SolidN2and CO in nanoporous glasses

Abstract

N 2 and CO condensed into nanoporous glasses have been investigated by vapor pressure measurements and x-ray diffraction as a function of temperature and fractional filling. The pore material is stable with respect to bulk condensation for any fractional filling. The adsorbate on the pore walls has to be distinguished from the capillary condensate in the center of the pores. This distinction can be made not only in the liquid, but also in the solid state. The solid capillary condensate is quasi-hcp at higher temperatures. N 2 remains in this state down to the lowest temperatures, even in pores as large as 130 Å diameter, whereas CO transforms into the orientationally ordered Pa3 phase. On heating an orientationally disordered intermediate phase with a fcc center-of-mass lattice is observed. ͓S0163-1829͑99͒08941-9͔

Freezing and melting of Ar in mesopores studied by optical transmission

“…Previously we have argued that this is so because on cooling the system is brought onto the adsorption branch due to condensation of vapor out of the dead volume whereas on heating some pore filling evaporates into the dead volume, hence the system is on the desorption branch. 4 The present results show that this view is too naive, but that it is the rearrangement of the material upon freezing that produces a state with a vapor pressure and a partition of the pore material that is similar to what is obtained by isothermal desorption. Thus the coincidence of T m and T f with the intersections of the mentioned coexistence lines of the -T diagram is by no means accidental.…”

“…The individual quantities show approximately a linear T dependence, the slopes ͑or entropies͒ being close to the bulk reference states. 4 The lines ⌬ a l (T) and ⌬ a s (T) and the lines ⌬ d l (T) and ⌬ d s (T) intersect at temperatures that agree well with the T f and T m , respectively. Previously we have argued that this is so because on cooling the system is brought onto the adsorption branch due to condensation of vapor out of the dead volume whereas on heating some pore filling evaporates into the dead volume, hence the system is on the desorption branch.…”

“…4 This latter fraction usually shows, in contrast to the ''dead'' wall coating, the phase transitions of its bulk counterpart, though with lower transition temperatures and thermal hysteresis, 4,5 the freezing and melting transition being the most prominent example. Our recent studies of vapor pressure isotherms, x-ray diffraction, 4 and heat capacity 6 on Ar in Vycor glass and xerogels have given some further insights. The temperatures T f and T m are related, respectively, to the vapor pressures p a of capillary condensation and p d of capillary evaporation.…”

Capillary condensation, freezing, and melting in silica nanopores: A sorption isotherm and scanning calorimetry study on nitrogen in mesoporous SBA-15

“…The wall coating up to 2-3 monolayers does not take part in a collective phase transformation. Presumably, it forms strongly bound, "dead" layers of shortrange, triangular lattices on the heterogeneous silica wall of the main pores as it was inferred from x-ray diffraction experiments on N 2 condensed in Vycor 43,44 and for N 2 physisorbed on aerosil silica surfaces 45 .…”

“…No high cost grain boundaries have to be formed and a vanishing amount of material is left for homogeneous freezing, in agreement with our caloric experiment. By contrast in the case of Ar the nanoscale Bridgman mechanism is less effective, leaving a large part of the condensate decoupled from the externally triggered crystallization for homogeneous nucleation, presumably by arrests of the crystallization front along the channels, the formation of empty pore space due to the volume shrinkage upon solidification and mismatching grain boundaries 44,52,56 .…”