Materials with high surface areas and small particle size (nanophases), metastable polymorphs, and hydrated oxides are increasingly important in both materials and environmental science. Using modifications of oxide melt solution calorimetry, we have developed techniques to study the energetics of such oxides and oxyhydroxides, and to separate the effects of polymorphism, chemical variation, high surface area, and hydration. Several generalizations begin to emerge from these studies. The energy differences among different polymorphs (e.g., various zeolite frameworks, the - and -alumina polymorphs, manganese and iron oxides and oxyhydroxides) tend to be small, often barely more than thermal energy under conditions of synthesis. Much larger contributions to the energetics come from oxidation-reduction reactions and charge-coupled substitutions involving the ions of basic oxides (e.g., K and Ba). The thermodynamics of hydration involve closely balanced negative enthalpies and negative entropies and are very dependent on the particular framework and cage or tunnel geometry.
Polymer molecules have contour lengths which may exceed the dimension of microphases. Especially in semicrystalline samples
a single molecule may traverse several phase areas, giving rise to structures in the nanometer region. While microphases have
properties that are dominated by surface effects, nanometer-size domains are dominated by interaction between opposing surfaces.
Calorimetry can identify such size effects by shifts in the phase-transition temperatures and shapes, as well as changes in
heat capacity. Specially restrictive phase structures exist in drawn fibers and in mesophase structures of polymers with alternating
rigid and flexible segments. On several samples shifts in glass and melting temperatures will be documented. The proof of
rigid amorphous sections at crystal interfaces will be given by comparison with structure analyses by X-ray diffraction and
detection of motion by solid state NMR. Finally, it will be pointed out that nanophases need special attention if they are
to be studied by thermal analysis since traditional ‘phase’ properties may not exist.
Authors:Fan Sun, Denis Laillé, and Thierry Gloriant
In this study, the thermal analysis of the ω nanophase transformation from a quenched metastable β Ti–12Mo alloy composition
(mass%) was investigated by electrical resistivity and dilatometry measurements. The activation energy was observed to be
121 ± 20 kJ mol−1 (from resistivity measurements) and 114 ± 12 kJ mol−1 (from dilatometry measurements) during the early stage of the transformation process. The kinetic of the ω nanophase transformation
was modelized by using the classical Johnson–Mehl–Avrami (JMA) theory and a modified Avrami (MA) analysis. An Avrami exponent
close to 1.5 was found at the early stage of the transformation suggesting a pure growth mechanism from pre-existing nucleation
sites. Nevertheless, it was observed a decrease of the Avrami exponent to 0.5 at higher transformed fraction demonstrating
a dimension loss in the growth mechanism due to the existence of the high misfit strain at the interface β/ω.
Contributions of modern, temperature-modulated calorimetry
are qualitatively and quantitatively discussed. The limitations are summarized,
and it is shown that their understanding leads to new advances in instrumentation
and measurement. The new thermal analysis experiments allow to separate reversing
from irreversible processes. This opens the irreversible states and transitions
to a description in terms of equilibrium and irreversible thermodynamics.
Amorphous systems can be treated frommacroscopic to nanometer sizes with weak
to strong coupling between neighboring phases. Semicrystalline, macromolecular
systems are understood on the basis of modulated calorimetry as globally metastable,
micro-to-nanophase-separated systems with locally reversible transitions.
microphase to nanophase dimensions.
The modulation amplitudes of temperature-modulated DSC, TMDSC, in frequent use for the last 20 years, are also indicated in Fig. 1 . They show the usefulness of TMDSC to test the reversibility of phase changes
Thermogravimetric (TG) and differential thermal analysis (DTA) curves of methyltributylammonium smectite (MTBAS), methyltrioctylammonium
smectite (MTOAS), and di(hydrogenatedtallow)dimethylammonium smectite (DHTDMAS), and also corresponding sodium smectite (NaS)
and tetraalkylammonium chlorides (TAAC) were determined. The TAACs was decomposed exactly by heating up to 500°C. The adsorbed
water content of 8.0% in the pure NaS was decreased down to 0.2% depending on the size of the non-polar alkyl groups in the
tetraalkylammonium cations (TAA+). The thermal degradation of the organic partition nanophase formed between 2:1 layers of smectite occurs between 250–500°C.
Activation energies (E) of the thermal degradations in the MTBAS, MTOAS and DHTDMAS are 13.4, 21.9, and 43.5 kJ mol−1, respectively. The E value increases by increasing of the interlayer spacing along a curve depending on the size of the alkyl groups in the TAA+.
based on the aluminosilicate glass-matrix with the nano-phase of fluoride
is an interesting material for optoelectronics. A new glass from the SiO2–B2O3–Na2O–LaF3 system
in which nanocrystallization of LaF3 could be obtained
as well is presented.
Thermal stability of glass and the crystalline
phases formed upon heat treatment were determined by DTA/DSC and XRD methods,
respectively. The effect of the glass composition on thermal stability was
investigated by the SEM method.
It has been found that the addition
of LaF3 increases the tendency to decomposition of
the borosilicate glass. In glasses with the ratio B2O3/(Na2O+3La2F6)<1 it is possible to obtain the immersed crystallization
of LaF3 in transparent glassy matrix. The process is
preceded by LaOF formation. Glasses with the composition B2O3/(Na2O+3La2F6)≥1 revealed the tendency to La(BSiO5)
Calorimetry deals with the energetics of atoms, molecules, and phases and can be used to gather experimental details about
one of the two roots of our knowledge about matter. The other root is structural science. Both are understood from the microscopic
to the macroscopic scale, but the effort to learn about calorimetry has lagged behind structural science. Although equilibrium
thermodynamics is well known, one has learned in the past little about metastable and unstable states. Similarly, Dalton made
early progress to describe phases as aggregates of molecules. The existence of macromolecules that consist of as many atoms
as are needed to establish a phase have led, however, to confusion between colloids (collections of microphases) and macromolecules
which may participate in several micro- or nanophases. This fact that macromolecules can be as large or larger than phases
was first established by Staudinger as late as 1920. Both fields, calorimetry and macromolecular science, found many solutions
for the understanding of metastable and unstable states. The learning of modern solutions to the problems of materials characterization
by calorimetry is the topic of this paper.