Enhancing thermal conductivity of TiC nanopowder with cryogenic treatment

Introduction

Titanium is an inorganic material, and carbide is an interstitial metallic material. TiC is a binary compound. TiC is selected in this research work. TiC is a hard wear-resistant and corrosion resistant material [1]. The applications of TiC nanopowder are cutting tools, bearings, wear resistant coatings, nozzles, optics, electronics industry, coal industry, aircraft, machining, powder metallurgy, etc. [2]. Titanium carbide nanopowder is used as a nanofluid for heat exchange applications.

Deep cryogenic treatment is an inexpensive method to change the behaviour of materials. This treatment cools the samples from room temperature to −196 °C and then soaks them for 24 h, bringing them back to room temperature. The liquid nitrogen is used as a working fluid to lower the temperature. This treatment is generally applicable to tool steel, die steel, carburised steel, medium carbon steel and other steels. It enhances mechanical properties like hardness, tensile strength, wear resistance and impact behaviour. The compressive residual stress of the surface of the steel is improved drastically. So that the life of the steel components is improved. This treatment is also applied to TiC nanopowder and the thermal conductivity and diffusivity are found. Cryogenic treatment is not only applicable to steel but also to nanopowders like SiC and TiC for enhancing thermal behaviour. The enhancement of thermal conductivity leads to the saving of energy.

Nadig et al. [3] investigated the effect of cryogenic treatment on the thermal conductivity of copper. It has been found that the thermal conductivity of copper is enhanced due to deep cryogenic treatment. The reason for the improvement is the refinement of the atomic structure after deep cryogenic treatment. Pingwang et al. [4] compared the thermal conductivity of cryogenically treated Cu_76.12Al_23.88 alloy with conventional alloy. It has been found that the thermal conductivity and thermal diffusion coefficient have improved after cryogenic treatment.

The cold treatment improved the cutting tool life by 10–40% [5]. Esad Kaya [6] investigated the influence of cryogenic treatment on the properties of AISI O2 steel. It has been found that the retained austenited in the steel is reduced after cryogenic treatment. It is also reported that the distance between the carbides is reduced and the carbide density is increased. The tribological behaviour of steel is also improved drastically. The main aim of this research work is to find out the thermal diffusivity and thermal conductivity of conventional and cryogenically treated TiC nanopellets.

Experimental investigations

The research methodology of the present research work is shown in Figure 1. TiC nanopowder is procured. The nanopowder is divided into two parts. One part is named as conventional TiC nanopowder and other one is named as cryo-treated TiC nanopowder. Then the cryo-treated TiC nanopowder is subjected to deep cryogenic treatment.

OPEN IN VIEWER

Figure 1

Research methodology.

Deep cryogenic treatment is a cold treatment carried out on Titanium Carbide nanopowder at a temperature of −196 °C. The material is slowly cooled from room temperature to −196 °C and held there for 24 h before being slowly heated to room temperature. The medium used for cooling the material is liquid nitrogen. Deep cryogenic treatment modifies the molecular structure of TiC nanopowder. The carbides are distributed homogenously after the deep cryogenic treatment.

The TiC pellets are prepared from the TiC nano powder for testing. The pelletizer is used for the preparation of pellets. Both conventional TiC and cryo-treated TiC pellets are prepared. Deb Ratul and Ahmed Abdul Baquee [7] stated that the binder is used to bind the nano power. Here, 10% polyvinyl alcohol (PVA) is used as a binder, with 2 drops of PVA per 10 g of powder. The binder mixture is prepared in a beaker by stirring with a glass rod. For making pellets in laboratory scale processing i.e. 2–5% PVA solution in distilled water (2 gm PVA in 98 ml of water) and to get a clear solution, the water is boiled on a hot plate. The small amount of powder (∼10–15 gm) is taken for pelletization in an agate mortar (5–6-inch diameter) and 5–10 drops of PVA solution are sprinkled on the powder with a thin glass rod. Grinding is done on the powder with the help of a pestle for a few minutes (5–10 min) till a fine powder is obtained. A few drops of PVA are added and the process is repeated. Depending upon the type and fineness of the powder, this process is repeated a few times till the formation of agglomerates is eliminated. When the powder is ready for pelletization, pellets are prepared immediately before it dries. The pelletizer is shown in Figure 2. The pellets are dried in the atmosphere and then subjected to the test. Figures 3 and 4 show the Titanium Carbide nanopowder and pellet.

OPEN IN VIEWER

Figure 2

Pelletizer.

OPEN IN VIEWER

Figure 3

Titanium carbide nanopowder.

OPEN IN VIEWER

Figure 4

Pellet.

A laser flash apparatus is used to find out the thermal diffusivity of TiC nanopellet and cryogenically treated TiC nanopellet. This test is a fast, non-destructive process, and the samples are tested without contact. The short energy light pulse is used to heat the front surface of the samples, and the temperature rise is measured with an infra-detector. The reference specimen is also used to compare the value of thermal diffusivity. The data taken from laser flash apparatus is directly obtained and further calculations are made to find the thermal diffusivity (α), specific heat capacity (Cp) and thermal conductivity (λ).

Results and discussion

The thermal diffusivity of conventional TiC nanopowder and cryogenically treated TiC nanopowder was found using laser flash apparatus. The thermal diffusivity values of both conventional and cryogenic treated TiC nanopowder are given in Table 1. The experimental properties are given below.

Temperature: 30 °C

OPEN IN VIEWER

Table 1

Diffusivity of conventional TiC nanopellets and cryotreated TiC nanopellet.

S.No	Sample	Diffusivity (mm2 s−1)	Average diffusivity (mm2 s−1)
1	Conventional TiC nanopellet	2.577	2.578 ± 0.002
2		2.577
3		2.58
4	Cryotreated TiC nanopellet	3.12	3.12 ± 0.003
5		3.122
6		3.121

(1)

(2)

K – Thermal conductivity

ρ – Density

Cp – Specific heat capacity

E – Energy pulse

m – Mass of specimen

θ – Temperature

Mass (m) of titanium carbide pellet = 7.494g = 0.007494 kg

Density (ρ) of titanium carbide pellet = 4.93 g cm⁻³ =4930 kg m⁻³.

The following details are obtained from Laser Flash Apparatus:

α = Diffusivity = 2.578 mm²= 0.00257 m²;

Pulse = 6 J;

Pulse width = 0.3 ms;

Temperature = 30 °C = 303 K;

Laser Voltage = 230 V.

The pulse voltage for the sample can be adjusted from (0–18 J). For this test, the pulse voltage is maintained at 6 J/pulse. From the pulse voltage, the specific heat capacity value is calculated using Equation (2) and found that Cp = 2.64 J kgK⁻¹.

The thermal diffusivity of conventional TiC Pellet is 2.578, whereas diffusivity of Cryotreated TiC Pellet is 3.12. Due to deep cryogenic treatment, the thermal diffusivity has increased in TiC pellets.

Thermal conductivity of cryogenically treated TiC nanopellet

The thermal conductivity of cryogenically treated TiC pellet is also calculated on similar lines as follows:

From the above results, the graph is compared between the thermal conductivity of, conventional titanium carbide nanopellet and cryogenically treated titanium carbide nanopellet, as shown in Figure 6.

OPEN IN VIEWER

Figure 6

Thermal conductivity of TiC nanopellet and cryotreated TiC nanopellet.

Percentage improvement

From the above results, the percentage increase in thermal conductivity is 21.39%.

The thermal conductivity of cryogenically treated Titanium carbide nanopellet has improved by 21.39% compared to that of conventional titanium carbide nanopellet. The homogenous distribution of the nanoparticles during the cryogenic treatment is the reason for the improvement of thermal conductivity in titanium carbide nanopowder. Our earlier studies show [8] that the particles are distributed homogenously after deep cryogenic treatment. Figure 7(a,b) shows the titanium carbide particles before and after deep cryogenic treatment. The particle distribution is homogenous in deep cryogenically treated TiC nanopowder, as shown in Figure 7(b) as compared with conventional TiC nanopowder, as shown in Figure 7(a). The agglomeration of TiC powder is decreased due to deep cryogenic treatment. The lattice vibrational waves (phonons) occurring in the materials are attributed to the increase in thermal conductivity. Free electrons in the material gain some kinetic energy, which transfers to atoms that cause vibrational energy to make collisions with phonons or imperfections in the crystal [9], which increases the thermal conductivity due to deep cryogenic treatment.

OPEN IN VIEWER

Figure 7

(a) Microstructure of TiC nanopowder at a magnification of 500×. (b) Microstructure of CryotreatedTiC nanopowder at a magnification of 500× [8].

Conclusion

The research work leads to the following conclusion:

Cryogenic treatment has improved the characteristics of titanium carbide nanopowder.