Erich Meyer

Adiabatic Nucleation in large molecule liquids

Abstract Zanotto recently demonstrated that the Adiabatic Nucleation Theory predicts very well whether different inorganic glasses show homogeneous nucleation and at which temperature homogeneous nucleation is expected to occur. Using two sets of approximations in the Classical (Isothermal) Nucleation Theory, he could also indicate upper and lower limits for the temperature of homogeneous nucleation, arriving at similar results. In the present work, Zanottos analysis of the Adiabatic Nucleation Theory is confirmed, but his predictions with the help of Classical (Isothermal) Nucleation Theory are criticized. At least in those cases where no homogeneous nucleation occurs, his analysis leads to serious contradictions. Indirect arguments indicate that the Adiabatic Nucleation Theory is also valid for polymers.

Simple technique for measuring the superconducting critical temperature of small (≥10 μg) samples

A simple technique for measuring the superconducting critical temperature of small (≥10 μg) samples is described. The apparatus is built in the form of a probe, which can be introduced directly into a liquid‐He storage Dewar and permits the determination of the critical temperature, with an imprecision of ±0.05 K above 4.2 K, in about 10 min.

Adiabatic nucleation and glasses

Abstract A recently developed Adiabatic Nucleation Theory (ANT), which has been shown to be in good agreement with the largest experimental supercoolings of pure liquid elements (mostly metals), is applied to organic, inorganic and metallic glasses. It is demonstrated that, in order to obtain a glass by cooling from the melt, the (normal, slow cooling) glass transition temperature (Tg) has to be systematically: (i) higher or (ii) only slightly lower than the maximum supercooling temperature (T−), predicted by ANT. In the former case, rapid quenching is sometimes necessary in order to avoid (or reduce) crystallization initiated by heterogeneous nucleation and in the second case, rapid quenching is always necessary in order to also avoid homogeneous nucleation. The latter case can be explained by the fact that Tg can be “raised” above T− by fast quenching. Only polymers containing co-polymers may show Tgs which are considerably lower than T−, because these co-polymers may stabilize (eventually small) amorphous domains of the principal polymer below T−. Other materials with low Tgs (Tg ⪡ T−) just do not form glasses by quenching from the liquid, but can eventually be obtained in the amorphous, glassy form by condensation from the vapour phase on a cold substrate. In this latter case, the glass is really “composed” well below T− and Tg. In conclusion it can be stated that the glass forming ability is well characterized by Tg/T−.

Adiabatic nucleation and crystallization of gels

Abstract Adiabatic Nucleation Theory (ANT) has been successfully applied to pure liquid metals, oxide glasses, metallic glasses and polymers. This paper shows that ANT gives an interesting correlation between the crystallization data of gels of both reluctant and good (dense) glass forming systems. For reluctant glass formers, one finds that Tch T14−. T−14 is the temperature predicted by ANT at which nucleation starts in cooling experiments or ceases during heating and Tch is the experimental temperature of crystallization on heating. For systems with Tch considerably lower than t−14, as observed for reluctant glass formers, it is doubtful that one can obtain dense glasses by heat treatment of gels. This conclusion is in diagreement with the often advanced idea that the gel route can lead to dense glasses of unusual, reluctant glass forming compositions.

Adiabatic nucleation and metallic glasses

Abstract A recently developed Adiabatic Nucleation Theory (ANT) has been applied to pure liquid metals, inorganic (ceramic) glasses and polymers. In this paper, ANT is shown to be in good agreement with experimental data for metallic glasses of eutectic or nearly eutectic composition. The glass transition temperatures ( T g ) of these metallic glasses are about coincident with the maximum (liquid) supercooling temperatures ( T −) predicted by ANT. During slow cooling, the liquid samples crystallize easily by heterogeneous nucleation. During fast quenching, however, the heterogeneous nucleation centers can grow only slightly and almost the whole sample remains amorphous when T g is attained. It is well known that T g is an increasing function of the quenching speed. Therefore, a fast quenching increases T g above T − and improves the stabilization of the metallic glass, because then also homogeneous nucleation is avoided.

Numerical approximations to adiabatic nucleation

Abstract A recently developed adiabatic nucleation model uses the approximation of constant and equal specific heat of the liquid and solid phase. This approximation is substituted in the present work by introducing the temperature dependence with two different numerical approaches. The results, however, are not significantly different, indicating that the initial approximation is realistic.

Test of the adiabatic nucleation theory in metallic and chalcogenide glasses

Adiabatic nucleation theory (ANT) has been successfully applied to pure liquid metals, polymers, oxide, chalcogenide, metallic and gel-derived glasses. All thirty seven chalcogenide glasses examined show a glass transition temperature (Tg) above the stability limit, indicated by ANT. This must be the case, because otherwise, when cooled, these glasses (undercooled liquids) would crystallize by generalized nucleation near the stability limit and no Tg could be observed. We conclude that ceramics, which do not form glasses, have a normally unknown Tg below the stability limit. This explains why these materials crystallize above or at the stability limit. Eighteen metallic glasses examine show a similar behavior, however at the average, the normal, low cooling speed Tgs are lower, closer to the stability limit and in some cases, slightly below. Metallic glasses, at the average, are structurally less stable and with few exceptions, must be obtained by fast quenching. It can be suggested that more chalcogenide glasses could be found, using fast quenching techniques.

Adiabatic nucleation in glasses, polymers and liquid metals

Abstract Successful application of the isothermal (classical) nucleation theory depends critically on the choice of interfacial tension, which usually cannot be determined independently. Use of Tolman correction, for interfacial tension dependence on radius, may change the nucleation frequency by a factor of 10 59 when the reduced interface tension parameter is modified by a factor of less than 0.5. Similar results can also be obtained by proposing empirical temperature dependencies of the interfacial tension. The parameters used must be chosen individually for each material. In contrast to this, the adiabatic nucleation theory shows it is in agreement with experimental results with a fixed set of parameters, as r = 3 δ , where r is the radius of the nucleus and δ is Tolmans parameter which is estimated to be about half of the intermolecular distance.

Adiabatic nucleation in the liquid–vapor phase transition

The fundamental difference between classical (isothermal) nucleation theory (CNT) and adiabatic nucleation theory (ANT) is discussed. CNT uses the concept of isothermal heterophase fluctuations, while ANT depends on common fluctuations of the thermodynamic variables. Applications to the nonequilibrium liquid to vapor transition are shown. However, we cannot yet calculate nucleation frequencies. At present, we can only indicate at what temperatures and pressures copious homogeneous nucleation is expected in the liquid to vapor phase transition. It is also explained why a similar general indication cannot be made for the inverse vapor to liquid transition. Simultaneously, the validity of Peng–Robinson’s equation of state [D.-Y. Peng and D. B. Robinson, Ind. Eng. Chem. Fundam. 15, 59 (1976)] is confirmed for highly supersaturated liquids.

Adiabatic nucleation in supersaturated liquids

Abstract Adiabatic nucleation theory (ANT) is shown to be in good agreement with experimental data of superheated liquids (boiling). Pure superheated liquids nucleate just a few K before reaching the spinodal, calculated by the Peng–Robinson equation of state. In spite of the correlation between the nucleation curve and the spinodal, nucleation is usually explained by the (isothermal) classical nucleation theory (CNT), which does not take into account the proximity of the spinodal. An alternative explanation is given by ANT. When the liquid is close to the spinodal, the spinodal is reached and overpassed by volume fluctuations and the nucleus appears as the result of spinodal phase separation.