Preparation method of pyrochlore-type rare earth hafnium salt high-entropy ceramics by two-step synthesis using ultra-concentrated slurry

Abstract

This invention belongs to the field of inorganic ceramic powder synthesis technology and discloses a two-step method for synthesizing pyrochlore-type rare earth hafnium salt high-entropy ceramics using an ultra-concentrated slurry. Deionized water, excess ammonia, and concentrated nitric acid are added to HfCl4 powder to obtain a hafnium nitrate solution. Rare earth nitrates and a combustion agent are then added, followed by concentration and self-propagating combustion to obtain a fluffy powder. This powder is then heat-treated to remove residual carbon and crystallize. This invention overcomes the technical bottleneck of synthesizing pure-phase pyrochlore-structured rare earth hafnium salt high-entropy ceramics at low temperatures using conventional combustion methods. Based on a synergistic strategy of ultra-concentrated slurry and two-step heat treatment, a three-dimensional nano-carbon network formed by excess combustion agent is used to achieve spatial confinement and chemical bonding of metal ions. Simultaneously, the molar fraction of free water in the slurry is controlled at a low level to construct a microenvironment that inhibits Ce and Pr valence-induced phase separation. This strategy ensures that the carbon template is no longer a residual impurity but is embedded within the powder, playing a role in spatial confinement and auxiliary crystallization.

Description

Technical Field

[0001] This invention belongs to the field of inorganic ceramic powder synthesis technology, specifically a two-step method for synthesizing pyrochlore-type rare earth hafnium salt high-entropy ceramics using ultra-concentrated slurry. Background Technology

[0002] The increasing thrust-to-weight ratio and thermal efficiency of advanced aero-turbine engines place stringent demands on the temperature resistance and thermal insulation performance of thermal barrier coatings (TBCs). Traditional coatings, such as yttrium-stabilized zirconia (YSZ), are prone to phase transformation and sintering when exposed to temperatures above 1200°C for extended periods, making them unsuitable for the thermal insulation requirements of future high thrust-to-weight ratio engines. Rare-earth hafnium salts (RE2Hf2O7) ceramics with a pyrochlore structure exhibit extremely low thermal conductivity, excellent high-temperature phase stability, and a thermal expansion coefficient matching that of the metal matrix, and are widely recognized as one of the most promising next-generation thermal barrier coating materials. High-entropy ceramics (HECs) are inorganic compound solid solutions in which multiple elements share one or more Wyckoff sites in equal or near-equal proportions. HECs possess four core effects similar to high-entropy alloys, giving them good phase stability, high hardness and strength, extremely low thermal conductivity, and excellent corrosion resistance. Currently, existing methods for preparing HECs mainly include high-energy ball milling, co-precipitation, and spray pyrolysis, which generally suffer from bottlenecks such as high energy consumption, long cycle time, poor compositional uniformity, and high equipment dependence, severely restricting their engineering applications. Therefore, how to achieve low-temperature, rapid, and controllable synthesis of rare-earth hafnium salt high-entropy ceramics has become a key technological direction that urgently needs to be broken through in the field of thermal barrier coatings.

[0003] Literature "KB Zhang, WW Li, JJ Zeng, et al., Preparation of (La 0.2 Nd 0.2 Sm 0.2 Gd 0.2 Yb 0.2"2Zr2O7 high-entropy transparent ceramic using combustion synthesized nanopowder [J]. Journal of Alloys and Compounds, 2020. 817:153328." Using glycine as a combustion agent, a fluffy sample was obtained by combustion at 300℃, followed by calcination at 1200℃. After high-energy ball milling, a high-entropy ceramic powder with uniform particle size was obtained. Patent CN106747428A discloses a one-step method for synthesizing yttrium hafnium oxide ceramic powders with different grain sizes. This process uses urea, glucose, citric acid, ethylenediaminetetraacetic acid, etc. as combustion agents to prepare yttrium hafnium oxide nanocrystalline powder with grain sizes in the range of 5~200nm.

[0004] However, existing methods for preparing high-entropy ceramics, such as co-precipitation, high-energy ball milling, and spray pyrolysis, all have significant limitations: co-precipitation involves high calcination and sintering temperatures, and the introduction of precipitants can easily lead to excessively high local concentrations, resulting in agglomeration and uneven composition of the prepared ceramic particles; high-energy ball milling requires not only high calcination and sintering temperatures and long holding times, but also prolonged ball milling during the mixing stage, consuming a large amount of energy and extending the process cycle; spray pyrolysis places extremely demanding requirements on equipment, significantly increasing production costs. The main problems and shortcomings of conventional combustion methods are: firstly, the combustion process has defects; the 300℃ combustion temperature mentioned in the literature is too low, resulting in incomplete reactions and easy retention of intermediate phases; patent CN106747428A mentions a 200℃ combustion temperature... The 800℃ combustion temperature range is too wide, making combustion behavior difficult to control and easily leading to excessive or incomplete reactions. Secondly, the subsequent processing and structural limitations are prominent. Literature requires high-temperature calcination at 1200℃ for compensation, and high-energy ball milling is necessary to refine the particle size, which easily introduces impurities and increases process complexity. The one-step combustion method of patent CN106747428A is only suitable for fluorite-type yttrium hafnium and cannot obtain the ordered pyrochlore structure required for high-entropy systems. In addition, for high-entropy rare earth hafnium salt ceramics containing variable-valence rare earth elements (such as Ce and Pr), the change in ion valence state during combustion easily leads to phase separation, making it difficult to obtain a pure pyrochlore structure. The combustion temperature and crystallization temperature are often coupled, making it impossible to simultaneously achieve both grain size and phase purity. Furthermore, the residual carbon produced by excessive combustion agent is only regarded as an impurity, and its potential template effect is not utilized. Summary of the Invention

[0005] The purpose of this invention is to provide a two-step method for synthesizing pyrochlore-type rare earth hafnium salt high-entropy ceramics using ultra-concentrated slurry, in order to solve the problems mentioned in the background art.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a two-step method for synthesizing pyrochlore-type rare earth hafnium salt high-entropy ceramics using ultra-concentrated slurry, the specific steps of which are as follows: Step 1: Take a certain mass of HfCl4 powder, slowly add an appropriate amount of deionized water to it multiple times, and then add excess ammonia water to the resulting solution to generate a white flocculent precipitate; repeatedly wash the precipitate to remove residual chloride ions, then add excess concentrated nitric acid, and heat and stir in a water bath at 60~80℃ until the reaction is complete to obtain hafnium nitrate solution. Step 2: Add five rare earth nitrates to the solution obtained in Step 1. The rare earth elements are selected from any five of La, Ce, Pr, Nd, Sm, Eu and Gd, and must contain at least one of Ce or Pr. The molar ratio of rare earth cations to hafnium ions is 1:1. Add an appropriate amount of mixed combustion agent and adjust the pH of the mixed solution to 1-5. Step 3: Force the evaporation of the solution obtained in Step 2 and monitor it in real time by mass loss method. When the free water mole fraction in the solution is controlled at 15% to 20%, a non-flowing ultra-concentrated slurry with a viscosity of 800 to 2000 mPa·s is obtained. Step 4: Place the ultra-concentrated slurry obtained in Step 3 in an environment of 400~500℃ to carry out a self-propagating combustion reaction. The combustion duration is 60s~120s, and a fluffy powder containing a carbon skeleton is obtained. Step 5: Heat-treat the fluffy powder obtained in Step 4 at 800~1200℃ for 1~3h to remove residual carbon and crystallize it, thereby obtaining a single-phase rare earth hafnium salt high-entropy ceramic with pyrochlore structure.

[0007] As a preferred technical solution of the present invention, the method for removing residual chloride ions in step one is as follows: transfer the precipitate to a centrifuge tube, add water to about two-thirds of the volume, and repeatedly centrifuge and wash 4 to 5 times; after each washing, take a small amount of supernatant, add AgNO3 solution to test for chloride ions, until no white flocculent precipitate is formed.

[0008] As a preferred embodiment of the present invention, the excess ammonia water mentioned in step one refers to the OH- ions contained in the ammonia water. - The amount of substance is Hf 4+ 4 to 5 times; the excess concentrated nitric acid refers to the NO3 contained in the concentrated nitric acid. - The mass of the substance is Hf 4+ 1.5 to 1.8 times that.

[0009] As a preferred technical solution of the present invention, after adding an appropriate amount of mixed combustion agent in step two, the mixed solution is placed at 70~90°C and heated and stirred until the solution is clear, and then the pH value of the solution is adjusted to 1~5.

[0010] As a preferred embodiment of the present invention, the combustion agent in step two is a mixture of urea and glucose, wherein the ratio of the total reduction value provided by the combustion agent to the total oxidation value provided by the oxidant is δ=1.2~1.8, and the molar ratio of urea to glucose is 5:1.

[0011] As a preferred technical solution of the present invention, the forced evaporation in step three adopts reduced pressure evaporation, with a pressure of 0.05~0.08 MPa and a temperature of 80~95℃; the free water mole fraction is determined by the following method: take an ultra-concentrated slurry sample and dry it at 100~120℃ to constant weight, measure the free water mass fraction, and then convert it into free water mole fraction by combining the molar number of each raw material.

[0012] As a preferred technical solution of the present invention, the combustion products of the self-propagating combustion reaction in step four include a three-dimensional interconnected nano-carbon network formed by the decomposition of excess combustor. This carbon network undergoes exothermic oxidation to assist crystallization during the heat treatment in step five and acts as a physical barrier to restrict grain growth, thereby controlling the particle size of the resulting powder to be 80~120nm.

[0013] As a preferred embodiment of the present invention, the five rare earth nitrate ions in step two contain at least Ce. 3+ or Pr 3+ In step three, the free water molar fraction in the ultra-concentrated slurry is controlled at 15% to 20% to suppress the valence state change of Ce / Pr ions during combustion.

[0014] The rare earth hafnium salt high-entropy ceramic powder prepared by the above method has a single pyrochlore structure, two clear superlattice diffraction peaks in the XRD pattern, a particle size of 80~120nm, and does not contain free hafnium dioxide or fluorite impurities.

[0015] The beneficial effects of this invention are as follows: 1. This invention overcomes the technical bottleneck of synthesizing pure-phase pyrochlore-structured rare-earth hafnium salt high-entropy ceramics at low temperatures using conventional combustion methods. Based on a synergistic strategy of ultra-concentrated slurry and two-step heat treatment, it utilizes a three-dimensional nano-carbon network formed by excess combustor to achieve spatial confinement and chemical bonding of metal ions. Simultaneously, the free water molar fraction in the slurry is controlled at a low level to construct a microenvironment that inhibits Ce and Pr valence-induced phase separation, so that the carbon template no longer acts as a residual impurity but is embedded inside the powder, playing a role in spatial confinement and assisting crystallization. The decoupled design of combustion and crystallization alleviates the contradiction between grain size and phase purity, exhibiting extremely high structural stability in multiple thermal cycles. The resulting powder has a uniform and fine particle size, possessing the characteristics of high-entropy ceramics with a pure pyrochlore structure, completely avoiding the drawbacks of impurities and variable-valence elements introduced by high-energy ball milling in conventional methods, which lead to the incorporation of impurities. 2. This invention significantly improves the phase purity and particle size controllability of rare earth hafnium salt high-entropy ceramic powder, while greatly reducing synthesis energy consumption and shortening the production cycle, thereby effectively reducing the preparation cost and process complexity of thermal barrier coating materials. In addition, it breaks the limitation of existing combustion methods that cannot stably prepare Ce and Pr pyrochlore high-entropy ceramics, and strongly supports the domestic application of high-end thermal barrier coating materials. Attached Figure Description

[0016] Figure 1 XRD patterns of rare earth hafnium salt high-entropy ceramics with pyrochlore structure synthesized by a two-step method using ultra-concentrated slurry. Figure 2 High-magnification SEM image of rare-earth hafnium salt high-entropy ceramic powder with calcined chlorite structure; Figure 3 XRD patterns of rare earth hafnium salt high-entropy ceramics prepared from non-concentrated slurries containing only Ce ions; Figure 4 XRD patterns of rare earth hafnium salt high-entropy ceramics were prepared by a one-step method for concentrating slurry; Figure 5 XRD patterns of rare earth hafnium salt high-entropy ceramics were prepared using a two-step method with non-concentrated slurry. Figure 6 High-magnification SEM images of rare earth hafnium salt high-entropy ceramic powder prepared by a two-step method for non-concentrated slurry. Detailed Implementation

[0017] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] Example 1: like Figure 1-2 As shown in the figure, this invention provides a two-step method for synthesizing pyrochlore-type rare earth hafnium salt high-entropy ceramics using ultra-concentrated slurry. The specific steps are as follows: Step 1: Take a certain mass of HfCl4 powder, slowly add an appropriate amount of deionized water to it multiple times, and then add excess ammonia water to the resulting solution to generate a white flocculent precipitate; repeatedly wash the precipitate to remove residual chloride ions, then add excess concentrated nitric acid, and heat and stir in a water bath at 60~80℃ until the reaction is complete to obtain hafnium nitrate solution. Step 2: Add five rare earth nitrates to the solution obtained in Step 1. The rare earth elements are selected from any five of La, Ce, Pr, Nd, Sm, Eu and Gd, and must contain at least one of Ce or Pr. The molar ratio of rare earth cations to hafnium ions is 1:1. Add an appropriate amount of mixed combustion agent and adjust the pH of the mixed solution to 1-5. Step 3: Force the evaporation of the solution obtained in Step 2 and monitor it in real time by mass loss method. When the free water mole fraction in the solution is controlled at 15% to 20%, a non-flowing ultra-concentrated slurry with a viscosity of 800 to 2000 mPa·s is obtained. Step 4: Place the ultra-concentrated slurry obtained in Step 3 in an environment of 400~500℃ to carry out a self-propagating combustion reaction. The combustion duration is 60s~120s, and a fluffy powder containing a carbon skeleton is obtained. Step 5: Heat-treat the fluffy powder obtained in Step 4 at 1000℃ for 2 hours to remove residual carbon and crystallize, thereby obtaining a single-phase rare earth hafnium salt high-entropy ceramic with a pyrochlore structure.

[0019] By employing a synergistic mechanism of spatial confinement and two-step heat treatment, a technological leap from conventional dilute solution combustion to controlled incomplete combustion is achieved. This involves utilizing the strong coordination bonds formed between glucose and rare earth and hafnium ions, which condense into a high-viscosity three-dimensional cross-linked network during dehydration. Simultaneously, the combustion temperature is controlled within a range where urea and glucose decompose to form a uniform carbon skeleton without densification or phase transformation. High-temperature crystallization is then performed, simultaneously removing carbon and inducing the ordering of pyrochlore. This decoupled design optimizes carbon template confinement and phase transformation driving forces, yielding small-particle-size, high-purity powders. Furthermore, addressing the fluorite phase tendency caused by the easily changing valence states of Ce and Pr, precise control of combustion and heat treatment parameters suppresses valence state changes, ensuring the formation of an ordered pyrochlore structure. Ultimately, this achieves the preparation of low-energy-consumption, ball-mill-free, high-purity, small-particle-size, elementally uniform rare earth hafnium salt high-entropy pyrochlore ceramic powders.

[0020] The method for removing residual chloride ions in step one is as follows: transfer the precipitate to a centrifuge tube, add water to about two-thirds of the volume, and centrifuge and wash repeatedly 4 to 5 times; after each washing, take a small amount of supernatant and add AgNO3 solution to test for chloride ions until no white flocculent precipitate is formed.

[0021] For high-entropy ceramics, trace amounts of chloride ions can hinder the uniform solid solution of multiple elements, leading to impurities. By strictly controlling the number of cleaning cycles, the success of subsequent combustion and crystallization is ensured.

[0022] In step one, the excess ammonia water refers to the OH- ions contained in the ammonia water. - The amount of substance is Hf 4+ 4 to 5 times; excessive concentrated nitric acid refers to the NO3 content in concentrated nitric acid. - The mass of the substance is Hf 4+ 1.5 to 1.8 times that.

[0023] This formulation allows for strict control of ion purity and reaction process, providing a reliable precursor for the subsequent homogeneous phase formation of high-entropy ceramics.

[0024] In step two, after adding an appropriate amount of mixed fuel, the mixed solution is placed at 70~90℃ and heated while stirring until the solution becomes clear. Then the pH value of the solution is adjusted to 1~5.

[0025] By heating and stirring simultaneously, the propellant and metal salt can be fully dissolved and evenly dispersed, avoiding uneven local concentrations.

[0026] In step two, the combustion agent is a mixture of urea and glucose. The ratio of the total reduction value provided by the combustion agent to the total oxidation value provided by the oxidant is δ=1.2~1.8, and the molar ratio of urea to glucose is 5:1.

[0027] Glucose can form extremely strong coordination bonds with rare earth ions and hafnium ions. As water is removed, glucose undergoes initial dehydration and condensation to form a high-viscosity three-dimensional cross-linked network. This network forcibly fixes five different rare earth ions in a nanoscale space, preventing them from segregating due to different diffusion rates during combustion. Glucose is no longer a simple combustion agent; it can provide a "confining" effect. Increasing the amount of urea can form a significant and controllable carbon template in the ultra-concentrated slurry, enabling the regulation of the combustion wave path and grain size.

[0028] In step three, forced evaporation is carried out under reduced pressure, with a pressure of 0.05~0.08 MPa and a temperature of 80~95℃. The free water mole fraction is determined by the following method: take an ultra-concentrated slurry sample and dry it at 100~120℃ until constant weight, measure the free water mass fraction, and then convert it into the free water mole fraction by combining the molar number of each raw material.

[0029] Reduced pressure evaporation allows for gentle and rapid dehydration, avoiding damage to the ion coordination structure caused by high temperatures; drying at 100~120℃ to constant weight and measuring free water enables precise control of the slurry concentration, ensuring the formation of ultra-concentrated slurry with the target viscosity, laying a key foundation for subsequent controlled incomplete combustion and carbon skeleton construction.

[0030] In step four, the combustion products of the self-propagating combustion reaction include a three-dimensional interconnected nano-carbon network formed by the decomposition of excess combustor. This carbon network undergoes exothermic oxidation to assist crystallization during the heat treatment process in step five and acts as a physical barrier to limit grain growth, thereby controlling the particle size of the resulting powder to be between 80 and 120 nm.

[0031] The three-dimensional carbon network reduces crystallization energy consumption and avoids grain agglomeration, allowing uniform and fine powder to be obtained without high-energy ball milling, thus solving the problems of uneven particle size and impurity introduction in traditional processes.

[0032] In step two, at least Ce is included among the five rare earth nitrate ions. 3+ or Pr 3+ In step three, the free water molar fraction in the ultra-concentrated slurry is controlled at 15% to 20% to suppress the valence state change of Ce / Pr ions during combustion.

[0033] This low free water microenvironment can isolate the oxidizing atmosphere, block the Ce / Pr ion valence state transition pathway, prevent phase separation caused by abnormal valence state, avoid the formation of fluorite impurity phases, and ensure that the system containing easily variable valence rare earth elements can also stably form a pure pyrochlore structure.

[0034] The rare earth hafnium salt high-entropy ceramic powder prepared by the above method has a single pyrochlore structure, two clear superlattice diffraction peaks in the XRD pattern, a particle size of 80~120nm, and does not contain free hafnium dioxide or fluorite impurities.

[0035] Figure 1 The XRD pattern of pyrochlore rare earth hafnium salt high-entropy ceramics prepared by self-propagating combustion reaction and heat treatment at 1000℃ for 2 h from ultra-concentrated slurry is shown. The two superlattice diffraction peaks are respectively Figure 1 The diffraction peaks (111) and (311) in the image are typical pyrochlore structure peaks, and no other impurity peaks are present.

[0036] Example 2: Verification of the effect of concentrated slurry on obtaining pure pyrochlore phase by combustion method in a system containing Ce and Pr. Step 1: Same as Step 1 in Implementation Example 1; Step 2: Modify "and contains both Ce and Pr rare earth elements" in the content of Step 2 corresponding to Example 1 to "only contains one rare earth element, either Ce or Pr". Do not modify any other statements. Step 3: Place the mixed solution obtained in Step 2 in an environment of 400~500℃ to carry out a self-propagating combustion reaction. The combustion duration is 300s~600s, and a fluffy powder containing a carbon skeleton is obtained. Step 4: Heat-treat the fluffy powder obtained in Step 3 at 1000℃ for 2 hours to remove residual carbon and crystallize, thus preparing rare earth hafnium salt high-entropy ceramics.

[0037] Figure 3 Rare earth hafnium salt high-entropy ceramics are obtained by self-propagating combustion and heat treatment of a non-concentrated slurry containing only Ce. Figure 3 It can be seen that after heat treatment at 1000℃ for 2 hours, the hafnium high-entropy ceramics exhibited a distinct fluorite structure, and no (111) and (311) characteristic diffraction peaks related to the pyrochlore structure were observed.

[0038] Note: This embodiment omits the crucial slurry concentration experiment and replaces it with a dilute solution to determine whether a dilute solution containing one of the rare earth atoms, Ce or Pr, can yield a rare earth hafnium salt high-entropy ceramic with a pyrochlore structure through combustion and subsequent heat treatment.

[0039] Example 3: One-step combustion method, where only slurry concentration and single heating cannot obtain the pyrochlore phase. Step 1: Same as Step 1 in Implementation Example 1; Step Two: The content corresponds to Step Two in Implementation Example One; Step 3: Same as Step 3 in Example 1; Step 4: Place the ultra-concentrated slurry obtained in Step 3 in an environment of 500~600℃ to carry out a self-propagating combustion reaction. The combustion duration is 300s~600s, resulting in fluffy ceramic powder.

[0040] Figure 4 XRD patterns of rare earth hafnium salt ceramic powder that has undergone only self-propagating combustion without heat treatment and is a concentrated slurry. Figure 4 It can be seen that a small amount of hafnium dioxide impurity phase exists in the obtained high-entropy ceramics, and the peak intensity of the (311) characteristic diffraction peak related to the pyrochlore structure is also relatively weak. This proves that regardless of whether the precursor is concentrated, the one-step method cannot obtain the pure pyrochlore phase.

[0041] Example 4: Concentrated slurry + two-step method to obtain pyrochlore structure at a lower heat treatment temperature, resulting in ceramic powder with smaller and more uniform grain size. Step 1: Same as Step 1 in Implementation Example 1; Step Two: The content corresponds to Step Two in Implementation Example One; Step 3: Place the mixed solution obtained in Step 2 in an environment of 400~500 ℃ to carry out a self-propagating combustion reaction. The combustion duration is 300s~600s, and a fluffy powder containing a carbon skeleton is obtained. Step 4: Heat-treat the fluffy powder obtained in Step 3 at 1000℃ for 2 hours to remove residual carbon and crystallize, thus preparing rare earth hafnium salt high-entropy ceramics.

[0042] Figure 5 XRD patterns of rare-earth hafnium salt high-entropy ceramics were obtained by heat-treating the non-concentrated slurry at 1000℃ for 2 hours using a two-step method. Figure 5 It can be seen that the high-entropy ceramic obtained by processing at this temperature exhibits a fluorite structure. Compared with the sample prepared by the same process using concentrated slurry, it requires a higher heat treatment temperature to obtain a pyrochlore structure. Figure 6 High-magnification SEM images of rare earth hafnium salt high-entropy ceramic powder were obtained by a two-step heat treatment method at 1000℃ for 2 hours on a non-concentrated slurry. Figure 6It can be seen that, compared with concentrated slurry, the ceramic powder obtained by non-concentrated slurry under the same preparation process has a larger particle size, uneven distribution, and a certain degree of agglomeration.

[0043] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0044] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A two-step method for synthesizing pyrochlore-type rare earth hafnium salt high-entropy ceramics using ultra-concentrated slurry, characterized in that, The specific steps are as follows: Step 1: Take a certain mass of HfCl4 powder, slowly add an appropriate amount of deionized water to it multiple times, and then add excess ammonia water to the resulting solution to generate a white flocculent precipitate; repeatedly wash the precipitate to remove residual chloride ions, then add excess concentrated nitric acid, and heat and stir in a water bath at 60~80℃ until the reaction is complete to obtain hafnium nitrate solution. Step 2: Add five rare earth nitrates to the solution obtained in Step 1. The rare earth elements are selected from any five of La, Ce, Pr, Nd, Sm, Eu and Gd, and must contain at least one of Ce or Pr. The molar ratio of rare earth cations to hafnium ions is 1:

1. Add an appropriate amount of mixed combustion agent and adjust the pH of the mixed solution to 1-5. Step 3: Force the evaporation of the solution obtained in Step 2 and monitor it in real time by mass loss method. When the free water mole fraction in the solution is controlled at 15% to 20%, a non-flowing ultra-concentrated slurry with a viscosity of 800 to 2000 mPa·s is obtained. Step 4: Place the ultra-concentrated slurry obtained in Step 3 in an environment of 400~500℃ to carry out a self-propagating combustion reaction. The combustion duration is 60s~120s, and a fluffy powder containing a carbon skeleton is obtained. Step 5: Heat-treat the fluffy powder obtained in Step 4 at 800~1200℃ for 1~3h to remove residual carbon and crystallize it, thereby obtaining a single-phase rare earth hafnium salt high-entropy ceramic with pyrochlore structure.

2. The method for preparing pyrochlore-type rare earth hafnium salt high-entropy ceramics using a two-step ultra-concentrated slurry synthesis according to claim 1, characterized in that: The method for removing residual chloride ions described in step one is as follows: transfer the precipitate to a centrifuge tube, add water to about two-thirds of the volume, and centrifuge and wash repeatedly 4 to 5 times; after each wash, take a small amount of supernatant and add AgNO3 solution to test for chloride ions until no white flocculent precipitate is formed.

3. The preparation method of pyrochlore-type rare earth hafnium salt high-entropy ceramics by two-step synthesis of ultra-concentrated slurry according to claim 1, characterized in that: The excess ammonia water mentioned in step one refers to the OH- ions contained in the ammonia water. - The amount of substance is Hf 4+ 4 to 5 times; the excess concentrated nitric acid refers to the NO3 contained in the concentrated nitric acid. - The mass of the substance is Hf 4+ 1.5 to 1.8 times that.

4. The preparation method of pyrochlore-type rare earth hafnium salt high-entropy ceramics by two-step synthesis of ultra-concentrated slurry according to claim 1, characterized in that: After adding an appropriate amount of mixed fuel as described in step two, place the mixed solution at 70~90℃ and heat while stirring until the solution becomes clear, and then adjust the pH value of the solution to 1~5.

5. The method for preparing pyrochlore-type rare earth hafnium salt high-entropy ceramics using a two-step ultra-concentrated slurry synthesis according to claim 1, characterized in that: The combustion agent mentioned in step two is a mixture of urea and glucose, wherein the ratio of the total reduction value provided by the combustion agent to the total oxidation value provided by the oxidant is δ=1.2~1.8, and the molar ratio of urea to glucose is 5:

1.

6. The method for preparing pyrochlore-type rare earth hafnium salt high-entropy ceramics using a two-step ultra-concentrated slurry synthesis according to claim 1, characterized in that: The forced evaporation in step three is carried out under reduced pressure, with a pressure of 0.05~0.08 MPa and a temperature of 80~95℃. The free water mole fraction is determined by the following method: take an ultra-concentrated slurry sample and dry it at 100~120℃ until constant weight, measure the free water mass fraction, and then convert it into free water mole fraction by combining the molar number of each raw material.

7. The method for preparing pyrochlore-type rare earth hafnium salt high-entropy ceramics using a two-step ultra-concentrated slurry synthesis according to claim 1, characterized in that: The combustion products of the self-propagating combustion reaction described in step four include a three-dimensional interconnected nano-carbon network formed by the decomposition of excess combustor. This carbon network undergoes exothermic oxidation to assist crystallization during the heat treatment process in step five and acts as a physical barrier to limit grain growth, thereby controlling the particle size of the resulting powder to be between 80 and 120 nm.

8. The method for preparing pyrochlore-type rare earth hafnium salt high-entropy ceramics using a two-step ultra-concentrated slurry synthesis according to claim 1, characterized in that: The five rare earth nitrate ions mentioned in step two contain at least Ce. 3+ or Pr 3+ In step three, the free water molar fraction in the ultra-concentrated slurry is controlled at 15% to 20% to suppress the valence state change of Ce / Pr ions during combustion.

9. The rare earth hafnium salt high-entropy ceramic powder prepared by the two-step synthesis of pyrochlore-type rare earth hafnium salt high-entropy ceramics using ultra-concentrated slurry according to any one of claims 1-8 is characterized in that: The powder has a single pyrochlore structure, two clear superlattice diffraction peaks in the XRD pattern, a particle size of 80~120nm, and does not contain free hafnium dioxide or fluorite impurities.

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