X-Ray Diffraction

Dynamic Temperature X-Ray Diffraction

                  Analysis of Glasses

Glasses or amorphous materials are frozen-in liquids, i.e. atoms / molecules are distributed
haphazardly throughout the material, without any periodic order as shown in Fig. 1(b).

                      (a) Crytalline                         (b)  Amorphous
                                                Fig. 1

The glasses may be catagorized into 3 distinct species as to their ease of formation:

    :-  Metallic  glasses: are obtained by rapidly quenching from metallic melts,
           e.g. Mg-Zn, Fe-B glasses etc; if the metallic melts are not cooled fastly,
           they crystallize to form crystalline solids.
    :- Inorganic glasses: are obtained by moderately cooling from the melts of
           inorganic materials, .e.g. silicates. For crystallization, their melts have to
           be cooled very slowly, otherwise they will solidify as amorphous.
    :- Organic glasses: are obtained from organic material melts some of which
            never crystallize whatever the cooling rate.

The crystallization behaviour ( phase analysis at crystallization) is usually studied
by thermal analysis methods, e.g. differential thermal analysis (DTA), differential
scanning calorimetry (DSC). In these methods the heat of crystallization stored in
glass is given up at the crystallization temperature, which is then registered as a peak
in the dT versus time (t) or Cp versus temperature (T) curves, where dT = Ts-Ti.
Ts and Ti are temperatues of the sample and an innert substance.  A disadvantage
of the thermal analysis methods in phase analysis is that no structural information
about the crystallization products is deciphered.

Structural information about the crystallization products may be secured by taking
at room temperature x-ray diffraction patterns of the glass samples tempered at
different temperatures. However, by this static temeprature x-ray diffraction analysis
a lot of valuable information about the crystallization mechanism remains hidden.

To decipher the phase formation at and after crystallization of a glass, the best
method would be to continuously register the x-ray diffraction pattern during
continuously heating the glass sample, i.e. recording a dynamic temperature x-ray
diffraction (DTXD) pattern of the glass sample under study. The author has
developed a DTXD apparatus [cf. J. Phys: Sci. Instru. 85 (1985)1054]  in his
laboratory and investigated a number of metallic and inorganic glasses.
DTXD patterns of important glasses e.g.
    :- MgZn30  (Liquid Quenched)
    :- CoB18  (Liquid Quenched)
  :- CoB18  (Vapor Deposited)
    :- CoB25  (Liquid Quenched)
    :- FeB25  (Liquid Quenched)
    :- PdSi13 (Liquid Quenched)
   :- Sm2(CoB25)98   (Liquid Quenched)
    :- Sm4(CoB25)96  (Liquid Quenched)
    :- ZrFe12Si2B  (Liquid Quenched)
    :- Zr1.67Fe12Si2BZr (Liquid Quenched)
    :- Bi4SrSCa2Cu4O14 (Liquid Quenched)

demonstrate the usefulness of this technic for structural analysis.

The potential of the DTXD method lies in the fact that continuity of the X-ray
diffraction lines reveals a lot of minute, hidden details about the crystallization
and post crystallization behaviour of glasses and melts not accessible by other
means, e.g. it is believed that X-ray diffraction halo of glasses is not structured,
but smooth and homogeneous in intensity. DTXD analyses show that it is only
true for those glasses which do not contain X-ray absorbing atoms, i.e. the
structure of the amorphous halo depends upon the absorption coefficient of the
constituents of the glass under study; the higher the absortion coefficient of the
glass constituents, the more structured is the halo, see for example DTXD
patterns of CoB25, Sm2(CoB25)98 and Sm4(CoB25)96; it can be seen that halo
is structured into bands, structuring increases with increasing content of Sm
due to L-absorption edge of Sm at CoKa wavelength. This effect can be used
to find atomic pair distances in amorphous materials.

Another important result of DTXD analysis of glasses is that a glass crystallizes
in the reverse order of the phase diagram, i.e. the phase or phases crystallizing
first from the liquid state also crystallize from the amorphous state as can be from
the above mentioned DTXD photographs, leading to the conclusion that it is hardly
conceiveable to accept that glasses are undercooled liquids. DXTD results obtained
so far support the scenario: liquids, which tend to undercool, cannot form glasses.

Question arises why some melts soldifiy to form glasses other crystallize what ever
the cooling rate. A detailed DTXD study of Mg-Zn glasses revealed that those
systems are prune to glass formation, which show  multimorphy ( polymorphy and
merimorphy  taken together), i.e. a phase exits in a large number of crystal structures
of the same type as a function of state parameters temperature, pressure and concen-
tration. The greater is the multimorphicity of a compound, the higher is the glass-
forming ability of that compound. If the melt of a multimorphous compound is cooled,
one or the modification forms depending upon the state parameters. In case of Mg-Zn
system, a new phase modification forms, if the temperature of the melts is raised a few
degrees above the regular melting temperature. Further details are given in the referen-
ces cited in the headings of the DTXT photos discussed above.

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