Thermal Properties of MaterialsA SCIMATP Presentation

OutlineI. Thermal ConductivityII. Thermal DiffusivityIII. Thermal expansionIV. Coefficient of thermal expansionV. Seebeck coefficientVI. Specific heatVII. Heat of vaporizationVIII. Heat of fusionIX. Glass transition temperatureX. Melting pointXI. Boiling pointXII. Triple pointXIII. Curie point

Thermal Conductivity

Thermal conduction refers to the ability of a material to transfer thermal energy because of difference in temperatures

How to compute for thermalconductivity

Rate T equals the ratio of the cross section A of the object to its length l, multiplied by the temperature difference (T2 - T1) and by the thermal conductivity of the material, designated by the constant k.

H = - k(A/l)(T2 - T1)

The minus sign arises because heat flows always from higher to lower temperature.

Thermal Diffusivity

o A measure of the rate at which a temperature disturbance at one point in a body travels to another point.

o It is expressed by the relationship K/dCp, where K is the coefficient of thermal conductivity, d is the density, and Cp is the specific heat at constant pressure. 

Diffusivity is the rate of how fast a body can change temperatures. It increases with an object’s heat conductivity and decreases with the amount of heat needed for the body to change temperature.

Thermal expansion

Thermal expansion It is the tendency of matter to change in

volume in response to a change in temperature. (Wikipedia)

Usually expressed as a fractional change in length or volume per unit temperature change.

Bond energy is decreased during thermal expansion.

Thermal expansion The degree of expansion divided by the change

in temperature is called the material's coefficient of thermal expansion.

Liquids expand greater than solids. Glasses’ thermal expansion is greater than

crystals. Solids tend to keep their shape during thermal

expansion, though they still expand. “All materials have this tendency.” (Wikipedia)

Coefficient of thermal expansion

Materials expand because an increase in temperature leads to greater thermal vibration of the atoms in a material, and hence to an increase in the average separation distance of adjacent atoms.

The linear coefficient of thermal expansion a (Greek letter alpha) describes by how much a material will expand for each degree of temperature increase, as given by the formula:where:dl = the change in length of material in the direction being measuredl = overall length of material in the direction being measureddT = the change in temperature over which dl is measured

Comparison of materials: Coefficient of thermal expansion

Take note Coefficient of Thermal Expansion is

rarely linear and should be quoted either at a specific temperature or as an average over a given temperature range.

Applications Most large bridges include expansion joints, which look rather like

two metal combs facing one another, their teeth interlocking. When heat causes the bridge to expand during the sunlight hours of a hot day, the two sides of the expansion joint move toward one another; then, as the bridge cools down after dark, they begin gradually to retract. Thus the bridge has a built-in safety zone; otherwise, it would have no room for expansion or contraction in response to temperature changes. As for the use of the comb shape, this staggers the gap between the two sides of the expansion joint, thus minimizing the bump motorists experience as they drive over it.

Expansion joints of a different design can also be found in highways, and on "highways" of rail. Thermal expansion is a particularly serious problem where railroad tracks are concerned, since the tracks on which the trains run are made of steel. Steel, as noted earlier, expands by a factor of 12 parts in 1 million for every Celsius degree change in temperature, and while this may not seem like much, it can create a serious problem under conditions of high temperature.

Examples An everyday example of thermal expansion can be seen in the

kitchen. Almost everyone has had the experience of trying unsuccessfully to budge a tight metal lid on a glass container, and after running hot water over the lid, finding that it gives way and opens at last. The reason for this is that the high-temperature water causes the metal lid to expand. On the other hand, glass—as noted earlier—has a low coefficient of expansion. Otherwise, it would expand with the lid, which would defeat the purpose of running hot water over it. If glass jars had a high coefficient of expansion, they would deform when exposed to relatively low levels of heat.

Another example of thermal expansion in a solid is the sagging of electrical power lines on a hot day. This happens because heat causes them to expand, and, thus, there is a greater length of power line extending from pole to pole than under lower temperature conditions. It is highly unlikely, of course, that the heat of summer could be so great as to pose a danger of power lines breaking; on the other hand, heat can create a serious threat with regard to larger structures.

Demo It is tempting to think of such small CTE figures as

meaningless for our purposes, until that is you remember the bimetallic strip experiment that you probably carried out at school. This uses a strip made of two different strips of metal, typically brass and steel, which are sandwiched together. Though straight at the temperature at which they were joined together (usually room temperature) the strip bends quite dramatically when place in a flame or in a dewar flask of liquid nitrogen, but returns to its rest state when the source of heat or cold is removed. This principle is used in applications such as cooker and fridge thermostats .Most structures that you will design have materials with different CTEs sandwiched together or soldered/bolted on. When temperature excursions occur, because of changes in either ambient conditions or the power dissipated by the circuit, these materials will expand differently, leading to the creation of stresses. In severe cases, we may get warping of an entire board, or solder joint fracture. These are issues to which we will return in Failure mechanisms.

Seebeck Coefficient

Seebeck coefficient Insulators have very high Seebeck

coefficients Examples:

Bismuth telluride (a semiconductor)Uranium dioxide (oxide of uranium)Constantan (copper-nickel alloy)

Metals have lowers values

Seebeck coefficient Also called “thermopower.” is a measure of the magnitude of an induced

thermoelectric voltage in response to a temperature difference across that material

Has units of volts per kelvin (V/K), or microvolts per kelvin (μV/K)

Note: It (V/K) is different from unit of power!

Seebeck coefficient

The Seebeck effect is the conversion of temperature differences directly into electricity.

Can be defined as:

...if temperature difference between two ends of materials is small.

Note: ΔV = the thermoelectric voltage, ΔT = temperature difference

Specific Heat

Definition

It is the amount of heat required to change a unit mass/quantity of a substance by one degree in temperature.

It can also be considered as a measure of how well a substance resists changing its temperature when it absorbs or releases heat.

Is also sometimes called specific heat capacity.

Equation

wherein:Q = Heat addedc = specific heatm = massΔT = change in temperature

The heat capacity indicates how much thermal energy ΔQ a physical body can absorb for a change in temperature ΔT. It refers to a specific body, and gives no indication of the amount of substance or composition of the body.

SOLIDS – Specific HeatProduct Specific heat capacity (kJ/kg K)

Brick, hardBrick, common

10.9

Chalk 0.9

Glass 0.84

Gold 0.13

Wood, oakWood, balsa

22.9

LIQUIDS – Specific HeatProduct Specific heat capacity (kJ/kg K)

Alcohol, ethyl 32oF (ethanol)Alcohol, ethyl 104oF (ethanol)

2.32.72

Mercury 0.14

Milk 3.93

Sodium, 200oFSodium, 1000oF

1.381.26

Water, freshWater, sea 36oF

4.193.93

Example # 1 How much energy does it take to raise

the temperature of 50 g of copper by 10 0C?

Example # 2 If we add 30 J of heat to 10 g of

aluminum, by how much will its temperature increase?

Heat of FusionHeat of Vaporization

Heat of fusion (ΔH˚fus)

Solid -> Liquid -> Solid Latent heat used when melting a solid or

freezing a liquid. (Encyclopedia Britannica) The process can be endothermic or exothermic.

Figure III.B. Temperature change of a solid to liquid (Wisc-Online.com)

Water = 80 cal/g at 0 ˚C.

Ethyl alchohol = 25 cal/g at -112 ˚C melting point.

Heat of fusion

“Latent heat” - heat that does not result in a temperature change (Wikipedia)

Melting point – temperature at which change occurs.

Solubility prediction

Heat of vaporization (Δhvap) Liquid -> Gas Heat absorbed by a unit mass of a

material at its boiling point in order to convert the material into a gas at the same temperature (The Free Dictionary)

Figure III.A. Temperature change of a solid to vapor (Wisc-Online.com)Water = 540 cal/g at 100

˚C

Ethyl alchohol = 204 cal/g at 78 boiling point ˚C

Heat of vaporization

Heat is absorbed by molecules as they turn into their gaseous state.

Expressed as the amount of heat (in Joules) that is required to change 1 gram of liquid into gas.

Dependent on different factors such as: PRESSURE and EXTERNAL TEMPERATURE.

Clausius-Clapeyron equation

Glass Transition Temperature

What happens when you put a rubber

band into a container of liquid

nitrogen?

Glass Transition Temperature (Tg ) The Tg of a non-crystalline material is the

critical temperature at which the material separates its behavior from being ‘glassy’ to ‘rubbery’.

Tg only applies to non-crystalline solids, which are mostly either glasses or rubbers.

Non-crystalline solids are also known as 'amorphous materials'.

Amorphous materials are materials that do not have their atoms or molecules arranged on a lattice that repeats periodically in space.

Below Tg & Above Tg Rubbery State Elastic Flexible Rubber elastomers

like polyisoprene and polyisobutylene

Glassy state Hard Brittle solid Easy to break Hard plastics like

polystyrene and poly(methyl methacrylate)

Properties that change around (Tg ) Density Specific Heat Dialectric coefficient Rates of gas/liquid diffusion through the

polymer Conductivity Change mobility

Tg vs. Melting The glass transition

is a transition which happens to amorphous polymers

that is, polymers whose chains are not arranged in ordered crystals, even though they are in the solid state.

Melting is a transition which occurs in crystalline polymers.

Melting happens when the polymer chains fall out of their crystal structures, and become a disordered liquid

Measuring TgDifferential Scanning

Calorimetry defines the glass transition as a change in the heat capacity as the polymer matrix goes from the glass state to the rubber state. Measures the heat effect

Thermo-Mechanical Analysis

defines the glass transition in terms of the change in the coefficient of thermal expansion (CTE) as the polymer goes from glass to rubber state with the associated change in free molecular volume Measures the physical effect

Boiling PointMelting Point

What does Boiling Point mean? The temperature at which a liquid boils at a

fixed pressure, especially under standard atmospheric conditions.

Boiling point is the temperature at which the liquid is transformed into vapour when heat is supplied.

The boiling point of the liquid varies according to the characteristics of the liquid.

A boiling point of a certain liquid will not increase, but it will definitely have a change of state.

When water turns into gas

What is Melting Point? The temperature at which the solid and

liquid phases of a compound are in equilibrium at a certain pressure.

When heat is applied to a certain solid object, it slowly melts and turns into a liquid state.

Melting points are usually the same as the freezing point of a certain object.

Melting points are often used to characterize organic and inorganic compounds and to ascertain their purity

Triple Point Curie Point

Triple Point The phase diagram is a

plot of pressure versus temperature at a constant volume of a substance

The triple point is the point where vapor, liquid and solid phases of a substance can coexist in equilibrium

The triple point of water is 273.16 K at 4.58 mmHg

Curie PointThe Curie point is the temperature above which a ferromagnet loses its ferromagnetic ability to possess a net (spontaneous) magnetization in the absence of an external magnetic field. At temperatures below the Curie point, magnetic moments are partially aligned within magnetic domains in ferromagnetic materials. As the Curie point is approached, thermal fluctuations increasingly destroy this alignment, until the net magnetization becomes zero at and above the Curie point. Above the Curie point, the material is purely paramagnetic.

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21, 2010, from Specific Heat and Heat Capacity: http://www.iun.edu/~cpanhd/C101webnotes/matter-and-energy/specificheat.html

Jones, L. (2009, December 23). Specific Heat. Retrieved November 21, 2010, from Journey into Science: http://www.sciencebyjones.com/specific_heat1.htm

The Engineering Toolbox. (n.d.). Solids - Specific Heat Capacity. Retrieved November 21, 2010, from The Engineering Toolbox: http://www.engineeringtoolbox.com/specific-heat-solids-d_154.html

The Engineering Toolbox. (n.d.). The Engineering Toolbox. Retrieved November 21, 2010, from Liquids - Specific Heat Capacity: http://www.engineeringtoolbox.com/specific-heat-fluids-d_151.html

Thermal Properties of

Materials

End of Presentation