Exploring Material Properties: Thermal, Electric, Magnetic

• The presentation is about exploring the physical properties of materials
• The focus is on thermal, electric, and magnetic characteristics

Physical Properties

• Specific weight, also called specific gravity or relative density, is the ratio of a material's density to water's density for reference.
• Density is mass per unit volume
• Density’s formula is ρ = m / V
• Density’s units are kg/m³
• Specific heat is the amount of heat needed to raise the temperature of a unit mass of material by 1 Kelvin (or 1 degree Celsius).
• Specific heat is measured in J/kg·K
• Specific heat formula is S = Q / (m * Δt)
• Melting point is one of the three state change temperatures
• Melting point is the temperature (in °C or K) at which a substance changes from solid to liquid.
• Fusion latent heat is the heat required or released when a unit mass of material changes from one state to another during a phase change
• Fusion latent heat is denoted by 'L'
• Fusion latent heat is measured in J/kg
• Fusion latent heat forumla is L = Q / m
• Thermal conductivity represents how easily a material conducts heat
• Thermal conductivity is defined as the amount of heat transmitted by a unit thickness of material, normal to a unit area surface in unit time, when the temperature gradient across the material is unity under steady state conditions.
• Thermal conductivity is measured in W/m·K

Thermal Properties

• Thermal properties describe how materials respond to temperature changes.
• Understanding thermal properties is crucial in engineering, construction, and everyday life.
• Metals have definite melting points
• Alloys melt over a range of temperatures, depending on their composition.
• Melting point affects manufacturing operations like annealing, heat treatment, and hot working.
• Melting point is important in selecting tool and die materials
• Melting point considerations are important for casting

Specific Heat Capacity

• Specific heat capacity is the heat needed raise the temperature of 1 kg of a substance by 1 °C or 1 Kelvin
• Specific heat capacity is defined as Cp = dQ/dT
• Applications such as water are ideal for cooling because of high specific heat capacity.
• Raising the tempertature of a unit mass by one degree, which is measured in J/kgK
• This can result is excessive temperature rise in a work piece
• Excessive temperatures can decrease product quality (surface finish and dimensional accuracy), cause tool and die wear, and cause undesirable material changes

Thermal Expansion

• Thermal expansion is inversely proportional to melting point.
• Thermal expansion is important in forging processes
• Thermal expansion can lead to cracking, warping, or loosening of components.
• Low expansion alloys exist such as iron nickel alloys
• Linear expansion - the change in length of a solid material with temperature changes is expressed as ΔL / L₀ = αLΔT.
• Volumetric Expansion - the change in volume of a material with temperature changes is expressed as ΔV / V₀ = αVΔT.
• Thermal expansion coefficient is when a material is heated, its dimensions change
• Thermal expansion coefficient represents how much a material expands with temperature increase
• The coefficient is measured in K⁻¹
• Coefficient of Linear Thermal Expansion - Denoted by “αL”, it relates the change in length of an object to temperature change.
• Coefficient of Volume Thermal Expansion - Denoted by “αV”, it relates the change in volume of an object to temperature change.

Thermal Conductivity

• Thermal conductivity is the rate at which heat flows through a material.
• Metals have high thermal conductivity values while plastics and ceramics have poor conductivity
• High thermal conductivity is needed for cooling fins, cutting tools, and die casting molds.
• Low thermal conductivity is desired for furnace linings, insulation, and handles for pots and coffee cups.
• Thermal conductivity measures how easily heat flows through a material
• Metals have high thermal conductivity and readily transfer heat
• Insulators have low thermal conductivity and resist heat flow
• Liquids and gases have low thermal conductivity because their particles are far apart.
• Thermal conductivity indicates a material's ability to conduct heat.
• Thermal conductivity is represented by k and measured in watts per kelvin per meter (W⋅K⁻¹⋅m⁻¹).
• Thermal resistivity is the reciprocal of thermal conductivity.
• The Law of Heat Conduction is also known as Fourier's Law developed by Joseph Fourier
• Heat flows from hot to cold
• Fourier's Law is expressed as q/A = -k(dT/dx) where:
  • q/A is heat flux in W/m²
  • k is thermal conductivity in W/m*K
  • dT/dx is the temperature difference in °C
• The rate of heat flow through a conductor is proportional to the cross sectional area of the conductor and the temperature gradient
• Thermal conductivity k is the constant of proportionality
• Heat flows down a temperature gradient: ΔQ/ΔT = -kA(Δθ/Δx)
• So k is defined as the rate of heat flow of heat through unit area of cross section of 1m of material when the temperature difference between the surfaces is 1K.
• Steady state methods for measuring thermal conductivity
• Transient state methods for measuring thermal conductivity
• Steady state methods are used when materials are in equilibrium, meaning their temperature is constant
• Accurate readings can be taken and steady state implies consistent signals
• A disadvantage is that the method is slow because the material takes a long time to reach equilibrium
• These methods involve measurements where the temperature of the material does not change over time
• Analysis is relatively straightforward since, a key advantage of the steady-state techniques Examples include the Searle's bar method and Lee's disc method
• Transient state methods are used during the heating of a material
• Transient thermal conductivity methods don't need a constant signal
• Readings can be taken during heating
• Readings are not accurate
• Advantage involves that these measurements can be taken relatively fast
• Difficulties involve mathematical analysis
• Examples include transient plane source method, the transient line source method, and the laser flash method.
• Abrupt changes in a material's phase can change heat conductivity
• Differences in phonon coupling along a crystal axis cause different thermal conductivity values along different crystal axes.
• Thermal anisotropy means the direction of heat flow may not match the temperature gradient.
• The Wiedemann-Franz law relates electrical and thermal conductivity in metals
• The electrical conductivity does not affect heat conductivity of non-metals
• The Maggi-Righi-Leduc effect says that magnetic fields change a conductor's thermal conductivity
• Applying magnetic fields develops an orthogonal temperature gradient.
• Isotopic purity affects heat conductivity via the following example:
  • Type IIa diamond (98.9% carbon-12) has a thermal conductivity of 10,000 Wm⁻¹·k⁻¹
  • 99.9% enriched diamond has a thermal conductivity of 41,000 Wm⁻¹·k⁻¹.

Thermal Conductivity values

• Silver has a k of 422 W m-1 K-1, at room temperature metals feel cold
• Copper has a k of 391 W m-1 K-1, great for pulling away heat
• Gold has a k of 295 W m-1 K-1
• Aluminum has a k of 205 W m-1 K-1
• Stainless Steel has a k of 10-25 W m-1 K-1, cookware uses S.S.
• Glass Concrete, Wood has a k of 0.5-3 W m-1 K-1, suitable for buildings
• Many Plastics has a k of ~0.4 W m-1 K-1, at room temperature plastics feel warm
• G-10 fiberglass has a k of 0.29 W m-1 K-1, the strongest insulator choice
• Stagnant Air has a k of 0.024 W m-1 K-1 but is usually moving
• Styrofoam has a k of 0.01-0.03 W m-1 K-1 and is better than air

Conductivity through Walls

• Higher thermal resistance means less heat flow and better insulation and thermal resistance due to the air in the pores of insulation
• Insulation materials have high thermal resistance to prevent heat loss.
• Understanding thermal conductivity is crucial for energy-efficient building design.
• Applying Fourier's Law: Q = kA * (T₁ - T₂) / L, where:
  • Q is heat transfer
  • k is thermal conductivity
  • A is the perpendicular area to heat transmission
  • ΔT is temperature difference
• Composite structures: more than one insulation layer is placed in the path of heat flow by conduction.
• Fourier's Law: Qx = -KA*(Tf-Ti)/L
• The temperature gradient exists in a stationary medium will cause heat transfer through conduction
• Energy is transferred from high to lower molecular energy collisions