Relationship between electrical and thermal conductivity in graphene-based The thermal and electrical conductivity of graphene-based thin films strongly .. The goal of this project is to investigate the ambipolar character promoted by.  The Thermal and Electrical Conductivity Probe (TECP) is a component of . [ 22] The science objectives of the Phoenix mission are (1) to study the history of . The relationship between the water content of a soil and dielectric permittivity is. moisture content) with electrical and thermal conductivity were investigated purpose. Many of these engineering features show significant.
Albeit the values of thermal conductivity of liquids and gasses are very small and hard to precisely and practically calculate due to convection for an experiment explaining convection this see our balloon experimenta qualitative comparison can easily be done. Using common cooking supplies, a comparison can be made by simply heating a container s containing each liquid and monitoring the change in temperature. For specific vales for the thermal conductivity of liquids check out our Materials Database Introduction The goal of the experiment is to gain a better understanding of thermal conductivity and how this concept applies to fluids.
This concept is quite important as it plays an important role in heat management, electronics and various other industries.
Qualitative testing of the Thermal Conductivity of liquids
Background Information It is important to take all necessary safety precautions while performing this experiment and it is important to know the boiling point of each liquid before advancing. Below is a list of the boiling points of the various liquid used in this experiment. If at any moment the liquid within the container begins to boil, remove the container from the heat and remove the Styrofoam lid.
One could measure the temperature using an infrared thermometer or simple thermometer Once steady state reached, place the container on the heat source Allow the liquid to heat up for minutes Record the temperature of each thermometer at the end of the minutes Repeat this process for the other two liquids Observation Upon completion of this experiment and the data recorded, an idea of which liquid is the better thermal conductor can be formed.
Vegetable oil consists of mainly non-metals hydrocarbons. Since non-metals are poor thermal conductors it may not be a stretch to hypothesize that vegetable oil will be the worst thermal conductor.
Because they conduct by a diffusion like mechanism, higher temperatures lead to higher conductivity, the reverse of what the simple Drude model would predict.
Breakdown voltage There is an important, and potentially lethal mechanism by which an insulator can become conductive. In air, it may be commonly recognised as lightning. Gases are commonly ionised in domestic lighting devices. The most common are fluorescent tubes and neon lights. To initially excite the mercury vapour in a fluorescent tube type light, a voltage spike exceeding the breakdown voltage is needed. This can be noticed when switching such a light on as a sudden ignition, with an associated radio interference spike.
A faulty tube may not fully ionise, leading to only a small glow at the ends. Under high voltages, even plexiglass may conduct. The temporarily ionised path is opaque on cooling, giving a Lichtenberg figure in this case. For non metals, there are relatively few free electrons, so the phonon method dominates. Heat can be thought of as a measure of the energy in the vibrations of atoms in a material. As with all things on the atomic scale, there are quantum mechanical considerations; the energy of each vibration is quantised and proportional to the frequency.
A phonon is a quantum of vibrational energy, and by the combination superposition of many phonons, heat is observed macroscopically. The energy of a given lattice vibration in a rigid crystal lattice is quantised into a quasiparticle called a phonon.
This is analogous to a photon in an electromagnetic wave; thermal vibrations in crystals can be described as thermally excited phonons, which can be related to thermally excited photons.
Phonons are a major factor governing the electrical and thermal conductivities of a material. A phonon is a quantum mechanical adaptation of normal modal vibration in classical mechanics. A key property of phonons is that of wave-particle duality; normal modes have wave-like phenomena in classical mechanics but gain particle-like behaviour under quantum mechanics. This is defined as the lowest possible energy that the system possesses and is the energy of the ground state.
If a solid has more than one type of atom in the unit cell, there will be two possible types of phonons: The frequency of acoustic phonons is around that of sound, and for optical phonons, close to that of infrared light. They are referred to as optical because in ionic crystals they are excited easily by electromagnetic radiation. If a crystal lattice is at zero temperature, it lies in its ground state, and contains no phonons.
When the lattice is heated to and held at a non-zero temperature, its energy is not constant, but fluctuates randomly about some mean value. These energy fluctuations are caused by random lattice vibrations, which can be viewed as a gas of phonons. Because the temperature of the lattice generates these phonons, they are sometimes referred to as thermal phonons.
Thermal phonons can be created or destroyed by random energy fluctuations. It is accepted that phonons also have momentum, and therefore can conduct energy through the lattice. Unlike electrons, there is a net movement of phonons - from the hotter to the cooler part of the lattice, where they are destroyed.
Electrons must maintain charge neutrality in the lattice, so there is no net movement of electrons during thermal conduction. The following simulation shows schematic optical and acoustic phonons in a 2D lattice, and has the option to animate a 2D wavevector defined by clicking inside the green box.
Umklapp scattering When two phonons collide, the resulting phonon has the vector sum of their momenta. The way of treating particles moving in a lattice quantum mechanically under the reduced zone scheme which is beyond the scope of this TLP but is explored in more depth in the Brillouin Zones TLPleads to a conceptually strange effect.
If the momentum is too great outside the first Brillouin zone then the resulting phonon moves in almost the opposite direction. This is Umklapp scattering, and is dominant at higher temperatures- acting to reduce thermal conductivity as the temperature increases. Applications Silicon chips As electrical properties vary with microstructure, a type of computer memory called phase-change random-access memory PC-RAM has been developed.
The amorphous state is semiconducting, while in a poly crystalline form it is metallic. Heating above the glass transition, but below the melting point, crystallises a previously semiconducting amorphous cell. Likewise, fully melting, then rapidly cooling a cell leaves it in the metallic crystalline state. This variation of resistivity with microstructure is crucial to the operation of such devices. This allows for multiple distinguishable levels of resistance per cell, increasing the storage density, and reducing the cost per megabyte.
The more common problem with silicon devices is dissipating heat. A modern processor has a thermal design power of above 70w Intel i722 nm process. It is common for heat sinks to have a copper block attached to the microprocessor casing by thermal paste, and pressure. The bulk of the heat sink is usually made from much cheaper aluminium, though the high thermal conductivity of copper is necessary for the interface. Thermal paste, whilst a better thermal conductor than air, is much worse than most metals, so it is only used as a thin layer to replace air gaps.
Conduction is not the most efficient method to carry heat to a separate heat sink, so convection and the latent heat of evaporation can be used.
Heat pipes, typically made from copper are filled with a low boiling point liquid, which boils at the hot end, and condenses at the cool end of the pipe. This is a much faster way of transferring heat over longer distances. Space There are many applications of thermal insulators, with development coming from attempts to improve bulk mechanical properties, while retaining insulating properties, i.
They are such good insulators, that the outside may glow red-hot, while inside the shuttle the astronauts are still alive. One of the best thermal insulators is silica aerogel. An aerogel is an extremely low-density solid-state material made from a gel where the liquid phase of the gel has been replaced with gas. The result is an extremely low density solid, which makes it effective as a thermal insulator.
One use of aerogels is for a lightweight micrometeorite collector, aerogel was used. While extremely light, it is strong enough to capture micrometeors. Matches stay cool millimetres from a blowtorch, a large array of aerogel bricks is ready to be launched into space, and the resulting space dust is photographed upon return to earth Aerogels can be made from a variety of materials, but share a universal structure style. However, a common material used is silicate. Silica aerogels were first discovered in Aerogels have extreme structures and extreme physical properties.
The highly porous nature of an aerogel structure provides a low density. Aerogels are good thermal insulators because they eliminate the three methods of heat transfer convection, conduction and radiation. They are good convective insulators due to the fact that air cannot circulate throughout the lattice. Silica aerogel is an especially good conductive insulator because silica is a poor conductor of heat - a metallic aerogel, on the other hand, would be a less effective insulator.
Carbon aerogel is an effective radiative insulator because carbon is able to absorb the infrared radiation that transfers heat. Hence, for maximum thermal insulation, the best aerogel is silica doped with carbon.
DoITPoMS - TLP Library Introduction to thermal and electrical conductivity
Power transmission One of the largest scale uses of electrical conductors is in power transmission. Unfortunately, the properties that are desirable for a strong cable seem opposed to those for a good conductor. There are a huge variety of steels, but again, the interstitial carbon atoms increase the resistance compared to pure iron.
This means that a larger diameter cable is needed, which, due to the density of steel, ends up being very heavy and expensive. Heavier cable also means we must construct additional pylons, which is a large component of the cost.
Copper, while appropriate for home wiring, is dense, and increasingly expensive. For most overhead power cables, the solution is to use two materials — a steel core, surrounded by many individual aluminium cores. This achieves light, high strength, and acceptable conductivity cables.
Superconductors have been trialled for power transmission, though only underground, and at a considerably higher cost and efficiency! Thermoelectric effect The thermoelectric effect is the direct conversion of a difference in temperature into electric voltage and vice versa. Simply put, a thermoelectric device creates a voltage when there is a different temperature on each side of the device.
This effect can be used to generate electricity, to measure temperature, to cool objects, or to heat them. Because the sign of the applied voltage determines the direction of heating and cooling, thermoelectric devices make very convenient temperature controllers. The Peltier effect is that when a direct current flows through a metal-semiconductor junction, and heat is either absorbed or released.
This is because the average energy of electrons in the two materials is different, and heat makes up this difference. A fuller understanding requires knowledge of the band structure, explored further in the TLP on Semiconductors. Summary We have now gone over the foundation behind electrical and thermal conduction, as well as some of the more common applications.
You should understand the role of electrons and phonons in thermal conduction, as well as how the interactions between them lead to changes in electrical conductivity with temperature. You should appreciate that metals have more heat transfer mechanisms than their non-metal counterparts, therefore explaining why they have higher thermal conductivity.