FIELD OF THE INVENTION
The present invention relates to semiconducting metal oxide nanotube based highly thermal conducting insulating nanofluid. Metal oxide nanotubes dispersed in transformer oil based nanofluid posses highest enhancement in thermal conductivity with low enhancement in electrical conductivity.
BACKGROUND OF THE INVENTION
The conventional heat transfer fluids have poor heat transfer properties, and they lack in cooling capabilities to desired extent. Nanofluid is a two-phase mixture, composed of a continuous liquid phase, known as the base fluid and dispersed nanoparticles in the suspension [1]. Nanofluids have potentials in heat transfer enhancement applications which arise mainly due to their intriguing properties such as considerable increase in thermal conductivity, long-term stability, and prevention of clogging in microchannels. Low thermal conductivity is a primary limitation in the development of energy efficient heat transfer fluids that are required in numerous industrial sectors, including power transmission and electronics industries. Transmission and distribution transformers constitute a crucial highly loaded and expensive part of the electricity generation and distribution network. Any kind of transformer failure can be quite deleterious. Metal oxide nanotube dispersed transformer oil helps improving its dielectric and thermal properties. The insulation fluids in power transformers perform two main functions insulating and cooling. Typically used transformers oils have low thermal conductivity and thus perform low-efficiency cooling. The micron sized particles added to the oil to enhance its properties decrease its dielectric strength. Nanomaterials on the other hand, are believed to be effective in improving the thermal and dielectric strength of the oil based nanofluids.
OBJECTS OF THE INVENTION
It is therefore, an object of the invention to propose a process to produce highly thermal conducting insulating nanofluid without substantial enhancement in electrical conductivity for insulating/dielectric applications. The main objective of the invention is to develop semiconducing metal oxide nanotube dispersed transformer oil based nanofluids. A still another object of the invention is to render hydrophobic surface for metal oxide nanotube to increase their solubility in non-polar transfoemer oil based fluid.
SUMMARY OF THE INVENTION
Thus, in the present invention, semiconducting metal oxide nanotubes are dispersed in transformer oil since these nanotubes when properly functionalized exhibits electric insulation property. Metal oxide nanotube dispersed transformer oil can improve the lifetime of the transformer and can increase the loads/cooling capacity.
Metal oxide nanotubes viz. one dimensional manganese di-oxide (MnO2 nanotube is prepared by hydrothermal synthesis technique [2]. For synthesis of MnO2 nanotubes (MNTs), KMnO4, HCl (37 wt.%) and distilled water were mixed together and the solution is transferred to a teflon-lined, stainless-steel autoclave. The autoclave is kept in an oven at 140 °C for 12h, and subsequently cooled down to room temperature. The resulting brown precipitates are collected, rinsed and filtered to pH 7. The as-prepared powders are then dried at 80 °C in air. MNTs are rendered hydrophobic by treatment with n-hexane and oleic acid for proper dispersion in t he transformer oil. Morphology and structure of the prepared MNTs were validated by X-ray diffractorgrams (XRD), Field
emission Scanning Electron Microscopy (FESEM), Transmission Electron Microscopy (TEM), Fourier transform infrared (FTIR) & Raman spectral analysis. Thermal and electrical conductivity measurements of these nanofluids for different volume fraction have been measured at varying temperature. The % volume fraction varies from 0.02 to 0.06 for thermal conductivity and 0.005 to 0.06% for electrical conductivity and the temperature ranges from 25 °C to 50 °C. The experiments have been repeated a minimum of 8 times for consistency and the conclusions have been drawn.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - Shows formation of a-MnO2 phase in MNTs in a x-ray diffractogram (XRD) study.
Figure 2 - Shows field emission scanning electron microscopy and transmission electron microscopy images of MNTS.
Figure 3 - Shows fourier transform infrared spectral analysis of MNTS.
Figure 4 - Shows vibrational properties of MNTS using Raman spectroscopy.
Figure 5 - Shows the variation of thermal conductivity of the nanofluid with the base fluid as a function of temperature for MNTs-based nanofluids.
Figure 6 - Shows electrical conductivity plot of pure transformer oil and MNTS.
DETAILED DESCRIPTION OF THE INVENTION
The MNTS in the present invention to act as an electrically insulating transformer oil based nanofluid is prepared by hydrothermal technique. The as synthesized MNTs were characterized by XRD, FESEM, TEM, FTIR and Raman spectroscopy.
XRD revealed the formation of a-MnO2 phase in MNTs (Figure 1)
Figure 2 shows the FESEM and TEM images of MNTs. Tubular nature of MNTs is clearly visible from the micrographs. Surface morphology of MNTs revealed the average length and diameters to be 1.16 µm and 58 mm, respectively. This approves large increment in thermal conductivity of MNTs as they posses higher aspect ratio (length to diameter ratio).
Figure 3 encompasses the FTIR spectral analysis of MNTs. The peaks of MNT at 3446 cm"1 can be due to the stretching vibrations of O-H vibrations. The three peaks at 1632, 1372 and 1035 cm-1 can be due to the bending vibrations of O-H. The sharp peaks at 534 cm-1and 724 cm-1 should be the contribution of the Mn-O vibrations in MNTs [3].
The vibrational properties of MNTs have been accessed using Raman spectroscopy (Figure 4). A sharp, low frequency band at 659 cm-1 is observed which is attributed to the symmetric stretching vibration Mn-O of MnO2 groups [4].
In order to form stable transformer oil based nanofluids, it is essential to make the MNTs hydrophobic. The nanotubes were treated with n-hexane and dispersant oleic acid by ball milling for 6h followed by ultrasonication for 1h and filtration. The filtrated particles were then dispersed in transformer oil by ultrasonication for nearly 1h. At final stage, n-hexane was dried off by rotary vacuum evaporator.
The thermal conductivity of the nanofluids was measured using KD2 Pro thermal properties analyzer from Decagon devices, Inc. The working principle follows the transient hot-wire technique [5,6]. Figure 5 shows the variation of thermal conductivity of the nanofluid with base fluid as a function of temperature for MNTs-based nanofluids in transformer oil. In the range from 25 to 50°C, the absolute thermal conductivity of the nanofluid and base fluids decreases with the increase in temperature. This is in accordance with the reported literature for non-aqueous based nanofluids [7]. The enhancement in thermal conductivity was nearly 3% with MNTs based nanofluids at room temperature for a volume fraction of 0.024%. It is observed from Figure 5(a) that as the volume fraction of nanoparticles increased, the effective thermal conductivity also enhanced. The % enhancement with thermal conductivity was 7% at 0.059% volume fraction of MNTs.
ELICO, EC-TDS ANALYSER CM 183 was used for measuring the electrical conductivity of the nanofluids. This microprocessor based analytical instrument enables easy and fast analysis of electrolyte conductivity (EC) in liquid samples. The working principle of the EC-TDS analyzer is based on the fact that current flowing between the electrodes immersed in conducting fluid varies inversely with resistance and directly with the conductance of electrolyte. The conduction of electric current in fluids is due to migration of ions under the influence of an
electric field in line with Ohm's law, analogous to electrons in metallic conductors.
The electrical conductivity of pure transformer oil and 0.059% MNT nanofluids at different temperatures is compared in Figure 6. It is observed that with increase in temperature, electrical conductivity decreases for both the samples. Also, the difference in electrical conductivity of the nanofluids is very small as compared with its base fluid. The % enhancement in electrical conductivity of MNT nanofluids was ~ 1% compared to transformer oil over the temperature ranges analyzed. This demonstrates the ability of the MNT based nanofluids as potential insulating fluids in transformers. The % enhancement with electrical conductivity was ~ 1% for MNTs. This demostrates the ability of the current metal oxides based nanofluids as potential insulating fluids in transformers.
MnO2 nanotubes in oil based nanofluids possess highest enhancement in thermal conductivity with low enhancement in electrical conductivity. The increase in thermal conductivity is remarkable considering the small volume fraction of MNTs. The observed increase of thermal conductivity, to the best knowledge of the author, not be predicted by any model and falls into a whole new area of research.
WE CLAIM :
1. A process for producing semiconducting metal oxide nanotube based highly thermal conducting Transformer oil, the process comprising the steps of:
Synthesising of MnO2 nanotubes (MNTs) by mixing KMnO4, Hcl (37% dilution), and distilled water;
transferring the synthesized solution to a teflon-lined stainless steel autoclave;
disposing the autoclave in an oven at 140 °C for at least twelve hours;
allowing the autoclave to cool down to room temperature;
collecting the brown precipitants from the autoclave, and rinsing including filtering the precipitants to pH 7;
drying the as prepared powder at 80 °C in air; and
treating the produced MnO2 nanotubes with n-hexane and oleic acid for dispersion in the transformer oil.
2. The process as claimed in claim 1, where the average length and diameter of the MNTS is 1.16 µm and 58 nm respectively.
3. The process as claimed in claim 1, wherein the nanotubes are treated with n-hexane for a period of 6 hours followed by ultrasonication for 1 hour.
4. The process as claimed in any of the proceeding claims, wherein the enhancement with thermal conductivity is 7% at 0.059% volume fraction of the MNTs.

ABSTRACT

The invention relates to a process for producing semiconducting metal oxide nanotube based highly thermal conducting Transformer oil, the process comprising the steps of : Synthesising of MnO2 nanotubes (MTN) by mixing KMnO4,Hcl (37% dilution), and distilled water; transferring the synthesized solution to a teflon-lined stainless steel autoclave; disposing the autoclave in an oven at 140 °C for at least twelve hours; allowing the autoclave to cool down to room temperature; collecting the brown precipitants from the autoclave, and rinsing including filtering the precipitants to pH 7; drying the as prepared powder at 80°C in air; and treating the produced MnO2 nanotubes with n-hexane and oleic acid for dispersion in the transformer oil.