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Multiple DNA repair pathways may be involved in the removal of the same DNA lesion caused by endogenous or exogenous agents. Although distinct DNA repair machinery fulfill overlapping roles in the repair of DNA lesions, the mechanisms coordinating different pathways have not been investigated in detail. Here, we show that Ku70, a core protein of nonhomologous end-joining (NHEJ) repair pathway, can directly interact with DNA polymerase-β (Pol-β), a central player in the DNA base excision repair (BER), and this physical complex not only promotes the polymerase activity of Pol-β and BER efficiency but also enhances the classic NHEJ repair. Moreover, we find that DNA damages caused by methyl methanesulfonate (MMS) or etoposide promote the formation of Ku70-Pol-β complexes at the repair foci. Furthermore, suppression of endogenous Ku70 expression by small interfering RNA reduces BER efficiency and leads to higher sensitivity to MMS and accumulation of the DNA strand breaks. Similarly, Pol-β knockdown impairs total-NHEJ capacity but only has a slight influence on alternative NHEJ. These results suggest that Pol-β and Ku70 coordinate 2-way crosstalk between the BER and NHEJ pathways.—Xia, W., Ci, S., Li, M., Wang, M., Dianov, G. L., Ma, Z., Li, L., Hua, K., Alagamuthu, K. K., Qing, L., Luo, L., Edick, A. M., Liu, L., Hu, Z., He, L., Pan, F., Guo, Z. Two-way crosstalk between BER and c-NHEJ repair pathway is mediated by Pol-β and Ku70.

As a carbon-free and sustainable fuel, ammonia serves as high-energy-density hydrogen-storage material. It is important to develop new reactions able to utilize ammonia as a hydrogen source directly. Herein, we report an electrochemical hydrogenation of alkenes, alkynes, and ketones using ammonia as the hydrogen source and carbon electrodes. A variety of heterocycles and functional groups, including for example sulfide, benzyl, benzyl carbamate, and allyl carbamate were well tolerated. Fast stepwise electron transfer and proton transfer processes were proposed to account for the transformation.

The first direct aziridination of triaryl-substituted alkenes was achieved by means of an electrochemical process that could extend to multisubstituted styrenes. Specifically, hexafluoroisopropanol sulfamate was used as a nucleophilic nitrogen source. Mechanistic experiments suggest that this electrochemical process proceeds by stepwise formation of two C-N bonds through reactions between cationic carbon species and the sulfamate.

A facile method to synthesize silicon oxide clad uranium oxide nanowires is presented. U3Si2, used as a precursor, was oxidized to produce uranium oxide nanocrystallites and amorphous silicon oxide under hydrothermal conditions at 300°C and a pressure of 7.8 × 106 Pa. The growth of uranium oxide nanowires was assisted by silicon oxide via assembling the uranium oxide nanocrystallites in an amorphous silicon oxide matrix. The microstructure and composition of silicon oxide clad uranium oxide nanowires were characterized by XRD, SEM, TEM, and EDS. The uranium oxide in the nanowires was determined as UO2.34 with a fluoride cubic structure.

A two-step processing was developed to prepare Yb2Si2O7-SiC nanocomposites. Yb2Si2O7-Yb2SiO5-SiC composites were first fabricated by a solid-state reaction/hot-pressing method. The composites were then annealed at 1250°C in air for 2 hours to activate the oxidation of SiC, which effectively transformed the Yb2SiO5 into Yb2Si2O7. The surface cracks purposely induced can be fully healed during the oxidation treatment. The treated composites have improved flexural strength compared to their pristine composites. The mechanism for crack healing and silicate transformation have been proposed and discussed in detail.

The thermal conductivity of nanocrystalline ceria films grown by unbalanced magnetron sputtering is determined as a function of temperature using laser-based modulated thermoreflectance. The films exhibit significantly reduced conductivity compared with stoichiometric bulk CeO2. A variety of microstructure imaging techniques including X-ray diffraction, scanning and transmission electron microscopy, X-ray photoelectron analysis, and electron energy loss spectroscopy indicate that the thermal conductivity is influenced by grain boundaries, dislocations, and oxygen vacancies. The temperature dependence of the thermal conductivity is analyzed using an analytical solution of the Boltzmann transport equation. The conclusion of this study is that oxygen vacancies pose a smaller impediment to thermal transport when they segregate along grain boundaries.

Bioactive tricalcium silicate ceramics (Ca3SiO5) were fabricated by spark plasma sintering (SPS), and their sinterability and mechanical properties were examined. The bioactivity and in vitro biocompatibility of Ca3SiO5 ceramics were evaluated. Ca3SiO5 ceramics show higher density and superior mechanical properties compared with those prepared by conventional pressureless sintering. In addition, hydroxyapatite was induced to form on the surface of Ca3SiO5 ceramics when soaked in simulated body fluid and bone marrow mesenchymal stem cells were attached and spread well on the ceramics. Ca3SiO5 ceramics fabricated by SPS possess excellent mechanical properties, bioactivity, and biocompatibility and are promising bone repaired materials.

Ti-doped Zr(2)[Al(Si)](4)C(5) solid solutions were prepared by an in situ hot-pressing method and its effect on the mechanical properties and high-temperature oxidation behavior were investigated. The solid solutions show comparable hardness, strength, and fracture toughness with Zr(2)[Al(Si)](4)C(5) except modulus, which decreases with Ti dopant content. The stiffness is maintained up to 1600 degrees C, which derives from the clean grain boundaries without glassy phases. The oxidation resistance of [Zr(1-x)(Ti)(x)](2)[Al(Si)](4)C(5) solid solutions at 1000 degrees-1300 degrees C is improved remarkably. The improved oxidation resistance is mostly due to the formation of a more protective oxide scale consisting of (Zr,Ti)O(2), Al(2)O(3), and mullite.

The reciprocating sliding friction and wear properties of two novel materials of Zr(2)[Al(Si)](4)C(5) ceramic and Zr(2)[Al(Si)](4)C(5)-30 vol% SiC composite against Si(3)N(4) ball were investigated. The sliding friction process of Zr(2)[Al(Si)](4)C(5) against Si(3)N(4) experiences two different stages under constant normal load and involves friction and wear mechanism transition. The static coefficient of friction increases with an increasing normal load. The friction force mainly comes from the interfacial shear between Si(3)N(4) ball and Zr(2)[Al(Si)](4)C(5), which changes with varied sliding distances and normal loads. In contrast, the friction process of the composite experiences one stage and the friction coefficient is not related to the test durations and normal loads. The friction force between Zr(2)[Al(Si)](4)C(5)-30 vol% SiC composite and Si(3)N(4) is mainly from the plough between SiC particles and Si(3)N(4) ball, which appears not to be influenced significantly by different normal load and sliding distance. In addition, microfracture induced mechanical wear is the rate-control wear mechanism in both Zr(2)[Al(Si)](4)C(5) and Zr(2)[Al(Si)](4)C(5)-30 vol% SiC composite. Adding SiC improves the wear resistance of the single-phase material, because the second phase bears normal load and slows down material removal.

The oxidation behavior of Hf-Al-C ceramics containing 37.5 wt% Hf(3)Al(3)C(5), 30.5 wt% Hf(2)Al(4)C(5), and 32.0 wt% Hf(3)Al(4)C(6) has been investigated at 900 degrees-1300 degrees C in air. The oxidation kinetics approximately follows a linear law with the activation energy of 194 +/- 12 kJ/mol. The oxidation resistance of Hf-Al-C ceramics at high temperature is superior to HfC. The oxide scale is porous and composed of well-mixed t-HfO(2) and Al(2)O(3) as well as residual free carbon. The stabilization mechanisms of t-HfO(2) and free carbon have been discussed. The simultaneous oxidation of Hf and Al in Hf-Al-C ceramics can be attributed to their close oxygen affinity as well as the strong coupling between Hf-C blocks and Al-C units in the crystal structures.

Hydrothermal oxidation of bulk Ti(3)SiC(2) in continuous water flow was studied at 500 degrees-700 degrees C under a hydrostatic pressure of 35 MPa. The oxidation was weak at 500 degrees-600 degrees C and accelerated at 700 degrees C due to the formation of cracks in oxides. The kinetics obeyed a linear time-law. Due to the high solubility of silica in hydrothermal water, the resulting oxide layers only consisted of titanium oxides and carbon. Besides general oxidation, two special modes are very likely present in current experiments: (1) preferential hydrothermal oxidation of lattice planes perpendicular to the c-axis inducing cleavage of grains and (2) uneven hydrothermal oxidation related to the occurrence of TiC and SiC impurity inclusions. Nonetheless the resistance against hydrothermal oxidation is remarkably high up to 700 degrees C.

HfAl(4)C(4), a new ternary aluminum carbide, was discovered and its crystal structure was determined by a combination of X-ray diffraction, transmission electron microscopy, and first-principles calculations. The crystal structure is trigonal belonging to the P (3) over bar m1 space group. The refined lattice constants are a50.3308 nm, c=2.190 nm. First-principles method was used to calculate the theoretical second-order elastic constants, bulk modulus, shear modulus, and the Young's modulus of HfAl(4)C(4). It shows that HfAl(4)C(4) has relatively high elastic stiffness.

ZrO(2)-Al(2)O(3) nanocrystalline powders have been synthesized by oxidizing ternary Zr(2)Al(3)C(4) powders. The simultaneous oxidation of Al and Zr in Zr(2)Al(3)C(4) results in homogeneous mixture of ZrO(2) and Al(2)O(3) at nanoscale. Bulk nano- and submicro-composites were prepared by hot-pressing as-oxidized powders at 1100 degrees-1500 degrees C. The composition and microstructure evolution during sintering was investigated by XRD, Raman spectroscopy, SEM, and TEM. The crystallite size of ZrO(2) in the composites increased from 7.5 nm for as-oxidized powders to about 0.5 mu m at 1500 degrees C, while the tetragonal polymorph gradually converted to monolithic one with increasing crystallite size. The Al(2)O(3) in the composites transformed from an amorphous phase in as oxidized powders to theta phase at 1100 degrees C and alpha phase at higher temperatures. The hardness of the composite increased from 2.0 GPa at 1100 degrees C to 13.5 GPa at 1400 degrees C due to the increase of density.

TaC-TaSi(2) composites were fabricated at 1700 degrees C by an in situ reaction/hot pressing method using Ta, Si, and graphite as initial materials. TaSi(2) content was 0-100 vol%. The microstructure and mechanical properties of the composites were investigated. It was found that the relative densities of composites were above 97.5% when the volume content of TaSi(2) was above 10%. The TaC/10 vol% TaSi(2) composite presented the highest flexural strength of 376 MPa. When the TaSi(2) content was 30-50 vol%, the composites showed the highest fracture toughness of about 4.3MP . am(1/2). In addition, the composites could retain high Young's modulus up to at least 15251 degrees C.

The mechanical and thermophysical properties of quaternarylayered carbides, Zr2[Al(Si)]4C5 and Zr3[Al(Si)]4C6 have been investigated and compared with those of Zr2Al3C4 and Zr3Al3C5. These four carbides are generally alike in mechanical and thermophysical properties due to their similar crystal structures that consisting of alternatively stacked ZrC layers and Al3C2/[Al(Si)]4C3 slabs. The layer thickness of zirconium carbide and aluminum carbide has effects on their properties. Thicker layer of zirconium carbide and/or thinner layer of aluminum carbides are in favor of stiffness, hardness, thermal, and electrical conductivities, but go against density, specific stiffness, Debye temperature, and coefficient of thermal expansion. Nomenclature l wavelength of CuKa radiation (nm). L sample length for the measurement of Young’s modulus(m). w sample wide for the measurement of Young’s modulus (m). t sample thickness for the measurement of Young’s modulus (m). m sample weight for the measurement of Young’s modulus (kg). ff flexural resonant frequency (Hz). Q � 1 internal friction. k exponential decay parameter of the amplitude of the flexural vibration component.

In this work, reciprocating ball-on-flat sliding friction and wear tests as well as two-body abrasive wear tests were performed on Zr2Al3C4, a new type of ceramic material. In the sliding wear tests, a Si3N4 ball and an AISI 52100 steel ball were used as counter materials. When Zr2Al3C4 was slid against an AISI 52100 ball, the coefficient of friction (COF) was as low as 0.20–0.42, being independent of normal loads, and the wear rates of Zr2Al3C4 and AISI 52100 steel were in the range of 10−4–10−5 mm3/m. A tribofilm between the tribopair was presumed to be responsible for the low COF and wear rate. According to Raman and energy-dispersive spectroscopy analysis, the tribofilm consists of a mixture of oxides of Zr, Al, and Fe as well as amorphous carbon. When Zr2Al3C4 was slid against a Si3N4 ball, a transition from mild wear to severe wear was observed as the normal load increased. The transition occurs under certain contact stresses after a damage-accumulating period. In the two-body abrasive wear test, surface chipping and fragmentation resulting from coalescence of surface and subsurface microcracks were the main material removal mechanisms of Zr2Al3C4.

A dense V2AlC ceramic was synthesized by an in situ reaction/hot pressing method using V, Al, and C powders as starting materials. The reaction path was investigated. It was found that V2AlC was produced by a reaction between Al8V5 and C. Single -phase V2AlC could be prepared using the optimized molar ratio of V:Al:C=2:1.2:0.9. The grain sizes of as-prepared V2AlC samples were temperature dependent. On increasing the temperature from 1400° to 1700°C, the mean grain size of V2AlC increased from 49 μm in length and 19 μm in width to 405 μm in length and 106 μm in width. The V2AlC sample sintered at 1500°C exhibited the highest flexural strength of 289 MPa and a fracture toughness of 5.7 MPa·m1/2, while the V2AlC sample prepared at 1400°C showed the highest compressive strength of 742 MPa. The sample sintered at 1700°C possessed the highest damage tolerance. Additionally, V2AlC could retain a high degree of Young's modulus up to 1200°C. Below 800°C, V2AlC showed excellent thermal shock resistance.

In situ synthesis of bulk Al3BC3 was achieved via a reactive hot-pressing method using Al, B4C, and graphite powders at 1800°C for 2 h. The reaction path for synthesizing Al3BC3 was investigated. It was found that Al3BC3 formed via the reaction of C, B4C, and Al4C3 above 1180°C. Dense Al3BC3 was prepared with a little B4C and graphite remained. Microstructure observations revealed the plate-like morphology of Al3BC3 grains. Moreover, the mechanical properties of Al3BC3 were characterized (Vickers hardness of 11.1 GPa, bending strength of 185 MPa, fracture toughness of 2.3 MPa·m1/2, and Young's modulus of 163 GPa). Young's modulus decreased slowly with increasing temperature, and at 1600°C remained 79% of that at ambient temperature. These results show that Al3BC3 is a promising lightweight high temperature structural material.

In this work, a bulk Nb4AlC3 ceramic was prepared by an in situ reaction/hot pressing method using Nb, Al, and C as the starting materials. The reaction path, microstructure, physical, and mechanical properties of Nb4AlC3 were systematically investigated. The thermal expansion coefficient was determined as 7.2 × 10−6 K−1 in the temperature range of 200°–1100°C. The thermal conductivity of Nb4AlC3 increased from 13.5 W·(m·K)−1 at room temperature to 21.2 W·(m·K)−1 at 1227°C, and the electrical conductivity decreased from 3.35 × 106 to 1.13 × 106Ω−1·m−1 in a temperature range of 5–300 K. Nb4AlC3 possessed a low hardness of 2.6 GPa, high flexural strength of 346 MPa, and high fracture toughness of 7.1 MPa·m1/2. Most significantly, Nb4AlC3 could retain high modulus and strength up to very high temperatures. The Young's modulus at 1580°C was 241 GPa (79% of that at room temperature), and the flexural strength could retain the ambient strength value without any degradation up to the maximum measured temperature of 1400°C.

In order to improve the mechanical properties of Ti3AlC2, near-fully dense Ti3AlC2/TiB2 composites were synthesized using Ti, Al, graphite, and B4C powders as the initial materials. Compared with monolithic Ti3AlC2, the composites exhibit a much higher strength (for the compressive strength, from initial 723 MPa to maximal 2205 MPa; for flexural strength, from initial 340 MPa to maximal 861 MPa), and the strengthening effect can be held at least up to 1100 degrees C. Moreover, besides the enhancement of the elastic modulus and hardness of Ti3AlC2, the introduction of a TiB2 phase makes a positive contribution to its electrical conductivity.

Bulk Ta4AlC3 ceramic was prepared by an in situ reaction synthesis/hot-pressing method using Ta, Al, and C powders as the starting materials. The lattice parameter and a new set of X-ray diffraction data were obtained. The physical and mechanical properties of Ta4AlC3 ceramic were investigated. Ta4AlC3 is a good electrical and thermal conductor. The flexural strength and fracture toughness are 372 MPa and 7.7 MPa center dot m(1/2), respectively. Typically, plate-like layered grains contribute to the damage tolerance of Ta4AlC3. After indentation up to a 200 N load, no obvious degradation of the residual flexural strength of Ta4AlC3 was observed, demonstrating the damage tolerance of this ceramic. Even at above 1200 degrees C in air, Ta4AlC3 still retains a high strength and shows excellent thermal shock resistance, which renders it a promising high-temperature structural material.

An in situ reactive hot-pressing process using zirconium (zirconium hydride), aluminum, and graphite as staring materials and Si and Y(2)O(3) as additives was used to synthesize bulk Zr(3)Al(3)C(5) ceramics. This method demonstrates the advantages of easy synthesis, lower sintering temperature, high purity and density, and improved mechanical properties of synthesized Zr(3)Al(3)C(5). Its electrical and thermal properties were measured. Compared with ZrC, Zr(3)Al(3)C(5) has a relatively low hardness (Vickers hardness of 12.5 GPa), comparable stiffness (Young's modulus of 374 GPa), but superior strength (flexural strength of 488 GPa) and toughness (fracture toughness of 4.68 MPa.m(1/2)). In addition, the stiffness decreases slowly with increasing temperature and at 1600 degrees C remains 78% of that at ambient temperature, indicating that Zr(3)Al(3)C(5) is a potential high-temperature structural ceramic.

Polycrystalline Zr2Al3C4 was fabricated by an in situ reactive hot-pressing process using zirconium (zirconium hydrides), aluminum, and graphite as starting materials. The investigation on reaction path revealed that the liquid Al played an important role in triggering the formation of ternary zirconium aluminum carbides. The mechanical properties of Zr2Al3C4 at room temperature were measured (Vickers hardness of 10.1 GPa, Young's modulus of 362 GPa, flexural strength of 405 MPa, and fracture toughness of 4.2 MPa.m(1/2)). The electrical resistivity and thermal expansion coefficient were determined as 1.10 mu Omega.m and 8.1 x 10(-6) K-1, respectively.

Avoparcin is a macrocyclic glycopeptide antibiotic structurally related to vancomycin, teicoplanin, and ristocetin A. When attached to 5-μm spherical silica gel, the avoparcin proved to be an effective chiral stationary phase (CSP) that could be used in the reversed-phase, normal-phase, and polar-organic modes. The avoparcin CSP was complimentary to the other macrocyclic glycopeptide CSPs in that it could resolve some racemates that the others could not, and vice versa. Some important compounds resolved on the avoparcin CSP include verapamil, thyroxine, mephenytoin, and 2-imidazolidone-4-carboxylic acid. The use of this CSP and the optimization of separations on it are discussed. Avoparcin appears to be a useful addition to this family of CSPs. Chirality 10:627–660, 1998. © 1998 Wiley-Liss, Inc.

Ti(3)SiC(2) shows a unique combination of the properties of both metals and ceramics. However, its stiffness and strength lose rapidly above 1050 degrees C, which is the main obstacle for the high-temperature application of this material. To improve the high-temperature mechanical properties of Ti(3)SiC(2), Zr, Hf, or Nb were used as dopants in Ti(3)(SiAl)C(2). At room temperature, the Zr-, Hf-, or Nb-doped Ti(3)(SiAl)C(2) ceramics have comparable stiffness, hardness, strength, and fracture toughness with those of Ti(3)(SiAl)C(2). At high temperatures, however, a significant improvement in stiffness and strength has been achieved for (Ti(1-x)T(x))(3)(SiAl)C(2) (T=Zr, Hf, or Nb). (Ti(1-x)T(x))(3)(SiAl)C(2) can retain high degrees of stiffness and strength up to 1200 degrees C, which is 150 degrees C higher than those for Ti(3)(SiAl)C(2).