Eine Entdeckungsreise durch die Analytik

Unter dem Motto „A Voyage of Discovery”︁ lud die 52. Pittsburgh Conference (Pittcon) Anfang März zu einer Entdeckungsreise durch die Analytische Chemie nach New Orleans, der Hauptstadt des Jazz, ein. Wiederum wurde ein reichhaltiges wissenschaftliches Programm mit der größten jährlichen Ausstellung analytisch‐chemischer Geräte verknüpft.     

Darstellung,Kristallstrukturen und Schwingungsspektren neuer ternärer Verbindungen mit dem Anion [N–B–N]3–

Ternary Nitridoborates. 2. Synthesis, Crystal Structure, and Vibrational Spectra of New Ternary Compounds with the [N–B–N]^3– Anion The isotypic compounds LiM₄[BN₂]₃ (M = Ca, Sr, Ba, Eu) and NaM₄[BN₂]₃ (M = Sr, Ba) are formed as colorless to pale yellow prismatic crystals (black with Eu) by reaction of the binary components Li₃N, M₃N₂, EuN and Na, NaN₃, Ba₃N₂ and BN in sealed niobium ampoules at 1375 and 1275 K, respectively. The linear anions [N–B–N]^3– have bond lengths d(B–N) between 132.6 and 136.6 pm. Vibrational frequencies and force constants f(B–N) = 7.25–7.89 Ncm^–1 reveal significant drifts related to bond length and effective anionic charge. The cubic crystal structures (Im3m (No. 229), Z = 2; LiM₄[BN₂]₃: a(Ca) = 711.5 pm; a(Sr) = 745.6 pm; a(Eu) = 742.5 pm, a(Ba) = 788.0 pm and NaM₄[BN₂] ${}_{\text{3}\text{: a(Sr) = 756.8 pm; a(Ba) = 791.7 pm)) are stuffed derivatives of the β‐PtHg4}}$ structure type, and the range of existence of this cubic structure is derived from the molar volume and the ionic radii. The cations form a partial structure of centered cubes E1(E2)₈ which are condensed to a [E1(E2)_8/2] network (E1 = Li, Na; E2 = Ca, Sr, Ba, Eu). The remaining open cubes are filled by the [BN₂]^3– anions yielding two interpenetrating [E1(BN₂)_6/2] networks. Periodic Nodal Surfaces (PNS) of Im3m symmetry show the regions of different interactions.     

Computer simulation of network formation via crosslinking copolymerization

The topology of networks formed by radical crosslinking copolymerization is strongly dependent on the type of crosslinker chosen. By a series of computer simulations, the influence of the relative reactivities of crosslinker and monomer on the network properties was investigated. Basic condition of the simulation is the postulation of a diffusion process control of the polymerization reaction. The resulting networks are analyzed with respect to cycle rank, loop formation, dangling ends, sol fraction and crosslinker molecules reacted only by one double bond. The results of the computer simulations indicate that the reactivity of the crosslinker decisively affects the properties of the network. Particularly the cycle rank, which determines the elastic properties of the network, is influenced strongly by the reactivity of the crosslinker. A change of the cycle rank of approximately one decade was observed. The influence of the reactivity was found to be most important in the region where the reactivity of the crosslinker is less than the reactivity of the monomer.     

Crystallization and decomposition of amorphous silicon-aluminium films

The crystallization and decomposition of vacuum-deposited amorphous silicon-aluminium films have been examined by means of transmission electron microscopy. Depending on the aluminium concentration, the transformation of the metastable amorphous phase into the stable phases of aluminium and silicon proceeds by different reactions such as pre-crystallization of aluminium, polymorphous transformation into supersaturated crystalline solid solutions or eutectic decomposition. The temperature dependence of the eutectic crystallization was measured. The results are discussed in terms of the thermodynamics of amorphous-to-crystalline transformation.