eng2000chapter9thermalpropertiesofmaterials.pptVIP

ENG2000 Chapter 9Thermal Properties of Materials Thermal properties of materials The problem of heat generation in ICs is becoming significant as there are more transistors per chip and increasing numbers of layers of metals and reduced dimensions The power (energy per unit time) delivered into a conductor is P = IV = I2R = V2/R and the resistance is given by R = rL/A So longer tracks with a smaller area are more susceptible to heating The conductor heats up until the temperature is such that the heat lost per unit time balances the power supplied Convection, radiation, conduction Conduction is the flow of heat through a solid, analogous to electronic transport but with the driving force being DT instead of DV the constant of proportionality is the thermal conductivity rather than the electrical conductivity Radiation is the loss of energy by the emission of electromagnetic radiation in the infra-red wavelengths Convection is the transfer of heat away from a hot object because the gas next to the object heats up and becomes less dense hence it rises, setting up currents of gas flow Clearly, the more isolated the conductor is, the hotter it gets So consider a conductor in a chip: But how hot does the conductor get? How fast is heat transported away from the region of heating? How do we optimise the design to overcome these issues? Read on! We will first cover a few basics and then talk about heat capacity, thermal conductivity and their origins Thermodynamics Pretty much everything to do with heat and heat flow is covered by thermodynamics When two bodies of different temperatures are brought in contact heat, Q, flows from the hotter to the cooler Alternatively, a temperature increase can be achieved by doing work, W, on the system e.g. electrical heating, friction, … In either case, there is a change of energy of the “system” DE = W + Q where Q is the heat received from the environment The first law of thermodynamics Energy, heat and work all have the same units