Low-energy Ga2O3 polymorphs with low electron effective masses.

We predict three Ga2O3 polymorphs with P21/c or Pnma symmetry. The formation energies of P21/c Ga2O3, Pnma-I Ga2O3, and Pnma-II Ga2O3 are 57 meV per atom, 51 meV per atom, and 23 meV per atom higher than that of ß-Ga2O3, respectively. All the polymorphs are shown to be dynamically and mechanically stable. P21/c Ga2O3 is a quasi-direct wide band gap semiconductor (3.83 eV), while Pnma-I Ga2O3 and Pnma-II Ga2O3 are direct wide band gap semiconductors (3.60 eV and 3.70 eV, respectively). Simulated X-ray diffraction patterns are provided for experimental confirmation of the predicted structures. The polymorphs turn out to provide low electron effective masses, which is of great benefit to high-power electronic devices.

Structure and Properties of Poly-α-olefins Containing Quaternary Carbon Centers.

In this paper, poly-α-olefins (PAO) containing quaternary carbon centers were prepared by two-step oligomerization using a metallocene catalyst followed by a Ziegler-Natta catalyst. First, the 1-decene dimer was oligomerized with [t-BuN(Me)2C(η5-C5H4)]ZrCl2, and the effects of the oligomerization temperature, Al/Zr molar ratio, and catalyst loading on the oligomerization were investigated. In the second step, the obtained 1-decene dimers were copolymerized with 1-decene with TiCl4/Et2AlCl, and the effects of the catalysts, monomer/dimer ratio, and α-olefin species on the copolymerization were investigated. The composition and structure of the dimers and copolymerization products were characterized by gas chromatography (GC) and 1H NMR and 13C NMR spectroscopy. The results of GC and 13C NMR analyses indicated that the metallocene catalyzed the formation of the 1-decene oligomerization product, resulting in the branched olefin dimer being the major product, and the existence of quaternary carbons in the 1-decene/1-decene dimer copolymerization product could also be found. The polymerization mechanism for the formation of the quaternary carbon centers is proposed. The 1-decene/1-decene dimer copolymerization product containing quaternary carbon centers has a kinematic viscosity of 10.8 mm2/s at 100 °C, a viscosity index of 165, and a pour point of -52 °C; thus, the product with quaternary carbon centers has a better viscosity-temperature performance and low-temperature fluidity than those of the 1-decene oligomerization product and typical PAO products, but the kinematic viscosity is similar.