An investigation of various hydrocarbon sources in the production of carbon nanoparticles via a plasma enhanced chemical vapour deposition technique.

38 FIGURE 3-3: PICTURE OF A HOLE FORMED IN THE PLASMA CHAMBER AS A RESULT OF THE SIDE OF THE CHAMBER. CARBON IS VISIBLE IN THE UPPER RIGHT SIDE OF THE MICROGRAPH. SCALE IS 2µM. 75 FIGURE 5-6: TEMPERATURE DIFFERENCE DURING COOLING OF A BRASS DISC FROM STAND.

LIST OF TABLES

INTRODUCTION TO CARBON NANOSTRUCTURES

Organisation of this dissertation
Carbon nanotubes

Structure
Properties
Possible applications

Synthesis methods

Arc Discharge
Laser Ablation
Chemical Vapour Deposition
Summary of synthesis methods

The blue dot indicates a single carbon atom that can be found on the surface of the tube. The combination of hexagons and pentagons is essential to the formation of the spherical shape (and is actually similar to the way old-fashioned football outers were constructed). In this model, Louchev proposes that the growth of the tubular structures occurs due to the surface diffusion of carbon atoms (which had been adsorbed on the surface of the growing structure) towards the open ends.

In their detailed analysis of the effect of temperature on carbon nanotube formation, Li et al. The remainder of this section is therefore devoted to the discussion of the PECVD method.

Figure 1-1: The hexagonal structure of graphene. A carbon atom lies at each of the vertices - as indicated by the dark dots

EXPERIMENTAL: PRODUCTION OF NANOSTRUCTURES

Introduction
Apparatus

Microwave Source
Plasma Chamber
Aerial
Process control elements

Process inputs
Physical setup
Experimental process

The glass tubes attached tangentially to the sides of the chamber are used as the inlet, with the opening at the bottom as the outlet. shown, the hole is on the surface to allow gases to be drawn into the hollow core of the tube; The 24/25 stop section and the 8mm extension. At the very top of the tube there is a surface which acts as a stand for an antenna to be placed.

Thiophene contains sulfur and sulfur improves the structure of the nanotubes grown in the CVD process. The ethanol-thiophene solution is shaken to aid in proportional transfer of thiophene and ethanol in the gas drawn into the plasma chamber. The flange is connected to the pump inlet with suitable seals and clamps.

The outlet tube of the plasma chamber lies directly above the pump inlet to ensure that there is minimal kinking of the LPG tube. The valve controlling the flow of hydrocarbon vapor into the plasma chamber was fully opened. After all of the above was done, the hydrogen flow (if required) was adjusted.

Normally, the period from ignition of the plasma to regulation of the hydrogen flow would not take more than 10 seconds.

Figure 2-1: Image of the plasma chamber. The glass tubes attached tangentially to the sides of the chamber are used as inlets, with the opening at the bottom used as an outlet

PRODUCTION OF NANOSTRUCTURES

Experiment Set A (Ethanol; 0% Thiophene) .1 Results of Experiment A0 (0 Lt/min Hydrogen)

Results of Experiment A1 (1 Lt/min Hydrogen)
Results of Experiment A3 (7.5 Lt/min Hydrogen)

Experiment Set B (Ethanol; 0.5% Thiophene) .1 Results of Experiment B0 (0 Lt/min Hydrogen)

Results of Experiment B1 (1 Lt/min Hydrogen)
Results of Experiment B3 (7.5 Lt/min Hydrogen)

Experiment Set C (Ethanol; 1.5% Thiophene) .1 Results of Experiment C0 (0 Lt/min Hydrogen)

Results of Experiment C1 (1 Lt/min Hydrogen)
Results of Experiment C3 (7.5 Lt/min Hydrogen)

Experiment Set D (Xylene)

Results of Experiment D0 (0 Lt/min Hydrogen)
Results of Experiment D1 (1 Lt/min Hydrogen)

Experiment Set E0 (Toluene)

Results of Experiment E0 – (0 Lt/min Hydrogen)
Results of Experiment E1 (1 Lt/min Hydrogen)
Results of Experiment E3 (7.5 Lt/min Hydrogen)

Summary of Carbon Nanostructure Results

The amount of amorphous carbon decreased significantly; perhaps due to the increase in the hydrogen flow rate. The darker area is a result of the density of the sample, rather than a collection of amorphous carbon. TEM analysis of the sample showed that the structures formed in this experiment (Figure 3-12) are a closer match to CNTs than those of Experiment B1.

In this experiment, the thiophene content in the hydrocarbon solution was increased to 1.5% by weight. In this experiment, product was only found along the length of the Nilo K aerial tip. It was found that the darkening occurred as a result of a large accumulation of product on the walls of the plasma chamber.

The air, plasma chamber and the part of the tube exposed to the plasma are quickly covered in the product. The plasma is well confined to the air point, but again the product forms on the air, the part of the tube exposed to the plasma and the side of the plasma chamber. Analysis of the product formed on the aerial photograph shows that ONSs are again produced in abundance, as shown in Figure 3-29.

The tube, antenna and sides of the plasma chamber quickly become covered with the deposit, turning the plasma chamber black. TEM analysis of the formed product again shows that ONS is produced in abundance. The remainder of this paper is therefore devoted to investigating the thermal conductivity of onion-like nanostructures.

Figure 3-1: TEM micrograph of two carbon nanotubes found in Experiment A0 - these tubes are approximately 40 nm in diameter and were the only such tubes found in this sample

THERMAL CONDUCTIVITY

Introduction
Fundamental law of heat transfer
Thermal conductivity applications of nanostructures .1 Thermal management

Hydrogen storage

Measurement methods

Searle’s bar
Lee’s disk

Summary

It has been shown that the addition of only a small amount of SWNTs can significantly increase the thermal conductivity of the interface material. Q is calculated from the mass of the water and the temperature difference, T4 – T3, by the equation. The determined values for A, x1, x2, T1, T2, Q and t are then substituted into Equation (4-4) to determine the steady-state thermal conductivity of the material.

It is therefore assumed that the temperatures determined by the thermocouples approach that of the sample surfaces. Disc surface area is calculated from the diameter of the disc, which is easily measured. This single brass disc is now heated and the temperature of the disc is monitored using a thermocouple.

The slope of the temperature versus time is determined at the appropriate temperature for use in the calculation of the thermal conductivity. Indeed, it has been observed that carbon nanotubes greatly increase the thermal conductivity of epoxies and silicone elastomers, especially if the nanotubes are aligned in the direction of heat flow. It is expected that although the addition of onion-like nanostructures to thermal interface materials, such as thermal grease, will increase the thermal conductivity of the thermal interface material.

A version of Lee's disc method, similar to that used by Hickson [135] and was used to measure the thermal conductivity of ONSs added to epoxy resin to form a disc.

Figure 4-1: Illustration of the parameters relating to the fundamental law of heat conduction

THERMAL CONDUCTIVITY OF ONION-LIKE NANOSTRUCTURES

Experimental .1 Samples

Setup
List of experiments

Results

The non-joining ends of the thermocouples are connected to the corresponding pins of the AD595AQ thermocouple amplifier. Analysis of thermal conductivity experiments revealed that an increase in carbon powder or ONS content in a resin disk results in increased thermal conductivity. To calculate the thermal conductivity of the samples, the heat flow through the brass disk must be known.

Again, the data in Table 5-2 and the graphs of Figure 5-8 show that the thermal conductivity of the samples increases with an increase in the additive carbon powder, or ONS, content. In Figure 5.8, an almost constant shift is observed between the thermal conductivities of the carbon powder and the ONS at corresponding content levels. The behavior of ONS conductivity compared to carbon power conductivity is revealed by analyzing the percentage increase in thermal conductivity of ONS compared to carbon powder at the same carbon loading levels.

5 Level identifier numbers have been introduced to facilitate the comparison of the percentage increase analysis. To roughly predict the thermal conductivity of a disc made entirely of ONSs, the contribution of the ONSs to the total mass of the sample was calculated (Table 5-4) and κ in Figure 5-11 plotted. It is clear that the thermal conductivity of the resin-epoxy is increased by the addition of carbon powder, but an even more significant increase is observed by adding ONS to the epoxy resin.

The thermal conductivity of the ONSs initially increases by more than 12% compared to carbon powder, but this percentage increase itself begins to decrease as the loading levels increase.

Figure 5-1: Image of an epoxy-carbon sample after curing and filing.

CONCLUSIONS AND FUTURE WORK

The thermal conductivity of ONSs is nowhere near as high as that of CNTs measured at 3000 W/m·K for a single MWNT [140]. When evaluating the PECVD process used here, it should be noted that only some of the parameters have been analyzed and 'optimized' and there remain other parameters that must also be looked at before we can draw firm conclusions about the process. One parameter is of course the plasma temperature effect, which can be controlled by adjusting the power of the microwave oven.

The suspicion is that, in Experiments C2 to Experiment C4, the reaction temperature may be a little too high - leading to the deformation of the tubes. Attempts to construct a Langmuir probe to measure the temperature of the plasma were made, but were ultimately plagued with problems such as the introduction of air leaks into the system which had a knock-on effect on the plasma itself. Proper integration of the Langmuir probe will require some redesign of the plasma chamber and tube, but the results may justify the effort expended.

Another parameter would be the effect of pressure – while pressure is directly affected by the hydrogen and precursor flow rates, there is currently no measure of the pressure in the plasma chamber. It would also be interesting to investigate whether the yield of the nanotubes could be increased by exposing a larger surface area of the catalyst to the plasma and the dissociated precursor, although this may require redesigning the antenna or this could be amenable to redesigning the tube and chamber. when installing the Langmuir probe in the system. There are a number of experimental scenarios that could be modified and could have a significant impact on the products of this PECVD system.

The study of the thermal conductivity of ONS is just the first step in the study of the properties of these structures.

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