A highly stable magnetic liquid and its preparation method

Abstract

This invention discloses a highly stable magnetic liquid and its preparation method, belonging to the field of magnetic fluid nanomaterials related to shipbuilding. The magnetic liquid comprises a metal-doped ferrite nanoparticle core, a bilayer surface-modified layer composed of oleic acid and a silane coupling agent, and a composite carrier liquid composed of polyalphaolefin synthetic oil and perfluoropolyether with added nanodiamonds. In preparation, metal-doped ferrite nanoparticles are first obtained through co-precipitation and calcination, then modified by bilayer coating, and finally mixed with the composite carrier liquid, ball-milled, ultrasonically treated, and magnetically separated and purified. This magnetic liquid achieves a saturation magnetization of 92-95 emu / g, can operate for extended periods under high-temperature conditions, exhibits a 2-3 fold increase in thermal conductivity, and demonstrates excellent redispersibility. It solves the problems of low magnetic properties, easy failure at high temperatures, and poor dispersion stability of traditional magnetic fluids, making it suitable for extreme scenarios such as aerospace, high-end manufacturing, and ship propulsion.

Description

Technical Field

[0001] This invention belongs to the field of ship-related magnetofluid nanomaterials, specifically a highly stable magnetic liquid and its preparation method. Background Technology

[0002] Magnetic fluids (also known as magnetorheological fluids, ferrofluids, or magnetically responsive fluids) are nanocolloidal fluids that can be controlled by a magnetic field. As a novel smart material, they show broad application prospects in mechanical engineering, biomedicine, optics, and other fields. Particularly in marine engineering, magnetohydrodynamic (MHD) propulsion technology is considered a crucial development direction for next-generation marine power systems due to its advantages such as the absence of mechanical transmission components, low noise, and high energy conversion efficiency. For example, MHD thrusters generate thrust through the interaction of a magnetic field and a conductive fluid, enabling silent navigation of ships, making them particularly suitable for military vessels and high-end research vessels. However, current applications of MHD in marine engineering face numerous technical bottlenecks, severely restricting performance improvements and expansion into high-end scenarios.

[0003] First, the saturation magnetization of magnetic fluids is generally below 70 emu / g, making it difficult to meet the requirements of many complex applications. From a materials perspective, commonly used magnetic particles such as iron(III) oxide (Fe3O4) suffer from surface and quantum size effects at the nanoscale, leading to unsaturated surface atomic coordination and disordered surface magnetic moments, thus weakening the overall magnetic properties. Furthermore, impurities and defects are easily introduced during the fabrication process, further reducing their intrinsic magnetism. For example, the saturation magnetization of conventional magnetic fluids (<70 emu / g) is insufficient to meet the high-load propulsion requirements of large ships. For magnetic bearings, the low saturation magnetization limits the magnetic levitation force generated by the magnetic fluid, making it difficult to support heavy, high-speed rotating components, significantly restricting the development and application of high-load magnetic drive equipment. Moreover, in weak magnetic field environments, fluids with low magnetic properties cannot respond quickly and accurately to changes in the geomagnetic field, resulting in decreased control precision of related equipment.

[0004] Secondly, the performance of magnetic fluids deteriorates significantly when the ambient temperature exceeds 180℃. On the one hand, the organic coatings (such as oleic acid and polyvinylpyrrolidone) on the surface of magnetic particles are prone to thermal decomposition at high temperatures, causing the coatings to lose their protective effect on the particles, leading to particle aggregation and oxidation, and thus weakening the fluid's magnetism. On the other hand, the base fluid (such as mineral oil and water) will accelerate volatilization or undergo chemical degradation reactions at high temperatures. Taking water-based magnetic fluids as an example, the rapid evaporation of water at high temperatures will change the composition ratio of the fluid, causing a sharp increase in viscosity and damaging its rheological properties; while organic base fluids may undergo oxidation and polymerization reactions at high temperatures, producing colloids and precipitates, affecting the normal use of the fluid. For example, the high temperatures (often exceeding 200℃) generated during the operation of ship engines and propulsion systems can cause the magnetic fluid base fluid (such as mineral oil) to evaporate and oxidize rapidly, and even carbonize. Taking a certain type of ship magnetic fluid sealing device as an example, the evaporation loss rate of the base fluid exceeds 15% per month under operating conditions above 180℃, requiring frequent shutdowns for maintenance and seriously affecting the ship's endurance.

[0005] Furthermore, although the addition of surfactants can improve the dispersibility of magnetic particles to some extent, the problem of dispersion stability in magnetic fluids remains fundamentally unresolved. From a physical mechanism perspective, under gravity, magnetic particles gradually sink due to the density difference between themselves and the base fluid; Brownian motion causes particles to undergo continuous random movement, increasing the probability of collisions and aggregation. During long-term static storage, van der Waals forces and magnetic forces between particles promote the aggregation of small particles into large particle clusters, ultimately leading to sedimentation. For example, in biomedical targeted drug delivery applications, if particle sedimentation occurs during storage or transportation, it will result in uneven distribution of drug-loaded particles, affecting the targeted therapy effect. In magnetohydrodynamic polishing technology, poor dispersion stability leads to uneven distribution of abrasive (magnetic particles) in the polishing fluid, resulting in inconsistent polishing quality of the workpiece surface and failing to meet high-precision machining requirements. In addition, particle sedimentation can also lead to uneven spatial distribution of the fluid's magnetic response performance, affecting the stability and reliability of related equipment.

[0006] Meanwhile, the magnetization process of magnetic fluids exhibits a time delay; that is, when the magnetic field strength changes, the fluid's magnetic response cannot be synchronized in real time, resulting in a significant hysteresis. This is mainly attributed to the rotational inertia of the magnetic particles, the viscous drag within the fluid, and the interactions between particles. In a rapidly alternating magnetic field environment, the hysteresis effect causes the fluid response frequency to lag behind the magnetic field change frequency, leading to a decrease in the dynamic performance of magnetically controlled equipment. For example, in high-frequency magnetorheological dampers, magnetic response hysteresis weakens the real-time adjustment capability of the damping force, reduces the equipment's rapid vibration suppression effect, and affects its application effectiveness in fields such as automotive suspension and vibration isolation of precision instruments.

[0007] Furthermore, once magnetic particles agglomerate and settle, conventional methods are insufficient to re-disperse them uniformly. The chemical bonds or physical adsorption forces formed by agglomerated particles are strong, and simple stirring or ultrasonic treatment cannot effectively break up the agglomerate structure. In industrial production, if particle settling occurs in the magnetic fluid during storage or use, reprocessing is costly and inefficient, severely impacting production continuity. For instance, in large-scale magnetohydrodynamic polishing operations, if the polishing fluid becomes ineffective due to particle settling, re-preparing the fluid is time-consuming and labor-intensive, leading to prolonged production line downtime and increased production costs.

[0008] Finally, the viscosity of magnetic fluids is influenced by a combination of factors, including magnetic field strength, temperature, and particle concentration, making precise control difficult. In practical applications, even minor fluctuations in environmental parameters can lead to significant changes in fluid viscosity. For example, in magnetic fluid sealing devices, changes in operating conditions, such as increased temperature or fluctuations in magnetic field strength, can cause sudden increases or decreases in fluid viscosity. Increased viscosity can cause sealing components to jam, affecting normal equipment operation; decreased viscosity, on the other hand, reduces the sealing effect, leading to leakage. Furthermore, uneven particle concentration distribution can also result in inconsistent viscosity within the fluid, causing instability in equipment operation and making it difficult to meet the stringent requirements of precision equipment for fluid performance stability. These technical bottlenecks severely limit the promotion and use of magnetic fluids in high-end applications, necessitating solutions through technological innovation and material optimization. Summary of the Invention

[0009] The present invention aims to at least partially solve one of the technical problems in the related art.

[0010] This invention improves the magnetic properties (saturation magnetization, etc.), high-temperature resistance, dispersion performance, redispersibility and viscosity control of magnetic liquids through innovative design of transition metal doped particles and composite carrier liquids, and alleviates the magnetic response hysteresis phenomenon; at the same time, it provides a method for preparing highly stable magnetic liquids.

[0011] The purpose of this invention is to provide a highly stable magnetic liquid, characterized by comprising core particles, a surface modification layer, and a composite carrier liquid;

[0012] The core particles are metal-doped iron oxide nanoparticles, and the surface modification layer includes a first coating layer and a second coating layer. The first coating layer is oleic acid, and the second coating layer is KH550.

[0013] The composite carrier fluid includes polyalphaolefin synthetic oil (PAO) and perfluoropolyether (PFPE).

[0014] Furthermore, the doping metal element is at least one of cobalt, nickel, and chromium;

[0015] Furthermore, the doping amount of the metal element is 3-5%;

[0016] Furthermore, the composite carrier liquid is a mixture of polyalphaolefin synthetic oil (PAO) and perfluoropolyether (PFPE) in a mass ratio of (6-8):(2-4);

[0017] Furthermore, the composite carrier liquid also includes nanodiamond particles with a mass fraction of 0.1-1% and a particle size of 5-10 nm;

[0018] The present invention also aims to provide a method for preparing a highly stable magnetic liquid, characterized in that:

[0019] Step S1:

[0020] A mixed solution of doped metal salts, ferrous salts, and ferric salts was prepared at a mass ratio of 1:(8-12):(18-20). The mixed solution was poured into a three-necked flask and magnetically stirred at 25-80℃ under an inert atmosphere. A precipitant was slowly added dropwise while stirring continuously, and the pH of the solution was adjusted to 9-12. During the dropwise addition, a black or dark brown precipitate gradually formed. After the precipitation reaction was completed, the solution was stirred and aged at a constant temperature of 60-100℃ for 12-24 hours. The precipitate was separated, the supernatant was removed, and the precipitate was washed with deionized water and ethanol. The washed precipitate was placed in an oven and dried at 60-120℃ for 12 hours. The dried powder was calcined at 300-600℃ for 1-3 hours under an inert atmosphere to obtain metal-doped iron tetroxide nanoparticles.

[0021] Step S2:

[0022] Metal-doped iron oxide nanoparticles were added to anhydrous ethanol and ultrasonically dispersed for 10-15 minutes to form a uniform suspension. Oleic acid was added at a ratio of 5-15 mL per gram of particles. The oleic acid was mixed with the suspension and then refluxed in an oil bath at 60-80℃ with stirring to make the oleic acid uniformly coat the particle surface and form a hydrophobic protective layer.

[0023] Prepare a 1-5 wt% KH550 ethanol solution, and slowly add the KH550 solution dropwise into the oleic acid-coated suspension, controlling the addition time to 10-15 minutes; heat to 70-90℃, and continue stirring the reaction for 3-5 hours at a speed of 500-1500 r / min to obtain metal-doped iron oxide nanoparticles coated with a surface-modified layer.

[0024] Step S3:

[0025] Polyalphaolefin synthetic oil (PAO) and perfluoropolyether (PFPE) are compounded at a mass ratio of (6-8):(2-4) and stirred at room temperature for 15-30 minutes until a uniform and transparent mixture is formed. 5-10 nm nanodiamonds are weighed and added to anhydrous ethanol and ultrasonically dispersed for 10-15 minutes to form a diamond suspension with a nanodiamond mass fraction of 0.1-1%. This suspension is then slowly poured into the PAO / PFPE mixture and stirred for another 10-15 minutes to obtain the composite carrier liquid.

[0026] Step S4:

[0027] Metal-doped iron oxide nanoparticles coated with a surface-modified layer, a dispersant, and a composite carrier liquid are mixed at a mass fraction ratio of (30-35):(2-6):(58-63), poured into a ball mill jar, and zirconia balls with a diameter of 5 mm are added. The mixture is then sealed and milled for 4-8 hours. The milled mixture is then subjected to low-temperature pulsed ultrasonic treatment for 15-30 minutes. The mixture is then stirred in an oil bath at 120-160℃ for 1.5-3.5 hours at a speed of 100-300 r / min. After cooling to room temperature, magnetic separation is performed using a strong magnet. The mixture is allowed to stand for 10-15 minutes, and impurities in the supernatant are discarded to obtain a high-purity magnetic liquid.

[0028] Furthermore, the doping metal in step S1 is at least one of cobalt, nickel, and chromium.

[0029] Furthermore, in step S2, the ratio of oleic acid added is 10 mL of oleic acid per gram of granules.

[0030] Furthermore, in step S3, the diameter of the nanodiamond is one of 5nm, 8nm, or 10nm.

[0031] Furthermore, in step S4, the temperature of the low-temperature pulse ultrasonic treatment is -20℃ to 20℃, the frequency is 20-100kHz, and the duty cycle is 1:1 to 1:5.

[0032] Step S1 provides the metal ion source required for synthesizing the target product. Proportion control is crucial, determining the doping amount and the final crystal structure / magnetic properties. An inert gas atmosphere (N2 / Ar) is used for protection against ferrous ions (Fe2+). 2+In an alkaline environment, the precursor is oxidized to ferric iron, ensuring the formation of strongly magnetic Fe3O4 or doped Fe3O4 structures, rather than weakly magnetic γ-Fe2O3. Stirring and aging promote the maturation of the precursor particles, resulting in more uniform particle size, a more complete crystal structure, and improved crystallinity. The precursor is transformed into well-crystallized spinel ferrite nanoparticles. Calcination promotes crystal growth and ordered internal atomic arrangement, significantly improving saturation magnetization. Optimizing the position and valence state of the doped metal ions in the crystal lattice further enhances magnetocrystalline anisotropy. High temperature helps remove residual organic matter and hydroxyl groups, improving particle purity.

[0033] Step S1 optimizes the crystal structure and magnetic moment alignment at the molecular / atomic level through precise control of doping (Co / Ni / Cr) and calcination processes. Doping with metals (especially Co) significantly increases the magnetocrystalline anisotropy constant (K) of the material, directly leading to higher saturation magnetization (Ms). This is the core foundation for improving the magnetic response of the magnetic liquid. The combination of co-precipitation, aging, and calcination enables the synthesis of nanoparticles with relatively uniform particle size and high crystallinity. High crystallinity reduces lattice defects, further contributing to high saturation magnetization and reduced hysteresis loss. The optimized crystal structure and doping help lower the energy barrier for magnetic domain flipping, making it easier and faster for the magnetic particles to achieve magnetic moment alignment and demagnetize after field removal under an applied magnetic field, thus effectively mitigating hysteresis. The calcination process gives the particles themselves high thermal stability (far higher than the uncalcined precursor), laying part of the foundation for the high-temperature performance of the final magnetic liquid (the carrier liquid is another part).

[0034] In step S2, the carboxyl group (-COOH) of oleic acid reacts with the metal ions (Fe) on the particle surface. 3+ Co 2+Oleic acid forms carboxylate bonds through chemisorption. The long carbon chains (hydrophobic chains) of oleic acid extend outward, forming a steric hindrance layer on the particle surface, preventing direct contact between particles and imparting hydrophobicity to the particle surface. The ethoxy group (-OC2H5) in KH550 hydrolyzes to generate silanol (-Si(OH)3). The silanol group undergoes a condensation reaction with the hydroxyl or carboxyl groups remaining on the surface of the oleic acid coating, forming stable Si-OC or Si-O-Si covalent bonds. Simultaneously, the silanol groups also condense to form a siloxane network. The amino group (-NH2) at the other end of KH550 is a strongly polar / reactive group with excellent affinity and reactivity. The surface-modified coating layer formed through the above steps consists of an inner layer of chemisorbed oleic acid (providing basic hydrophobicity and steric hindrance) and an outer layer of a siloxane network linked by chemical bonds and exposed amino groups (providing stronger steric hindrance and compatibility with the carrier liquid). The double coating provides a dual protection mechanism. The resulting surface-modified coating significantly enhances steric hindrance through a double-layer coating. The oleic acid layer provides the first layer of steric hindrance, while the siloxane layer formed by KH550 further thickens the coating, greatly increasing the steric repulsion between particles. The double-layer coating also significantly reduces interfacial tension and improves compatibility. The amino groups (-NH2) of KH550 can form hydrogen bonds or van der Waals forces with the composite carrier liquid (especially PAO), significantly improving the compatibility / affinity between the particle surface and the carrier liquid molecules, and reducing interfacial energy. This allows the particles to suspend more stably in the carrier liquid, significantly extending storage stability and reducing hard sedimentation.

[0035] Compared to traditional single-layer coatings (oleic acid only), which may be difficult to redisperse after particle settling due to close contact caused by van der Waals forces, KH550 provides a chemically bonded layer and stronger steric hindrance, ensuring that settled particles maintain a certain distance and repulsive force. This allows for rapid and complete redispersement with only gentle stirring or short-term ultrasonication. This is crucial for long-term use and reliability. Oleic acid may desorb at high temperatures. The covalently bonded siloxane layer formed by KH550 possesses extremely high thermal and chemical stability, maintaining the integrity of the coating layer at high temperatures (far exceeding the oleic acid desorption temperature), preventing particle sintering and agglomeration. This is one of the core innovations ensuring the high-temperature stability of magnetic liquids. Good dispersibility and particle-liquid compatibility contribute to a more homogeneous system, reducing localized high-viscosity areas caused by agglomeration, resulting in more controllable and stable overall viscosity.

[0036] In step S3, PAO (polyalphaolefin synthetic oil) provides excellent low-temperature fluidity, a high viscosity index (viscosity changes little with temperature), good lubricity, low volatility, and relatively low cost, forming the base carrier fluid. PFPE (perfluoropolyether), as the core high-temperature component, provides extreme thermal stability (decomposition temperature >400°C), extremely low evaporation (virtually non-volatile), excellent chemical inertness (resistant to strong acids and alkalis), a wide liquid range (extremely low pour point), and non-flammability. Room temperature stirring ensures thorough and uniform mixing of the two oils with different viscosities, forming a homogeneous and transparent solution (indicating good compatibility). The addition of nanodiamonds (0.1-1wt%, 5-10nm) significantly improves the thermal conductivity and wear resistance of the system; the uniformly dispersed nanodiamonds (high thermal conductivity ~2000W / mK) form additional highly efficient thermal conduction pathways in the carrier fluid. This significantly improves the overall thermal conductivity of magnetic fluids, facilitating the rapid dissipation of Joule heat or frictional heat generated in magnetic fluid applications (such as seals and dampers), preventing localized overheating, and enhancing system reliability and power density. The high hardness of the nanodiamond particles allows them to act as micro-bearings and polishes when the magnetic fluid flows through friction pairs such as sealing gaps, reducing friction and wear and extending the service life of components such as seals.

[0037] The introduction of PFPE breaks through the thermal stability limits of traditional mineral oil, silicone oil, or ester oil carrier fluids. This allows the magnetic fluid to operate stably for extended periods in high-temperature environments >300°C and even approaching 400°C, without issues such as carrier fluid decomposition, carbonization, significant evaporation, or a dramatic increase in viscosity, solving the core problem of traditional magnetic fluid failure at high temperatures. The presence of PAO ensures good fluidity at room temperature. The high viscosity index of PAO and the excellent viscosity-temperature characteristics of PFPE itself allow the composite carrier fluid to maintain relatively stable viscosity over a very wide temperature range (e.g., -50°C to >300°C), ensuring the predictability and reliability of the magnetic fluid's performance under different operating conditions. Simultaneously, the extremely low vapor pressure of PFPE and the low volatility of PAO significantly reduce evaporation losses during use, extending the service life and maintenance cycle of the magnetic fluid, making it particularly suitable for vacuum or closed systems. PFPE possesses exceptional chemical inertness, resisting the erosion of most chemicals (acids, alkalis, oxidants, solvents). PAO also exhibits good oxidation resistance. This significantly expands the application environment of magnetic fluids, enabling them to operate stably in harsh chemical environments.

[0038] In step S4, the powerful mechanical dispersion combination of ball milling and ultrasound, combined with the subsequent high-temperature stirring and chemical bonding enhancement process, ensures that the modified particles and nanodiamonds achieve near-atomic-level uniform dispersion and stable anchoring in the composite carrier liquid. This is a key process guarantee for obtaining high-performance, long-life products. Magnetic separation, as a purification step, can efficiently remove all non-target impurities such as large agglomerates, grinding ball debris, and unreacted substances. These impurities are the root cause of deterioration in magnetic fluid performance (such as clogging of flow channels, increased friction, reduced thermal conductivity, and accelerated aging). By precisely controlling the particle concentration (30-35%) and dispersant dosage (2-6%), the viscosity of the final magnetic fluid can be finely adjusted while maintaining high magnetic properties to meet the needs of different application scenarios. Through powerful dispersion and purification processes, the final product is ensured to have ultra-high uniformity, purity, colloidal stability, and controllable viscosity.

[0039] Low-temperature pulsed ultrasound treatment of magnetic fluids is a special treatment method that combines a low-temperature environment with pulsed ultrasound technology. The low-temperature environment reduces the thermal kinetic energy of particles, decreasing the tendency for aggregation caused by random collisions. The cavitation effect generated by pulsed ultrasound can produce instantaneous high pressure and strong shear force locally, dispersing existing agglomerates. The combination of these two factors effectively maintains the uniform dispersion of particles. Low temperature reduces the evaporation rate of the solvent, preventing particle concentration increases and aggregation due to solvent reduction. At the same time, low temperature can prevent phase changes in the solvent system (such as solidification or vaporization), maintaining the liquid-phase stability of the magnetic fluid. Low-temperature treatment can affect the lattice structure and magnetic moment arrangement of magnetic particles, while the mechanical action of pulsed ultrasound can introduce local stress, changing the direction or intensity of magnetic anisotropy of the particles, thereby optimizing parameters such as saturation magnetization and coercivity of the magnetic fluid. By refining particle size (ultrasonic dispersion effect) and improving the particle surface coating (more controllable surface reaction at low temperature), the magnetic relaxation time of magnetic particles can be reduced, making the response of the magnetic fluid under the action of an external magnetic field more rapid. Cavitation impact from pulsed ultrasound can enhance the bonding force between the modifier and the particle surface, forming a more stable coating layer and reducing direct contact and agglomeration between particles. The synergistic effect of cryogenics and ultrasound can alter the interfacial tension between magnetic fluids and substrates or other media, improving their wettability on specific surfaces. This is crucial for the application of magnetic fluids in coatings, sensors, and other fields.

[0040] Through the combined effect of the above steps, a synergistic effect is generated, which makes the final magnetic liquid far surpass the level of traditional magnetic liquids in terms of magnetic properties, high temperature resistance limit, long-term dispersion stability, redispersion convenience, viscosity controllability, thermal conductivity, wear resistance, and chemical stability. It is particularly suitable for extreme or high-requirement applications such as shipbuilding, aerospace, high-end manufacturing, vacuum equipment, high-temperature reactors, and high-power electronic heat dissipation.

[0041] Beneficial effects:

[0042] 1. Through optimization of metal doping with cobalt, nickel, and other metals and calcination processes, the magnetic moments in the crystal structure are more ordered, magnetocrystalline anisotropy is enhanced, and the saturation magnetization is significantly improved compared to traditional magnetic liquids, alleviating the magnetic response hysteresis phenomenon. The high crystallinity and optimized magnetic domain structure lower the domain flipping energy barrier, allowing for faster magnetic moment alignment of magnetic particles under varying applied magnetic fields, significantly reducing the hysteresis effect. This results in an increased response frequency and improved dynamic control precision in high-frequency magnetorheological dampers.

[0043] 2. The PFPE carrier fluid and the KH550 siloxane coating layer work together to enable the magnetic fluid to operate for a long time at temperatures above 300°C, solving the problem of failure at temperatures above 180°C in traditional systems.

[0044] 3. The double-layer coating (oleic acid + silane coupling agent) and ball milling-ultrasonic dispersion process inhibit particle sedimentation, significantly improve the uniformity maintenance time, and facilitate redispersibility. The composite carrier liquid (PAO / PFPE) combined with nanodiamond reduces viscosity fluctuation by 40%, increases thermal conductivity by 2-3 times, and enhances wear resistance and chemical inertness. Attached Figure Description

[0045] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments 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.

[0046] Figure 1 This is a schematic diagram of the structure of the highly stable magnetic liquid in this application.

[0047] 1. Core particle; 2. First coating layer; 3. Second coating layer; 4. Composite carrier liquid; 5. Nanodiamond; Detailed Implementation

[0048] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0049] With regard to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0050] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0051] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0052] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0053] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0054] Example 1

[0055] A method for preparing a highly stable magnetic liquid includes the following steps:

[0056] Step S1:

[0057] A mixed solution of cobalt chloride (CoCl2·6H2O), ferrous chloride (FeCl2·4H2O), and ferric chloride (FeCl3·6H2O) was prepared at a mass ratio of 1:10:19. The solution was poured into a three-necked flask and magnetically stirred at 50°C under an inert atmosphere. A precipitant was slowly added dropwise while stirring continuously, and the pH of the solution was adjusted to 10. During the addition, a black or dark brown precipitate gradually formed. After the precipitation reaction was complete, the solution was aged at a constant temperature of 80°C for 12 hours. The precipitate was separated, the supernatant was removed, and the precipitate was washed with deionized water and ethanol. The washed precipitate was placed in an oven and dried at 80°C for 12 hours. The dried powder was calcined at 450°C for 2 hours under an inert atmosphere to obtain metal-doped iron oxide nanoparticles with a particle size of 12 nm and a doping amount of 4 wt%.

[0058] Step S2:

[0059] Metal-doped iron oxide nanoparticles were added to anhydrous ethanol and ultrasonically dispersed for 15 minutes to form a uniform suspension. Oleic acid was added to the suspension at a ratio of 10 mL per gram of particles. The mixture was then refluxed in an oil bath at 80°C for 2 hours and stirred to allow the oleic acid to uniformly coat the particle surface and form a hydrophobic protective layer.

[0060] Prepare a 3wt% KH550 ethanol solution, and slowly add the KH550 solution dropwise into the oleic acid-coated suspension, controlling the addition time to 15 minutes; heat to 80℃, and continue stirring for 4 hours at a speed of 1000 r / min to obtain metal-doped iron oxide nanoparticles coated with a surface-modified layer.

[0061] Step S3:

[0062] Polyalphaolefin synthetic oil (PAO) and perfluoropolyether (PFPE) were compounded at a mass ratio of 7:3 and stirred at room temperature for 15-30 minutes until a uniform and transparent mixture was formed. 8 nm nanodiamonds were weighed and ultrasonically dispersed in anhydrous ethanol for 10-15 minutes to form a diamond suspension with a nanodiamond mass fraction of 0.5%. This suspension was then slowly poured into the PAO / PFPE mixture and stirred for another 10-15 minutes to obtain the composite carrier liquid.

[0063] Step S4:

[0064] Metal-doped iron oxide nanoparticles coated with a surface-modified layer, a dispersant, and a composite carrier liquid were mixed at a mass ratio of 32:4:62. The mixture was poured into a ball mill jar with 5 mm diameter zirconia balls and sealed for ball milling for 6 hours. The milled mixture was then subjected to low-temperature pulsed ultrasonic treatment for 25 minutes. The mixture was then stirred in a 140℃ oil bath for 2.5 hours at a speed of 200 r / min. After cooling to room temperature, magnetic separation was performed using a strong magnet. After standing for 10-15 minutes, impurities in the supernatant were discarded to obtain a high-purity magnetic liquid.

[0065] Example 2

[0066] Step S1:

[0067] Nickel sulfate (NiSO4) and chromium chloride (CrCl3·6H2O) were mixed at a mass ratio of 1:1. A mixed solution of nickel-chromium salt, ferrous chloride (FeCl2·4H2O), and ferric chloride (FeCl3·6H2O) was prepared at a mass ratio of 1:12:18. The mixed solution was poured into a three-necked flask and magnetically stirred at 50°C under an inert atmosphere. A precipitant was slowly added dropwise while stirring continuously, and the pH of the solution was adjusted to 10. During the addition process, a black or dark brown precipitate gradually formed. After the precipitation reaction was complete, the solution was stirred and aged at a constant temperature of 80°C for 12 hours. The precipitate was separated, the supernatant was removed, and the precipitate was washed with deionized water and ethanol. The washed precipitate was placed in an oven and dried at 80°C for 12 hours. The dried powder was calcined at 500°C for 1.5 hours under an inert atmosphere to obtain metal-doped iron oxide nanoparticles with a particle size of 12 nm.

[0068] Step S2:

[0069] Metal-doped iron oxide nanoparticles were added to anhydrous ethanol and ultrasonically dispersed for 15 minutes to form a uniform suspension. Oleic acid was added to the suspension at a ratio of 10 mL per gram of particles. The mixture was then refluxed in an oil bath at 80°C for 2 hours with stirring to allow the oleic acid to uniformly coat the particle surface and form a hydrophobic protective layer.

[0070] Prepare a 3wt% KH550 ethanol solution, and slowly add the KH550 solution dropwise into the oleic acid-coated suspension, controlling the addition time to 15 minutes; heat to 80℃, and continue stirring for 4 hours at a speed of 1000 r / min to obtain metal-doped iron oxide nanoparticles coated with a surface-modified layer.

[0071] Step S3:

[0072] Polyalphaolefin synthetic oil (PAO) and perfluoropolyether (PFPE) were compounded at a mass ratio of 8:2 and stirred at room temperature for 15-30 minutes until a uniform and transparent mixture was formed. 5 nm nanodiamond particles were weighed and added to anhydrous ethanol and ultrasonically dispersed for 10-15 minutes to form a diamond suspension with a nanodiamond mass fraction of 0.3%. This suspension was then slowly poured into the PAO / PFPE mixture and stirred for another 10-15 minutes to obtain the composite carrier liquid.

[0073] Step S4:

[0074] Metal-doped iron oxide nanoparticles coated with a surface-modified layer, a dispersant, and a composite carrier liquid were mixed at a mass ratio of 35:4:62. The mixture was poured into a ball mill jar with 5 mm diameter zirconia balls and sealed for 8 hours of ball milling. The milled mixture was then subjected to low-temperature pulsed ultrasonic treatment for 30 minutes. The mixture was then stirred in a 150℃ oil bath for 3 hours at a speed of 250 r / min. After cooling to room temperature, magnetic separation was performed using a strong magnet. After standing for 10-15 minutes, impurities in the supernatant were discarded to obtain a high-purity magnetic liquid.

[0075] Example 3

[0076] A method for preparing a highly stable magnetic liquid includes the following steps:

[0077] Step S1:

[0078] A mixed solution of cobalt chloride (CoCl2·6H2O), ferrous chloride (FeCl2·4H2O), and ferric chloride (FeCl3·6H2O) was prepared at a mass ratio of 1:10:19. The solution was poured into a three-necked flask and magnetically stirred at 50°C under an inert atmosphere. A precipitant was slowly added dropwise while stirring continuously, and the pH of the solution was adjusted to 10. During the addition, a black or dark brown precipitate gradually formed. After the precipitation reaction was complete, the solution was stirred and aged at a constant temperature of 80°C for 12 hours. The precipitate was separated, the supernatant was removed, and the precipitate was washed with deionized water and ethanol. The washed precipitate was placed in an oven and dried at 80°C for 12 hours. The dried powder was calcined at 600°C for 2 hours under an inert atmosphere to obtain metal-doped iron oxide nanoparticles with a particle size of 12 nm and a doping amount of 5 wt%.

[0079] Step S2:

[0080] Metal-doped iron oxide nanoparticles were added to anhydrous ethanol and ultrasonically dispersed for 15 minutes to form a uniform suspension. Oleic acid was added to the suspension at a ratio of 10 mL per gram of particles. The mixture was then refluxed in an oil bath at 80°C for 2 hours and stirred to allow the oleic acid to uniformly coat the particle surface and form a hydrophobic protective layer.

[0081] Prepare a 5wt% KH550 ethanol solution, and slowly add the KH550 solution dropwise into the oleic acid-coated suspension, controlling the addition time to 15 minutes; heat to 80℃, and continue stirring for 4 hours at a speed of 1000 r / min to obtain metal-doped iron oxide nanoparticles coated with a surface-modified layer.

[0082] Step S3:

[0083] Polyalphaolefin synthetic oil (PAO) and perfluoropolyether (PFPE) were compounded at a mass ratio of 6:4 and stirred at room temperature for 15-30 minutes until a uniform and transparent mixture was formed. 8 nm nanodiamonds were weighed and ultrasonically dispersed in anhydrous ethanol for 10-15 minutes to form a diamond suspension with a nanodiamond mass fraction of 1%. This suspension was then slowly poured into the PAO / PFPE mixture and stirred for another 10-15 minutes to obtain the composite carrier liquid.

[0084] Step S4:

[0085] Metal-doped iron oxide nanoparticles coated with a surface-modified layer, a dispersant, and a composite carrier liquid were mixed at a mass ratio of 32:4:62. The mixture was poured into a ball mill jar with 5 mm diameter zirconia balls and sealed for ball milling for 6 hours. The milled mixture was then subjected to low-temperature pulsed ultrasonic treatment for 25 minutes. The mixture was then stirred in a 140℃ oil bath for 2.5 hours at a speed of 200 r / min. After cooling to room temperature, magnetic separation was performed using a strong magnet. After standing for 10-15 minutes, impurities in the supernatant were discarded to obtain a high-purity magnetic liquid.

[0086] Comparative Example 1:

[0087] Unlike Example 1, ferrous chloride (FeCl2·4H2O) and ferric chloride (FeCl3·6H2O) were prepared into a mixed metal salt solution. The mixed metal salt solution was poured into a three-necked flask and magnetically stirred at 50°C under an inert atmosphere. A precipitant was slowly added dropwise while stirring continuously to adjust the pH of the solution to 10. During the dropwise addition, a black or dark brown precipitate gradually formed. After the precipitation reaction was completed, the solution was stirred and aged at a constant temperature of 80°C for 12 hours. The precipitate was separated, the supernatant was removed, and the precipitate was washed with deionized water and ethanol. The washed precipitate was placed in an oven and dried at 80°C for 12 hours. The dried powder was calcined at 450°C for 2 hours under an inert atmosphere to obtain undoped iron(III) oxide nanoparticles.

[0088] Comparative Example 2:

[0089] Unlike Example 1, in step S2: metal-doped iron oxide nanoparticles are added to anhydrous ethanol and ultrasonically dispersed for 15 minutes to form a uniform suspension. Oleic acid is added to the suspension at a ratio of 10 mL per gram of particles. The mixture is then refluxed in an oil bath at 80°C for 2 hours with stirring to allow the oleic acid to uniformly coat the particle surface and form a hydrophobic protective layer. Metal-doped iron oxide nanoparticles with a surface-modified layer are obtained.

[0090] Comparative Example 3:

[0091] Unlike Example 1, in step S3: polyalphaolefin synthetic oil (PAO) and dimethyl silicone oil are compounded at a mass ratio of 7:3 and stirred at room temperature for 15-30 minutes until a uniform and transparent mixture is formed; thus, a composite carrier liquid is obtained.

[0092] The magnetic fluids obtained in Examples 1-3 and Comparative Examples 1-3 were subjected to performance tests.

[0093] Referring to ASTM A894 "Test Method for Saturation Magnetization of Ferrite Permanent Magnets" and IEC 60404-5 "Methods for Measurement of Magnetic Properties of Soft Magnetic Materials", a vibrating sample magnetometer (VSM) was used to measure the magnetic field strength at room temperature, with a range of ±20 kOe. The sample was a dry powder (product of step S1) to eliminate the dilution effect of the carrier liquid. A complete hysteresis loop was plotted using the VSM, and the coercivity (Hc) and remanence ratio (Mr / Ms) were calculated to evaluate the magnetic response hysteresis. A rotational rheometer was used to measure the viscosity change in the range of -40℃ to 300℃ at a heating rate of 10℃ / min, and the viscosity index (VI) was calculated. A 10g sample of magnetic liquid was placed in a constant-weight crucible and tested at 250℃ ± 2℃ for 100 hours (forced convection oven), and the evaporation loss was calculated. After aging at 250℃ for 100h, the magnetic properties (Ms retention rate), viscosity change rate, and dispersion state (particle size analysis) of the samples were retested. The samples were then placed in a 200℃ oven for 7 days, and the particle aggregation state was observed using transmission electron microscopy (TEM) and dynamic light scattering (DLS). The thermal conductivity of the magnetic liquid containing nanodiamonds was measured using the transient hot-wire method (room temperature to 200℃). Sealing materials (such as fluororubber) were immersed in the magnetic liquid (150℃ / 168h), and their volume expansion rate and hardness change were tested. The sedimentation stability of the samples was tested by vertically placing them in standard sedimentation tubes (Φ25×150mm) at 25℃ and 60℃ for 30 days.

[0094] The test results are shown in Table 1 below.

[0095] Table 1

[0096]

[0097] The experimental results in Table 1 show that using cobalt, nickel, and chromium-doped Fe3O4 nanoparticles with a doping concentration of 3-5% is effective. This range improves magnetocrystalline anisotropy through lattice optimization while avoiding crystal structure distortion due to excessive doping. Regarding material compatibility, transition metal ions exhibit high lattice matching with Fe3O4. In the examples, the doped particles prepared via co-precipitation-calcination achieved a saturation magnetization of 92-95 emu / g, significantly higher than the undoped sample (78 emu / g). The bilayer coating structure of oleic acid and silane coupling agent (KH550) functionalizes the particle surface through chemical adsorption (carboxylate bonds) and covalent bonds (Si-OC / Si-O-Si). Experimental data show that the oleic acid monolayer-coated sample exhibited a sedimentation rate >40% at 200℃ / 7d, while the bilayer-coated sample showed a sedimentation rate <5% and excellent redispersibility, verifying the effectiveness of bilayer coating in inhibiting particle aggregation. Single-layer oleic acid coatings suffer from severe aggregation (sedimentation >40%) and desorption (evaporation loss) at high temperatures, while the KH550 layer provides covalent anchoring, improving high-temperature dispersion stability by more than 10 times. PAO and PFPE are blended in a 7:3 mass ratio, combined with 0.5% nanodiamond particles, to form a stable system over a wide temperature range. PAO provides low-temperature fluidity (pour point -50℃), PFPE ensures high-temperature stability (decomposition temperature >400℃), and nanodiamonds (thermal conductivity ~2000W / mK) construct thermal conductivity pathways. Performance tests show that the evaporation loss of this carrier liquid at 250℃ / 100h is only 0.5-1.2%, significantly lower than that of silicone oil-based carrier liquids (23%). Silicone oil completely decomposes and fails at 250℃, while the PFPE-based carrier liquid maintains ultra-low evaporation (<1%) and viscosity stability, demonstrating its irreplaceable role in extreme high-temperature scenarios.

[0098] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0099] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0100] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A highly stable magnetic liquid, characterized in that: It includes core particles, a surface modification layer, and a composite carrier liquid; The core particles are metal-doped iron oxide nanoparticles, and the surface modification layer includes a first coating layer and a second coating layer. The first coating layer is oleic acid, and the second coating layer is KH550. The composite carrier fluid includes polyalphaolefin synthetic oil (PAO) and perfluoropolyether (PFPE).

2. The highly stable magnetic liquid according to claim 1, characterized in that: The composite carrier fluid is a mixture of polyalphaolefin synthetic oil (PAO) and perfluoropolyether (PFPE) in a mass ratio of (6-8):(2-4).

3. The highly stable magnetic liquid according to claim 1, characterized in that: The metals are cobalt, nickel, and chromium, with a doping level of 3-5%.

4. The highly stable magnetic liquid according to claim 1, characterized in that: The composite carrier liquid also includes nanodiamond particles, with a mass fraction of 0.1-1% and a particle size of 5-10 nm.

5. A method for preparing a highly stable magnetic liquid, characterized in that: Step S1: A mixed solution of doped metal salts, ferrous salts, and ferric salts was prepared at a mass ratio of 1:(8-12):(18-20). The mixed solution was poured into a three-necked flask and magnetically stirred at 25-80℃ under an inert atmosphere. A precipitant was slowly added dropwise while stirring continuously, and the pH of the solution was adjusted to 9-12. During the dropwise addition, a black or dark brown precipitate gradually formed. After the precipitation reaction was completed, the solution was stirred and aged at a constant temperature of 60-100℃ for 12-24 hours. The precipitate was separated, the supernatant was removed, and the precipitate was washed with deionized water and ethanol. The washed precipitate was placed in an oven and dried at 60-120℃ for 12 hours. The dried powder was calcined at 300-600℃ for 1-3 hours under an inert atmosphere to obtain metal-doped iron tetroxide nanoparticles. Step S2: Metal-doped iron oxide nanoparticles were added to anhydrous ethanol and ultrasonically dispersed for 10-15 minutes to form a uniform suspension. Oleic acid was added at a ratio of 5-15 mL per gram of particles. The oleic acid was mixed with the suspension and then refluxed in an oil bath at 60-80°C with stirring to make the oleic acid uniformly coat the particle surface and form a hydrophobic protective layer. Prepare a 1-5 wt% KH550 ethanol solution, and slowly add the KH550 solution dropwise into the oleic acid-coated suspension over a period of 10-15 minutes. Heat to 70-90℃ and continue stirring for 3-5 hours at a speed of 500-1500 r / min to obtain metal-doped iron oxide nanoparticles coated with a surface-modified layer. Step S3: Polyalphaolefin synthetic oil (PAO) and perfluoropolyether (PFPE) are compounded at a mass ratio of (6-8):(2-4) and stirred at room temperature for 15-30 minutes until a uniform and transparent mixture is formed. 5-10 nm nanodiamonds are weighed and added to anhydrous ethanol and ultrasonically dispersed for 10-15 minutes to form a diamond suspension with a nanodiamond mass fraction of 0.1-1%. This suspension is then slowly poured into the PAO / PFPE mixture and stirred for another 10-15 minutes to obtain the composite carrier liquid. Step S4: Metal-doped iron oxide nanoparticles coated with a surface-modified layer, a dispersant, and a composite carrier liquid are mixed at a mass fraction ratio of (30-35):(2-6):(58-63), poured into a ball mill jar, and zirconia balls with a diameter of 5 mm are added. The mixture is then sealed and milled for 4-8 hours. The milled mixture is then subjected to low-temperature pulsed ultrasonic treatment for 15-30 minutes. The mixture is then stirred in an oil bath at 120-160℃ for 1.5-3.5 hours at a speed of 100-300 r / min. After cooling to room temperature, magnetic separation is performed using a strong magnet. The mixture is allowed to stand for 10-15 minutes, and impurities in the supernatant are discarded to obtain a high-purity magnetic liquid.

6. The method for preparing a highly stable magnetic liquid according to claim 5, characterized in that: The doping metal in step S1 is at least one of cobalt, nickel, and chromium.

7. The method for preparing a highly stable magnetic liquid according to claim 5, characterized in that: The ratio of oleic acid added in step S2 is 10 mL of oleic acid per gram of granules.

8. The method for preparing a highly stable magnetic liquid according to claim 5, characterized in that: In step S3, the diameter of the nanodiamond is one of 5nm, 8nm, or 10nm.

9. The method for preparing a highly stable magnetic liquid according to claim 5, characterized in that: In step S4, the temperature of the low-temperature pulse ultrasonic treatment is -20℃ to 20℃, the frequency is 20-100kHz, and the duty cycle is 1:1 to 1:5.

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