CN115650757B - Electrothermal ceramic with high thermal stability and preparation process thereof - Google Patents
Electrothermal ceramic with high thermal stability and preparation process thereof Download PDFInfo
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Abstract
The invention relates to the field of ceramics, in particular to an electrothermal ceramic with high thermal stability and a preparation process thereof. The invention uses yttrium fluoride and Y 4 Al 2 O 9 The silicon sol and the silicon sol are jointly used as sintering aids, wherein the silicon dioxide can fill gaps among silicon carbide particles, but the thermal conductivity of the silicon dioxide is poor; y is 4 Al 2 O 9 Can form Y with better heat conductivity with silicon dioxide 2 Si 2 O 7 But it attacks silicon carbide; and yttrium fluoride is just capable of inhibiting Y 4 Al 2 O 9 Attack on silicon carbide, therefore, yttrium fluoride, Y are used 4 Al 2 O 9 The silica sol is used as a sintering aid of the silicon carbide ceramic, not only the density of the ceramic is increased, but also a second phase Y with better heat conductivity is generated 2 Si 2 O 7 Thereby obtaining the multiphase silicon carbide ceramic with excellent heat conductivity. The higher the thermal conductivity of the ceramic matrix is, the faster the heat transfer rate is, the less heat energy is consumed on the ceramic matrix, so that the electric-heat conversion rate is also improved, and the electric heating ceramic with high thermal conductivity, high electric-heat conversion rate and high thermal stability is obtained.
Description
Technical Field
The invention relates to the field of ceramics, in particular to an electrothermal ceramic with high thermal stability and a preparation process thereof.
Background
Common electric heating materials can be divided into metal electric heating materials and nonmetal electric heating materials. The metal electrothermal materials mainly comprise: noble metals, heavy metals and their alloys, nickel-based alloys, iron-based alloys, copper-based alloys, and the like. The nonmetal electric heating material mainly comprises: silicon carbide, molybdenum silicide, lanthanum chromate, barium titanate (PTC material), carbon/ceramic composite materials, and the like. The traditional electric heating material mainly comprises metal materials, and mainly comprises nickel-chromium alloy and iron-aluminum alloy which are widely applied. When the electric heating wire is in a working state, the electric heating wire is at a high temperature and is easy to generate oxidation reaction in the air to be blown; the heating wire is often used in a spiral state, and an inductive reactance effect can be generated when the heating wire is electrified; analyzed in terms of conversion of electric heat energy, energy is lost due to generation of a part of visible light. In addition, the exhausted non-renewable metal deposits are difficult to meet the requirements, and the metal materials with extremely large consumption can cause serious environmental pollution from the mining, smelting and forming processes. Therefore, from the perspective of sustainable development, people have been exploring new non-metallic electric heating materials.
Among the electric heating materials, ceramic electric heating materials having high melting point and high temperature resistance have attracted interest. The prior widely applied nonmetal electric heating materials mainly comprise: silicon carbide materials, lanthanum chromate materials, PTC ceramic materials, carbon ceramic composite materials and the like. The former two are mainly used for high temperature heating, and the latter two are mainly used for medium and low temperature heating.
Silicon carbide (SiC) has extremely excellent thermodynamic properties, namely, the melting point is 3100K, the specific heat capacity is 0.69J/g DEG C, the intrinsic thermal conductivity of the crystalline SiC ceramic is up to 490W/((m.K)), while the thermal conductivity of polycrystal is often lower than that of single crystal, and the maximum value of the thermal conductivity of the polycrystalline SiC ceramic prepared in the laboratory at present is 270W/((m.K)) at room temperature. The average thermal expansion coefficient of the silicon carbide is 4.4 multiplied by 10 when the temperature is 25 to 1400 DEG C -6 /° c, and the coefficient of thermal expansion of silicon (4.4 × 10) -6 /° c), both are approximately equal. In addition, silicon carbide has excellent chemical stability owing to its special oxidation properties. Silicon carbide itself is easily oxidized to silicon dioxide, but when its surface is oxidized to form a silicon dioxide film, its oxidation process is hindered. In a neutral medium or a reducing atmosphere, siC is hardly oxidized and is still stable under the high-temperature condition of 2200 ℃; when in an oxidizing atmosphere, siC is oxidized as the temperature increases, but its oxidation resistance can be maintained at up to 150 ℃.
SiC is a covalent compound, electrons are bound, it mainly depends on phonons to transfer heat energy, and the mechanism affecting phonon mean free path is mainly divided into two types: one is the collision between phonons and phonons. The collision among the phonons is greatly influenced by the temperature, and the collision among the phonons is less at low temperature, so the influence of other functions is more important; however, for most ceramic materials, as the temperature increases, the phonon movement increases, the collision increases, and the mean free path decreases significantly, so that the phonon interaction and scattering caused by the point defect become the main contradiction. Secondly, phonon collides with various impurities, defects and crystal boundaries. Thermal conductivity analysis of polycrystalline SiC is more complex than that of single crystal SiC. The thermal conductivity of single crystal SiC is about 490W/(m · K) at room temperature, whereas in general the thermal conductivity of polycrystalline ceramics is much lower than that of its corresponding single crystal, and the thermal conductivity of polycrystalline SiC experimentally produced at present is about 270W/(m · K) at room temperature at the most.
Because the monocrystalline SiC is difficult to prepare, low in efficiency and extremely high in cost, how to improve the thermal conductivity of the polycrystalline SiC becomes an important direction for current research.
Disclosure of Invention
Aiming at the defects in the prior art, the invention provides an electrothermal ceramic with high thermal stability and a preparation process thereof.
A preparation process of electrothermal ceramic with high thermal stability comprises the following steps:
(1) Respectively weighing 80-90 parts by weight of silicon carbide powder, 10-20 parts by weight of zirconia powder, 0.9-11 parts by weight of yttrium fluoride powder and 0.9-11 parts by weight of Y 4 Al 2 O 9 Powder, 1-8 parts of magnesium oxide powder, 0.2-0.3 part of boric acid, 8-12 parts of silica sol and 100-150 parts of absolute ethyl alcohol;
(2) Mixing the silica sol obtained in the step (1) with absolute ethyl alcohol according to a formula, stirring at the rotating speed of 160-200r/min for 10-20min at normal temperature, and then adding the silicon carbide powder, the zirconium oxide powder, the magnesium oxide powder, the yttrium fluoride powder and the Y powder obtained in the step (1) 4 Al 2 O 9 Continuously stirring the powder and boric acid at the rotating speed of 160-200r/min for 20-40min, finally transferring the powder and boric acid into a planetary ball mill, and ball-milling the powder and boric acid at the rotating speed of 160-200r/min for 4-8h at normal temperature to obtain mixed slurry;
(3) Injecting the mixed slurry into a mold, embedding the heating wire, and placing in a hot press for hot pressing for 40-60min to obtain a blank;
(4) Placing the biscuit in a tubular atmosphere furnace, introducing nitrogen for protection, heating to 600-800 ℃ at the normal temperature at the speed of 4-6 ℃/min, preserving heat for 1-2h, heating to 1100-1400 ℃ at the speed of 1-2 ℃/min, preserving heat for sintering for 2-4h, and cooling to room temperature to obtain the electrothermal ceramic with high thermal stability.
The average grain diameter of the silicon carbide powder in the step (1) is 20-40 μm, the average grain diameter of the zirconium oxide powder is 20-40 μm, the average grain diameter of the magnesium oxide powder is 20-40 μm, the average grain diameter of the yttrium fluoride powder is 30-50 μm, and the average grain diameter of the boric acid is 60-80 μm.
And (3) the hot-pressing treatment conditions of the hot press in the step (3) are that the temperature is 80-90 ℃ and the pressure is 160-200MPa.
The nitrogen gas introducing speed in the step (4) is 160-200mL/min.
The invention firstly prepares the electrothermal ceramic according to the conventional sintering process of the silicon carbide ceramic, and the thermal conductivity is found to be poor, which shows that the electrothermal ceramic has slow heating rate and much heat dissipation and is not very suitable for being used as the electrothermal ceramic. Therefore, the invention further adds a certain amount of yttrium fluoride into the conventional polycrystalline silicon carbide ceramic formula, and the heat conductivity of the conventional polycrystalline silicon carbide ceramic formula is obviously improved 2 Si 2 O 7 The second phase is obtained, fills the voids between the SiC grains, and diffuses along the grain boundaries, thereby reducing phonon scattering at the grain boundaries and increasing the thermal conductivity. Thereby achieving the purpose of purifying the crystal lattice of SiC, and the generated liquid phase can also promote the progress of sintering and reduce the reaction temperature. According to the invention, yttrium fluoride is replaced by silica sol, silica contained in the silica sol can fill up gaps of ceramics, the density of the ceramics is increased, and pores and defects are reduced, so that the scattering of phonons is reduced, and the thermal conductivity of the ceramics is improved to a certain extent. The invention also adopts Y 4 Al 2 O 9 Instead of silica sol, use is made of Y 4 Al 2 O 9 Reacting with silicon dioxide and magnesium oxide to generate Y 2 Si 2 O 7 Or Mg 2 Al 4 Si 5 O 12 ，Mg 2 Al 4 Si 5 O 12 Has the properties of negative expansion coefficient material, and has reduced shrinkage volume at high temperature and increased expansion volume at low temperatureLarge, thereby reducing porosity, increasing density of the ceramic, and increasing thermal conductivity. A second phase is obtained, but Y 4 Al 2 O 9 Is an oxide that reacts with the silicon dioxide on the surface of the silicon carbide and then contacts the silicon carbide, causing further oxidation of the silicon carbide and resulting impure silicon carbide crystal lattice.
Furthermore, the invention tries to adopt yttrium fluoride and silica sol as sintering aids together, namely the yttrium fluoride and the silica sol have the function of filling the ceramic pores and can also react with the silica to generate Y with better thermal conductivity 2 Si 2 O 7 Thereby enhancing the thermal conductivity of the ceramic. However, silicon fluoride gas is generated in the reaction process of yttrium fluoride and silica sol, so that the density of the obtained ceramic is reduced, and the thermal conductivity of the ceramic is influenced to a certain extent. The present invention again attempts to adopt Y 4 Al 2 O 9 Silica sol is used as a sintering aid, and due to the increase of the amount of silica, Y is prevented to a certain extent 4 Al 2 O 9 The heat conductivity of the silicon carbide is improved by the corrosion of the silicon carbide. Using yttrium fluoride, Y 4 Al 2 O 9 Together as a sintering aid, due to the presence of yttrium fluoride, and Y 4 Al 2 O 9 Form a competitive relationship, resulting in Y 4 Al 2 O 9 The erosion of silicon carbide is reduced, but the density of the ceramic formed is lower, affecting the thermal conductivity.
In order to pursue a silicon carbide ceramic having more excellent properties, yttrium fluoride and Y are added to the ceramic 4 Al 2 O 9 The silicon sol and the silicon sol are jointly used as sintering aids, wherein the silicon dioxide can fill gaps among silicon carbide particles, but the thermal conductivity of the silicon dioxide is poor; y is 4 Al 2 O 9 Can form Y with better heat conductivity with silicon dioxide 2 Si 2 O 7 But it attacks silicon carbide; and yttrium fluoride is just capable of inhibiting Y 4 Al 2 O 9 Attack on silicon carbide, therefore, yttrium fluoride, Y are used 4 Al 2 O 9 The silica sol is used as a sintering aid of the silicon carbide ceramic, not only increases the density of the ceramic, but also generatesA second phase Y with better thermal conductivity 2 Si 2 O 7 Thereby obtaining the multiphase silicon carbide ceramic with excellent heat conductivity.
The invention has the beneficial effects that:
the invention uses yttrium fluoride and Y 4 Al 2 O 9 The silicon sol and the silicon sol are jointly used as sintering aids, wherein the silicon dioxide can fill gaps among silicon carbide particles, but the thermal conductivity of the silicon dioxide is poor; y is 4 Al 2 O 9 Can form Y with better heat conductivity with silicon dioxide 2 Si 2 O 7 But it attacks silicon carbide; and yttrium fluoride is just capable of inhibiting Y 4 Al 2 O 9 Attack on silicon carbide, therefore, yttrium fluoride, Y are used 4 Al 2 O 9 The silica sol is used as a sintering aid of the silicon carbide ceramic, not only the density of the ceramic is increased, but also a second phase Y with better heat conductivity is generated 2 Si 2 O 7 Thereby obtaining the multiphase silicon carbide ceramic with excellent heat conductivity. The higher the thermal conductivity of the ceramic matrix is, the faster the heat transfer rate is, the less heat energy is consumed on the ceramic matrix, so that the electric-heat conversion rate is also improved, and the electric heating ceramic with high thermal conductivity, high electric-heat conversion rate and high thermal stability is obtained.
Detailed Description
The average grain diameter of the silicon carbide powder is 20-40 mu m, the average grain diameter of the zirconia powder is 20-40 mu m, Y 4 Al 2 O 9 The average grain diameter of the powder is 20-40 μm, the average grain diameter of the magnesium oxide powder is 20-40 μm, the average grain diameter of the yttrium fluoride powder is 30-50 μm, and the average grain diameter of the boric acid is 60-80 μm.
Silica sol, cat No.: BJN-1430HP, water glass factory in Foshan.
Electric heating wire, brand: 0Cr27AL7Mo2, melting point: 1520 deg.C, jiangsu Huanyuan Su-SpA.
Example 1
A preparation process of electrothermal ceramic with high thermal stability comprises the following steps:
(1) Respectively weighing 90 parts of silicon carbide powder, 10 parts of zirconia powder, 3 parts of magnesia powder, 0.2 part of boric acid and 120 parts of absolute ethyl alcohol according to parts by weight;
(2) Mixing the silicon carbide powder, the zirconium oxide powder, the magnesium oxide powder, the boric acid and the absolute ethyl alcohol in the step (1) according to a formula, stirring for 30min at a rotating speed of 200r/min, finally transferring into a planetary ball mill, and ball-milling for 6h at a rotating speed of 200r/min at normal temperature to obtain mixed slurry;
(3) Injecting the mixed slurry into a mold, embedding the heating wire, and placing the mold in a hot press for hot pressing for 50min to obtain a blank;
(4) And (3) placing the biscuit in a tubular atmosphere furnace, introducing nitrogen for protection, heating to 700 ℃ at the normal temperature at the speed of 5 ℃/min, preserving heat for 1.5h, heating to 1300 ℃ at the speed of 1 ℃/min, preserving heat for sintering for 3h, and cooling to the room temperature to obtain the electrothermal ceramic with high thermal stability.
And (3) carrying out hot pressing treatment on the hot press in the step (3) under the conditions of the temperature of 90 ℃ and the pressure of 180MPa.
The nitrogen gas introducing speed in the step (4) is 180mL/min.
Example 2
A preparation process of electrothermal ceramic with high thermal stability comprises the following steps:
(1) Respectively weighing 90 parts of silicon carbide powder, 10 parts of zirconia powder, 5 parts of yttrium fluoride powder, 3 parts of magnesia powder, 0.2 part of boric acid and 120 parts of absolute ethyl alcohol according to parts by weight;
(2) Mixing the silicon carbide powder, the zirconium oxide powder, the magnesium oxide powder, the yttrium fluoride powder, the boric acid and the absolute ethyl alcohol in the step (1) according to a formula, stirring for 30min at a rotating speed of 200r/min, finally transferring to a planetary ball mill, and ball-milling for 6h at a rotating speed of 200r/min at normal temperature to obtain mixed slurry;
(3) Injecting the mixed slurry into a mold, embedding the heating wire, and placing the mold in a hot press for hot pressing for 50min to obtain a blank;
(4) And (3) placing the biscuit in a tubular atmosphere furnace, introducing nitrogen for protection, heating to 700 ℃ at the normal temperature at the speed of 5 ℃/min, preserving heat for 1.5h, heating to 1300 ℃ at the speed of 1 ℃/min, preserving heat for sintering for 3h, and cooling to the room temperature to obtain the electrothermal ceramic with high thermal stability.
And (3) carrying out hot pressing treatment on the hot press in the step (3) under the conditions of the temperature of 90 ℃ and the pressure of 180MPa.
The nitrogen gas introducing speed in the step (4) is 180mL/min.
Example 3
A preparation process of electrothermal ceramic with high thermal stability comprises the following steps:
(1) Respectively weighing 90 parts of silicon carbide powder, 10 parts of zirconia powder, 3 parts of magnesia powder, 0.2 part of boric acid, 10 parts of silica sol and 120 parts of absolute ethyl alcohol according to parts by weight;
(2) Mixing the silica sol and the absolute ethyl alcohol in the step (1) according to a formula, stirring at the rotating speed of 200r/min for 20min at normal temperature, then adding the silicon carbide powder, the zirconium oxide powder, the magnesium oxide powder and the boric acid in the step (1), continuously stirring at the rotating speed of 200r/min for 30min, finally transferring to a planetary ball mill, and ball-milling at the rotating speed of 200r/min for 6h at normal temperature to obtain mixed slurry;
(3) Injecting the mixed slurry into a mold, embedding the heating wire, and placing the mold in a hot press for hot pressing for 50min to obtain a blank;
(4) And (3) placing the biscuit in a tubular atmosphere furnace, introducing nitrogen for protection, heating to 700 ℃ at the normal temperature at the speed of 5 ℃/min, preserving heat for 1.5h, heating to 1300 ℃ at the speed of 1 ℃/min, preserving heat for sintering for 3h, and cooling to the room temperature to obtain the electrothermal ceramic with high thermal stability.
And (3) carrying out hot pressing treatment on the hot press in the step (3) under the conditions of the temperature of 90 ℃ and the pressure of 180MPa.
The nitrogen gas introducing speed in the step (4) is 180mL/min.
Example 4
A preparation process of electrothermal ceramic with high thermal stability comprises the following steps:
(1) Respectively weighing 90 parts by weight of silicon carbide powder, 10 parts by weight of zirconia powder and 5 parts by weight of Y 4 Al 2 O 9 Powder, 3 parts of magnesium oxide powder, 0.2 part of boric acid and 120 parts of absolute ethyl alcohol;
(2) According to the formula, the silicon carbide powder, the zirconium oxide powder, the magnesium oxide powder and the Y in the step (1) are mixed 4 Al 2 O 9 Mixing the powder, boric acid and absolute ethyl alcohol, stirring at the rotating speed of 200r/min for 30min, finally transferring to a planetary ball mill, and ball-milling at the rotating speed of 200r/min for 6h at normal temperature to obtain mixed slurry;
(3) Injecting the mixed slurry into a mold, embedding the heating wire, and placing the mold in a hot press for hot pressing for 50min to obtain a blank;
(4) And (3) placing the biscuit in a tubular atmosphere furnace, introducing nitrogen for protection, heating to 700 ℃ at the normal temperature at the speed of 5 ℃/min, preserving heat for 1.5h, heating to 1300 ℃ at the speed of 1 ℃/min, preserving heat for sintering for 3h, and cooling to the room temperature to obtain the electrothermal ceramic with high thermal stability.
And (3) carrying out hot pressing treatment on the hot press in the step (3) under the conditions of the temperature of 90 ℃ and the pressure of 180MPa.
The nitrogen gas introduction rate in the step (4) is 180mL/min.
Example 5
A preparation process of electrothermal ceramic with high thermal stability comprises the following steps:
(1) Respectively weighing 90 parts of silicon carbide powder, 10 parts of zirconia powder, 5 parts of yttrium fluoride powder, 3 parts of magnesia powder, 0.2 part of boric acid, 10 parts of silica sol and 120 parts of absolute ethyl alcohol according to parts by weight;
(2) Mixing the silica sol and the absolute ethyl alcohol in the step (1) according to a formula, stirring at a rotating speed of 200r/min for 20min at normal temperature, then adding the silicon carbide powder, the zirconium oxide powder, the magnesium oxide powder, the yttrium fluoride powder and the boric acid in the step (1), continuously stirring at a rotating speed of 200r/min for 30min, finally transferring to a planetary ball mill, and ball-milling at a rotating speed of 200r/min for 6h at normal temperature to obtain mixed slurry;
(3) Injecting the mixed slurry into a mold, embedding the heating wire, and placing in a hot press for hot-pressing treatment for 50min to obtain a blank;
(4) And (3) placing the biscuit in a tubular atmosphere furnace, introducing nitrogen for protection, heating to 700 ℃ at the normal temperature at the speed of 5 ℃/min, preserving heat for 1.5h, heating to 1300 ℃ at the speed of 1 ℃/min, preserving heat for sintering for 3h, and cooling to the room temperature to obtain the electrothermal ceramic with high thermal stability.
And (3) carrying out hot pressing treatment on the hot press in the step (3) under the conditions of the temperature of 90 ℃ and the pressure of 180MPa.
The nitrogen gas introducing speed in the step (4) is 180mL/min.
Example 6
A preparation process of electrothermal ceramic with high thermal stability comprises the following steps:
(1) Respectively weighing 90 parts by weight of silicon carbide powder, 10 parts by weight of zirconia powder and 5 parts by weight of Y 4 Al 2 O 9 Powder, 3 parts of magnesium oxide powder, 0.2 part of boric acid, 10 parts of silica sol and 120 parts of absolute ethyl alcohol;
(2) Mixing the silica sol and the absolute ethyl alcohol in the step (1) according to a formula, stirring at the rotating speed of 200r/min for 20min at normal temperature, and then adding the silicon carbide powder, the zirconium oxide powder, the magnesium oxide powder and the Y in the step (1) 4 Al 2 O 9 Continuously stirring the powder and boric acid at the rotating speed of 200r/min for 30min, finally transferring the powder and boric acid into a planetary ball mill, and ball-milling the powder and boric acid at the rotating speed of 200r/min for 6h at normal temperature to obtain mixed slurry;
(3) Injecting the mixed slurry into a mold, embedding the heating wire, and placing the mold in a hot press for hot pressing for 50min to obtain a blank;
(4) And (3) placing the biscuit in a tubular atmosphere furnace, introducing nitrogen for protection, heating to 700 ℃ at the normal temperature at the speed of 5 ℃/min, preserving heat for 1.5h, heating to 1300 ℃ at the speed of 1 ℃/min, preserving heat for sintering for 3h, and cooling to the room temperature to obtain the electrothermal ceramic with high thermal stability.
And (3) carrying out hot pressing treatment on the hot press in the step (3) under the conditions of the temperature of 90 ℃ and the pressure of 180MPa.
The nitrogen gas introducing speed in the step (4) is 180mL/min.
Example 7
A preparation process of electrothermal ceramic with high thermal stability comprises the following steps:
(1) Respectively weighing 90 parts by weight of silicon carbide powder and 10 parts by weight of oxideZirconium powder, 5 parts of yttrium fluoride powder, 5 parts of Y 4 Al 2 O 9 Powder, 3 parts of magnesium oxide powder, 0.2 part of boric acid and 120 parts of absolute ethyl alcohol;
(2) According to the formula, the silicon carbide powder, the zirconium oxide powder, the magnesium oxide powder, the yttrium fluoride powder and the Y in the step (1) are mixed 4 Al 2 O 9 Mixing the powder, boric acid and absolute ethyl alcohol, stirring at the rotating speed of 200r/min for 30min, finally transferring to a planetary ball mill, and ball-milling at the rotating speed of 200r/min for 6h at normal temperature to obtain mixed slurry;
(3) Injecting the mixed slurry into a mold, embedding the heating wire, and placing the mold in a hot press for hot pressing for 50min to obtain a blank;
(4) And (3) placing the biscuit in a tubular atmosphere furnace, introducing nitrogen for protection, heating to 700 ℃ at the speed of 5 ℃/min at normal temperature, preserving heat for 1.5h, heating to 1300 ℃ at the speed of 1 ℃/min, preserving heat for sintering for 3h, and cooling to room temperature to obtain the electrothermal ceramic with high thermal stability.
And (3) carrying out hot pressing treatment on the hot press in the step (3) under the conditions of the temperature of 90 ℃ and the pressure of 180MPa.
The nitrogen gas introducing speed in the step (4) is 180mL/min.
Example 8
A preparation process of electrothermal ceramic with high thermal stability comprises the following steps:
(1) Respectively weighing 90 parts by weight of silicon carbide powder, 10 parts by weight of zirconia powder, 5 parts by weight of yttrium fluoride powder and 5 parts by weight of Y 4 Al 2 O 9 Powder, 3 parts of magnesium oxide powder, 0.2 part of boric acid, 10 parts of silica sol and 120 parts of absolute ethyl alcohol;
(2) Mixing the silica sol obtained in the step (1) with absolute ethyl alcohol according to a formula, stirring at the normal temperature at the rotating speed of 200r/min for 20min, and then adding the silicon carbide powder, the zirconium oxide powder, the magnesium oxide powder, the yttrium fluoride powder and the Y obtained in the step (1) 4 Al 2 O 9 Continuously stirring the powder and boric acid at the rotating speed of 200r/min for 30min, finally transferring the powder and boric acid into a planetary ball mill, and ball-milling the powder and boric acid at the rotating speed of 200r/min for 6h at normal temperature to obtain mixed slurry;
(3) Injecting the mixed slurry into a mold, embedding the heating wire, and placing the mold in a hot press for hot pressing for 50min to obtain a blank;
(4) And (3) placing the biscuit in a tubular atmosphere furnace, introducing nitrogen for protection, heating to 700 ℃ at the normal temperature at the speed of 5 ℃/min, preserving heat for 1.5h, heating to 1300 ℃ at the speed of 1 ℃/min, preserving heat for sintering for 3h, and cooling to the room temperature to obtain the electrothermal ceramic with high thermal stability.
And (3) carrying out hot pressing treatment on the hot press in the step (3) under the conditions of the temperature of 90 ℃ and the pressure of 180MPa.
The nitrogen gas introducing speed in the step (4) is 180mL/min.
Test example 1
Thermal conductivity test
In the experiment, a thermal conductivity analyzer is adopted to measure the thermal diffusion coefficient and the thermal conductivity of the high-thermal-stability electrothermal ceramic prepared by the invention, a laser pulse method is adopted to measure the thermal diffusion coefficient of the same sample at the thickness of 2 mm,2.5 mm and 3 mm under the condition of 25 ℃, the average number is taken, and the thermal conductivity of the sample is calculated.
The test method comprises the following steps: and placing the polished sample in a tray. During testing, the pulse laser emits a beam of laser pulse to the lower surface of the sample, and the lower surface of the sample absorbs heat and conducts the heat to the upper surface of the sample, so that the temperature of the upper surface is increased to reach the maximum temperature increment. Assuming that the heat absorbed by the sample is not dissipated to the surroundings, a one-dimensional longitudinal heat flow is formed over the sample. Measuring the thermal diffusion coefficient of the sample by measuring the temperature rise time;
substituting alpha into the relational expression of density and thermal conductivity, the thermal conductivity of the sample can be obtained as follows:
where ρ is the actual density of the sample (g/cm) 3 ) (ii) a c is the specific heat capacity (J/K) of the sample; l is the thickness (mm) of the sample;𝑡 1/2 the time(s) required for the upper surface of the sample to heat up to half the maximum temperature; alpha is the thermal diffusivity (mm) of the sample 2 S); λ is the thermal conductivity of the sample (W/(m · K)).
As can be seen from Table 1, the electrothermal ceramic with high thermal stability prepared in example 1 of the present invention has the worst thermal conductivity, which indicates that it has a slow temperature-rising rate and high heat dissipation, and is not very suitable for use as an electrothermal ceramic. In example 2, a certain amount of yttrium fluoride is added to the scheme of example 1, and it is found that the thermal conductivity of the yttrium fluoride is obviously improved compared with that of example 1, and the reason is considered that the yttrium fluoride can react with silicon dioxide on the surface of silicon carbide to generate yttrium oxide and silicon tetrafluoride, and further, the yttrium oxide reacts with the silicon dioxide to generate Y 2 Si 2 O 7 The second phase is obtained, fills the voids between the SiC grains, and diffuses along the grain boundaries, thereby reducing phonon scattering at the grain boundaries and increasing the thermal conductivity. Thereby achieving the purpose of purifying the crystal lattice of SiC, and the generated liquid phase can also promote the progress of sintering and reduce the reaction temperature.
In the embodiment 3 of the invention, the silica sol is added into the scheme of the embodiment 1, and the silica contained in the silica sol can fill up the gaps of the ceramic, increase the density of the ceramic, and reduce the generation of air holes and defects, so that the scattering of phonons is reduced, and the thermal conductivity of the ceramic is improved to a certain extent.
In the embodiment 4 of the invention, Y is added on the basis of the embodiment 1 4 Al 2 O 9 By using Y 4 Al 2 O 9 Reacting with silicon dioxide and magnesium oxide to generate Y 2 Si 2 O 7 Or Mg 2 Al 4 Si 5 O 12 ，Mg 2 Al 4 Si 5 O 12 The material has the property of a negative expansion coefficient, the shrinkage volume is reduced at high temperature, and the expansion volume is increased at low temperature, so that the porosity is reduced, the density of the ceramic is improved, and the thermal conductivity is increased. But Y 4 Al 2 O 9 Is an oxide that reacts with the silicon dioxide on the surface of the silicon carbide and then contacts the silicon carbide, causing further oxidation of the silicon carbide and resulting impure silicon carbide crystal lattice.
Furthermore, in the embodiment 5 of the present invention, yttrium fluoride and silica sol are used as the sintering aid, which has the function of silica filling up the ceramic pores, and can also react with yttrium fluoride and silica to generate Y with better thermal conductivity 2 Si 2 O 7 Thereby enhancing the thermal conductivity of the ceramic. However, silicon fluoride gas is generated in the reaction process of yttrium fluoride and silica sol, so that the density of the obtained ceramic is reduced, and the thermal conductivity of the ceramic is influenced to a certain extent.
Example 6 use of Y 4 Al 2 O 9 Silica sol is used as a sintering aid, and due to the increase of the amount of silica, Y is prevented to a certain extent 4 Al 2 O 9 The heat conductivity of the silicon carbide is improved compared with that of the silicon carbide in the embodiments 3 and 4. Example 7 use of Yttrium fluoride, Y 4 Al 2 O 9 Together as a sintering aid, due to the presence of yttrium fluoride, and Y 4 Al 2 O 9 Form a competitive relationship, resulting in Y 4 Al 2 O 9 The erosion of silicon carbide is reduced, but the density of the ceramic formed is lower, affecting the thermal conductivity. Example 8 the invention provides yttrium fluoride, Y 4 Al 2 O 9 The silicon sol and the silicon sol are jointly used as sintering aids, wherein the silicon dioxide can fill gaps among silicon carbide particles, but the thermal conductivity of the silicon dioxide is poor; y is 4 Al 2 O 9 Can form Y with better heat conductivity with silicon dioxide 2 Si 2 O 7 But it attacks silicon carbide; and yttrium fluoride is just capable of inhibiting Y 4 Al 2 O 9 Attack on silicon carbide, therefore, yttrium fluoride, Y are used 4 Al 2 O 9 The silica sol is used as a sintering aid of the silicon carbide ceramic, not only the density of the ceramic is increased, but also a second phase Y with better heat conductivity is generated 2 Si 2 O 7 Thereby obtaining the multiphase silicon carbide ceramic with excellent heat conductivity.
Test example 2
The electric-thermal conversion rate of the electrothermal ceramic with high thermal stability prepared by the invention is obtained by testing with reference to method A of 17.1 in GB/T7287-2008 & lttest method for infrared radiation heaters'.
Table 2 shows the heat conversion rate of the high thermal stability electrothermal ceramics prepared in each example and comparative example of the present invention, which has substantially the same tendency as in test example 1, and it is considered by the present invention that this is caused by the change in the thermal conductivity, the higher the thermal conductivity of the ceramic matrix, the faster the heat transfer rate thereof, the less the thermal energy is consumed on the ceramic matrix, and thus the higher the electricity-heat conversion rate.
Claims (6)
1. A preparation process of electrothermal ceramic with high thermal stability is characterized by comprising the following steps:
(1) Respectively weighing 80-90 parts by weight of silicon carbide powder, 10-20 parts by weight of zirconia powder, 0.9-11 parts by weight of yttrium fluoride powder and 0.9-11 parts by weight of Y 4 Al 2 O 9 Powder, 1-8 parts of magnesium oxide powder, 0.2-0.3 part of boric acid, 8-12 parts of silica sol and 100-150 parts of absolute ethyl alcohol;
(2) Mixing the silica sol obtained in the step (1) with absolute ethyl alcohol according to a formula, stirring at the rotating speed of 160-200r/min for 10-20min at normal temperature, and then adding the silicon carbide powder, the zirconium oxide powder, the magnesium oxide powder, the yttrium fluoride powder and the Y powder obtained in the step (1) 4 Al 2 O 9 Continuously stirring the powder and boric acid at the rotating speed of 160-200r/min for 20-40min, finally transferring the powder and boric acid into a planetary ball mill, and carrying out ball milling at the rotating speed of 160-200r/min for 4-8h at normal temperature to obtain mixed slurry;
(3) Injecting the mixed slurry into a mold, embedding the heating wire, and placing in a hot press for hot pressing for 40-60min to obtain a blank;
(4) Placing the biscuit in a tubular atmosphere furnace, introducing nitrogen for protection, heating to 600-800 ℃ at the normal temperature at the speed of 4-6 ℃/min, preserving heat for 1-2h, heating to 1100-1400 ℃ at the speed of 1-2 ℃/min, preserving heat for sintering for 2-4h, and cooling to room temperature to obtain the electrothermal ceramic with high thermal stability.
2. The process for preparing electrothermal ceramic with high thermal stability according to claim 1, wherein the average particle size of the silicon carbide powder in step (1) is 20 to 40 μm, the average particle size of the zirconia powder is 20 to 40 μm, the average particle size of the magnesia powder is 20 to 40 μm, the average particle size of the yttrium fluoride powder is 30 to 50 μm, and the average particle size of the boric acid is 60 to 80 μm.
3. The process for preparing electrothermal ceramics with high thermal stability according to claim 1, wherein the hot pressing conditions of the hot press in the step (3) are 80-90 ℃ and 160-200MPa.
4. The process for preparing electrothermal ceramic with high thermal stability according to claim 1, wherein the nitrogen gas is introduced at a rate of 160-200mL/min in the step (4).
5. A process for preparing a high thermal stability electrothermal ceramic according to claim 1, comprising the steps of:
(1) Respectively weighing 90 parts by weight of silicon carbide powder, 10 parts by weight of zirconia powder, 5 parts by weight of yttrium fluoride powder and 5 parts by weight of Y 4 Al 2 O 9 Powder, 3 parts of magnesium oxide powder, 0.2 part of boric acid, 10 parts of silica sol and 120 parts of absolute ethyl alcohol;
(2) Mixing the silica sol obtained in the step (1) with absolute ethyl alcohol according to a formula, stirring at the rotating speed of 200r/min for 20min at normal temperature, and then adding the silicon carbide powder, the zirconium oxide powder, the magnesium oxide powder, the yttrium fluoride powder and the Y obtained in the step (1) 4 Al 2 O 9 Continuously stirring the powder and boric acid at the rotating speed of 200r/min for 30min, finally transferring the powder and boric acid into a planetary ball mill, and ball-milling the powder and boric acid at the rotating speed of 200r/min for 6h at normal temperature to obtain mixed slurry;
(3) Injecting the mixed slurry into a mold, embedding the heating wire, and placing the mold in a hot press for hot pressing for 50min to obtain a blank;
(4) Placing the biscuit in a tubular atmosphere furnace, introducing nitrogen for protection, heating to 700 ℃ at the normal temperature at the speed of 5 ℃/min, preserving heat for 1.5h, heating to 1300 ℃ at the speed of 1 ℃/min, preserving heat for sintering for 3h, and cooling to the room temperature to obtain the electrothermal ceramic with high thermal stability;
the hot pressing condition of the hot press in the step (3) is that the temperature is 90 ℃ and the pressure is 180MPa;
the nitrogen gas introducing speed in the step (4) is 180mL/min.
6. A high thermal stability electric heating ceramic, which is prepared by the preparation process of the high thermal stability electric heating ceramic according to any one of claims 1 to 5.
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