Thermal Conductivity

It is well-known that the low thermal conductivity of PCM increases its thermal response time for storage and release of latent heat. Thus, enhancing the thermal conductivity of PCMs is a major aim in designing a nano/microencapsulated material which attracts a vast attention. Measuring these thermo-physical properties was a challenge for a long time since different methods and techniques presented different results. Thus, the method which is going to be used is significant to lower the measurement error and uncertainty as much as possible. In this study, two thermal conductivity measurement methods were employed including Transient hot-wire techniques and laser flash method.

Generally, Fourier’s law for conduction heat transfer can be utilized to measure thermal conductivity of a material. A temperature difference can cause heat transfer through materials which is known as conductive heat transfer. Thermal conductivity is the property of the material which relates heat flux to temperature gradient and can differ from one material to another. It can be calculated from the following equation:

k = (q A ) (∆T L) (‎3-11)

Where, k is thermal conductivity, q is magnitude of heat transmission, ∆T is temperature difference, A is cross sectional area and L is the length (Ehsan Bitaraf et al., 2012). To simplify calculations, a one-dimensional temperature field and steady state flow are considered.

3.4.7.1 Transient Hot-Wire Method (THW)

This is the most regular and the oldest method of measuring thermal conductivity. This technique has been well developed and widely used for measurements of the thermal conductivity of solids, and in some cases, the thermal diffusivity of fluids with a high degree of accuracy. Comparing other methods, the most

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attractive advantage of the technique is its very fast measurement with high accuracy and the simplicity of the conceptual design of the apparatus of it as well. The transient hot-wire technique was first introduced by (Carslaw & Jaeger, 1986). It has been used to measure thermal conductivity of powders at the beginning. However, it has been improved by many researchers. The THW method is based upon using a long, thin platinum wire as a dual line heat source and temperature sensor. This is possible due to the relatively unique relationship between the temperature and thermal characteristics of platinum. The platinum filament is fully placed within the material for which the thermal conductivity is to be determined, and a step increase in the electrical power supplied to the wire is introduced. This allows the platinum wire to heat up due to resistive heating. The excess heat from the hot filament is rejected to the surrounding material through conduction. Simultaneously, while this heating occurs, the relative change in resistivity of the wire is being measured through a two or four wire resistive measurement system. The surface temperature of the hot wire and therefore the temperature of the immediate surrounding can be calculated based upon this approach. The measured value of total input power, and power lost to the surrounding, combined with the overall change in resistivity of the platinum wire due to electrical heating are collected; these values, in conjunction with the measured experimental time and dimensional parameters of the hot wire setup, can be used to back calculate the thermal conductivity of the material of interest.

In this study a KD2-Pro system was used to measure the thermal conductivity of some NE/MEPCMs. The KD2 Pro resolves to 0.001℃ in temperature by utilizing special algorithms to analyze measurements made during a heating and a cooling interval. Algorithms also separate out the effects of the heat pulse from ambient temperature changes. The prepared samples were placed in a proper container in a

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circulating bath while the temperature was controlled and a dual needle apparatus was used to measure the thermal conductivity of the samples.

3.4.7.2 Laser Flash Technique (LFA)

For the precise and reliable measurement of thermophysical properties like the thermal diffusivity and thermal conductivity, the Laser Flash technique (LFA) has proven itself as a fast, versatile and precise absolute method. The Laser Flash technique is based on the measurement of the thermal transient of the rear surface of the sample when a pulsed laser illuminates the front and it avoids interferences between the thermal sensor and the heat source. The physical model of the Laser Flash measurement supposes to have a single pulsed heat source (delta like), for example a laser shot, on the sample front surface. The study of the thermal transient of the rear surface provides the desired thermal information. The temperature of the rear face is measured with an infrared detector and it can be expressed mathematically as a function of several variables that are grouped into dimensionless parameters. These variables include sample geometry, thermal diffusivity and heat loss from the sample (Mirmohammadi & Behi, 2012).

The laser flash technique (Netzsch LFA 447 NanoFlash) was used to measure the thermal diffusivity of samples at 35 and 80℃. The thermal conductivity may also be derived from the thermal diffusivity when the specific heat and the sample bulk density are known as shown in equation (‎3-12):

k = α. ρ. C_{p (‎3-12)}

Where k is the thermal conductivity (W/ (m.K)), α is thermal diffusivity (m2/s), ρ is density (kg/m³) and Cp is specific heat capacity (J/(kg.K)).

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