Materials
The Materials element stores information about any materials used in the simulation. To use one of VSim’s pre-defined materials, highlight the Materials element and then switch from 3D View to Database in the Geometry View (see User Guide: Setup Window for Visual-setup Simulations for a picture of the Geometry View). To add one of the pre-defined materials from the table, highlight the material then press Add To Simulation button in upper right hand corner of the VSim Composer window. The material will now be listed under the Materials element. To access a wider selection of materials you may load the emthermal.vmat file by right-clicking on the Materials element and selecting Import Materials. You may also import your own customized material (see Customizing Materials below).
The editable properties of dielectric materials are:
- kind (not editable)
The kind of material (eg dielectric, conductor, particle absorber, permeable, etc), as defined in the .vmat file.
- conductivity
The conductivity of the material.
- relative permittivity
The relative permittivity of the material. Note that when a material is assigned to a geometry, the relative permittivity can be used in both the electrostatic and electromagnetic field solves. However, a VSim PD license is required to assign a dielectric material to a geometry when using the electrostatic field solve.
Drude-Lorentz and Debye-Lorentz Materials
VSim includes the ability to model frequency dependent dielectrics, using either the Drude-Lorentz or Debye-Lorentz models.
The Drude model focuses on the zero frequency limit of conductance, while the Debye Relaxation focuses on the zero frequency limit of dielectric. The Drude model assumes unbound charge carriers which undergo collisions, resulting in a frequency dependant conduction current. The Debye Relaxation Model assumes bound charge carriers whose dielectric response relaxes at higher frequencies.
In general Drude models are more advantageous for conductivity dominated dielectrics while Debye models are for permittivity dominated dielectrics.
Each model makes use of Lorentz resonances to handle the middle frequencies of the dielectric. The Lorentz model assumes bound charge carriers whose response is resonant at a material specified frequency and line width. Any number of Lorentz resonances may be specified.
The total conduction current of the dielectric is given by the specified Infinity Limit Current + Drude/Debye Current + Sum of all Lorentz Currents.
These materials are most commonly used in plasmonics and photonics simulation problems.
Drude-Lorentz Material Parameters
- collision function
A function describing the collision frequency of the unbound charge carriers used in the Drude model.
- conductivity function
A spatial function describing the conductivity of the material.
- lorentz oscillator strength
A vector describing the oscillation at each lorentz resonance, the density of bound charge carriers seeing the lorentz resonance. Given in units of 1/(ohms*meters*seconds)
- lorentz frequency
A vector of frequencies of the Lorentz resonances, in Hz.
- lorentz line width
A vector of the line widths (bandwidths) of the Lorentz resonances, in 1/seconds.
- relative permittivity at infinite frequency
The permittivity of the material at infinite frequencies. Used to keep the simulation stable.
- conductivity at infinite frequency
The conductivity of the material at infinite frequencies. Used to keep the simulation stable.
- background conductivity
The conductivity of the simulation space outside of this material.
Debye-Lorentz Material Parameters
- relaxation function
A function describing the time scale at which dielectric response relaxes from the specified permittivity function to the specified relative permittivity at infinite frequency.
- permittivity function
A spatial function describing the relative permittivity of the material.
- lorentz oscillator strength
A vector describing the oscillation at each lorentz resonance, the density of bound charge carriers seeing the lorentz resonance. Given in units of 1/(ohms*meters*seconds)
- lorentz frequency
A vector of frequencies of the Lorentz resonances, in Hz.
- lorentz line width
A vector of the line widths (bandwidths) of the Lorentz resonances, in 1/seconds.
- relative permittivity at infinite frequency
The permittivity of the material at infinite frequencies. Used to keep the simulation stable.
- conductivity at infinite frequency
The conductivity of the material at infinite frequencies. Used to keep the simulation stable.
- background conductivity
The conductivity of the simulation space outside of this material.
PEC Materials
PEC is an acronym for Perfect Electric Conductor. It should be used when modeling any primary conductor within VSim. For example while copper or stainless steel do have some resistivity to them, it is not worth attempting to model that resistivity within the timsescale that FDTD simualtions operate in.
Particle Absober Materials
The material absorbium exists within VSim for use as a particle absorber. If a shape has this material assigned to it there will be no impact on the field solve of the simulation, however cut-cell boundary conditions can then be applied to the shape so that particles may be absorbed.
Customizing Materials
Custom materials can be created within the simulation environment, or added to the materials database for reuse in any simulation.
Material Creation within Simulation
To create a new dielectric within the simulation space import the Material named Custom
The name of this material can then be changed, and a conductivity and relative permittivity set to the desired values. If conductivity is changed from 0 the material will be modeled as a lossy dielectric.
Material Addition to Database
Custom materials properties can be created in a text editor and imported into VSim. To import a custom material:
1. Go to the “data” folder at the top level of the VSim installation directory.
2. Create a new text file that ends in the .vmat extension. For example: emthermalcustom.vmat.
3. Open the default materials file emthermal.vmat.
4. Copy a block from emthermal.vmat to emthermalcustom.vmat to use as a sample.
5. Edit the title and properties of the block as needed. For example:
<Material FreshWater> <strings> kind = "dielectric" </strings> <params> heat capacity = 4.184 conductivity = 0.005 relative permittivity = 80.4 thermal conductivity = 0.6065 </params> </Material>6. Save the new file.
7. Back in VSim, click on the Materials element in your simulation and then click Add –> Import Materials. A file browser will appear with the data directory already open.
8. Select your custom vmat file. If you do not see your vmat file in the directory, navigate to where you saved it.
9. The view will change to the Database tab and the materials added will be available here.