SELECTING A MAGNET

Title: Selecting a magnet

In order to design the magic mat, the type of magnet to be used must be consider. Here are the properties for different types of magnets.

Contain:

1) Ferrite Magnet

Physical and Thermal Properties
Property	Typical Value
Coefficient of thermal expansion (25°C to 450°C)
Perpendicular to orientation	10 x 10-6 cm/cm/°C
Parallel to orientation	14 x 10-6 cm/cm/°C
Thermal conductivity	0.007 cal/cm-sec°C
Reversible temperature coefficient of residual induction	-0.2% /°C
Reversible temperature coefficient of intrinsic coercive force	0.2 to 0.5% /°C
Curie temperature	450°C
Maximum service temperature* (without metallurgical change)	800°C

2) SmCo Magnet

Physical and Thermal Properties
Description	SmCo 1-5 Alloys	SmCo 2-17 Alloys	NdFeB
Mechanical Properties:
Modulus of elasticy	23 x 106 psi	17 x 106 psi	22 x 106 psi
Ultimate tensile strength	6 x 103 psi	5 x 103 psi	12 x 103 psi
Density	8.2 g/cc	8.4 g/cc	7.4 g/cc
Coefficient of thermal expansion:
Perpendicular to orientation	13 x 10-6/°C	11 x 10-6/°C	-4.8 x 10-6/°C
Parallel to orientation	6 x 10-6/°C	8 x 10-6/°C	3.4 x 10-6/°C
Electrical resistivity	5µ ohm cm	86µ ohm cm	160µ ohm cm
Magnetic Properties:
Curie temperature	750°C	825°C	310°C
Reversible temperature coefficient of residual induction (-100°C to + 100°C)	-0.043% / °C	-0.03% /°C	-0.09 to -0.13% /°C
Recoil permeability	1.05	1.05	1.05
Max. service temperature*	250°C	300°C	150°C
* Maximum Service Temperature depends on permeance coefficient of magnetic circuit. Temperatures shown here are guidelines only

3) NdFeB Magnet

Physical and Thermal Properties
Description	SmCo 1-5 Alloys	SmCo 2-17 Alloys	NdFeB
Mechanical Properties:
Modulus of elasticy	23 x 106 psi	17 x 106 psi	22 x 106 psi
Ultimate tensile strength	6 x 103 psi	5 x 103 psi	12 x 103 psi
Density	8.2 g/cc	8.4 g/cc	7.4 g/cc
Coefficient of thermal expansion:
Perpendicular to orientation	13 x 10-6/°C	11 x 10-6/°C	-4.8 x 10-6/°C
Parallel to orientation	6 x 10-6/°C	8 x 10-6/°C	3.4 x 10-6/°C
Electrical resistivity	5µ ohm cm	86µ ohm cm	160µ ohm cm
Magnetic Properties:
Curie temperature	750°C	825°C	310°C
Reversible temperature coefficient of residual induction (-100°C to + 100°C)	-0.043% / °C	-0.03% /°C	-0.09 to -0.13% /°C
Recoil permeability	1.05	1.05	1.05
Max. service temperature*	250°C	300°C	150°C
* Maximum Service Temperature depends on permeance coefficient of magnetic circuit. Temperatures shown here are guidelines only Conclusion: Selecting a magnet should consider the above criteria such as its maximum temperature and so on. The best magnet would be NdFeB type which have high modulus of elasticy along with high electrical resistivity.