4.3. The Characteristic Equation of Carbon Concentration and Resistance
Through several experiments, derived the characteristic equation of graphite concentration and resistance as follows:
In Formula (1), C
0 is the lowest doping concentration for using polymers as practicality resistance material; ρ
0 is the resistivity of the lowest doping concentration; C
C is the doping critical concentration of the steadying conducting state; C
S is the doping saturation concentration of saturated adsorption; ρ
S is the resistivity of saturation concentration in saturated adsorption; Q
m is the quality factor of material, which is dimensionless. The Q
m of graphite is 4. When temperature raise to 300k, the C
0 of polyurethanes rubber which doping with graphite, is 75M, the C
0 of silica is 45M, the C
C of polyurethanes rubber is 80M, the C
C of silica is 50M, the C
S of polyurethanes rubber is 90M, the C
S of silica is 60M, the ρ
0 of polyurethanes rubber is 2.08×10
6Ω-cm, the ρ
0 of silica is 6.16×10
7Ω-cm, the ρ
s of polyurethanes rubber is 3.04×10
2Ω-cm, the ρ
s of silica is 5.60×10
2Ω-cm. When C
D is smaller than C
C, Formula (1) can simplify as follows:
4.4. The Characteristic Equation of Resistance and Temperature Coefficient
Physisorption is not a chemical reaction, needed no activation energy, therefore it can react easily and quickly respond at low temperature. Adsorption is exothermic, so the resistance, which caused by the physisorption of polymer, proportional to temperature. The weak Van der Waals force is the main force of physisorption. Conservation of energy dictates that electron transfer between an electrode and a redox system in solution or adsorbed on the electrode surface is fastest when the energy of the electron is equal in the metal and in the thermally activated redox system [
28,
29]. The physisorption easily affected by temperature, so the resistance of temperature coefficient varies greatly. When the polyurethanes rubber doping whit 80M graphite, the characteristics of resistance temperature as
Figure 9. The characteristic equation of resistivity (ρ) and temperature as follows:
In Formula (3), T0 is the temperature of starting desorption; ρ0 is the resistivity at T0, Tpl is the starting temperature of positive temperature coefficient linear region, ρpl is the resistivity at Tpl, Tc is the critical temperature of positive and negative temperature coefficient of rubber polymers, ρC is the resistivity at TC, Tnl is the termination temperature of negative temperature coefficient linear region, ρnl is the resistivity at Tnl, Tm is the melt temperature of rubber polymers, ρm is the resistivity at Tm, Te is the environment temperature. QTd is the characteristic of material thermodynamics is dimensionless. The QTd of graphite is 2. Dm is the characteristic of material desorption, is dimensionless. The Dm of graphite is 2. When polyurethanes rubber doping with 80M graphite, the T0 =267K, Tpl =312K, TC = 319K, Tnl = 322.5K, Tm = 343K, ρ0 = 1.296×103Ω-cm, ρpl = 4.88×104Ω-cm, ρC = 5.712×104Ω-cm, ρnl = 4.936×104 Ω-cm , ρm = 7.04×103Ω-cm.
When the silica doping whit 60M graphite, the characteristics of resistance temperature as
Figure 10. By linear segments can explain the relationship between resistivity (ρ) and temperature, as piecewise-linear equivalent. Because the linear are close to the actual curve, the characteristic equation as follows:
In Formula (4), T0 is the temperature of starting desorption; ρ0 is the resistivity at T0, Tcl is the first section critical temperature of positive temperature coefficient linear region, ρcl is the resistivity at Tcl, Tc2 is the second section critical temperature of positive temperature coefficient linear region of rubber polymers, ρc2 is the resistivity at Tc2, Tm is the melt temperature of rubber polymers, ρm is the resistivity at Tm, Te is the environment temperature. QTd is the characteristic of material thermodynamics is dimensionless. The QTd of graphite is 2. Dm is the characteristic of material desorption, is dimensionless. The Dm of graphite is 2. When polyurethanes rubber doping with 60M graphite, the T0 = 283K, Tcl = 350.5K, Tc2 = 368K, Tm = 373K, ρ0 = 1.83×101 Ω-cm, ρcl = 3.74×102Ω-cm, ρc2 = 7.16×102Ω-cm, ρm = 9.85× 102 Ω-cm.