Orthorhombic-to-hexagonal transition

The orthorhombic-to-hexagonal transition has been carefully studied from measurements of electrical conductivity by Ishii et al. 63), who concluded that the transition temperature in a-U30g, increased with decreasing O/U ratio, and that in the nearly stoichiometric UjOg region in... 

Since the orthorhombic-to-hexagonal transition in the a-U,Og phase is considered to be a second-order transition characterized by a A-type anomaly in the specific heat capacity curve, it is surmised that the transition is of order-disorder type based on a configurational change of U(V) and U(VI) ions, similarly to the case of transition in the U40<., phase. Further studies on the origin of this phase transition arc called for. 

Besides the orthorhombic-to-hexagonal transition in the a-U O phase, another transition from, to U,Oh +, . has been found to take place in the lower O/U ratio region, from work by Ishii cl al. (63) based on electrical conductivity measurements. The phase relation betw een the U,0,3 + and UjOg, phases suggested by Ishii el al. is also shown in Fig. 1. In contrast to the orthorhombic-to-hexagonal transition, this transition is thought to be a common first-order phase transition. The differences is the characteristics of the crystal structure between those of the U,0, and phases have... 

In fact, the hexagonal form of PE may be also obtained above a critical pressure, by heating y irradiated PE samples above a certain temperature [230]. The orthorhombic to hexagonal transition temperature is a function of y ray dose, and a sufficiently high irradiation dose makes the radiation induced hexagonal phase of PE stable even at atmospheric pressure [228,230]. The radiation-induced hexagonal phase shows IR spectra with the characteristic bands of kink defects [228]. 

Figure 18 Molecular weight dependencies of the phase transition temperature (T,) from orthorhombic to hexagonal phase and the melting temperature Tm) of the hexagonal phase of PE. O = phase transition from orthorhombic to hexagonal phase A A = melting of the hexagonal phase. (From Ref. 131.)...

Stmctural changes in the orthorhombic-to-hexagonal phase transition of PE crystals were investigated in the course of heating to the melting point. The IR and spectral patterns characteristic of the hexagonal phase were confirmed. In particular, the bands characteristic of the disordered short trans segments (shorter than... 

No. 23, 4th Nov.1996, p.7460-9 STRUCTURAL INVESTIGATION OF ORTHORHOMBIC-TO-HEXAGONAL PHASE TRANSITION IN POLYETHYLENE CRYSTAL. EXPERIMENTAL CONFORMATION OF THE CONFORMATIONALLY DISORDERED STRUCTURE BY X-RAY DIFFRACTION AND INFRARED-RAMAN SPECTROSCOPIC MEASUREMENTS Tashiro K Sasaki S Kobayashi M Osaka,University... 

Structural changes in the orthorhombic-to-hexagonal phase transition of PE crystal was investigated by... 

Figure 16. Temperature dependences of an intensity, half-width and angular position of the diffusive maximum on the equator of the X-ray pattern of as-spun CPE-2 fibers. The arrow indicates transition temperature from orthorhombic to hexagonal modification.

Another example of the use of NMR to discriminate between motional modes involves the solid-solid phase transition of nonadecane. The transition involves a change from orthorhombic to hexagonal packing of the alkane. The orthorhombic phase gives rise to a static powder spectrum (Figure 6A) Transition to a... 

Figure 20 shows the phase diagram of polyethylene119). The existence range of the condis crystals increases with pressure and temperature. The enthalpy of the reasonably reversible, first order transition from the orthorhombic to the hexagonal condis phase of polyethylene is 3.71 kJ/mol at about 500 MPa pressure 121) which is about 80 % of the total heat of fusion. The entropy of disordering is 7.2 J/(K mol), which is more than the typical transition entropy of paraffins to their high temperature... 

We note a minor error by these authors who claim that Eq. 1 applies as written also to the orthorhombic/hexagonal transition. It does not but requires to be modified, as in Eq. 11, on account of the different densities of the two phases. 

The evidence that lamellae 15 nm thick can enter the hexagonal phase came from electron diffraction of specimens annealed as low as 236 °C at 0.54 GPa, 3 K lower than the maximum of the corresponding differential thermal analysis peak recorded for bulk polymer under the same conditions. At this annealing temperature lamellae were still entire, i.e. essentially of unaltered thickness although with some internal variation. No attempt was then made to anneal at lower temperatures and determine the orthorhombic/hexagonal transition temperature for this thickness. Such experiments offer, nevertheless, a direct approach by which the quantities of Eq. 9 may be determined. 

The depression of the orthorhombic/hexagonal transition temperature for lamellae thickness k follows from equating the differential surface and compensating volume contributions to the free energy according to the geometry of Fig. 4. Thus... 

Type I belite grains are those that have crystallized from the liquid at temperatures above about I420°C and thus as a-C2S. The primary striations arise on cooling through the a to a transition, in which the symmetry decreases from hexagonal to orthorhombic, each set of striations thus representing a different orientation of the a structure. The transition to a -CjS that occurs on further cooling does not further increase the number of orientations, but that from o l to P, in which the symmetry falls to monoclinic, causes each orientation to split into two. This is the cause of the secondary sets of striations described above. 

---