Characteristics of Neodymium NdFeB Magnets

This section provides information of the physical properties of Neodymium Iron Boron (NdFeB) magnets.

As already discussed in previous sections, the NdFeB magnet has various grades:- each grade has its own magnetic properties (relating to strength of magnetic field output and resistance to demagnetisation, maximum recommended operating temperature and temperature coefficients).

The grades have other physical properties which are similar between the grades. Below is an overview of these properties:-

Summary of Physical Properties of Neodymium Iron Boron, NdFeB, magnets

Characteristic	Symbol	Unit	Value
Density	D	g/cc	7.5
Vickers Hardness	Hv	D.P.N	570
Compression Strength	C.S	N/mm2	780
Coefficient of Thermal Expansion	C//	10-6/°C	3.4
	C^	10-6/°C	-4.8
Electrical Resistivity	r	µ Ω.cm	150
Temperature Coefficient of Resistivity	a	10-4/°C	2
Electrical Conductivity	s	106S/m	0.667
Thermal Conductivity	k	kCal/(m.h.°C)	7.7
Specific Heat Capacity	c	kCal/(kg.°C)	0.12
Tensile Strength	σUTS, or SU	kg/mm2	8
Young’s Modulus	l / E	1011N/m2	1.6
Flexural Strength	b	10-12m2/N	9.8
Compressibility	s	10-12m2/N	9.8
Rigidity	E.I	N/m2	0.64
Poisson’s Ratio	n		0.24
Curie Temperature	Tc	°C	310

Structural use of Neodymium Iron Boron, NdFeB, magnets
There is a risk of chipping or breaking the magnets because all magnets are inherently brittle. The Neo magnets are less brittle than SmCo. It is advised to not put magnets in conditions of mechanical stress e.g. in load bearing situations.

The Effects of Radiation on Neodymium Iron Boron, NdFeB, magnets
The NdFeB magnets may be demagnetised by radiation. The Neodymium Rare Earth magnets do not perform as well as SmCo Rare Earth magnets. E.W. Blackmore, (TRIUMF, 1985) and A.F. Zeller & J.A. Nolen (National Superconducting Cyclotron Laboratory, 09/87) demonstrated SmCo having a better performance, with Sm²Co¹⁷ offering 2-40 times better radiation resistance than NdFeB. Some NdFeB grade are demagnetised to half their maximum performance with a proton beam radiation of 4 x 10⁶ rads and are completely demagnetised with a proton beam radiation of 7 x 10⁷ rads. A rule of thumb is to select magnets with higher Hci values, designed to operate at high Pci and, where possible, to have radiation shielding protecting them when being subjected to any levels of radiation. The user of the magnets would need to test for effectiveness of the magnets as the magnet suppliers do not have the equipment to test for suitability of magnet grades for environments with raised levels of radiation.

Neodymium Iron Boron, NdFeB, magnets and corrosion resistance
The NdFeB magnets require a protective coating / surface finish to minimize the effects of corrosion. Iron within the structure can ‘rust’ which causes a permanent structural change in NdFeB which results in a permanent weakening of the magnetic performance – the worst case scenario is a total loss of magnetism.

A NdFeB magnet kept in dry conditions will not corrode and will retain its performance theoretically for ever (if not subjected to excessive heat, radiation or strong external magnetic fields). If the conditions are wet, it is recommended that alternative magnets be considered for use of that the magnet design try to protect the magnet from moisture (e.g. encasing, modified coatings such as zinc plus rubber, etc). The plating / surface finish should be hermetic for best corrosion protection – scratched or damaged surfaced may render the affected region more prone to corrosion. Marine environments (salt sprays, sea water) are particularly corrosive and far from ideal for NdFeB. In critical applications where corrosion and magnet failure are unacceptable, magnets such as ferrite or SmCo may be more suitable. Please note that any claims that a NdFeB magnet will not corrode is misleading. It is claimed that higher Hci magnets resist corrosion better although the empirical results are not so conclusive (a trend suggesting an improvement in corrosion resistance exists but it is not guaranteed). It is the application and the overall design that determines how well the magnet will perform in damp environments.

Table comparing main coating types

COATING APPLIED		NICKEL		EPOXY RESIN		Ni + EPOXY
		Electroless		Powder Spray Coating	E-Coating	Nickel plating + Epoxy E-Coating
Coating Thickness	Range (microns)	12 to 25	25 to 40	20 to 40	15 to 25	25 to 40
	Homogeneity	Excellent	Good	Poor	Excellent	Good
Effectiveness versus Magnet Size	Small (<20 grams)	Excellent	Good	Fair	Good	Good
	Large (>20 grams)	Fair to Good	Good	Fair	Good	Good
Hours before coating is likely to fail	Temp. & Humidity (60ºC, 95%RH)	>2500		>500	>1500	>2500
	Temp. & Humidity (85ºC, 85%RH)	>500		>100	>300	>500
	Salt Spray (35ºC, 5% NaCl)	>48		<24	>100	>200
Coating Colour		Silver	Silver	Black	Black	Black
Heat Cycle		Fair	Fair	Fair	Fair	Fair
Heat Resistance		Poor	Poor	Poor	Poor	Poor
Collision Test		Fair	Fair	Fair	Fair	Fair
Film to material adhesion test		Fair	Fair	Fair	Fair	Fair
Glue adhesion test		Fair	Fair	Fair	Fair	Fair
Tolerance accuracy		Excellent	Excellent	Fair	Fair	Fair to Poor
Additional Remarks		15-30 microns Ni-Cu-Ni Standard coating		Epoxy resins are not hermetic		Thickness buildup can be a problem