KR101187759B1 - Magnetic nanofluids, manufacturing method and using method thereof - Google Patents

Abstract

The present invention relates to a method for producing a magnetic nanofluid to improve the heat transfer characteristics of various high viscosity liquid heat transfer media using magnetic nanopowder and alternating magnetic field. Disclosed is a step of putting a dispersant into a high viscosity liquid solvent and stirring, putting a magnetic nanoparticle into the liquid solvent to which the dispersant is added and stirring, and dispersing the stirred nanofluid with a high energy dispersing apparatus and Modifying the surface. When the alternating magnetic field is applied to the magnetic nanofluid manufactured through the above process, the thermal conductivity is improved, and thus the viscosity is relatively high, such as oil, so that the thermal properties of the sample itself are poor, and the thermal conductivity and heat transfer characteristics of the nanoparticles are added. It can be utilized to greatly improve the thermal characteristics in the low-effective liquid sample.

Description

Magnetic nanofluid, manufacturing method and method of use {MAGNETIC NANOFLUIDS, MANUFACTURING METHOD AND USING METHOD THEREOF}

The present invention relates to a magnetic nanofluid, a method of manufacturing the same, and a method of using the same, and in particular, to a magnetic nanofluid that improves heat transfer characteristics of various high viscosity liquid heat transfer media using magnetic nanopowders and alternating magnetic fields, and a method and a method of using the same. will be.

Nanofluid is a kind of solid and liquid mixed fluid in which a small amount of nano powder is uniformly and stably dispersed in various liquid cooling and lubricants such as water, ethylene glycol and engine oil. These nanofluids are attracting great attention worldwide as the next generation cooling and lubricating media that have much higher thermal conductivity than pure fluids due to endless irregular brown motion in the fluid.

However, the effect of improving the heat transfer characteristics of the liquid medium by the addition of nanopowder is characterized by disappearing when the viscosity of the fluid rises. This is interpreted as the Brownian movement of nanopowders in high viscosity fluids is greatly limited.

Therefore, in the low viscosity organic solvents such as water and ethylene glycol, high thermal conductivity and heat transfer characteristics are improved by adding nanopowders, but when applied to a higher viscosity heat transfer medium, there is no obvious thermal effect.

In fact, since oil has very low heat transfer characteristics compared to refrigerants such as water or ethylene glycol, an improvement effect of heat transfer characteristics by adding nano powder is required. Therefore, a special method for increasing the effect of adding nanopowders to a high viscosity liquid medium such as oil is required.

An object of the present invention is to solve the above problems, to provide a magnetic nanofluid, a manufacturing method and a method of using the nanofluid prepared for a high viscosity liquid solvent to improve the thermal conductivity and heat transfer characteristics. have.

In order to achieve the above object, the present invention provides a method for preparing a magnetic nanofluid, comprising: adding a dispersant to a high viscosity liquid solvent and stirring, adding and stirring a magnetic nanoparticle to the liquid solvent to which the dispersant is added, and stirred nanoparticles Dispersing the fluid into a high energy dispersing device and modifying the surface of the nanopowder.

In the method of manufacturing the magnetic nanofluid of the present invention, a step of adding a dispersant to a low-viscosity liquid solvent and agitating, adding a magnetic nanoparticle to the liquid solvent to which the dispersant is added and stirring the agitated nanofluid is a high energy dispersing apparatus. Dispersing and modifying the surface of the nanopowder, and substituting a low viscosity liquid solvent with a high viscosity liquid solvent to complete the magnetic nanofluid.

The nano powder is preferably a ferromagnetic material or paramagnetic material, and iron, nickel, cobalt, or an alloy thereof is used as the ferromagnetic material.

The dispersant is preferably contained 1 to 40% by weight based on the nanopowder.

The nano powder is added in 0.001 to 5.0% by weight relative to the liquid solvent, the diameter of the nano powder is preferably 1 ~ 100nm.

The high energy dispersing device includes a bead mill, an ultrasonic disperser or a high pressure homogenizer.

The method of manufacturing the magnetic nanofluid of the present invention further includes mixing the magnetic nanofluid with the nonmagnetic nanofluid to prepare a mixed nanofluid, and redispersing the mixed nanofluid.

The magnetic nanofluid of the present invention is removed by the above-described method for producing magnetic nanofluid of the present invention.

In the process of using the magnetic nanofluid of the present invention, an alternating magnetic field or ultrasonic wave is applied to the magnetic nanofluid from the outside.

In the process of acting the alternating magnetic field, it is preferable to adjust the three forces and the frequency of the external magnetic field to cause a resonance phenomenon according to the average particle size and density of the nano-powder dispersed in the liquid phase.

The magnetic force line direction of the alternating magnetic field is preferably directed toward the vertical direction of the heat transfer surface.

Ultrasonic waves may be provided to the sample to enhance the relative motility between the liquid solvent and the nanopowder.

According to the magnetic nanofluid according to the present invention, a manufacturing method and a method of using the same, when the nanofluid is manufactured using a high viscosity liquid medium such as oil, the thermal conductivity improvement range is limited to a predetermined level or less by the viscosity of the fluid. It has a characteristic that can increase significantly. That is, a nanofluid is prepared by mixing magnetic nanopowders with a high viscosity liquid solvent such as oil, and when the alternating magnetic field is applied to the magnetic nanofluid from the outside, the thermal conductivity is improved by several to several ten percent, so the viscosity is similar to that of oil. It is relatively high, so the thermal property of the sample itself is inferior, and in the liquid sample with low thermal conductivity and heat transfer characteristic effect due to the addition of nano powder, it can be utilized to greatly improve the thermal property.

In addition, according to the present invention, by using oil instead of water or ethylene glycol, the energy efficiency of parts that need to be cooled or given lubrication characteristics can be greatly improved, and the performance improvement of the parts and development of new products can be promoted.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

1 is a flow chart showing a manufacturing process of the magnetic nanofluid according to the present invention.

First, in the case of using a high-viscosity liquid solvent such as oil as a dispersion medium, the step of adding a nanopowder dispersant into a high-viscosity liquid solvent and stirring (S100), putting a magnetic nanoparticles in the liquid solvent to which the dispersant is added, stirring Step (S110), wet dispersing the stirred nanofluid with a high energy dispersing device and modifying the surface of the nanopowder (S120), mixing the prepared magnetic nanofluid with the nonmagnetic nanofluid (S130), and Re-dispersing the mixed nanofluid (S140).

On the other hand, the manufacturing process in the case of using a low viscosity liquid solvent, such as normal hexane as a dispersion medium, the nanopowder dispersing agent is added to the low viscosity liquid solvent (S200), the nano-powder is magnetic to the liquid solvent to which the dispersant is added Putting and stirring (S210), dispersing the stirred nanofluidic fluid with a high energy dispersing mechanism and modifying the surface of the nanopowder (S220), replacing the low viscosity liquid solvent with a high viscosity liquid solvent (S230), preparation Mixing the magnetic nanofluid with the nonmagnetic nanofluid (S240), and redispersing the mixed nanofluid (S250).

Hereinafter will be described in detail step by step the manufacturing process of the magnetic nanofluid according to the present invention.

Powder selection

In the manufacturing process of the magnetic nanofluid according to the present invention, both nanopowders prepared by a dry method and a wet method can be used.

Dispersant  Mixing and stirring step ( S100 , S200 )

In the process of preparing nanofluid using the nanopowder prepared by the dry method, the nanopowder dispersant is added to a high viscosity liquid solvent such as oil and subjected to a pre-stirring step (S100).

The reason why the nanopowder dispersant is premixed and stirred in the high viscosity liquid solvent is to cause the dispersant to act uniformly on the surface of the nanopowder. This is to improve the dispersibility of the nanopowder in the liquid solvent and to suppress the tendency of reaggregation between the nanoparticles over time. Dispersants (or surface modifiers) include Polyoxyethylene Alkyl Acid Ester, Alkyl Aryl Sulfonate and Sorbitan Monooleate, which may be used alone or in combination. It is added within 40% by weight by weight of the powder.

On the other hand, when the viscosity of the liquid solvent is higher than a certain level, in order to improve the dispersibility, a volatile low viscosity organic solvent may be used as the dispersion medium without directly using the desired high viscosity liquid solvent as the dispersion medium. When the bead mill is used as a high energy dispersing device, if the viscosity of the liquid solvent is too high, not only the dispersing effect is low, but also the problem of incorporating the beads into the sample occurs.

In this case, the nanopowder dispersant (or surface modifier) is put in a low viscosity volatile organic solvent such as normal hexane and subjected to a pre-stirring step (S200).

Nano powder  Mixing and stirring step ( S110 , S210 )

The nano-powder is to disperse the ferromagnetic or paramagnetic powder with an average diameter of 100nm or less at a concentration of 0.001 to 5.0% by weight relative to the weight of the liquid solvent. Iron, nickel, cobalt or alloys thereof are used as the ferromagnetic powder.

Dispersion and Reforming Steps ( S120 , S220 )

Nanopowder aggregates of several hundred nm to several tens of micrometers aggregate again over time. This is because the van der Waals attractive forces acting between the powders to reduce the powder surface energy and then re-agglomerate over time.

Therefore, in order to suppress this, powder lipophilic surface modification using a dispersant is performed simultaneously with or immediately after wet dispersion to form a coating layer on the surface of the nanopowder. Here, as a high energy dispersing device, a bead mill, a high pressure homogenizer, or an ultrasonic disperser is used.

After wet dispersing and surface modification, a paper filter is used to remove the foreign matter and micronized powder agglomerates that are not sufficiently ground during sample collection.

Liquid solvent substitution step ( S230 )

After mixing normal hexane containing wet dispersion and surface-modified nanopowder with high viscosity liquid solvent at an appropriate ratio, the solvent of the mixed sample is mixed in normal hexane using a vacuum concentrator at a temperature of 60 ° C. and high vacuum of 20 mmHg or more. Substitute in a high viscosity liquid solvent.

In order to increase the concentration efficiency of the sample, the sample flask is rotated at 50 rpm or more, and the vacuum degree starts at an initial 300 mmHg and gradually lowers to 10 mmHg, finally reaching 20 mmHg. After confirming that normal hexane is no longer collected while maintaining at the final vacuum for at least 4 hours, solvent substitution is terminated.

Mixed nanofluidic manufacturing step ( S130 , S240 )

When manufacturing multicomponent nanofluids using two or more kinds of nanopowders, first prepare nanofluids with a single powder composition in consideration of problems such as preventing bead contamination and dispersing agent, and then mixing them in an appropriate ratio. do.

That is, mixed nanofluids are prepared by mixing nanofluids prepared using magnetic nanopowders and nanofluids prepared using nonmagnetic nanopowders.

Mixed nanofluidic redispersion step ( S140 , S250 )

In order to achieve the synergistic effect of mixing each nanofluid at an appropriate ratio to take advantage of each nanofluid, it is not necessary to simply mix the nanofluids.

For example, after mixing each of the nanofluids, when the preparation of the mixed nanofluids by a simple stirring operation, rather than the disadvantages of each of the nanofluids is highlighted, the effect of improving the properties compared to pure nanofluids is not obtained at all.

Therefore, after mixing each nanofluid, it is very important to perform dispersion treatment after mixing the nanofluid so that each nanofluid can be perfectly mixed using a rather powerful dispersing device such as an ultrasonic disperser.

In the process of using the magnetic nanofluid according to the present invention, an alternating magnetic field or an ultrasonic wave is applied to the magnetic nanofluid manufactured by the method of manufacturing the magnetic nanofluid from the outside.

In the process of applying an alternating magnetic field to the magnetic nanofluid, the intensity and frequency of the externally acting magnetic field are controlled to cause resonance depending on the average particle size and density of the nanopowder dispersed in the liquid phase. At this time, the direction of the magnetic field lines of the alternating magnetic field is directed toward the vertical direction of the solid / liquid heat transfer surface.

The external alternating magnetic field acting on the sample to improve the mobility of the magnetic nanopowder dispersed in the high-viscosity liquid medium is generated by using solenoid and Helmholtz coils. Generate a magnetic field of 30 mT or more. As the strength of the external alternating magnetic field increases, the reactivity of the magnetic nanopowder contained in the fluid also increases, i) to increase the voltage applied to the coil in order to increase the strength of the magnetic field, or ii) such as a silicon steel sheet in the center of the coil. Install the iron core and ⅲ) Connect the upper and lower parts of the iron core with the metal member of the same material along the direction of magnetic force line. In addition, the frequency of resonance caused when the powder vibrates depends on the viscosity of the fluid, the size of the magnetic nanopowder dispersed in the fluid, the specific gravity and the degree of responsiveness to the alternating magnetic field. Adjust the AC frequency. At this time, the magnetic field lines of the alternating magnetic field should be as perpendicular to the heat transfer plane as possible. Finally, ultrasonic waves may be added to the sample to enhance the relative motility of the magnetic powder with respect to the liquid solvent.

Meanwhile, the thermal conductivity of a liquid sample such as nanofluid is measured by using a transient hot-wire method using a platinum wire (Pt Wire) having an insulating coating on its surface as a heating wire and a temperature sensor. This is because when the heating current flows to the platinum wire instantaneously, the amount of heat generated from the heating wire is used to raise the temperature of the heating wire itself and the fluid in radial contact with the heating wire. That is, if the thermal conductivity of the fluid surrounding the hot wire is high, most of the heat generated is transferred to the fluid, so that the temperature rise of the hot wire itself is small.However, if the fluid has low thermal conductivity, the amount of heat generated is not well transferred to the fluid. The rise is large. In addition, when a magnetic field is applied to a sample, thermal conductivity sensors and measuring devices are also affected by the magnetic field, which may cause errors in the data. Therefore, magnetic field shielding materials should be used for the equipment and wires except the sample to minimize the influence of the magnetic field.

Hereinafter, specific examples of the production of the magnetic nanofluid according to the present invention and the improvement of thermal conductivity and heat transfer characteristics of the nanofluid by an external alternating magnetic field will be described.

<Examples>

magnetism Nanofluidic  Produce

In order to prepare a magnetic nanofluid, a used Fe ₃ O ₄ nanopowder prepared by a dry method was used. Fe ₃ O ₄ The powder (Nanostructured and Amorphous Materials, Inc.) has an average particle size of 20 nm, a purity of 99.5%, and a spherical shape. As a liquid solvent to disperse the powder, a cooling oil (Xceltherm 500M, Radco Industries) having an insulating property widely used as a military solvent was used, which has a viscosity value of 5.1 cSt at 40 ° C.

For lipophilic surface modification of the Fe ₃ O ₄ nanopowder, 30% by weight of alkyl aryl sulfonate (Alkyl Aryl Sulfonate) based on the weight of the powder was first added to the cooling oil as a dispersant and sufficiently stirred (S100).

Fe ₃ O ₄ nanopowder was added to the cooling oil to which the dispersant was added at a concentration of 1% by weight, and then pre-stirred for 30 minutes using homogenizers (T25, 1KA) (S110).

After completing the pre-stirring, the magnetic nanofluid of excellent dispersibility was prepared by dispersing the sample for 2 hours by circulating the sample using ultrasonic disperser (UH600SR, SMT) of 600W output (S120). Dispersion stability data of the prepared magnetic nanofluid is shown in FIG. 2. As shown in FIG. 2, when a centrifugal force corresponding to 1147 times atmospheric pressure is applied to the sample, the precipitation of the nanopowder is small in the low concentration sample of 0.5 wt% or less, and the precipitation amount of the powder is not large even in the 1 wt% concentration sample. It can be seen that under atmospheric pressure, it has a very good dispersibility.

External AC magnetic field generation

The magnetic nanofluidic sample produced by the above process is operated with an alternating magnetic field using a magnetic field generator.

As shown in Fig. 3, the magnetic room generator is wound around a soft iron core having an outer diameter of 65 mm and a height of 150 mm with a winding number of enameled coils of 2 mm in diameter for 1000 turns, and the upper and lower portions of the iron core in the direction of the lines of magnetic force are the same iron core member (thickness). A solenoid coil (outer diameter 170 mm, height 150 mm) (100) connected by 8 mm and width 65 mm) was used.

The solenoid coil 100 is a tap (Tap) every 200 times the number of windings to enable the experiment for each of the five windings (220, 400, 600, 800, 1000). When supplying 220V, 60Hz power to the solenoid coil under the condition of 600 turns, a local magnetic field of up to 72mT was generated.

As shown in Table 1 below, the strength of the magnetic field was different for each part of the solenoid coil.

TABLE-1

A B C D 600 72.4 20.8 12.2 6.7 800 48.1 15.1 8.2 5.2 1000 35.0 12.0 6.8 4.0

 Accordingly, as shown in FIG. 4, the AC direction in which the magnetic field strength is the highest among the various parts of the solenoid coil 100 is the side of the magnetic fluid sample container 200, that is, the direction of the magnetic force line is perpendicular to the thermal conductivity measuring device platinum line. Locate it.

When the voltage value of the current supplied to the solenoid coil is varied from 50 to 200 V, the change in the magnetic field strength at the A and B positions of FIG. 3 is shown in FIG. 5. That is, it can be seen that as the voltage value of the current increases, the magnetic field strength also increases in proportion thereto.

Thermal conductivity measurement

When measuring the thermal conductivity of magnetic nanofluids by the unsteady heating method, the most caution is that not only the sample but also the thermal conductivity sensor and the measuring equipment are affected by the magnetic field at the same time. Therefore, magnetic field shielding material made of silicon steel sheet was installed around the solenoid coil in order to minimize the magnetic field influence on the measuring device and wires except the thermal conductivity sensor which can only be exposed to the external magnetic field with the sample.

In all the experiments, the thermal conductivity of the pure cooling oil was measured, and then the thermal conductivity and the thermal conductivity rise were calculated based on the values. The platinum wire used as a hot wire and a measuring sensor was used as an insulating coated diameter of 25㎛, and the supply voltage to the platinum wire was 3.5V, and 10000 data were acquired and analyzed for 10 seconds at a sampling rate of 1kHz. The thermal conductivity of the magnetic fluid was compared and measured while varying the voltage value of the current flowing through the solenoid coil at 50 to 200 V with respect to 1 wt% Fe ₃ O ₄ magnetic nanofluid under the condition of winding number 800 (FIG. 6). When the magnetic nanofluid was prepared by simply adding 1% by weight of Fe ₃ O ₄ nanopowder to the cooling oil without applying an alternating magnetic field from the outside, the thermal conductivity improvement for the pure cooling oil was negligibly small. As shown in FIG. 6, when the AC magnetic field is applied from the outside, the thermal conductivity of the magnetic fluid is 5 to 10% higher than that of the pure cooling oil.

As described above, the present invention has been described based on the preferred embodiments, but the present invention is not limited to the specific embodiments, and those skilled in the art can change the scope within the scope of the claims. have.

1 is a flow chart showing a manufacturing process of the magnetic nanofluid according to the present invention.

Figure 2 is a graph showing the dispersion stability of the magnetic nanofluid according to an embodiment of the present invention.

Figure 3 is a photograph showing a solenoid coil used for generating an alternating magnetic field.

Figure 4 is a photograph of the experimental apparatus for confirming the thermal conductivity of the high viscosity liquid sample by the magnetic nanofluid and external alternating magnetic field.

Figure 5 is a graph showing the magnetic field strength and current for each position according to the voltage magnitude of the current flowing in the solenoid coil.

Figure 6 is a graph showing the thermal conductivity rise of the magnetic nanofluid according to the voltage magnitude of the current flowing in the solenoid coil.

<Explanation of symbols for the main parts of the drawings>

100: solenoid coil 200: sample container

Claims (13)

Adding a dispersant to the high viscosity cooling oil and stirring; Adding a ferromagnetic or paramagnetic magnetic nanopowder to the cooling oil to which the dispersant is added and stirring; And Dispersing the agitated nanofluid with a high energy dispersing mechanism and modifying the surface of the nanopowder to act an alternating magnetic field externally to the prepared magnetic nanofluid. Method of using a magnetic nanofluid, characterized in that to control the intensity of the external magnetic field to 35 to 72.4 mT at a frequency of 60 Hz to cause resonance depending on the average particle size and density of the nanopowder. Stirring the dispersant in a low viscosity liquid solvent; Adding a ferromagnetic or paramagnetic magnetic nanopowder to the liquid solvent to which the dispersant is added and stirring; Dispersing the stirred nanofluid with a high energy dispersing device and modifying the surface of the nanopowder; And Substituting an alternating magnetic field externally to the prepared magnetic nanofluid, including the step of completing the magnetic nanofluid by substituting a low viscosity liquid solvent with a high viscosity cooling oil, Method of using a magnetic nanofluid, characterized in that to control the intensity of the external magnetic field to 35 to 72.4 mT at a frequency of 60 Hz to cause resonance depending on the average particle size and density of the nanopowder. delete 3. The method according to claim 1 or 2, The ferromagnetic material is a method of using a magnetic nanofluid, characterized in that iron, nickel, cobalt or alloys thereof. 3. The method according to claim 1 or 2, The dispersant is a method of using magnetic nanofluids, characterized in that 1 to 40% by weight based on the nanopowder. 3. The method according to claim 1 or 2, The nano powder is added in 0.001 to 5.0% by weight based on the cooling oil, the diameter of the nano powder is a method of using magnetic nano fluid, characterized in that 1 ~ 100nm. 3. The method according to claim 1 or 2, The high energy dispersing mechanism is a method of using magnetic nanofluids, characterized in that the bead mill, ultrasonic disperser or high pressure homogenizer. The method of claim 1 or claim 2, wherein the magnetic nanofluid is Mixing a magnetic nanofluid with a nonmagnetic nanofluid to prepare a mixed nanofluid; And Method of using a magnetic nanofluid, characterized in that the mixed nanofluid prepared further comprising the step of redispersing the mixed nanofluid. delete delete delete 3. The method according to claim 1 or 2, Magnetic field line direction of the alternating magnetic field is a direction of the magnetic nanofluid, characterized in that the direction of the heat transfer surface. 3. The method according to claim 1 or 2, Method of using a magnetic nanofluid, characterized in that to provide an ultrasonic wave to the sample to enhance the relative motility between the cooling oil and the nanopowder.

Images