultrasonic dispersion, a bar type ultrasonic machine is adopted, the ultrasonic frequency is 28-40 kHz, and the ultrasonic time is 30-240 min. The rest is the same as one of the first to sixth embodiments.

The specific implementation mode is eight: the present embodiment differs from one of the first to seventh embodiments in that: in the second step, the sintering aids are Al and Al₂O₃、ZrO₂、Y₂O₃、TiB₂、CrB₂、W₂B₅One or a mixture of more of SiC, TiC, graphene and carbon nano tubes according to any ratio. The rest is the same as one of the first to seventh embodiments.

The specific implementation method nine: the difference between this embodiment and the first to eighth embodiments is: and in the second step, the binder is water-soluble phenolic resin or PVA. The rest is the same as the first to eighth embodiments.

The detailed implementation mode is ten: the present embodiment differs from one of the first to ninth embodiments in that: and in the second step, the ball milling medium used for ball milling is zirconia balls, alumina balls, silicon carbide balls or boron carbide balls, the diameter of the balls is 1-20 mm, and the ball material ratio is (2-10): 1, the rotating speed of the ball mill is 10-100 rpm. The rest is the same as one of the first to ninth embodiments.

The following experiments were performed to verify the beneficial effects of the present invention:

example 1

The preparation method of the boron carbide granulation powder for pressureless sintering is completed according to the following steps:

firstly, raw material dispersion: mixing nanometer boron carbide powder (D)₅₀60nm) was put in deionized water containing a dispersant a and sonicated using a bar sonicator at a frequency of 40khz for 120min to disperse uniformly,obtaining a nanometer boron carbide mixed solution; mixing micrometer boron carbide powder (D)₅₀1.5 μm) in deionized water containing a dispersant B, and performing ultrasonic treatment for 120min at a frequency of 40khz by using a bar-type ultrasonic machine to uniformly disperse the mixture to obtain a micron boron carbide mixed solution; the mass ratio of the micron boron carbide powder to the nanometer boron carbide powder is 1: 0.1; the mass ratio of the nano boron carbide powder to the dispersant A is 1: 0.02; the mass ratio of the micron boron carbide powder to the dispersant B is 1: 0.02; the dispersant A is polyethyleneimine; the dispersant B is tetramethyl ammonium hydroxide;

secondly, mixing the raw materials: respectively adding water-soluble phenolic resin and CrB into the nanometer boron carbide mixed solution and the micrometer boron carbide mixed solution₂(D50 is 1.5 mu m), ball-milling and mixing for 24h by using a stirring ball mill, wherein the ball-milling medium is boron carbide balls, and the ball-material ratio is (1-20: 1); respectively obtaining nano boron carbide slurry and micron boron carbide slurry;

thirdly, drying the raw materials: feeding the micron boron carbide slurry and the nanometer boron carbide slurry into an atomizing disc of a spray granulation tower by using a peristaltic pump, respectively feeding the nanometer boron carbide slurry and the micron boron carbide slurry into two feed inlets, wherein the speed of the peristaltic pump is respectively 2r/min and 48r/min, the rotating speed of the atomizing disc is 8000-18000 r/min, the inlet temperature is 220 ℃, and the outlet temperature is 80 ℃, so as to obtain the micron and nanometer boron carbide mixed granulation powder.

Wherein the mass ratio of the nano boron carbide powder to the micron boron carbide powder to the water-soluble phenolic resin is 1: 0.1; nano boron carbide powder, micron boron carbide powder and CrB₂The quantity ratio is 1: 0.1.

the schematic structural diagram of the spray granulation tower in this embodiment is shown in fig. 1, and mainly shows the structure of the feeding portion, and the two feeding ports respectively feed the nano boron carbide slurry and the micron boron carbide slurry. Comprises a charging barrel 1, a peristaltic pump 2, feeding ports 3 and 4, an atomizing disc 5 and a spray granulation tower body 6;

FIG. 2 is a scanning electron microscope image of the product obtained in this example at a magnification of 400. As can be seen from FIG. 2, the obtained granulated powder has a round shape, a particle size distribution of 10-50 μm and a certain particle size ratio;

FIG. 3 shows the product obtained in this exampleThe scanning electron microscope image of (2) has a magnification of 7000 times. As can be seen from FIG. 3, the obtained granulated powder was uniformly mixed nano-B₄C particles and micron B₄The nano particles are filled in gaps of the wedge-shaped micron powder, so that the void ratio is reduced, and the density is improved.

The granulated powder prepared in this example was subjected to pressureless sintering, and the effect thereof was verified. Firstly, granulating powder is dry-pressed and molded by a four-column hydraulic press under the pressure of 200 MPa. And after glue discharging treatment at 1000 ℃ for 1.5h, placing the sample in a high-temperature sintering furnace, and sintering the sample at 2200 ℃ for 60min to obtain a pressureless sintered block sample. The sintered block produced had a relative density of 96.7% as measured using archimedes drainage. Fracture toughness of 3.89 MPa.m^1/2Bending strength is 473 MPa.

Claims

1. A preparation method of boron carbide granulation powder for pressureless sintering is characterized by comprising the following steps:

firstly, raw material dispersion: putting the nano boron carbide powder into deionized water containing a dispersant A, and performing ultrasonic treatment for 120min at the frequency of 40khz by using a rod-type ultrasonic machine to uniformly disperse the nano boron carbide powder to obtain a nano boron carbide mixed solution; putting the micron boron carbide powder into deionized water containing a dispersant B, and performing ultrasonic treatment for 120min at the frequency of 40khz by using a rod type ultrasonic machine to uniformly disperse the micron boron carbide powder to obtain a micron boron carbide mixed solution; the mass ratio of the micron boron carbide powder to the nanometer boron carbide powder is 1: 0.1; the mass ratio of the nano boron carbide powder to the dispersant A is 1: 0.02; the mass ratio of the micron boron carbide powder to the dispersant B is 1: 0.02; the dispersant A is polyethyleneimine; the dispersant B is tetramethyl ammonium hydroxide; the grain diameter of the mixed solution of the nanometer boron carbide is 60 nm; the grain size of the micron boron carbide powder is 1.5 mu m;

secondly, mixing the raw materials: respectively adding water-soluble phenolic resin and CrB into the nanometer boron carbide mixed solution and the micrometer boron carbide mixed solution₂Ball-milling and mixing for 24 hours by using a stirring ball mill, wherein a ball-milling medium is boron carbide balls, and the ball-to-material ratio is (1-20: 1); respectively obtaining nano boron carbide slurry and micron boron carbide slurry; CrB₂The particle diameter of (A) is 1.5 μm;

thirdly, drying the raw materials: feeding the micron boron carbide slurry and the nanometer boron carbide slurry into an atomizing disc of a spray granulation tower by using a peristaltic pump, respectively feeding the nanometer boron carbide slurry and the micron boron carbide slurry into two feed inlets, wherein the speed of the peristaltic pump is respectively 2r/min and 48r/min, the rotating speed of the atomizing disc is 8000-18000 r/min, the inlet temperature is 220 ℃, and the outlet temperature is 80 ℃, so as to obtain micron and nanometer boron carbide mixed granulation powder;

wherein the mass ratio of the nano boron carbide powder to the micron boron carbide powder to the water-soluble phenolic resin is 1: 0.1; nano boron carbide powder, micron boron carbide powder and CrB₂The quantity ratio is 1: 0.1; the mass ratio of the nano boron carbide powder to the micron boron carbide powder to the deionized water is 1: 1.