ALGUNAS DIFERENCIAS ENTRE LA TCT Y LA TRI

The binary iron-boron sample, GDF5761 (p. 108), has a higher magnetic saturation (138emu-g ’) than the Fe-Zr-B preparations, a lower boron content (26at.%), and a good analyte recovery. It shows various stages of boride contamination at 500°C and above (see

fig. in.D.7), and proved a vital reference when analysing the XRD data from the ternary

preparations. The boride pattern at 500®C has a significant Fe^B-like pattern (as well as Fe^B), in comparison with the Fe-Zr-B data which shows primarily FCjB boride. This might be expected from the greater overall iron to boron ratio of the FeB preparation. At 600°C the Fe^B strengthens, at the expense of the Fe^B. This might imply a migration of the iron fix)m the boride to enlarge the crystalline bcc iron, in the process enriching the

residual boride phase in boron. At 750°C the Fe^B has fully-formed, with fully sharp diffraction peaks perfectly matching library data.

GDF5761 /500°C GDF5761 /600°C

GDF5761 /750°C

[ intentionally blank ]

Fig. III.D.7: Diffraction patterns for the binary iron-boron preparation, following heat treatment. This series provided useful Fe-B reference patterns: GDF5761 /500°C, Fe^B with some Fe^B phases; GDF5761 /600°, (possibly nanocrystalline) Fe^B phase; GDF5761 /750°, well defined classic Fe^B pattern, perfectly matching the library data. Strong peaks at 19.7®, 27.9° and 34,4° are bcc iron.

vi. GDF5741 - FeZrB (salts solution added to borohydride)

From its diffraction patterns, Môssbauer spectra and DSC curve, sample GDF5741 (p. 103) immediately stands out as radically different from all the other iron-zirconium- boron preparations. In contrast to the rest, this is the one sample which was synthesised by adding the metal salt solution to the borohydride. The Môssbauer spectra shows a prominent doublet, attributed to an Fe(II) complex. DSC shows no clear crystallisation temperature, the magnetic saturation is less than half that of any other sample, and it shows an exceptionally low analyte recovery (55%). On heating, a completely different set of crystalline phases appears, while a large fraction of non-crystalline material continues to contribute a high background (see fig. lU.D.S). In this instance, the chemistry has clearly produced something different.

GDF5741 /750°C GDF5741 /600°C

Fig. ni.D.8: Dif&action patterns for the sample prepared by adding the metal salt solution to the borohydride solution after heat treatment. Note the complexity of the patterns, lack of bcc iron, and high background.

vii. GDF5881 - FeCoB

Finally, the iron-cobalt-boron preparation, GDF5881 (p. 109), shows lower boron content

(21%) and higher coercivity (600 Oe) than the other preparations, and there is indirect

evidence that the product (without being heavily compressed) has much lower density than the others - despite filling a DSC cell, this only constituted 6.1 mg, compared to ~20mg for most of the other samples in the same volume. Insufficient sample mass may explain the poor DSC (which was repeated, with similar results). This might imply a needle-like structure, for example that described by Oppegard e/ <a/ [1961].

viii. Melt spun ribbons ‘B2’ and ‘B6’

The ribbons (p.llO and 111) appear to be fully amorphous as made, and crystallise to a

pure nanocrystalline state (with small residual amorphous fraction) following heating to

600°C (for example see fig. III.C.3, p. 122). ‘B6’ shows a ferromagnetic hysteresis loop

with very low coercivity (<18 Oe) and high initial permeability, while ‘3 2 ’ is

paramagnetic at room temperature.

11. Chemistry

We have so far accepted that, although firm evidence is difficult to obtain, the likelihood is that many of the chemically-precipitated Fe-M-B ‘alloys’ reported in the literature probably do consist of a largely homogenous distribution of atoms with uniform

concentration. The variation of crystallisation temperature with M, reported by Inoue et al

[1988], and the evidence of an alloy sextet with a significantly higher hyperfine field

(365kOe) than iron in an aimealed sample of (FeQ7CoQ^)B reported by Li and Xue [1993]

are particularly convincing. In contrast the strong evidence for ZrO^ phases and the general lack of systematic variation of crystallisation temperature, Môssbauer spectra or

coercivity with the zirconium amount in ‘Fe-Zr-B’ preparations makes a truly homogeneous product look unlikely. Can this absence of mixing be related to the chemistry of the preparation?

Cobalt and nickel, present in the more widely-studied and accepted Fe-M-B chemically reduced alloys (and to a lesser extent chromium and manganese) exhibit chemical and

physical properties much more akin to iron than does zirconium (see table in.D.5). Since

the chemical conditions (concentration, pH, temperature etc) required for a given reaction will depend on the electrode potential, as the electrode potentials of the two metal ions become more different, the likelihood of their co-precipitation to pure metals under the same conditions might be expected to decrease.

Atom Reduction Electrode

potential Atomic radii /pm Iron Fe^+ Fe -0.447V 124.1 Cobalt Co —> Co -0.28V 125.3 Nickel Ni^^-»N i -0.257V 124.6 Manganese Mn^^ —> Mn -1.185V 136.7 Chromium Cr^^ Cr -0.91 124.9 Copper Cu^^ Cu 0.341V 127.8 Zirconium Zr'^^-^Zr -1.45 V 159.0

Table m.D.5: Comparison of some properties of iron with other transition metals commonly alloyed. (Note that chromium and iron are also commonly tri-valent)

Further evidence is that binary amorphous Co-B and Ni-B alloys have been precipitated

singly [Shen et al 1993]. This alone says nothing about how or the scale on which the

phases might mix during a co-precipitation, but confirms that such chemistry is realistic. By contrast, zirconium has a much greater electrode potential — which might be expected to influence the dominant chemical reactions or their optimum pH, furthermore zirconium has a reputation for attracting oxygen. From our records at UCL, Forster had attempted to make a binary Zr-B alloy, but it was not reckoned to be successful. The DSC trace was very peculiar, though unfortunately the corresponding XRD pattern cannot be found. If binary Zr-B alone cannot be formed under similar conditions to Fe-B, then a mixed Fe-Zr-B co-precipitation — although conceivably possible via some intermediate reaction — would seem unlikely.