17 Nanocomposites for Permanent Magnets
427
Sol-gel synthesis followed by high-temperature reduction-diffusion was reported to
lead to the formation of a mixture of Nd 2 Fe 14 B, Nd 2 Fe 17 and α-Fe [61]. However,
further development is needed to produce pure Nd 2 Fe 14 B nanoparticles of controllable size with high remanent magnetisation and coercivity values. The chemical
synthesis of nanocomposites based on RE-lean (e.g. SmFe 11-1 Ti) or RE-free (MnBi,
FeNi, Fe 16 N 2 ) phases having even mid-range magnetic properties is also very attractive. Their successful fabrication would free up Nd now used in bonded NdFeB-based
magnets, allowing it to be reserved for where it is really needed, in very high energy
product magnets [62].
17.3.2 Compaction of Hard-Soft Nanocomposites
A number of novel compaction routes exploiting either low temperatures (down to
room temperature), short duration (down to microseconds) or high pressure (up to
tens of GPa) have been used for the consolidation of bulk nanostructured permanent
magnets, in most cases working with powders produced by melt spinning or high
energy ball milling [21]. Careful control of the processing parameters resulted in a
restraint of grain growth and in some cases actual grain refinement. Two such techniques, namely spark plasma sintering (SPS) and high-pressure warm compaction
(HPWC), have been used for the consolidation of chemically synthesised nanocomposites. Spark plasma sintering was carried out on a mixture of fcc FePt and Fe 3 O 4
nanoparticles under a pressure of 100 MPa, for a duration of 600 s and in the temperature range 400–600 °C [63]. Chemical ordering of the FePt nanoparticles to form L1 0
started at 500°C and was almost complete at 600°C. The average grain size of FePt,
as estimated from XRD analysis, increased from about 7 nm in samples consolidated
at 400°C to 17 nm in those consolidated at 600 °C. Fe 2 O 3 remained in the samples
consolidated at 400 and 500 °C but it was suppressed in the sample consolidated at
600 °C (Fig. 17.24). The latter sample contained 13 vol.% Fe 3 Pt, formed by diffusion between thermally reduced Fe 2 O 3 and neighbouring FePt grains. The maximum
density measured in these FePt/Fe 3 Pt hard/soft nanocomposite was 70% of theoretical full density. It remains to be seen if SPS can be used to consolidate to full density
and with a higher soft phase volume content. High-pressure warm compaction was
also used to produce FePt/Fe 3 Pt by consolidation of a mixture of fcc FePt and Fe 3 O 4
nanoparticles [64]. In this case, consolidation at 600 °C under a pressure of 3.8 GPa
led to about 95% of the theoretical full density of an FePt/Fe 3 Pt composite with
15% volume content of soft phase. The achievement of higher density with HPWC
compared to SPS is attributed to significant plastic deformation of the nanoparticles
under the much higher pressure used in HPWC. Lessons learned from the compaction
of nanocrystalline powders produced by melt spinning or high energy ball milling
using a range of consolidation techniques (SPS, HPWC, shock wave compaction,
microwave compaction) [21] can guide the application of such techniques to the
fabrication of fully dense chemically synthesised nanocomposites.
427
Sol-gel synthesis followed by high-temperature reduction-diffusion was reported to
lead to the formation of a mixture of Nd 2 Fe 14 B, Nd 2 Fe 17 and α-Fe [61]. However,
further development is needed to produce pure Nd 2 Fe 14 B nanoparticles of controllable size with high remanent magnetisation and coercivity values. The chemical
synthesis of nanocomposites based on RE-lean (e.g. SmFe 11-1 Ti) or RE-free (MnBi,
FeNi, Fe 16 N 2 ) phases having even mid-range magnetic properties is also very attractive. Their successful fabrication would free up Nd now used in bonded NdFeB-based
magnets, allowing it to be reserved for where it is really needed, in very high energy
product magnets [62].
17.3.2 Compaction of Hard-Soft Nanocomposites
A number of novel compaction routes exploiting either low temperatures (down to
room temperature), short duration (down to microseconds) or high pressure (up to
tens of GPa) have been used for the consolidation of bulk nanostructured permanent
magnets, in most cases working with powders produced by melt spinning or high
energy ball milling [21]. Careful control of the processing parameters resulted in a
restraint of grain growth and in some cases actual grain refinement. Two such techniques, namely spark plasma sintering (SPS) and high-pressure warm compaction
(HPWC), have been used for the consolidation of chemically synthesised nanocomposites. Spark plasma sintering was carried out on a mixture of fcc FePt and Fe 3 O 4
nanoparticles under a pressure of 100 MPa, for a duration of 600 s and in the temperature range 400–600 °C [63]. Chemical ordering of the FePt nanoparticles to form L1 0
started at 500°C and was almost complete at 600°C. The average grain size of FePt,
as estimated from XRD analysis, increased from about 7 nm in samples consolidated
at 400°C to 17 nm in those consolidated at 600 °C. Fe 2 O 3 remained in the samples
consolidated at 400 and 500 °C but it was suppressed in the sample consolidated at
600 °C (Fig. 17.24). The latter sample contained 13 vol.% Fe 3 Pt, formed by diffusion between thermally reduced Fe 2 O 3 and neighbouring FePt grains. The maximum
density measured in these FePt/Fe 3 Pt hard/soft nanocomposite was 70% of theoretical full density. It remains to be seen if SPS can be used to consolidate to full density
and with a higher soft phase volume content. High-pressure warm compaction was
also used to produce FePt/Fe 3 Pt by consolidation of a mixture of fcc FePt and Fe 3 O 4
nanoparticles [64]. In this case, consolidation at 600 °C under a pressure of 3.8 GPa
led to about 95% of the theoretical full density of an FePt/Fe 3 Pt composite with
15% volume content of soft phase. The achievement of higher density with HPWC
compared to SPS is attributed to significant plastic deformation of the nanoparticles
under the much higher pressure used in HPWC. Lessons learned from the compaction
of nanocrystalline powders produced by melt spinning or high energy ball milling
using a range of consolidation techniques (SPS, HPWC, shock wave compaction,
microwave compaction) [21] can guide the application of such techniques to the
fabrication of fully dense chemically synthesised nanocomposites.