Hydrolytically stable fluorocarbon ether bibenzoxazole polymers

Hydrolytically and thermooxidatively stable fluorocarbon ether bibenzoxazole (FEB) polymers were synthesized by the acetic acid-promoted polycondensation of fluorocarbon ether bis(o-aminophenol) monomers with novel fluorocarbon ether dithioimidate ester monomers. The dithioimidate esters were derived from hexafluoropropylene oxide (HFPO) or, preferably, from a combination of HFPO and tetrafluoroethylene oxide (TFEO). The latter class of TFEO:HFPO-derived dithioimidate esters imparted improved hydrolytic stability to the resultant FEB polymers while minimizing undesirable, concomitant increases in polymer glass transition temperatures. The polymers exhibited inherent viscosities in the range of 0.20 to 0.41 dl/g. Polymer structures were verified by elemental and infrared (IR) spectral analyses. Lower glass transition temperatures were obtained with increased fluorocarbon ether content, a minimum value of ?48°C being achieved. High thermooxidative stability of the HFPO- and TFEO:HFPO-based FEB polymers was substantiated by thermogravimetric analysis and isothermal aging studies in air. After isothermal aging in air for 200 hr at 288 and 260°C, representative polymers exhibited weight losses of 17 and 7%, respectively. Hydrolytic stability of the polymers was evaluated by exposure to a 95% relative humidity, 93°C environment for prolonged periods of time. Based on comparative glass transition temperature, inherent viscosity, and IR spectral data, the HFPO- and TFEO:HFPO-based FEB polymers exhibited vastly improved hydrolytic stability over analogous TFEO-based FEB polymers. Optimum hydrolytic stability and low-temperature viscoelastic properties were achieved by the use of a TFEO:HFPO-derived dithioimidate ester that did not contain structural isomers that would lead to sites of hydrolytic instability in the polymer backbone.     

Nuclear structure effects in sub-barrier fusion cross sections

The total fusion cross sections around and down to ≈ 12 MeV below the Coulomb barrier in the c.m. system have been measured with the Munich heavy-ion recoil spectrometer for 30 projectile-target combinations: ^{32, 36}S + ^{92, 94, 96, 98, 100}Mo, ^{100, 101, 102, 104}Ru, ¹⁰³Rh, ^{104, 105, 106, 108, 110}Pd, The excitation functions can be reproduced with a one-dimensional barrier penetration model by increasing the nuclear radii by $\text{ΔR ? 0.255 ± 0.035}\text{fm}$ and introducing a gaussian distribution of the nuclear radius R with a standard deviation σ_fit(R). The σ_fit(R) can be explained as being due to quadrupole vibrational fluctuations of the surface-to-surface distance at the barrier and to two-neutron-transfer reactions with positive Q-values.     

Phase diagram of SrSi2 to 40 kbar and 1200°C

The pressure-temperature phase diagram of dimorphic SrSi₂ was investigated in the pressure range 10–40 kbar and temperature range 600–1200°C in a belt-type apparatus. The phase boundary between cubic SrSi₂I (SrSi₂-type) and tetragonal SrSi₂II (-ThSi₂-type) was studied by X-ray techniques in quenched samples. The curvature of the equilibrium line was not resolved. Molar volumes of both polymorphs have been measured in the temperature range 20–400°C at ambient pressure. From the slope of the equilibrium line in the pressure-temperature diagram and the change in molar volume, approximate values of the entropy and heat of transition SrSi₂(I–II) have been calculated.

In the silicon sublattice of both polymorphs of SrSi₂ the topologically simplest three-dimensional three-connected nets are found to be slightly distorted from their ideal configurations. The ideal SrSi₂-type net represents the three-connected analogy to the four-connected diamond net.     

Investigation of high spin states in23Na and23Mg and the12C+12C resonance atE c.m.=19.3 MeV

The resonance in the¹²C+¹²C system atE _c.m.=19.3 MeV has been studied in the neutron and proton decay channels. From(p, γ), (n, p) and (n, γ) coincidence measurements high spin states could be localized inA=23 nuclei, in²³Mg up to possibly 21/2⁺. These states were resonantly populated. The present data favorJ ^π=17/2⁺ for the 9.61 MeV state in²³Mg and therefore also for the probable parent state at 9.81 MeV in²³Na. AJ ^π=12⁺ assignment to the resonance is supported and evidence was found that the resonant excitation of the lowest 15/2⁺ and 17/2⁺ states in mass 23 nuclei is caused by a change of the intrinsic structure of the Yrast levels aroundJ ^π=15/2⁺ towards larger deformation.     

Spontaneous-emission suppression on arbitrary atomic transitions

We propose a very simple scheme to slow down the usual exponential decay of upper state population in an atomic two-level system considerably. The scheme uses an additional possibly intense field with frequency lower than the total decay width of the atomic transition. This allows for additional decay channels with the exchange of one or more low-frequency photons during an atomic transition. These channels may then interfere with each other. The intensity and frequency of the low-frequency field are shown to act as two control parameters modifying duration and amount of the population trapping. An extension of the scheme to include transitions to more than one lower state is straightforward.     

Properties in Reactive Structure Materials: ZrW2 and HfW2 – High Melting Temperatures,Densities, Hardness,and Exothermic Ignition Energies

ZrW₂ and HfW₂ were prepared by melting the constituent elements in an arc-furnace. By cooling through perictectic phase lines above approximately 2000 °C, samples of the nominal 1:2 composition were obtained as products consisting of two phases, of which approximately 95 % adopts the MgCu₂ type structure and 5 % the W structure. In ZrW₂ and in HfW₂, the MgCu₂ type structure enables efficient topological compact spacefilling of the smaller W atoms into the cavities of the three-dimensional net constructed by the larger tetravalent metals Zr and Hf. Correspondingly, very dense and hard alloys are obtained which burn at a crash in air in a strong exothermic ignition reaction. These materials are non-toxic.