109,720 research outputs found
A new bound for the smallest x with ?(x) > li(x)
No full textWe reduce the leading term in Lehman's theorem. This improved estimate allows us to refine the main theorem of Bays and Hudson [2]. Entering 2,000,000 Riemann zeros, we prove that there exists x in the interval [exp (727.951858), exp (727.952178)] for which ?(x) - li(x) > 3.2 × 10151. There are at least 10154 successive integers x in this interval for which ?(x) > li(x). This interval is strictly a sub-interval of the interval in Bays and Hudson, and is narrower by a factor of about 12
New performing GC columns with unmatched separation capabilities
No full textGas chromatography (GC) is widely used for qualitative and quantitative analysis in numerous fields, such as petroleum, chemical industry, agriculture, environmental protection, medicine, and so on, due to its high
versatility, high selectivity, simplicity of use, analysis speed, and low sample consumption. The column is the heart of a GC instrumentation, which allows the analyte separation and their recognition and quantification. Commercial columns do not always allow a complete peak separation when compounds (i.e., isomers) are very
similar in molecular weight, polarity, and vapor pressure. The choice of the correct stationary phase, with high selectivity towards target analytes, is the key to obtaining the required chromatographic separation and the subsequent qualitative and quantitative analysis.
Considering the rapid polymer science development and the growing demand for new columns with
improved resolution capabilities, in this work novel stationary phases for capillary GC have been designed, synthesized, and characterized in terms of polarity range, resolution, column efficiency, thermal stability, filmforming properties, and support-deactivating capacity1-5. The separation features of these novel stationary phases allow high-resolution performances for a wide range of compounds, like aromatic anilines, xylenes, aromatic amines, halogenated benzenes, and aromatic aldehydes, with marked capabilities toward isomer separations.References:
[1] T. Sun, M. Ba, Y. Song, W. Li, Y. Zhang, Z. Cai, S. Hu, X. Liu, D. Nardiello, M. Quinto, Analytica Chimica Acta, 2024, 1291, art. no. 342221.
[2] T. Sun, R. Chen, Q. Huang, M. Ba, Z. Cai, H. Chen, Y. Qi, H. Chen, X. Liu, D. Nardiello, M. Quinto, Analytica
Chimica Acta, 2023, 1251, art. no. 340979.
[3] T. Sun, R. Chen, Q. Huang, M. Ba, Z. Cai, S. Hu, X. Liu, D. Nardiello, M. Quinto, ACS Applied Materials and Interfaces, 2022, 14 50, pp. 56132-56142
[4] R. Chen, Z. Cai, W. Li, Q. Huang, D. Nardiello, M. Quinto, X. Liu, S. Hu, T. Sun, Chemistry and Biodiversity, 2022,
19, art. no. e202200829
[5] Q. Huang, Z. Cai, R. Chen, W. Zhang, D. Nardiello, M. Quinto, X. Liu, S. Hu, T. Sun, Microchemical Journal, 2022,
183, art. no. 10808
A 2 h periodic variation in the low-mass X-ray binary Ser X-1
No full textSpectroscopy of the low-mass X-ray binary Ser X-1 using the Gran Telescopio Canarias have revealed a ?2 h periodic variability that is present in the three strongest emission lines. We tentatively interpret this variability as due to orbital motion, making it the first indication of the orbital period of Ser X-1. Together with the fact that the emission lines are remarkably narrow, but still resolved, we show that a main-sequence K dwarf together with a canonical 1.4 M? neutron star gives a good description of the system. In this scenario, the most likely place for the emission lines to arise is the accretion disc, instead of a localized region in the binary (such as the irradiated surface or the stream-impact point), and their narrowness is due instead to the low inclination (?10°) of Ser X-1
The full gradient design in Li-rich cathode for high performance lithium ion batteries with reduced voltage decay
No full textLi-rich nickel cobalt manganese oxides (LNCM) cathode with full gradient compounds is prepared by using an ethanol assisted co-precipitation method. The rationally designed procedure involves gradient distributions of transition-metal ions, in which the Ni content increases continuously, Mn content decreases gradually and Co keeps at a low level. The employing ethanol during the synthesis adjusts the surface state to further improve the rate performance of the sample. The full gradient cathode not only provides superior cycling capacity, but also improves operating voltage and mitigates voltage decay due to the high Ni/Mn ratio of out layer with decreased Li2MnO3. The prepared material exhibits high specific capacity of 213.1 mAh g−1 after 100 cycling with voltage decay per cycle about 3.36 mV at 1 C. Even more notably, the voltage decay during cycling could still be significantly limited at high rates and capacity retention rates of 84% and 75% can be reached after 100 cycling at 2 C and 5 C, respectively. Hence, this state-of-art full gradient cathode provides an interesting model to promote a further application of LNCM
Tailoring of Gradient Particles of Li-Rich Layered Cathodes with Mitigated Voltage Decay for Lithium-Ion Batteries
No full textVoltage decay during cycling is still a major issue for Li-rich cathodes in lithium ion batteries. Recently, the increase of Ni content has been recognized as an effective way to mitigate this problem, although it leads to lower-capacity materials. To find a balance between voltage decay and high capacity, particles of Li-rich materials with concentration gradients of transition metals have been prepared. Since voltage decay is caused by oxygen loss and phase transition that occur mainly on the particle surface, the Ni content is designed with a negative gradient of concentration from the surface to the bulk of particles. To do so, microsized Li1.20Ni0.13Co0.13Mn0.54O2 particles are mixed with much smaller LiNi0.8Co0.1Mn0.1O2 particles to form deposits of small particles onto larger particles. The concentration gradient of Ni is achieved as the Ni ions in LiNi0.8Co0.1Mn0.1O2 penetrate into Li1.20Ni0.13Co0.13Mn0.54O2 during a calcination post-treatment. Gradient samples show superior cycling performance and voltage retention as well as improved safety. This systematic study explores a material model combining Li-rich and high-Ni layered cathodes that is shown to be effective in creating a balance between mitigated voltage decay and high energy density
Preparation of porous slag-based geopolymer spheres by direct template route for pH buffering applications
No full textPorous slag-based geopolymer spheres with a high degree of sphericity were produced by a facile direct template route. The porosity, cellular morphology, chemical structure, specific surface area, mechanical properties as well as pH buffering capacity of the obtained spheres was investigated. The obtained slag-based geopolymer spheres with high porosity and high degree of sphericity as well as acceptable strength have potential application as pH regulators
On Used Systems and Systems with Used Components
No full textConsider an n-component coherent system having random lifetime T_X, where X = (X1, . . . ,Xn) is the vector of the non-independent components' lifetimes. Stochastic comparisons of the residual life of T_X at a fixed time t ≥ 0, conditioned on {TX > t} or on {Xi > t, ∀i = 1, . . . , n}, are investigated. Sufficient conditions on the vector X that imply this comparison in the usual stochastic order are provided, together with sufficient conditions under which the lifetime T_X satisfies the NBU aging property
A three-dimensional TiO2-Graphene architecture with superior Li ion and Na ion storage performance
No full textIn this work, a three-dimensional TiO2-graphene composite with large specific surface area is designed by freeze drying. In this architecture, primary TiO2 nanoparticles (less than 10 nm in size) are wrapped with graphene homogeneously, forming spherical secondary particles (≈100 nm), and the spherical TiO2 particles further agglomerate into platelet-like particles with several micrometers in size. The TiO2-graphene composite delivers high de-lithiation and de-sodiation capacities of 312 mAh g−1 and 280 mAh g−1 at 17 mA g−1 as negative electrode materials in lithium and sodium cells, respectively, and excellent cycling performance with negligible capacity loss after 500 cycles at a specific current of 85 mA g−1. When coupled with an activated carbon positive electrode, it demonstrates high capacitance and long cycle life in sodium ion capacitors (81% after 2000 cycles at 1 A g−1) and lithium ion capacitors (95% after 5000 cycles at 1 A g−1). Its superior performance benefits from the designed 3D architecture, which combines the advantages of small primary particle size, a homogeneous carbon coating and an adequate contact area with the liquid electrolyte. The storage mechanisms of the superior TiO2-graphene composite in lithium and sodium cells are investigated by operando X-ray diffraction studies
The effect of compositional variations on characteristics of coarse intermetallic particles in overaged 7000 aluminium alloys
No full textTo provide an understanding of how compositional variations affect the characteristics of coarse intermetallic particles in 7xxx alloys, three Al-Zn-Mg-Cu-Zr aluminium alloy plates with different Zn, Mg and Cu contents were studied by optical microscopy based image analysis, scanning electron microscopy (SEM), energy disperse X-ray spectroscopy (EDS) and differential scanning calorimetry (DSC). These coarse intermetallic particles are detrimental, especially to the toughness of the alloy. The experimental observations were interpreted successfully on the basis of the phase diagram and the temperature dependent S phase solvus which was derived on the basis of the regular solution model. The temperature dependent S phase solvus indicates that some compositions in the composition windows of 7050 and 7x75 type alloys will give rise to the detrimental S phase that can not be dissolved during solution treatment. Also the T phase is analysed and conditions for its formation and dissolution are discussed. It is shown that the present results can produce useful information on alloy design and thermo-mechanical processing via microstructural control
Discovery of an optical counterpart to the hyperluminous X-ray source in ESO 243-49
No full textThe existence of black holes of masses similar to 10(2)-10(5)M(circle dot) has important implications for the formation and evolution of star clusters and supermassive black holes. One of the strongest candidates to date is the hyperluminous X-ray source (HLX1), possibly located in the S0- a galaxy ESO 243-49, but the lack of an identifiable optical counterpart had hampered its interpretation. Using the Magellan telescope, we have discovered an unresolved optical source with R = 23.80 +/- 0.25 mag and V = 24.5 +/- 0.3 mag within HLX1's positional error circle. This implies an average X-ray/optical flux ratio similar to 500. Taking the same distance as ESO 243-49, we obtain an intrinsic brightness M-R = -11.0 +/- 0.3 mag, comparable to that of a massive globular cluster. Alternatively, the optical source is consistent with a main-sequence M star in the Galactic halo (for example an M4.4 star at approximate to 2.5 kpc). We also examined the properties of ESO 243-49 by combining Swift/Ultraviolet/Optical Telescope (UVOT) observations with stellar population modelling. We found that the overall emission is dominated by a similar to 5-Gyr-old stellar population, but the UV emission at approximate to 2000 angstrom is mostly due to ongoing star formation at a rate of similar to 0.03M(circle dot) yr(-1). The UV emission is more intense (at least a 9 sigma enhancement above the mean) north-east of the nucleus, in the same quadrant as HLX1. With the combined optical and X-ray measurements, we put constraints on the nature of HLX1. We rule out a foreground star and a background AGN. Two alternative scenarios are still viable. HLX1 could be an accreting intermediate mass black hole in a star cluster, which may itself be the stripped nucleus of a dwarf galaxy that passed through ESO 243-49, an event which might have caused the current episode of star formation. Or, it could be a neutron star in the Galactic halo, accreting from an M4-M5 donor star
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