Activation Energy of Metastable Amorphous Ge2Sb2Te5 from Room Temperature to Melt

Resistivity of metastable amorphous Ge2Sb2Te5 (GST) measured at device level show an exponential decline with temperature matching with the steady-state thin-film resistivity measured at 858 K (melting temperature). This suggests that the free carrier activation mechanisms form a continuum in a large temperature scale (300 K - 858 K) and the metastable amorphous phase can be treated as a super-cooled liquid. The effective activation energy calculated using the resistivity versus temperature data follow a parabolic behavior, with a room temperature value of 333 meV, peaking to ~377 meV at ~465 K and reaching zero at ~930 K, using a reference activation energy of 111 meV (3kBT/2) at melt. Amorphous GST is expected to behave as a p-type semiconductor at Tmelt ~ 858 K and transitions from the semiconducting-liquid phase to the metallic-liquid phase at ~ 930 K at equilibrium. The simultaneous Seebeck (S) and resistivity versus temperature measurements of amorphous-fcc mixed-phase GST thin-films show linear S-T trends that meet S = 0 at 0 K, consistent with degenerate semiconductors, and the dS/dT and room temperature activation energy show a linear correlation. The single-crystal fcc is calculated to have dS/dT = 0.153 {\mu}V/K for an activation energy of zero and a Fermi level 0.16 eV below the valance band edge.Comment: 5 pages, 5 figure

Model copolymerization reactions. Determination of the relative rates of addition of styrene and acrylonitrile to the 1-phenylethyl radical

1,1'-Azobis(1-phenyl[1-^(13)C]ethane) (1) was prepared in 15% overall yield, starting from [1-^(13)C]acetic acid (99 atom %). Analysis of end-group concentrations in styrene-acrylonitrile copolymers prepared with 1 as initiator allows accurate determination of the relative rates of addition of these monomers to the 1-phenylethyl radical. We obtain k_s/k_A = 0.20 ± 0.02, a result consistent with the penultimate model treatment of the styrene-acrylonitrile copolymerization by Hill, O'Donnell, and O'Sullivan

Determination of the relative rates of addition of styrene and acrylonitrile to the 1-(1,3-diphenyl)propyl and 1-(3-cyano-1-phenyl)propyl radicals. Evidence for a penultimate effect in radical copolymerization

1,1’-Azobis( 1,3-[ 1-^(13)C]diphenylpropane) (1b) and 4,4’-azobis(4-phenyl[4-^(13)C]butyronitrile) (1C) were prepared in overall yields of 10% and 1.2%, respectively, both starting from Ba^(13)CO_3 (99 atom %). Analysis of end-group concentrations in styrene-acrylonitrile (SAN) copolymers prepared with 1b as initiator allows accurate determination of the relative rates of addition of these monomers (k_s/k_A) to the 1-( 1,3-diphenylpropyl) radical (2b); similarly, analysis of SAN copolymers prepared with IC allows determination of k_s/k_A for the 1-(3-cyano-l-phenylpropyl) radical (2c). Significantly different preferences of the radicals for addition of styrene and acrylonitrile were observed; we obtain k_s/k_A = 0.21 f±0.01 for 2b and k_s/k_A = 0.52 ± 0.03 for 2c. These results show that k_s/k_A for 1-(1-phenylalkyl) radicals is sensitive to substitution y to the radical center and provide strong support for the existence of a penultimate unit effect in SAN copolymerization. The results are consistent with the penultimate model analysis of SAN copolymerization by Hill, O’Donnell, and OSullivan

Model copolymerization reactions. Initiation of the copolymerization of methyl methacrylate and acrylonitrile by 1,1′-azobis(1-phenyl[1-^(13)C]ethane)

The copolymerization of methyl methacrylate (MMA) and acrylonitrile (A) was initiated by photolysis of 1,1′-azobis(1-phenyl[^(13)C]ethane) (1) at 33°C in benzene. Determination of relative endgroup concentrations in the resulting copolymers allowed determination of the relative rates of addition of MMA and A to the 1-phenylethyl radical derived from 1. We find k_(MMA)/k_A = 0.44 ± 0.03, a value consistent with known copolymerization behavior. Further analysis of 75 MHz ^(13)C NMR spectra led to the determination of k_(MMA)/k_A for the 2-(4-phenylpentanenitrile) radical derived from primary radical addition to A. The measured value of kMMA/kA supports only one of five reported studies of the radical copolymerization behavior of MMA and A

A method for the determination of the relative reactivities of monomers towards the 1-phenylethyl radical

Several monomers have been polymerized using 1,1′-azobis(1-phenyl [1-^(13)C] ethane as initiator; the initiator fragments incorporated in the polymers have been examined by ^(13)C-NMR. The chemical shift for the enriched site in the end-group depends upon the nature of the attached monomeric unit. It is concluded that acenaphthylene is suitable as a reference monomer in comparisons of the reactivities of monomers towards the 1-phenylethyl radical (regarded as a model for the polystyrene radical) by consideration of end-groups in copolymers

Reactivities of monomers towards the 1-phenylethyl radical: Monomers with low ceiling temperatures

The reactivities towards the 1-phenylethyl radical of 2-vinylnaphthalene and some monomers with low ceiling temperatures (α-methylstyrene, 2-isopropenylnaphthalene and α-methoxystyrene) have been assessed by ^(13)C-NMR examination of end-groups in terpolymers prepared at 100° using ^(13)C-enriched azobis-1,1′-phenylethane as initiator. In each case, acenaphthylene has been used with styrene (STY) or methyl methacrylate and a third monomer selected from the quoted list. The present and previous results suggest that, for many monomers, the reactivity towards the 1-phenylethyl radical is very similar to that predicted from data on copolymerizations with STY, regarding the initiating small radical as a model for the polystyrene radical. There are appreciable differences between the observed and predicted relative reactivities in cases where either the copolymerization of the monomer with STY may show quite pronounced penultimate group effects or the monomer has a low ceiling temperature