code: def _create_air():
      
      name = "Air"
      namel = name.lower()
      mm = 28.9645  # g/mol
  
      ds_dict = _create_ds_dict([
          "dataset-air-lienhard2015",
          "dataset-air-lienhard2018"])
      active_ds = "dataset-air-lienhard2018"
  
      # create polynomial models to describe material properties
      #   comment it out after model creation is complete, so that it does not
      #   run every time during use.
      # _create_polynomial_model(name, "Cp", 13, ds_dict[active_ds], ds_dict)
      # _create_polynomial_model(name, "k", 8, ds_dict[active_ds], ds_dict)
      # _create_polynomial_model(name, "mu", 8, ds_dict[active_ds], ds_dict)
      # _create_polynomial_model(name, "rho", 14, ds_dict[active_ds], ds_dict)
  
      # IgRhoT(mm, 101325.0).plot(ds_dict, _path(f"data/{namel}-rho-igrhot.pdf"))
  
      model_dict = {
          "rho": IgRhoT(mm, 101325.0),
          "beta": IgBetaT()}
  
      model_type = "polynomialmodelt"
      for property in ["Cp", "mu", "k"]:
          name = f"data/{namel}-{property.lower()}-{model_type}-{active_ds}.json"
          model_dict[property] = PolynomialModelT.read(_path(name))
  
      material = Material(name, StateOfMatter.gas, model_dict)
  
      return material, ds_dict
docs: Create a dictionary of datasets and a material object for air.
  
      :return: (Material, {str, DataSet})