chemical, thermal and electric properties of materials

8/12/2019 Chemical, Thermal and Electric Properties of Materials

1/46

Chemical, Thermal and

Electric Properties of

Materials


2/46

pH

Hygroscopy

Surface tension Specific internal surface area

Reactivity

Corrosion resistance

Chemical Properties


3/46

Thermal Properties

Thermal conductivity Thermal diffusity

Thermal expansion

Seebeck coefficient

Emissivity Specific heat

Heat of vaporization

Heat of fusion

Pyrophoricity

Flammability

Autoignition temperature

Inversion temperature Critical point

Glass transition

temperature

Eutectic point

Melting point Boiling point

Triple point

Flash point

Curie point


4/46

Electrical Properties

Electrical conductivity

Permittivity

Dielectric constant

Dielectric strength

Piezoelectric constants


5/46

Ph

pHis a measure of the acidity or basicity of a solution.Solutions with a pH less than 7 are said to be acidic and

solutions with a pH greater than 7 are said to

be basic or alkaline.

In a solution pH is the negative logarithm (base 10) ofthe molar concentration of dissolved hydrogen ions (H+);

a low pH indicates a high concentration of hydrogen

ions, while a high pH indicates a low concentration.


6/46


7/46

Hygroscopy

Hygroscopyis the ability of a substance to attract andhold water molecules from thesurrounding environment through either absorption oradsorption with the adsorbing or absorbing materialbecoming physically 'changed,' somewhat, increase in

volume, stickiness, or other physical characteristicchanges of the material as water molecules become'suspended' between the material's molecules in theprocess.

Hydroscopic substances include sugar, honey,

glycerol, ethanol, methanol, diesel fuel, sulfuricacid, methamphetamine, many salts (including tablesalt), and a huge variety of other substances.


8/46

What is the difference between absorption andadsorption?

Absorptionhappens when atoms pass through or enter abulky material like sponges.

Adsorptionhappens when the atoms settle or accumulateon the surface of a material rather than literally enteringor diffusing into that same material.

Many engineering polymers are hygroscopic. Theseinclude: nylon, ABS, polycarbonate, cellulose,and Poly(methyl methacrylate). Other polymers, such

as polyethylene and polystyrene, do not normally adsorbmuch moisture,

but are able to carry significant moisture on their surfacewhen exposed to liquid water.


9/46

Surface Tension

Surface tensionis a property of the surface of a liquid thatallows it to resist an external force. This property iscaused by cohesion of like molecules, and is responsiblefor many of the behaviors of liquids.

The cohesive forces among the liquid molecules areresponsible for this phenomenon of surface tension. Inthe bulk of the liquid, each molecule is pulled equally inevery direction by neighboring liquid molecules, resultingin a net force of zero. The molecules at the surface do

not have other molecules on all sides of them andtherefore are pulled inwards. This creates some internalpressure and forces liquid surfaces to contract to theminimal area.


10/46

Diagram shows, in cross-section, a needle floating on the

surface of water. Its weight, Fw, depresses the surface, and is

balanced by the surface tension forces on either side, Fs,

which are each parallel to the water's surface at the pointswhere it contacts the needle. Notice that the horizontal

components of the two Fs arrows point in opposite directions,

so they cancel each other, but the vertical components point in

the same direction and therefore add up to balance Fw.


11/46

Specific Surface Area

Specific surface area is a material property of solids whichmeasures the total surface area per unit of mass, solid orbulk volume, or cross-sectional area

It is a derived scientific value that can be used to determine

the type and properties of a material (e.g. soil). It isdefined either by surface area divided by mass (withunits of m/kg), or surface area divided by the volume

(units of m/m or m-1)

It has a particular importance in case of adsorption,

heterogeneous catalysis, and reactions on surfaces.


12/46

ReactivityReactivity then refers to the rate at which

a chemical substance tends to undergoa chemical reaction in time.

In pure compounds, reactivity is regulated by the

physical properties of the sample. For instance,

grinding a sample to a higher specific surfacearea increases its reactivity. In impure

compounds, the reactivity is also affected by the

inclusion of contaminants.

In crystalline compounds, the crystalline form can

also affect reactivity. However in all cases,

reactivity is primarily due to the sub-atomic

properties of the compound.


13/46

What do you mean by reactive?

Although it is commonplace to make statements

that substance 'X is reactive', all substances

react with some reagents and not others. For

example, in making the statement that 'sodiummetal is reactive', we are alluding to the fact that

sodium reacts with many common reagents

(including pure oxygen, chlorine, hydrochloric

acid, water) and/or that it reacts rapidly withsuch materials at either room temperature or

using a bunsen flame.


14/46

Corrosion Resistance

Some metals are more intrinsically resistant to corrosion thanothers, either due to the fundamental nature of theelectrochemical processes involved or due to the details ofhow reaction products form.

The materials most resistant to corrosion are those for whichcorrosion is thermodynamically unfavorable. Any corrosion

products of gold or platinum tend to decomposespontaneously into pure metal, which is why theseelements can be found in metallic form on Earth, and is alarge part of their intrinsic value. More common "base"metals can only be protected by more temporary means.

Some metals have naturally slow reaction kinetics, eventhough their corrosion is thermodynamically favorable.These include such metals as zinc, magnesium,and cadmium. While corrosion of these metals iscontinuous and ongoing, it happens at an acceptably slowrate.


15/46

Methods of protection from corrosionApplied coatings

Plating, painting, and the application of enamel are the most

common anti-corrosion treatments. They work by providing abarrier of corrosion-resistant material between the damagingenvironment and the (often cheaper, tougher, and/or easier-to-process) structural material.

Reactive coatings

If the environment is controlled (especially in recirculatingsystems), corrosion inhibitors can often be added to it. Theseform an electrically insulating and/or chemically impermeablecoating on exposed metal surfaces, to suppress electrochemicalreactions.

Cathodic protection

Cathodic protection (CP) is a technique to control the corrosionof a metal surface by making that surface the cathode ofan electrochemical cell.

Anodic protection

Anodic protection impresses anodic current on the structure to

be protected (opposite to the cathodic protection).


16/46

Thermal Conductivity

Thermal conductivity, k, is the property of a material

reflecting its ability to conduct heat.

Thermal conductivity is measured in watts per kelvin per

metre (W/Km). Multiplied by a temperature difference (in

kelvins, K) and an area (in square metres, m2), and

divided by a thickness (in metres, m), the thermal

conductivity predicts the rate of energy loss (in watts, W)

through a piece of material.

The reciprocal of thermal conductivity is thermal resistivity,

usually measured in kelvin-metres per watt (KmW1).When dealing with a known amount of material,

its thermal conductanceand the reciprocal

property, thermal resistance, can be described.


17/46

Thermal Diffusity

Thermal diffusivity is the thermal conductivity divided bythe volumetric heat capacity. It has the SI unit of m/s.

where:

k: thermal conductivity (SI units: W/(mK))

: density (kg/m)

cp: specific heat capacity (J/(kgK))

Substances with high thermal diffusivity rapidly adjust their

temperature to that of their surroundings, because they

conduct heat quickly in comparison to their volumetric heat

capacity or 'thermal bulk'.


18/46

Thermal Expansion

Thermal expansion is the tendency of matter to changein volume in response to a change in temperature. Allmaterials have this tendency.

When a substance is heated, its particles begin moving and

become active thus maintaining a greater averageseparation.

The change in the linear and volumetric dimension can beestimated to be:

L = coefficient of linear expansion

L = initial length

T= change in temperature

L=change in length

V= coefficient of volume expansion

V = initial volume

T= change in temperature

V=change in volume


19/46

Seebeck Coefficient

Seebeck coefficient(or thermopower) of a material is ameasure of the magnitude of an induced thermoelectric

voltage in response to a temperature difference across

that material. The thermopower has units

of volts per kelvin (V/K), although it is more often given

in microvolts per kelvin (V/K).

The Seebeck effect is the conversion of temperature

differences directly into electricity.

S = Seebeck coefficient

T = Temperature difference

V = Thermoelectric voltage seen at the

terminals


20/46

The emissivity of a material (usually written or e) is the

relative ability of its surface to emit energy by radiation. It

is the ratio of energy radiated by a particular material to

energy radiated by a black body at the same

temperature. A true black body would have an = 1

while any real object would have < 1. Emissivity isa dimensionless quantity.

In general, the duller and blacker a material is, the closer

its emissivity is to 1. The more reflective a material is,

the lower its emissivity.Emissivity depends on factors such as temperature,

emission angle, and wavelength.

Emissivity


21/46

Specific HeatHeat capacity(usually denoted by a capital C, often with

subscripts) is the measurable physical quantity thatcharacterizes the amount of heat required to change abody's temperature by a given amount. Inthe International System of Units, heat capacity isexpressed in units of joules per kelvin.

Derived quantities that specify heat capacity asan intensive property, independent of the size of asample, are the:

molar heat capacity, which is the heat capacity

per mole of a pure substance (J/mol-K),specific heat capacity, often called simply specific heat,

which is the heat capacity per unit mass of a material(J/kg-K).


22/46

Heat of vaporization

The heat of vaporization, also known as the enthalpy ofvaporization or heat of evaporation, is the energy

required to transform a given quantity of a substance into

a gas at a given pressure (often atmospheric pressure).

It is often measured at the normal boiling point of asubstance; although tabulated values are usually

corrected to 298 K, the correction is often smaller than

the uncertainty in the measured value.

Values are usually quoted in J/mol or kJ/mol (molar

enthalpy of vaporization), although kJ/kg or J/g (specific

heat of vaporization), and units like kcal/mol, cal/g

and Btu/lb are sometimes used, among others.


23/46

Heat of FusionThe enthalpy of fusion, also known as the heat of

fusion or specific melting heat, is the change in

enthalpy resulting from the addition or removalof heat from 1 mole of a substance to change its statefrom a solid to a liquid (melting) or the reverse processesof freezing. It is also called the latent heat of fusion, andthe temperature at which it occurs is called the melting

point.

The units of heat of fusion are usually expressed as:

kilojoules per mole (the SI unit)

calories per gram (old metric units now little used, except

for a different, larger calorie used in nutritional contexts) British thermal unit per pound or Btu per pound-mole


24/46

PyrophoricityA pyrophoricsubstance will ignite spontaneously in air.

Examples are iron sulfide and manyreactive metals including uranium, when powdered orsliced thinly. Pyrophoric materials are often waterreactive as well and will ignite when they contact wateror humid air. They can be handled safely in atmospheres

of argon or (with a few exceptions) nitrogen.

The creation of sparks from metals is based on thepyrophoricity of small metal particles. This can be useful,including: the sparking mechanisms in lighters and

various toys, using ferrocerium; starting fires withoutmatches, using a firesteel; the flintlock mechanism infirearms; and spark testing ferrous metals.


25/46

FlammabilityFlammabilityis defined as how easily something will burn

or ignite, causing fire or combustion. The degree ofdifficulty required to cause the combustion of asubstance is quantified through fire testing.

The ratings achieved are used in building codes, insurancerequirements, fire codes and other regulations governingthe use of building materials as well as the storage and

handling of highly flammable substances inside andoutside of structures and in surface and airtransportation.

Ratings: 0 Materials that will not burn.

1 Materials that must be preheated before they will ignite. 2 Materials that must be moderately heated or exposed to relatively high

ambient temperatures before they will ignite.

3 Liquids and solids that can ignite under almost all temperature conditions.

4 Materials which will rapidly vaporize at atmospheric pressure and normaltemperatures, or are readily dispersed in air and which burn readily.


26/46

Flammable vs Inflammable

The word inflammable came from Latin inflammre = to set

fire to, where the prefix 'in-' means in as in inside, ratherthan not as in invisible and ineligible. Nonetheless,

inflammable is often erroneously thought to mean non-

flammable. To avoid thissafety hazard, flammable, despite

not being the proper Latin-derived term, is now commonly

used on warning labels when referring to physicalcombustibility.


27/46

Autoignition Temperature

The autoignition temperature or kindling point of asubstance is the lowest temperature at which it

will spontaneously ignite in a normal atmosphere without

an external source of ignition, such as a flame or spark.

This temperature is required to supply the activation

energy needed for combustion.

Autoignition temperature of selected substances:

Diesel: 210 C (410 F)

Gasoline (Petrol): 246280 C (475536 F)Butane: 405 C (761 F)

Paper: 218-246C (424-474F)

Magnesium: 473 C (883 F)

Hydrogen: 536 C (997 F)


28/46

Inversion Temperature

The inversion temperature is the critical temperature belowwhich a non-ideal gas (all gases in reality) that is

expanded at constant enthalpy will experience atemperature decrease, and above which will experiencea temperature increase. This temperature change isknown as the Joule-Thomson effect, and is exploited inthe liquefaction of gases.


29/46

Critical Point

A critical point, also called a critical state, specifies the

conditions (temperature, pressure and sometimes

composition) at which a phase boundary ceases to exist.

For instance, as the vapor-liquid critical point is

approached, the properties of the gas and liquid phasesapproach one another, resulting in only one phase at the

critical point: a homogeneous supercritical fluid. The heat

of vaporization is zero at and beyond this critical point,

so there is no distinction between the two phases.

In water, the critical point occurs at around 647 K (374 C or 705 F)

and 22.064 MPa(3200 PSIA or 218 atm)


30/46

The vapor-liquid critical point in a pressure-temperature phasediagram is at the high-temperature extreme of the liquid-gas phaseboundary. The dotted green line gives the anomalous behavior ofwater.


31/46

Glass transition Temperature

The liquid-glass transition(or glass transitionfor short)is the reversible transition in amorphous materials (or inamorphous regions within semi-crystalline materials)from a hard and relatively brittle state into a moltenor rubber-like state.

The liquid-glass transition is an atypical phase transition inthat it is not connected with a discontinuous change instructure, and in that it does not have a sharp transitiontemperature. There is rather a glass transformation

range that extends over several Kelvin or more.


32/46

Eutectic Temperature

A eutectic system is a mixture of chemicalcompounds or elements that has asingle chemical composition that solidifies at alower temperature than any other composition.

This composition is known as the eutecticcompositionand the temperature is known asthe eutectic temperature.

Not all binary alloys have a eutectic point; forexample, in the silver-gold system the melttemperature (liquidus) and freeze temperature(solidus) both increase monotonically as the mixchanges from pure silver to pure gold.


33/46

Melting Point

The melting point of a solid is the temperature at whichthe vapor pressure of the solid and the liquid are equal.

At the melting point the solid and liquid phase exist inequilibrium. When considered as the temperature of the

reverse change from liquid to solid, it is referred to as thefreezing point or crystallization point.

Because of the ability of some substances to supercool, thefreezing point is not considered to be a characteristicproperty of a substance. When the "characteristic

freezing point" of a substance is determined, in fact theactual methodology is almost always "the principle ofobserving the disappearance rather than the formation ofice", that is, the melting point.


34/46

Boiling pointThe boiling pointof an element or a substance is the

temperature at which the vapor pressure of the liquidequals the environmental pressure surrounding theliquid.

The boiling point of liquids varies with and depends uponthe surrounding environmental pressure (which tends to

vary with elevation).The normal boiling point(also called the atmosphericboiling pointor the atmospheric pressure boilingpoint) of a liquid is the special case in which the vaporpressure of the liquid equals the defined atmosphericpressure at sea level, 1 atmosphere. Saturationtemperaturemeans boiling point.

The saturation temperatureis the temperature for acorresponding saturation pressure at which a liquid boilsinto its vapor phase. The liquid can be said to besaturated with thermal energy.


35/46

Melting Points, Boiling Points, Heat of Fusion and Heat of

Vaporization of Common Substances


36/46

Triple Point

The triple pointof a substance isthe temperature and pressure at whichthree phases (for example, gas, liquid, and solid)of that substance coexist in thermodynamicequilibrium.

The single combination of pressure andtemperature at which liquid water, solid ice,and water vapor can coexist in a stableequilibrium occurs at exactly 273.16 K (0.01 C)and a partial vapor pressure of 611.73 pascals(ca. 6.1173 millibars, 0.0060373057 atm).


37/46

Flash Point

The flash point of a volatile liquid is the

lowest temperature at which it can vaporize to form an

ignitable mixture in air. Measuring a liquid's flash point

requires an ignition source. At the flash point, the vapor

may cease to burn when the source of ignition isremoved.

The flash point is not to be confused with the autoignition

temperature, which does not require an ignition source.

The flash point is often used as a descriptive characteristicof liquid fuel, and it is also used to help characterize the

fire hazards of liquids.


38/46

Curie Point

the Curie temperature(Tc), or Curie point, isthe temperature at which a ferromagnetic or aferrimagnetic materialbecomes paramagnetic on heating; the effect is

reversible. An iron magnet will loseits magnetism if heated above the Curietemperature.

Below the Curie temperature, the ferromagnet isordered and above it, disordered. The saturation

magnetization goes to zero at the Curietemperature.


39/46

Electrical Conductivity

Electrical conductivityis a measure of a material's abilityto conduct an electric current. When an electrical

potential difference is placed across a conductor, its

movable charges flow, giving rise to an electric current.

The conductivity is defined as the ratio of thecurrentdensity Jto the magnitude of the electric (vector) field E:

Conductivity is the reciprocal (inverse) of

electrical resistivity, , and has the SI units

of siemens per metre (Sm-1) and CGSE

units of inverse second (s1):


40/46

Permittivity

Permittivityis the measure of how much resistance isencountered when forming an electric field in a medium.In other words, permittivity is a measure of howan electric field affects, and is affected bya dielectric medium.

In SI units, permittivity is measured infarads per meter(F/m).

Permittivity is determined by the ability of a materialto polarize in response to the field, and thereby reducethe total electric field inside the material. Thus,

permittivity relates to a material's ability to transmit (or"permit") an electric field.

Dielectricsmaterials possessing high electrical resistivites.


41/46

Dielectric Constant

The relative permittivity of a material for a frequency of zerois known as its static relative permittivityor asits dielectric constant.

The relative permittivityof a material under given

conditions reflects the extent to which itconcentrates electrostatic lines of flux. It is the ratio ofthe amount of electrical energy stored in a material by anapplied voltage, relative to that stored in a vacuum;similarly, it is the ratio of the capacitance of a capacitorusing that material as a dielectric, compared to a similarcapacitor which has a vacuum as its dielectric.


42/46

Dielectric strength

The dielectric strength of an insulating material, themaximum electric field strength that it can withstandintrinsically without breaking down, i.e., withoutexperiencing failure of its insulating properties.

Factors affecting dielectric strength

it increases with the increase in thickness of thespecimen. (Directly proportional)

it decreases with the increase in operating temperature.(Inversely proportional)

it decreases with the increase in frequency. (Inverselyproportional)

it decreases with the increase in humidity. (Inverselyproportional)


43/46

Piezoelectric Constant

Piezoelectric constantis the measure of charge which

accumulates in certain solid materials (notably crystals,

certain ceramics, and biological matter such as

bone, DNA and various proteins) in response to applied

mechanical strain. The wordpiezoelectricitymeanselectricity resulting from pressure.

Direct piezoelectricity of some substances like quartz, as

mentioned above, can generate potential differences ofthousands of volts.


44/46

Applications of Piezoelectricity

The best-known application is the electric cigarette lighter:pressing the button causes a spring-loaded hammer to hita piezoelectric crystal, producing a sufficiently high voltageelectric current that flows across a small spark gap, thusheating and igniting the gas.

Detection of pressure variations in the form of sound is the

most common sensor application, e.g.piezoelectric microphones (sound waves bend thepiezoelectric material, creating a changing voltage) andpiezoelectric pickups for Acoustic-electric guitars.

Loudspeakers: Voltage is converted to mechanical

movement of a piezoelectric polymer film. Inkjet printers: On many inkjet printers, piezoelectric

crystals are used to drive the ejection of ink from the inkjetprint head towards the paper.


45/46

THE END


46/46

chemical, thermal and electric properties of materials

Documents