A zirconium-based ceramic foam filter and a method of making the same

Abstract

The application relates to the technical field of foam ceramics, in particular to a zirconium-based foam ceramic filter and a preparation method thereof, which comprises the following steps: S1, ceramic aggregate is crushed, dried, mixed with a dispersing agent and deionized water, ball milled, and then functional additives are added and ball milled to obtain ceramic slurry, wherein the ceramic aggregate comprises zirconium oxide, zirconium silicate and aluminum oxide, and the ceramic aggregate comprises 40-60% zirconium silicate, 20-35% zirconium oxide and the balance of aluminum oxide in percentage by mass; S2, a polyurethane foam template is immersed in the ceramic slurry, and the polyurethane foam template is dried in sections to obtain a green body; and S3, the green body is subjected to temperature sintering under gas protection to obtain the zirconium-based foam ceramic filter, through replacement of part of the zirconium oxide by zirconium silicate, through utilization of the SiO2 and ZrO2 contained in the zirconium silicate to form a synergistic phase system, through cooperation of the coke discharging and sectional sintering processes, through avoidance of cracking caused by phase change stress and impurity phases, through stabilization and controllability of a phase structure, and through enhancement of mechanical properties.

Description

Technical Field

[0001] This invention relates to the field of foam ceramics technology, and in particular to a zirconium-based foam ceramic filter and its preparation method. Background Technology

[0002] Foam ceramic filters, as key purification components in the metal casting field, can efficiently trap oxide inclusions, slag, and gases in molten metal, significantly improving the density and mechanical properties of castings. Zirconium-based foam ceramics, thanks to the excellent high-temperature resistance, chemical stability, and fracture toughness of zirconium oxide (ZrO2), have become a core material for precision casting in aerospace, high-end equipment, and other fields.

[0003] However, existing zirconium-based foam ceramic filters have the following three problems: 1. High risk of cracking: Zirconia exhibits polymorphic transformation characteristics. At room temperature, it is a monoclinic phase (m-ZrO2, space group P21 / c, lattice parameters a=5.145Å, b=5.206Å, c=5.311Å, β=99.23°), which undergoes a martensitic transformation at around 1170℃, transforming into a tetragonal phase (t-ZrO2, space group P42 / nmc, lattice parameters a=5.094Å, c=5.166Å), accompanied by a volume shrinkage of about 3-5%. During cooling, the tetragonal phase reverses to the monoclinic phase, resulting in a volume expansion of about 2-3%. The repeated phase transformation volume effect leads to internal stress concentration during sintering, which easily causes defects such as cracking and microcracks. At the same time, the traditional aggregate system relies solely on zirconium oxide and does not consider the synergistic effect between inorganic phases, further aggravating the shrinkage cracking problem. 2. Insufficient mechanical strength: The theoretical fracture strength of monoclinic zirconium-based ceramics is approximately 200 MPa. However, in actual preparation, due to incomplete sintering densification, porosity defects caused by residual carbon in the organic template, and microcracks caused by phase transformation stress, the flexural strength of the finished product is generally lower than 20 MPa. It also exhibits poor impact resistance and is prone to brittle fracture during installation, high-temperature filtration, and repeated use. 3. Poor process adaptability: Traditional sintering processes lack targeted coking procedures. Residual carbon in the organic binder and polyurethane template (C content > 0.5%) reacts with zirconium oxide at the interface, generating impurity phases such as ZrC (melting point 3530℃), which disrupts the lattice integrity of ZrO2. Simultaneously, the porosity formed by residual carbon reduces the material's density, further deteriorating its mechanical properties and crack resistance.

[0004] In existing technologies, although there are solutions that involve adding yttrium oxide for partial stabilization, these solutions can only partially suppress the monoclinic-tetragonal phase transition and cannot fundamentally solve the volume shrinkage problem, making it difficult to achieve both crack resistance and high strength performance. Summary of the Invention

[0005] In view of this, the present invention proposes a zirconium-based foam ceramic filter and its preparation method, which improves the technical problems of high cracking risk and insufficient mechanical strength of existing zirconium-based ceramic filters due to the zirconium oxide phase transformation volume effect.

[0006] The technical solution of this invention is implemented as follows: A method for preparing a zirconium-based foam ceramic filter includes the following steps: S1, the ceramic aggregate is crushed and dried, mixed with dispersant and deionized water, ball-milled, and then functional additives are added and ball-milled to obtain ceramic slurry. The ceramic aggregate includes zirconium oxide, zirconium silicate and alumina, and by mass percentage, it includes 40%-60% zirconium silicate, 20%-35% zirconium oxide and the balance is alumina. S2, Immerse the polyurethane foam template in the ceramic slurry and dry it in sections to obtain the green body; S3, the green body is heated and sintered under gas protection to obtain a zirconium-based foam ceramic filter.

[0007] Based on this technical solution, preferably, step S1 includes: S1.1, Zirconium silicate, zirconium oxide, and alumina are pulverized to a particle size of 5-15μm, dried at 120℃ for 4h, and the moisture content is controlled to be ≤0.5%. Deionized water and dispersant are added to a ball mill and stirred for 10min. Then, the dried zirconium silicate, zirconium oxide, and alumina are added to the ball mill and dispersed for 30-60min. S1.2, add functional additives, and continue ball milling for 2-4 hours to obtain ceramic slurry.

[0008] Based on this technical solution, preferably, the functional additives include binders, plasticizers, and sintering aids. The viscosity of the ceramic slurry is 500-800 mPa·s, and the sedimentation rate of the ceramic slurry after standing for 24 hours is ≤3%. The binder is polyvinyl alcohol, the dispersant is sodium polyphosphate, the plasticizer is glycerol, and the sintering aid is a mixture of magnesium oxide and yttrium oxide in a mass ratio of 1:2-3.

[0009] Based on this technical solution, preferably, step S2 includes: S2.1, a polyurethane foam template with a pore density of 10-60 PPI is selected, cleaned with ethanol, dried at 80℃, immersed in ceramic slurry, vacuum impregnated, and then rolled to remove excess slurry. S2.2, the polyurethane foam template is placed in a 60℃ oven and dried for 4 hours, then the temperature is raised to 70℃ and dried for another 4 hours, and then the temperature is raised to 80℃ and dried for another 4 hours to obtain the green body.

[0010] Based on this technical solution, preferably, the vacuum impregnation conditions are: impregnation at -0.08MPa for 15 minutes, followed by roller pressing at 0.3MPa-0.5MPa to remove excess slurry, so that the coating thickness of the ceramic slurry on the polyurethane foam skeleton is 0.1mm-0.3mm; the moisture content of the green body is ≤1%.

[0011] Based on this technical solution, preferably, step S3 includes: The billet is placed in the kiln, inert gas is introduced, and the temperature is raised from room temperature to 300℃-350℃ at a rate of 5-10℃ / h, and held for 4h-6h. Then the temperature is raised to 800℃-1000℃ at a rate of 20-30℃ / h, and held for 2h-3h. Finally, the temperature is raised from 800℃-1000℃ to 1500℃-1650℃ at a rate of 30-50℃ / h, and held for 6h-8h. After cooling to room temperature, a zirconium-based foam ceramic filter is obtained.

[0012] Based on this technical solution, preferably, the flow rate of the inert gas is 0.5 L / min.

[0013] In addition, the present invention provides a zirconium-based foam ceramic filter, which is prepared by the method of preparing zirconium-based foam ceramic filter as described in any one of the first aspects, comprising ceramic aggregate composed of zirconium silicate, zirconium oxide and alumina, including zirconium silicate, zirconium oxide and alumina, and the mass ratio of zirconium silicate to zirconium oxide in the raw material feeding is 1.2-3.0:1.

[0014] Based on this technical solution, preferably, the phase composition of the filter includes tetragonal zirconium oxide, zirconium silicate and α-alumina, wherein the content of monoclinic zirconium oxide is ≤15wt% and the content of tetragonal zirconium oxide is 60-75wt%.

[0015] The zirconium-based foam ceramic filter and its preparation method described in this invention have the following advantages over the prior art: By replacing part of the zirconium oxide with zirconium silicate, and utilizing its SiO2 content to form a ZrO2-SiO2 synergistic phase system, the monoclinic zirconium content is controlled below 15%, and the sintering shrinkage rate is reduced to below 8%. With the optimized coke removal procedure and segmented sintering process, carbon residue (≤0.1%) is completely removed, avoiding cracking caused by phase transformation stress and impurity phases. No cracks are generated after 10 thermal shock cycles. The rigid ZrSiO4 framework, along with the t-ZrO2 reinforcing phase and the Al2O3 modified phase, forms a synergistic structure, increasing the fracture toughness to 1.8 MPa·m. 1 / 2 The bending strength is above 8.0MPa, the compressive strength is above 8.4MPa, which is more than 60% higher than the strength of traditional products and more than 40% higher than the impact resistance. By precisely controlling the raw material ratio and sintering parameters, the tetragonal zirconium content is stably controlled, avoiding the zirconium oxide phase transformation volume effect, ensuring the structural stability of the material under high temperature conditions, and the strength retention rate after high temperature cycling exceeds 92%. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 This is a schematic diagram comparing the zirconium-based foam ceramic filter prepared by the preparation method of Example 2 of the present invention with an ordinary zirconium oxide ceramic filter; Figure 2 Box plots of flexural strength of the zirconium-based foam ceramic filter prepared by the preparation method of Example 2 of the present invention and the filter of Comparative Example 1. Detailed Implementation

[0018] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0019] A method for preparing a zirconium-based foam ceramic filter includes the following steps: S1, the ceramic aggregate is crushed and dried, mixed with dispersant and deionized water, ball-milled, and then functional additives are added and ball-milled to obtain ceramic slurry. The ceramic aggregate includes zirconium oxide, zirconium silicate and alumina, and by mass percentage, it includes 40%-60% zirconium silicate, 20%-35% zirconium oxide and the balance is alumina. The ceramic aggregate consists of 40%-60% zirconium silicate, 20%-35% zirconium oxide, and the balance being alumina by mass percentage. In this composition, zirconium silicate provides high-temperature strength and corrosion resistance, zirconium oxide introduces a phase transformation toughening mechanism, and alumina improves slurry suspension and sintering density.

[0020] Zirconium silicate, with the molecular formula ZrSiO4, is used as the core aggregate. Its SiO2 content forms a ZrO2-SiO2 binary system with zirconium oxide, suppressing the monoclinic-tetragonal phase transformation of zirconium oxide through lattice matching, thus reducing volume shrinkage caused by the phase transformation. Simultaneously, zirconium silicate exhibits no phase transformation at high temperatures, forming a "rigid skeleton" to disperse internal stress during sintering, reducing the risk of cracking at its source. Tetragonal zirconium oxide powder is selected; its excellent fracture toughness fills the gaps in the zirconium silicate skeleton, forming a reinforcing phase and improving the overall mechanical properties of the material. Alumina (α-phase) integrates into the zirconium oxide lattice to form a solid solution, refining grain size, improving interfacial bonding strength, and further enhancing structural density. The mass ratio of zirconium silicate to zirconium oxide needs to be controlled between 1.2 and 3.0:1. This is mainly because: if the ratio is too low, the SiO2 content is insufficient, failing to form a continuous rigid skeleton and negating the phase transformation suppression effect; if the ratio is too high, the proportion of the zirconium oxide reinforcing phase is insufficient, leading to decreased fracture toughness and increased interfacial defects.

[0021] In a preferred embodiment, step S1 includes: S1.1, Zirconium silicate, zirconium oxide, and alumina are pulverized to a particle size of 5μm-15μm, dried at 120℃ for 4h, and the moisture content is controlled to be ≤0.5%. Deionized water and dispersant are added to a ball mill and stirred for 10min. Then, the dried zirconium silicate, zirconium oxide, and alumina are added to the ball mill and dispersed for 30min-60min. S1.2, add functional additives, and continue ball milling for 2-4 hours to obtain ceramic slurry.

[0022] Step S1.1: First, deionized water and dispersant sodium polyphosphate are mixed and dissolved, then aggregate is added and dispersed for 30-60 minutes. The electrostatic repulsion of sodium polyphosphate can break the agglomeration force between aggregate particles, achieving initial dispersion. Step S1.2: After adding functional additives, ball milling continues for 2-4 hours. On the one hand, the binder polyvinyl alcohol, plasticizer glycerol, and sintering aid uniformly coat the aggregate surface; on the other hand, mechanical force further refines the particle agglomerates, ensuring the uniformity of the slurry. The sintering aid is a mixture of magnesium oxide and yttrium oxide at a mass ratio of 1:2-3. Magnesium oxide can inhibit abnormal grain growth and avoid structural defects caused by coarse grains; yttrium oxide, as a stabilizer, can form a substitution solid solution with zirconium oxide, stabilizing the tetragonal zirconium oxide content and reducing the phase transformation volume effect. The synergistic effect of the two can stabilize the tetragonal zirconium oxide content in the finished product at 60-75 wt%, and the monoclinic zirconium oxide content ≤15 wt%.

[0023] S2, Immerse the polyurethane foam template in the ceramic slurry and dry it in sections to obtain the green body; In a preferred embodiment, step S2 includes: S2.1, a polyurethane foam template with a pore density of 10-60 PPI is selected, cleaned with ethanol, dried at 80℃, immersed in ceramic slurry, vacuum impregnated, and then rolled to remove excess slurry. S2.2, the polyurethane foam template is placed in a 60℃ oven and dried for 4 hours, then the temperature is raised to 70℃ and dried for another 4 hours, and then the temperature is raised to 80℃ and dried for another 4 hours to obtain the green body.

[0024] Open-cell polyurethane foam with a pore density of 10-60 PPI was selected as the template. After cleaning with ethanol and drying at 80℃, surface oil and impurities were removed, improving the compatibility between the template and the slurry. Vacuum impregnation (-0.08 MPa, 15 min) used negative pressure to expel air from the template pores, allowing the slurry to fully penetrate deep into the pores and ensuring the integrity of the skeleton coating. Subsequent roller pressing at 0.3 MPa-0.5 MPa removed excess slurry, controlling the coating thickness to 0.1 mm-0.3 mm. This thickness ensures a continuous ceramic skeleton after sintering and avoids uneven drying shrinkage caused by slurry accumulation. A segmented, stepped drying method was adopted, the core of which was controlling the moisture evaporation rate and mitigating humidity and temperature gradients during the drying process: free water was slowly evaporated at a low temperature of 60℃ to prevent rapid surface drying and the formation of a hard shell that would prevent internal moisture from escaping; the temperature was gradually increased in the medium and high temperature stages to evaporate bound water, ultimately achieving a green body moisture content ≤1%. This process can effectively reduce drying stress, prevent defects such as cracking and deformation in the green body, and provide a structurally complete green body for subsequent sintering.

[0025] S3, the green body is heated and sintered under gas protection to obtain a zirconium-based foam ceramic filter.

[0026] In a preferred embodiment, step S3 includes: The billet is placed in the kiln and inert gas is introduced at a flow rate of 0.5 L / min. The temperature is increased from room temperature to 300℃-350℃ at a rate of 5-10℃ / h and held for 4-6 hours. Then the temperature is increased to 800℃-1000℃ at a rate of 20-30℃ / h and held for 2-3 hours. Finally, the temperature is increased from 800℃-1000℃ to 1500℃-1650℃ at a rate of 30-50℃ / h and held for 6-8 hours. After cooling to room temperature, a zirconium-based foam ceramic filter is obtained.

[0027] In a preferred embodiment, the green body is placed in a kiln and an inert gas is introduced as a protective gas to isolate it from air, preventing the oxidation of zirconium oxide and alumina at high temperatures. This also prevents the gases generated from the decomposition of the organic phase from reacting with the ceramic matrix, ensuring the stability of the phase composition. The temperature is increased from room temperature to 300℃-350℃ at a rate of 5-10℃ / h and held for 4-6 hours. This is the coke removal stage. The low-rate heating and holding process slowly removes the organic phase from the polyurethane template and binder, reducing the residual carbon content to below 0.1wt%. This avoids the porosity and cracks caused by the rapid decomposition of the organic phase due to rapid heating, and also prevents residual carbon from reacting with zirconium oxide to form ZrC impurity phases, thus protecting the integrity of the zirconium oxide lattice. The temperature is then increased to 800℃-1000℃ at a rate of 20-30℃ / h and held for 2-3 hours. This is the preheating stage, where the temperature is gradually increased to alleviate thermal stress and prevent cracking of the green body caused by a sudden temperature rise. Simultaneously, a stable interfacial transition phase is formed between zirconium silicate and zirconium oxide. The lattice parameters of this transition phase are between the two, which can further alleviate the difference in interfacial stress and improve structural stability. The temperature is then increased from 800℃-1000℃ to 1500℃-1650℃ at a rate of 30-50℃ / h and held for 6-8 hours. This is the sintering stage, where the high temperature promotes diffusion between aggregate particles and the growth of sintering necks, achieving structural densification. At the same time, alumina fully integrates into the zirconium oxide lattice to form a solid solution, and the rigid framework of zirconium silicate and the reinforcing phase of zirconium oxide form a synergistic structure, ultimately obtaining a high-strength, low-porosity ceramic body. After cooling to room temperature, a zirconium-based foam ceramic filter is obtained.

[0028] It should be noted that the sintering temperature in this embodiment is 1500-1650℃. Zirconium silicate undergoes controlled partial decomposition within this temperature range. The decomposition reaction is: ZrSiO4 → ZrO2 + SiO2. Both the decomposition products and the undecomposed zirconium silicate are completely retained in the ceramic body, with no component loss or volatilization. At the same time, the decomposed SiO2 exists in the form of an amorphous glass phase and is not a crystalline phase material. XRD only detects crystalline phase components, so the SiO2 crystalline phase was not detected in the phase analysis. The zirconium silicate feed amount in the raw materials in Table 1 is 40%-60%. The zirconium silicate crystalline phase content in the finished product is reduced. The difference between the raw material ratio and the crystalline phase content in the finished product is due to the high-temperature phase transformation. Finally, the composite skeleton is composed of the continuous crystalline phase of ZrO2 generated by decomposition, the magnesium-stabilized zirconium oxide phase, and the residual ZrSiO4 hard particles.

[0029] In a preferred embodiment, the functional additives include a binder, a plasticizer, and a sintering aid. The viscosity of the ceramic slurry is 500 mPa·s-800 mPa·s, and the sedimentation rate of the ceramic slurry after standing for 24 hours is ≤3%. The binder is polyvinyl alcohol, the dispersant is sodium polyphosphate, the plasticizer is glycerol, and the sintering aid is a mixture of magnesium oxide and yttrium oxide in a mass ratio of 1:2-3.

[0030] The viscosity of the ceramic slurry is controlled between 500 mPa·s and 800 mPa·s, and the settling rate after standing for 24 hours is ≤3%. This performance index ensures the permeability and uniform coating of the slurry during the impregnation process of the polyurethane template: if the viscosity is too high, the slurry will not be able to fully penetrate into the template pores, resulting in local material shortage; if the viscosity is too low, the slurry will easily be lost, affecting the coating thickness of the skeleton; and the low settling rate ensures that the aggregate is evenly distributed in the slurry, avoiding component segregation and structural inhomogeneity after sintering.

[0031] In a preferred embodiment, the vacuum impregnation conditions are: impregnation at -0.08MPa for 15 minutes, followed by roller pressing at 0.3MPa-0.5MPa to remove excess slurry, so that the ceramic slurry coating thickness on the polyurethane foam skeleton is 0.1mm-0.3mm, and the moisture content of the green body is ≤1%.

[0032] In addition, the present invention provides a zirconium-based foam ceramic filter, which is prepared by the method of preparing zirconium-based foam ceramic filter as described in any one of the first aspects, comprising ceramic aggregate composed of zirconium silicate, zirconium oxide and alumina, including 40%-60% zirconium silicate, 20%-35% zirconium oxide, and the balance being alumina.

[0033] More specifically, the mass ratio of zirconium silicate to zirconium oxide in the raw material input is 1.2-3.0:1.

[0034] From the perspective of phase regulation mechanism, the phase equilibrium state of the ZrO2-SiO2 binary system formed by SiO2 and zirconium oxide in zirconium silicate is extremely sensitive to the component ratio: if the mass ratio is <1.2:1, i.e., when zirconium silicate is insufficient, the SiO2 content cannot form a continuous rigid skeleton, making it difficult to disperse the stress generated by the zirconium oxide phase transformation, the phase transformation suppression effect fails, and the material is still prone to cracking; if the mass ratio is >3.0:1, i.e., when zirconium silicate is excessive, the proportion of zirconium oxide reinforcing phase is insufficient, leading to a decrease in fracture toughness, and excessive zirconium silicate will cause interface defects due to lattice matching differences, reducing the material density; only in the specific range of 1.2-3.0:1 can a synergistic structure of zirconium silicate rigid skeleton, tetragonal zirconium oxide reinforcing phase, and alumina modified phase be formed: zirconium silicate blocks the phase transformation transmission between zirconium oxide grains through island silicate structure, reducing the overall volume effect; tetragonal zirconium oxide fills the skeleton gaps to play a reinforcing role; alumina is integrated into the zirconium oxide lattice to refine the grains and improve the interfacial bonding strength.

[0035] Furthermore, zirconium silicate and zirconium oxide are not simply physically mixed, but rather form a stable transition phase through interfacial reactions during sintering. The lattice parameters of this transition phase lie between the two, effectively mitigating interfacial stress differences. This synergistic interfacial effect relies on a specific ratio of the two components and a precise sintering process. If the ratio deviates from the range defined in this invention, not only will the transition phase fail to form, but performance will also deteriorate due to interfacial compatibility issues, fully demonstrating that this invention is an improvement based on phase regulation mechanisms.

[0036] More specifically, the ceramic slurry of this filter is made from the following components by weight percentage: Ceramic aggregate: 60-75%, composed of zirconium silicate, zirconium oxide, and alumina, specifically including: Zirconium silicate: 40%-60%, ceramic aggregate. Its SiO2 can form a ZrO2-SiO2 binary system with ZrO2, which can suppress the monoclinic-tetragonal phase transformation amplitude of ZrO2 and reduce the volume shrinkage rate. Moreover, it is stable at high temperature without phase transformation and can be used as a rigid skeleton to disperse sintering stress.

[0037] Zirconia: 20%-35%, reinforcing phase, using nano-sized tetragonal zirconium oxide with a particle size of 50nm-100nm, forming a synergistic reinforcing system with zirconium silicate to improve the fracture toughness of the material.

[0038] Alumina: Balance, modified phase (α-Al2O3), can form a solid solution with ZrO2, refine grains, and improve structural density.

[0039] Sintering aid: 3%-8%, composed of magnesium oxide and yttrium oxide mixed in a mass ratio of 1:2-3. Magnesium oxide inhibits abnormal grain growth, while yttrium oxide stabilizes the tetragonal ZrO2 phase and reduces the phase transformation volume effect.

[0040] Binder: 2%-5%, polyvinyl alcohol, provides molding strength to the preform, and completely decomposes without residue at high temperatures.

[0041] Dispersant: 0.5%-2%, sodium polyphosphate, which improves the dispersibility of slurry through electrostatic repulsion and avoids aggregate agglomeration.

[0042] Plasticizer: 0.3%-1.5%, glycerin, to improve the plasticity of the slurry and reduce drying shrinkage stress.

[0043] Deionized water: 15%-25%, used as a dispersion medium to adjust the viscosity of the slurry.

[0044] In a preferred embodiment, the phase composition of the filter includes tetragonal zirconium oxide, zirconium silicate and α-alumina, wherein the content of monoclinic zirconium oxide is ≤15wt% and the content of tetragonal zirconium oxide is 60-75wt%.

[0045] The zirconium-based foam ceramic filter provided by this invention comprises tetragonal zirconium oxide (60-75 wt%), zirconium silicate, and α-alumina, with a monoclinic zirconium oxide content ≤15 wt%. The high toughness of the tetragonal zirconium oxide synergistically with the rigid framework of the zirconium silicate results in a product with a flexural strength ≥8.0 MPa, a compressive strength ≥8.4 MPa, and a fracture toughness ≥1.8 MPa·m. 1 / 2 This product boasts a strength increase of over 60% compared to traditional zirconium-based foam ceramics. Through the phase transformation suppression effect of zirconium silicate, stress relief from segmented drying, and defect control from gradient sintering, the product exhibits no crack formation after 10 thermal shock cycles at 1200℃, maintaining a strength retention rate of ≥92%, thus improving the thermal shock cracking problem caused by the phase transformation volume effect in traditional products. The product's internal porosity is controlled at 30-35%, with uniform pore size distribution, effectively trapping oxide inclusions and slag from molten metal. Simultaneously, its dense interface structure and high mechanical strength ensure no chipping during filtration, achieving a filtration efficiency of ≥98%, making it suitable for filtration of molten steel at 1600℃.

[0046] Example 1

[0047] A method for preparing a zirconium-based foam ceramic filter includes the following steps: S1, by mass percentage, the ceramic aggregate consists of 40% zirconium silicate, 35% zirconium oxide, and the balance alumina. The three components are pulverized to a particle size of 8μm for zirconium silicate, 5μm for zirconium oxide, and 10μm for alumina. They are then dried in a 120℃ drying oven for 4 hours, with the moisture content controlled to be ≤0.5%. 120g of ceramic aggregate and 0.5% sodium polyphosphate as a dispersant are weighed and added to 50g of deionized water. The deionized water and dispersant are added together to a ball mill and stirred for 10 minutes to completely dissolve the dispersant. Then, the dried zirconium silicate, zirconium oxide, and alumina are added. Agate balls are used as the grinding medium with a ball-to-material ratio of 3:1, and the mixture is dispersed for 30 minutes. S1.2, by mass percentage, 2% polyvinyl alcohol as a binder, 0.3% glycerol as a plasticizer, and 3% sintering aid of the total ceramic aggregate are weighed out. The sintering aid is a mixture of magnesium oxide and yttrium oxide in a mass ratio of 1:2. The above functional aids are added to a ball mill and ball milled for 2 hours to obtain a ceramic slurry. The viscosity of the ceramic slurry is 500 mPa·s, and the sedimentation rate after standing for 24 hours is 1.2% ≤ 3%.

[0048] S2, select open-cell polyurethane foam with a pore density of 10 PPI as a template, clean it with ethanol to remove surface oil, and then dry it in an 80℃ drying oven for 2 hours; immerse the pretreated polyurethane foam template in the ceramic slurry prepared in step S1, and vacuum impregnate it for 15 minutes under a vacuum of -0.08 MPa to allow the slurry to fully penetrate into the template pores. Then, remove the excess slurry by rolling with a pressure of 0.3 MPa, and control the coating thickness of the ceramic slurry on the polyurethane foam skeleton to be 0.1 mm; The rolled polyurethane foam template was placed in a 60℃ oven and dried for 4 hours, then the temperature was raised to 70℃ and dried for another 4 hours, and finally the temperature was raised to 80℃ and dried for another 4 hours to obtain the green body; the moisture content of the green body was tested and found to be 0.6%≤1%.

[0049] S3. The dried green body is placed in an atmosphere furnace, and argon gas is introduced as an inert protective gas, with the argon gas flow rate controlled at 0.5 L / min. The temperature is increased from room temperature to 300℃ at a rate of 5℃ / h and held for 6h to remove the organic phase in the green body. The temperature is then increased to 800℃ at a rate of 20℃ / h and held for 3h to relieve thermal stress. The temperature is then increased from 800℃ to 1500℃ at a rate of 30℃ / h and held for 8h to promote grain densification. The green body is then naturally cooled to room temperature to obtain a zirconium-based foam ceramic filter.

[0050] Example 2

[0051] A method for preparing a zirconium-based foam ceramic filter includes the following steps: S1, by mass percentage, the ceramic aggregate consists of 50% zirconium silicate, 28% zirconium oxide, and the balance alumina. The three are pulverized to a particle size of 10μm for zirconium silicate, 8μm for zirconium oxide, and 12μm for alumina, and dried in a 120℃ drying oven for 4 hours, controlling the moisture content to ≤0.5%. Weigh 140g of ceramic aggregate, and simultaneously weigh sodium polyphosphate (0.8% of the total mass of ceramic aggregate) and sodium citrate (0.4% of the total mass of ceramic aggregate) as composite dispersants. Add 37.6g of deionized water, and add the deionized water and dispersant together to a ball mill. Stir for 10 minutes to completely dissolve the dispersant. Then add the dried zirconium silicate, zirconium oxide, and alumina. Use agate balls as the grinding medium, with a ball-to-material ratio of 3:1, and disperse for 45 minutes. By mass percentage, 3.5% polyvinyl alcohol as a binder, 0.9% glycerol as a plasticizer, and 5.5% sintering aid were weighed from the total ceramic aggregate. Magnesium oxide and yttrium oxide were mixed at a mass ratio of 1:2.5. The above functional aids were added to a ball mill and ball milled for 3 hours to obtain a ceramic slurry. 0.42g of γ-aminopropyltriethoxysilane was added to the ceramic slurry and the mixture was stirred at high speed for 20 minutes to modify the interface and improve the interfacial bonding force between the aggregate and the binder. The viscosity of the ceramic slurry was tested to be 650mPa·s, and the sedimentation rate after standing for 24 hours was 2.1%≤3%.

[0052] S2, select open-cell polyurethane foam with a pore density of 35 PPI as the template, clean it with ethanol to remove surface oil, and then dry it in an 80℃ drying oven for 2 hours; immerse the pretreated polyurethane foam template in the ceramic slurry modified in step S1, and vacuum impregnate it for 15 minutes at a vacuum degree of -0.08 MPa to allow the slurry to fully penetrate into the template pores. Then, use a roller with a pressure of 0.4 MPa to remove excess slurry, and control the coating thickness of the ceramic slurry on the polyurethane foam skeleton to be 0.2 mm. The rolled polyurethane foam template was placed in a 60℃ oven and dried for 4 hours, then the temperature was raised to 70℃ and dried for another 4 hours, and finally the temperature was raised to 80℃ and dried for another 4 hours to obtain the green body; the moisture content of the green body was tested and found to be 0.4%≤1%.

[0053] S3. The dried green body is placed in an atmosphere kiln, and nitrogen is introduced as an inert protective gas, with the nitrogen flow rate controlled at 0.5 L / min. The temperature is increased from room temperature to 320℃ at a rate of 8℃ / h and held for 5h to remove the organic phase in the green body. Then the temperature is increased to 900℃ at a rate of 25℃ / h and held for 2.5h to relieve thermal stress. The temperature is then increased from 900℃ to 1580℃ at a rate of 40℃ / h and held for 7h to promote grain densification. The green body is then naturally cooled to room temperature to obtain a zirconium-based foam ceramic filter.

[0054] like Figure 1 As shown, Figure 1 The left side shows the zirconium-based foam ceramic filter obtained in this embodiment, and the right side shows a common zirconium oxide filter. It can be seen that the common filter on the right has irregular pore morphology, and the local skeleton is thin and unevenly distributed. The zirconium-based foam ceramic filter on the left has nearly circular and uniformly connected pores, and the ceramic skeleton is robust and dense, with no obvious material shortage or weak areas. This reflects the preparation method of the zirconium-based foam ceramic filter in this embodiment. Through precise proportioning and gradient sintering process, the structural density and mechanical stability of the material are effectively improved.

[0055] Example 3

[0056] A method for preparing a zirconium-based foam ceramic filter includes the following steps: S1, by mass percentage, the ceramic aggregate consists of 60% zirconium silicate, 20% zirconium oxide, and the balance alumina. The three are pulverized to a particle size of 15μm for zirconium silicate, 12μm for zirconium oxide, and 14μm for alumina, and dried in a 120℃ drying oven for 4 hours, controlling the moisture content to ≤0.5%. Weigh 150g of ceramic aggregate, weigh 2% of sodium polyphosphate as a dispersant, add 30g of deionized water, and add the deionized water and dispersant together to a ball mill. Stir for 10 minutes to completely dissolve the dispersant. Then add the dried zirconium silicate, zirconium oxide, and alumina, using agate balls as the grinding medium at a ball-to-material ratio of 3:1, and disperse for 60 minutes. By mass percentage, 5% polyvinyl alcohol as a binder, 1.5% glycerol as a plasticizer, and 8% sintering aid were weighed from the total mass of the ceramic aggregate. Magnesium oxide and yttrium oxide were mixed at a mass ratio of 1:3. The above functional aids were added to a ball mill and ball milled for 4 hours to obtain a ceramic slurry. The viscosity of the ceramic slurry was tested to be 800 mPa·s, and the sedimentation rate after standing for 24 hours was 2.8% ≤ 3%.

[0057] S2, select open-cell polyurethane foam with a pore density of 60 PPI as a template, clean it with ethanol to remove surface oil, and then dry it in an 80℃ drying oven for 2 hours; immerse the pretreated polyurethane foam template in the ceramic slurry prepared in step S1, and vacuum impregnate it for 15 minutes at a vacuum degree of -0.08 MPa to allow the slurry to fully penetrate into the template pores. Then, use a roller with a pressure of 0.5 MPa to remove excess slurry, and control the coating thickness of the ceramic slurry on the polyurethane foam skeleton to be 0.3 mm; The rolled polyurethane foam template was dried in a 60℃ oven for 4 hours, then the temperature was raised to 70℃ and dried for another 4 hours, and finally the temperature was raised to 80℃ and dried for another 4 hours to obtain the green body. The moisture content of the green body was found to be 0.8%≤1%.

[0058] S3. The dried green body is placed in an atmosphere furnace, and helium is introduced as an inert protective gas, with the helium flow rate controlled at 0.5 L / min. The temperature is increased from room temperature to 350℃ at a rate of 10℃ / h and held for 4h to remove the organic phase in the green body. Then the temperature is increased to 1000℃ at a rate of 30℃ / h and held for 2h to relieve thermal stress. The temperature is then increased from 1000℃ to 1650℃ at a rate of 50℃ / h and held for 6h to promote grain densification. The green body is then naturally cooled to room temperature to obtain a zirconium-based foam ceramic filter.

[0059] Comparative Example 1

[0060] Compared with Example 2, the difference is that zirconium silicate was not added to the ceramic aggregate, and only 60% zirconium oxide and 40% alumina were used as ceramic aggregate. The rest of the preparation steps are completely the same as those in Example 2.

[0061] Comparative Example 2

[0062] Compared with Example 2, the difference is that the coke removal and heat preservation stage of 300-350℃ is not set in step S3. Instead, the temperature is directly raised from room temperature to 900℃ at a heating rate of 25℃ / h. The rest of the preparation steps are completely the same as in Example 2.

[0063] Comparative Example 3

[0064] Compared with Example 2, the difference is that the sintering aid is only magnesium oxide and no yttrium oxide is added, while the rest of the preparation steps are completely the same as in Example 2.

[0065] Comparative Example 4

[0066] Compared with Example 2, the difference is that the mass ratio of zirconium silicate to zirconium oxide is 3.5:1, while the rest of the preparation steps are completely the same as in Example 2.

[0067] Performance testing

[0068] To verify the performance of the zirconium-based foam ceramic filter prepared in this invention, the filters prepared in Examples 1-3 and Comparative Examples 1-4 were subjected to phase composition analysis, mechanical property testing, and crack resistance testing, respectively. The specific test methods and results are as follows: Phase composition analysis: X-ray diffractometer (CuKα target) was used. λ =1.5406 Å, scanning range 2θ=20°-80°) to detect the phase composition of the filter and calculate the content of monoclinic zirconium oxide and tetragonal zirconium oxide; mechanical property testing: the flexural strength and compressive strength of the filter were tested according to GB / T19654-2005 standard, and the fracture toughness was tested by hardness tester indentation method. The test environment was 25℃ and 55% humidity. The results of multiple measurements of Example 2 and Comparative Example 1 are shown in the box plot as follows. Figure 2 The test results are shown in Table 2. Crack resistance test: The crack resistance was tested by thermal shock test (room temperature to 1200℃, heat preservation for 30 min and then air cooling, 10 cycles). The surface crack condition was observed and the strength retention rate after high temperature cycling was tested simultaneously.

[0069] Table 1 Phase Composition Detection

[0070] As shown in Table 1, Examples 1-3 all exhibited phase characteristics of monoclinic zirconium oxide content ≤15wt%, tetragonal zirconium oxide content 65.7-72.8wt%, and no impurity phases. The ZrSiO4 content remained stable at 16.6-18.7wt%, which is consistent with the synergistic phase transformation suppression mechanism proposed in this invention. SiO2 in zirconium silicate reacts with zirconium oxide at the interface to form a ZrO2-SiO2 binary system. This system restricts the degree of freedom of zirconium oxide crystal structure transformation through lattice matching effect, significantly suppressing the monoclinic tetragonal phase transformation. Yttrium oxide, as a tetragonal phase stabilizer, forms a substitutional solid solution with zirconium oxide, locking the tetragonal phase structure through lattice distortion effect and reducing the phase transformation driving force. Aluminum oxide, as a grain refiner, disrupts the continuity of grain growth after incorporating into the zirconium oxide lattice, reducing the formation of coarse grains. These three components achieve precise control of phase composition from three dimensions: phase transformation suppression, structural stability, and grain regulation.

[0071] The monoclinic zirconium content in the examples was significantly lower than in all comparative examples, and there were no impurity phases, resulting in a more stable phase structure. Comparative Example 1, without the addition of zirconium silicate, failed to form the ZrO2-SiO2 binary system, leading to an unrestrained zirconium oxide phase transformation. The monoclinic phase content soared to 45.7 wt%, while the tetragonal phase content was only 48.2 wt%. Simultaneously, due to the lack of rigid support from zirconium silicate, lattice defects increased, generating 6.1 wt% impurity phases, and the phase structure became completely out of control. This result fully demonstrates the necessity of combining zirconium silicate and zirconium oxide in a specific ratio, and that the phase transformation suppression effect of zirconium silicate on zirconium oxide during sintering cannot be achieved simply by adding known materials, but rather relies on a unique synergistic mechanism.

[0072] Comparative Example 2 did not include a 300-350℃ coking stage. The residual carbon generated from the decomposition of the organic phase reacted with zirconium oxide to form a 1.3wt% ZrC impurity phase. Furthermore, the rapid temperature rise resulted in insufficient phase transformation suppression, with the monoclinic phase content increasing to 18.9wt%. Comparative Example 3 used only magnesium oxide as a sintering aid, lacking the synergistic stabilizing effect of yttrium oxide. The tetragonal zirconium oxide had insufficient thermodynamic stability, and some of it transformed into the monoclinic phase, resulting in a monoclinic phase content of 22.7wt% and a tetragonal phase content of 58.6wt%. This confirms that the stabilizing effect of yttrium oxide on the tetragonal phase structure is key to optimizing the phase composition. In Comparative Example 4, the mass ratio of zirconium silicate to zirconium oxide reached 3.5:1, exceeding the specified range of 1.2-3.0:1. The excessively high proportion of zirconium silicate led to an insufficient proportion of zirconium oxide reinforcing phase, with the ZrSiO4 content increasing to 24.5wt% and the tetragonal zirconium oxide content only reaching 59.2wt%. The phase equilibrium was disrupted, indicating that outside the specified ratio range of this invention, effective phase transformation suppression and phase stabilization cannot be achieved.

[0073] Table 2 Mechanical property tests

[0074] As shown in Table 2, the flexural strength of Examples 1-3 is ≥8MPa, the compressive strength is ≥8.4MPa, and the fracture toughness is ≥1.8MPa·m. 1 / 2 The sintering shrinkage rate is ≤7.8%, and the strength retention rate after 10 thermal shock cycles at 1200℃ is ≥91.7%, demonstrating significantly better mechanical properties and structural stability than the comparative example. Test data shows that the mechanical properties of the embodiments of the present invention are significantly improved, with the flexural strength of Example 2 reaching 8.6 MPa, a significant improvement over Comparative Example 1. The box plot of the flexural strength is shown below. Figure 2 As shown, this fully demonstrates that the rigid framework of zirconium silicate and the tetragonal zirconium oxide reinforcing phase have a synergistic structural strengthening effect. The mechanism is presumably as follows: zirconium silicate exhibits no phase transformation at high temperatures, forming a continuous rigid framework that effectively disperses internal stress during sintering, reducing structural damage caused by stress concentration; tetragonal zirconium oxide possesses excellent fracture toughness, enhancing the material's fracture resistance through crack deflection and bridging effects; after alumina refines the grains, the grain boundary area increases, increasing grain boundary slip resistance while simultaneously inhibiting porosity growth and improving structural density; the gradient sintering process, including the coke removal stage, avoids porosity and cracks caused by the rapid decomposition of the organic phase, further optimizing structural integrity.

[0075] Comparative Example 1, due to uncontrolled phase composition, exhibited a loose structure and significantly increased porosity caused by the volume effect of the zirconia phase transformation. Its flexural strength was only 5.2 MPa, compressive strength 5.9 MPa, and fracture toughness 1.0 MPa·m. 1 / 2 The sintering shrinkage rate of Comparative Example 1 was as high as 12.4%, and severe cracking occurred after thermal shock due to stress concentration, resulting in a low strength retention rate. Comparative Example 2 had multiple structural defects due to the presence of ZrC impurity phase and residual internal stress, leading to a significant decrease in mechanical properties and a cycle stability of only 78.5%. Comparative Example 3 had insufficient phase stability, resulting in abnormal grain growth and increased grain boundary defects, with both mechanical properties and cycle stability lower than those of the Example. Comparative Example 4 had a proportion that deviated from the optimal range, resulting in a reduced interface bonding area between zirconium silicate and zirconium oxide, increased interface defects, and a decrease in both flexural and compressive strength. Microcracks appeared after thermal shock, and the strength retention rate was poor, further demonstrating that if the proportion range defined by the Example is deviated from, the synergistic enhancement effect cannot be achieved.

[0076] Table 3 Cracking Resistance Test

[0077] As shown in Table 3, after 10 thermal shock cycles, Examples 1-3 showed no surface cracks, an internal porosity of 30.2-32.5%, and a high-temperature cycling strength retention rate ≥91.7%, demonstrating excellent thermal shock resistance and filtration stability. Experimental results show that the embodiments of the present invention still showed no cracks after 10 thermal shock cycles, with reasonable internal porosity, no chipping during filtration, high filtration efficiency, and crack resistance and filtration stability far exceeding the comparative examples. This indicates that zirconium silicate, through the ZrO2-SiO2 binary system, minimizes the volume change of zirconium oxide phase transformation, reducing phase transformation stress. The segmented drying process controls the moisture evaporation rate through gradient heating, avoiding the formation of humidity and temperature gradients during drying, effectively releasing drying stress. In the gradient sintering process, the coking stage slowly removes the organic phase, the preheating stage gradually alleviates thermal stress, and the sintering stage promotes structural densification. The synergistic effect of these three stages inhibits crack initiation and propagation from three levels: stress generation, stress release, and structural strengthening.

[0078] Comparative Example 1, lacking the phase transformation suppression effect of zirconium silicate, exhibited concentrated volumetric stress due to the zirconium oxide phase transformation, resulting in eight obvious cracks after thermal shock. The maximum crack width was 125 μm, the internal porosity was 45.7%, and the strength retention rate was only 45.8%. Compared to Example 1, which showed no cracking after 10 cycles, the phase transformation suppression effect achieved by the specific ratio of zirconium silicate and zirconium oxide in this invention is clearly evident. Comparative Example 2, lacking a coke removal stage, experienced rapid decomposition of the organic phase, generating a large amount of gas and forming numerous porosity defects. Simultaneously, the residual internal stress was not fully utilized. After thermal shock, four local cracks appeared in Comparative Example 3, with a maximum crack width of 83 μm and a porosity of 38.9%. Due to the unstable phase composition, some tetragonal phases in Comparative Example 3 transformed into monoclinic phases during thermal shock, generating phase transformation stress and resulting in three microcracks with a maximum crack width of 65 μm. Due to the deviation of the proportion from the optimal range, the synergistic effect between the zirconium silicate rigid skeleton and the zirconium oxide reinforcement phase was weakened, and the structural integrity decreased. Two microcracks appeared after thermal shock, with a maximum crack width of 42 μm. None of these results meet the requirements for use in high-temperature filtration scenarios.

[0079] In summary, this invention constructs a regulatory system for phase transformation suppression, structural strengthening, and stress release through the synergistic ratio of zirconium silicate-zirconium oxide-alumina, the composite stabilizing effect of magnesium oxide and yttrium oxide, and a gradient sintering process including a coke removal stage. This effectively solves the technical bottlenecks of high cracking risk and insufficient mechanical strength in traditional zirconium-based foam ceramic filters. The resulting zirconium-based foam ceramic filter is suitable for scenarios such as metal casting where high-temperature stability and structural integrity are required.

[0080] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a zirconium-based foam ceramic filter, characterized in that, Includes the following steps: S1. Ceramic aggregate is pulverized and dried, mixed with dispersant and deionized water, ball-milled, and then functional additives are added and ball-milled again to obtain ceramic slurry. The ceramic aggregate comprises 40%-60% zirconium silicate, 20%-35% zirconium oxide, and the balance alumina by mass percentage. The ceramic slurry contains 60%-75% ceramic aggregate by mass, dispersant by mass of 0.5%-2% of the total mass of ceramic aggregate, and functional additives by mass of 5.3%-14.5% of the total mass of ceramic aggregate. The functional additives include polyvinyl alcohol, glycerol, and a sintering aid composed of magnesium oxide and yttrium oxide mixed in a mass ratio of 1:2-3. S2, Immerse the polyurethane foam template in the ceramic slurry and dry it in sections to obtain the green body; S3, the green body is heated and sintered under gas protection to obtain a zirconium-based foam ceramic filter.

2. The method for preparing the zirconium-based foam ceramic filter as described in claim 1, characterized in that, Step S1 includes: S1.1, Zirconium silicate, zirconium oxide, and alumina are pulverized to a particle size of 5μm-15μm, dried at 120℃ for 4h, and the moisture content is controlled to be ≤0.5%. Deionized water and dispersant are added to a ball mill and stirred for 10min. Then, the dried zirconium silicate, zirconium oxide, and alumina are added to the ball mill and dispersed for 30min-60min. S1.2, add functional additives, and continue ball milling for 2-4 hours to obtain ceramic slurry.

3. The method for preparing the zirconium-based foam ceramic filter as described in claim 2, characterized in that, The viscosity of the ceramic slurry is 500 mPa·s-800 mPa·s; the sedimentation rate of the ceramic slurry after standing for 24 hours is ≤3%; wherein the dispersant includes sodium polyphosphate or sodium citrate.

4. The method for preparing the zirconium-based foam ceramic filter as described in claim 1, characterized in that, Step S2 includes: S2.1, a polyurethane foam template with a pore density of 10-60 PPI is selected, cleaned with ethanol, dried at 80℃, immersed in ceramic slurry, vacuum impregnated, and then rolled to remove excess slurry. S2.2, the polyurethane foam template is placed in a 60℃ oven and dried for 4 hours, then the temperature is raised to 70℃ and dried for another 4 hours, and then the temperature is raised to 80℃ and dried for another 4 hours to obtain the green body.

5. The method for preparing the zirconium-based foam ceramic filter as described in claim 4, characterized in that, The vacuum impregnation conditions are: impregnation at -0.08MPa for 15 minutes, followed by roller pressing at 0.3MPa-0.5MPa to remove excess slurry, so that the ceramic slurry coating thickness on the polyurethane foam skeleton is 0.1mm-0.3mm; the moisture content of the green body is ≤1%.

6. The method for preparing the zirconium-based foam ceramic filter as described in claim 1, characterized in that, Step S3 includes: The billet is placed in the kiln, inert gas is introduced, and the temperature is raised from room temperature to 300℃-350℃ at a rate of 5-10℃ / h, and held for 4h-6h. Then the temperature is raised to 800℃-1000℃ at a rate of 20-30℃ / h, and held for 2h-3h. Finally, the temperature is raised from 800℃-1000℃ to 1500℃-1650℃ at a rate of 30-50℃ / h, and held for 6h-8h. After cooling to room temperature, a zirconium-based foam ceramic filter is obtained.

7. The method for preparing the zirconium-based foam ceramic filter as described in claim 6, characterized in that, The flow rate of the inert gas is 0.5 L / min.

8. A zirconium-based foam ceramic filter, prepared by the method of any one of claims 1-7, characterized in that, The ceramic aggregate comprises zirconium silicate, zirconium oxide, and alumina, wherein the mass ratio of zirconium silicate to zirconium oxide in the raw material feed is 1.2-3.0:

1.

9. A zirconium-based foam ceramic filter as described in claim 8, characterized in that, The phase composition of the filter includes tetragonal zirconium oxide, zirconium silicate and α-alumina, wherein the content of monoclinic zirconium oxide is ≤15wt% and the content of tetragonal zirconium oxide is 60-75wt%.

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