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The Materials... list of the Device menu displays all of the materials defined for the device. (If more than one device is loaded in a layout, the same set of materials apply to all devices.) Only the materials assigned to components (in the Components list) will actually be used in thermal simulations. The color, description, and thermal properties of any material can be changed by double clicking on the material in the list to open the Material Properties dialog.
In the Material Properties dialog, the color of the material can be changed by pressing the color button next to the material name. The description of the material can be changed by editing the description box. The description string should not contain quotation marks, commas, or the greater than (>) or less than (<) signs. Temperature-dependent thermal properties can be modified by editing or entering new values in the tables. Save the changes by pressing the OK button, or discard them by pressing the Cancel button.
Thermal conductivity defines the rate of heat transfer in a material. Thermal conductivity values are given in watts per micron kelvin at each temperature in kelvin. Conductivity values are linearly interpolated between temperatures and given the nearest table entry for temperatures outside the range of temperatures entered. Entering a value at only one temperature creates a temperature-independent conductivity.
Note: While nominal thermal conductivities are already available in the generic template for many materials, these values might not include the requisite values over the whole temperature range of the simulation. If a material has highly temperature-dependent conductivity, the user should input the additional thermal conductivity values at other temperatures. Details on material properties provided by the basic device templates can be found in the Appendices.
Materials can have the same conductivity along the x-, y- and z-axes (isotropic), or have different conductivities in the three directions (orthotropic). When the Isotropic box is checked, only the kx values will be used to define the conductivities – values in the ky and kz columns will be ignored. The thermal conductivities of orthotropic materials can be rotated by using the >> button next to the isotropic check box.
Steady state solutions only require thermal conductivity values. To obtain transient solutions, specific heat capacities and mass densities must also be defined for each material used. Specific heat capacity is given in joules per milligram kelvin, and mass density is given in milligrams per cubic micron. These consistent units give a relatively easy way to translate SI units using exponential notation. For example, if a material (silver) has a thermal conductivity k=429 W/m•K, specific heat capacity c=232 J/kg•K, and mass density r=10500 kg/m3 at 300 K, values could be entered as shown in the following figure:
The tables can be navigated while editing by using the Tab and Enter keys. Tab moves to the next column, while Enter moves to the next row. Shift+Tab moves to the previous column in the table. The up arrow key moves to a previous table row, and the down arrow moves to the next one. The Insert key can be used to terminate editing which switches the up/down arrow keys to list navigation. When in list navigation mode, a new row can be inserted above the current row using the Insert key, and a row can be deleted using the Delete key. In this mode the Tab key will switch the keyboard focus to the next table or button in the dialog.
A property can be entered using a formula by preceding the formula with an equals sign, as shown below. First enter a temperature in the left hand column, then enter the formula in the property value column using the letter T to reference the temperature. When the formula is complete press Tab or Enter to have the formula evaluated and replaced with a real value. To use the same formula on multiple entries, copy the formula using Ctrl-C before it is evaluated. Paste the formula in subsequent entries using Ctrl-V.
The emissivity is a surface property of a material that indicates its effectiveness in losing heat as photon radiation. Emissivity is a scalar value defined as the ratio of energy radiated from the material's surface to the energy radiated by a blackbody at the same temperature. Hence, it is a value less than 1 (perfect emitter), but greater than zero (perfect reflector). For example, the emissivity of polished aluminum is about 0.05 while the emissivity of anodized aluminum is about 0.77. The emissivity property is only used for the thermal radiation boundary condition. If this type of bouundary condition is not used on a material, then that material does not require any values be entered for emissivity.
The viscosity property is not used in thermal simulation at all. This property is only used by the Heat Transfer Coefficient Calculator to assist in the selection of an appropriate heat transfer coefficient for a film boundary condition. Only materials with viscosities can be used as heat transfer fluids in the Calculator. For more information, please see Choosing Appropriate Boundary Conditions in the chapter on Analysis Methods.
New materials can be easily defined by double-clicking on the Add... item at the bottom of the Materials list. This will open a blank Material Properties dialog. Type a new, unique material name up to 15 characters long. A valid material name cannot contain quotation marks, spaces, commas, or the greater than (>) or less than (<) signs. Invalid characters will be replaced by underscores. To copy properties from an existing material, enter the name of that material and press the Enter key. The dialog will then be filled with the copied properties. Change the name of the material to a unique name before exiting the dialog or the changes will not be accepted.
To delete a material, highlight the material in the Materials list and press the Delete key. Only materials that are not found on any components can be deleted from the list.
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