J. A. Duffy

Phase transitions of CO2 confined in nanometer pores as revealed by positronium annihilation

The results of a comprehensive positronium annihilation study of the phase behaviour of carbon dioxide in porous Vycor glass are presented. Isobaric measurements of the 3 gamma :2 gamma annihilation ratio show that on cooling, the gas-liquid phase boundary is raised by ~5 K while the liquid-solid transition is depressed by ~12 K relative to the bulk. The resulting phase diagram suggests the existence of a `triple point` of the confined fluid at a temperature ~10 K and a pressure ~2 bar below the bulk triple point.

Hysteresis at the gas-liquid and liquid-solid phase transitions of capillary confined CO2: a positronium annihilation study

The hysteresis behaviour of capillary confined CO2 gas is studied through the positron/positronium annihilation technique. The arrangement of the confining medium (VYCOR glass) allowed simultaneous measurement of the bulk and pore confined phase behaviour. Isobaric temperature cycling showed marked hysteresis at the gas-liquid and liquid-solid phase boundaries for the confined gas. Observations at the gas-liquid transition are in agreement with capillary condensation theories and simple ideas are proposed to explain the liquid-solid transition behaviour.

Positron and positronium annihilation studies of the phase behaviour of fluids in Vycor

We summarize the results of a recent positron annihilation study of the phase behaviour of confined in Vycor glass. Particular emphasis is placed on the phase diagram of the confined fluid and on the usefulness of the positron annihilation technique in determining the mechanisms underlying phase transitions of fluids in porous solids.

Positron and positronium annihilation studies of phase behaviour in confined geometry

In this paper we present a brief review of the current state of positron annihilation research into the phase behaviour of fluids confined within restricted boundaries. We summarise, in the form of selected examples, the work done so far on: (1) fluids confined in the nanometer-size pores of VYCOR glass, with particular emphasis on the confined phase diagram and the mechanisms behind phase transitions compared to bulk. (2) The adsorption/physisorption of gases on internal surfaces of grafoil and the potential of positron technique for revealing physical properties, such as the intricate molecular arrangements during phase transitions of the “layered” fluid.

Hysteresis during isothermal scanning through the gas-liquid transition of carbon dioxide confined in nanometer pores

Abstract Scanning hysteresis behaviour at the gas-liquid transition of CO 2 confined in pores of average diameter ∼ 40 A has been studied by positronium annihilation spectroscopy. Closer inspection of positron 2γ annihilation reveals a significant difference on increasing and decreasing the pressure through the transition, which is explained in terms of a larger liquid-gas interface on pore filling. The results favour the network model as the origin of the observed hysteresis.

Phase behaviour near the triple point of carbon dioxide confined in mesoporous VYCOR glass

The phase behaviour of carbon dioxide confined in VYCOR glass at pressures below that of the bulk triple point (∼0.51 MPa) has been investigated. The temperature at which freezing occurs appears to be pressure dependent below 0.3 MPa. As experiments are performed at successively lower pressures, the confined phase transitions gradually disappear, due to either partial pore filling, or the proximity of the confined triple point.

Anomalous freezing and melting behaviour of capillary confined CO2

In this paper we present our recent positron annihilation study of the liquid»solid phase boundary for CO2 confined in nanometer pores of VYCOR glass. We find that CO2 remains liquid in the pores far below the bulk freezing temperature and there is pronounced hysteresis between freezing and melting compared to that seen at the gas-liquid boundary in the pores. On freezing we see evidence of open space created in the pores. This leads to complex melting behaviour possibly involving the formation of gas-liquid interfaces. We see that frezing in the pores is totally irreversible, so that any solid which forms (no matter how small) remains stable up to the higher melting temperature. In contrast melting is more reversible (possibly indicating nucleation centres which permit immediate re-freezing). Finally, the pre-frozen state in the pores is different to the post-melted state.

Defect annealing and the formation of Li-rich clusters in Al-Li alloys

Positron annihilation lineshape measurements are conducted on pure Al, Al-1.9 at.% Li and Al-(12.0-12.5) at.% Li alloys. The recovery of each sample following quench and/or cold-work treatments is discussed in terms of vacancy migration and clustering, dislocation annealing and precipitation. The results indicate precipitation in both alloy concentrations but no positron trapping at dislocations. All cold-worked alloys show large vacancy clustering effects, providing strong evidence of vacancy agglomeration at precipitates.