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[en] Cobalt nanoparticles anchored on graphene composites (Co/G composites) have been successfully synthesized by pyrolyzing the zeolitic imidazolate frameworks (ZIF-67)/graphene oxide composites. The structural characterization reveals that ZIF-67 derived Co nanoparticles with diameter of less than 100 nm are homogeneously distributed on the graphene sheets. When employed as anode for lithium ion batteries (LIBs), the composites deliver an initial discharge capacity of 670.8 mAh g−1 at a low current density of 50 mA g−1, and after 120 cycles, the capacity can be maintained at 562.4 mAh g−1. The composites can also display long cycling stability when measured at alternating high current. Even at 2000 mA g−1, a reversible discharge capacity of 248.8 mAh g−1 can be achieved after 1000 long cycles. The synergistic effect of Co nanoparticles and the excellent conductivity of the graphene matrix contribute to the superior cycle performance and rate capability of the composite electrodes. - Highlights: • Cobalt nanoparticles anchored on graphene composites are synthesized for LIBs. • The Co/G composites show superior cycling stability and rate capability. • A reversible discharge capacity of 248.8 mAh g−1 is achieved after 1000 cycles. • The electrochemical performance can be ascribed to the unique structure.
[en] Analyzing the imprint of relic gravitational waves (RGWs) on the cosmic microwave background (CMB) power spectra provides a way to determine the signal of RGWs. In this Letter, we discuss a statistical bias, which could exist in the data analysis and has the tendency to overlook the RGWs. We also explain why this bias exists, and how to avoid it. (orig.)
[en] Bismuth sulfide (Bi2S3) nanofibers have been successfully prepared by a photochemical method from an aqueous solution of bismuth nitrate (Bi(NO3)3) and thioacetamide (TAA) in the presence of complexing agents of nitrilotriacetic acid (NTA) at room temperature. It was found that the irradiation time, the pH of the solution, and the species of complexing agents play important roles in the morphology control of the bismuth sulfate (Bi2S3) nanomaterials. The nanofibers were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), electron diffraction (ED), high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectra (XPS), and UV-Visible absorption spectra (UV-Vis). Probable mechanisms for the photochemical formation of Bi2S3 nanofibers in aqueous solutions are proposed. The photochemical method is a convenient approach for controlling the shape for other metal sulfide semiconductor nanocrystals
[en] The purpose of this study was to compare the bonding characteristics of titanium porcelain Duceratin bonded to Ti–24Nb–4Zr–7.9Sn (TNZS) alloy and commercial pure titanium (cp Ti). The bond strengths between porcelain and TNZS were tested by a three-point flexural device. The same tests for the cp Ti were used as for the control. Coefficient of thermal expansion (CTE) of TNZS was evaluated with a push-rod dilatometer. Interfacial characterization was carried out by X-ray energy-dispersive spectrometry (EDS) analysis operating in line scan mode. Additionally, microstructure characterizations of TNZS and cp Ti after debonding fracture were analyzed by scanning electron microscope (SEM) and EDS. The porcelain bond strength of TNZS alloy was 31.51 MPa, showing a significant increase relatively to that of cp Ti (23.89 MPa) (P < 0.05). Mean CTE values of TNZS alloy was 9.51 × 10−6/°C exceeding the porcelain by 0.81 × 10−6/°C, attesting to a better mechanical performance. Interfacial characterization showed the mutual diffusion of Ti, Si, O and Sn along the TNZS–ceramic interface. Both SEM and EDS results revealed that fracture modes of TNZS specimens exhibited a mixed mode of cohesive and adhesive failures. The results demonstrated that TNZS could be a good alternative for the metal–ceramic restoration in the future. - Highlights: ► Comparing to cp Ti, TNZS alloy showed a significant improvement in bond strength. ► We checked the coefficient of thermal expansion (CTE) of TNZS. ► The fracture mode of TNZS–porcelain showed a mixed cohesive and adhesive failure. ► TNZS could be a good alternative for the metal–ceramic restoration in the future.
[en] In this paper, we study the possibility of building Yang-Mills (YM) field dark energy models with equation of state (EoS) crossing -1, and find that it cannot be realized by the single YM field models, no matter what kind of Lagrangian or initial condition. But the states of -1 < ω < 0 and ω < -1 all can be naturally got in this kind of model. The former is like a quintessence field, and the latter is like a phantom field. This states that one can build a model with two YM fields, one with the initial state of -1 < ω < 0, and the other with ω < -1. We give an example model of this kind, and find that its EoS is larger than -1 in the past and less than -1 at the present time. We also find that this change must be from ω > -1 to <-1, and it will go to the critical state of ω = -1 with the expansion of the universe, which character is the same as the single YM field models, and the big rip is naturally avoided
[en] By Polnarev's method we analytically calculate the polarization spectra of the cosmic microwave background radiation (CMB) generated by cosmic relic gravitational waves (RGW). In this analytic approach the physics involved in this generating process is more transparent. Consequently, the effects due to various elements of physics can be isolated easily. In solving the equation for evolution of RGW in the expanding universe, both the sudden transition and the WKB approximation for the scale factor during the radiation-matter transition have been taken. To describe more precisely the decoupling process, we have introduced an analytic expression for the visibility function, consisting of two pieces of half-Gaussian curves. We also include the damping on polarizations due to the photon diffusion up to the second order of the tight coupling. Analytic polarization spectra ClXX have been obtained with the following several improvements over the previous results. 1. The approximate analytic result is quite close to the numerical one evaluated from the cmbfast code, especially, for the first three peaks of the spectrum that are observable. By using the analytic exact solution of RGW in the sudden transition approximation, we have demonstrated the dependence of ClXX on the dark energy and the baryons. 2. Our analytic half-Gaussian approximation of the visibility function fits better than the usual Gaussian model, and its time integration yields an analytic damping factor, which is parameter-dependent. This improves the spectra by ∼30% around the second and third peaks. 3. The second order of tight coupling reduces the overall amplitude of ClXX by 58%, comparing with the tight-coupling limit. 4. The influences of inflation on RGW and on CMB polarization are explicitly demonstrated
[en] As strong evidence for inflation, relic gravitational waves (RGW) have been extensively studied. Although they have not been detected yet, some constraints have been achieved by observations. Future experiments for RGW detection are mainly of two kinds: CMB experiments and laser interferometers. In this paper, we study these current constraints and the detective abilities of future experiments. We calculate the strength of RGW Ωg(k) using two methods: the analytic method and the numerical method, by solving the inflationary flow equations. By the first method, we obtain a bound Ωg<3.89x10-16 at ν=0.1 Hz, where we have used the current constraints on the scalar spectral index and the tensor-scalar ratio; furthermore, we have taken into account the redshift-suppression effect, the accelerating expansion effect, and the neutrino damping effect on RGW. But the analytic expression of Ωg(k) depends on specific inflationary models and does not apply well for the waves with very high frequencies. The numerical method is more precise for the waves with high frequencies. It gives a bound Ωg<8.62x10-14, which is independent of the inflationary parameters, and applies to any single-field slow-roll inflationary model. After considering the current constraints on the inflationary parameters, this bound becomes Ωg<2x10-17. These two methods give consistent conclusions: the current constraints on RGW from LIGO, big bang nucleosynthesis, and pulsar timing are too loose to give any constraint for the single-field inflationary models, and the constraints from WMAP are relatively tighter. Future laser interferometers are more effective for detecting RGW with the smaller tensor-scalar ratio, but the CMB experiments are more effective for detecting the waves with the larger ratio. These detection methods are complementary to each other for the detection of RGW
[en] In this paper, we investigate a kind of special quintom model, which is made of a quintessence field φ1 and a phantom field φ2, and the potential function has the form of V(φ12-φ22). This kind of quintom field can be separated into two kinds: the hessence model, which has the state of φ12>φ22, and the hantom model with the state φ12<φ22. We discuss the evolution of these models in the ω-ω' plane (ω is the state equation of the dark energy, and ω' is its time derivative in units of Hubble time), and find that according to ω>-1 or <-1, and the potential of the quintom being climbed up or rolled down, the ω-ω' plane can be divided into four parts. The late time attractor solution, if existing, is always quintessencelike or Λ-like for hessence field, so the big rip does not exist. But for hantom field, its late time attractor solution can be phantomlike or Λ-like, and sometimes, the big rip is unavoidable. Then we consider two special cases: one is the hessence field with an exponential potential, and the other is with a power law potential. We investigate their evolution in the ω-ω' plane. We also develop a theoretical method of constructing the hessence potential function directly from the effective equation-of-state function ω(z). We apply our method to five kinds of parametrizations of equation-of-state parameter, where ω crossing -1 can exist, and find they all can be realized. At last, we discuss the evolution of the perturbations of the quintom field, and find the perturbations of the quintom δQ and the metric Φ are all finite even at the state of ω=-1 and ω'≠0
[en] It has been suggested that the Yang-Mills (YM) field can be a kind of candidate for the inflationary field at high energy scales or dark energy at very low energy scales, which can naturally give the equation of state -1 < ω < 0 or ω < -1. We discuss the zero order and first order Einstein equations and YM field kinetic energy equations of the free YM field models. From the zero order equations, we find that ω + 1 α a-2, from which it follows that the equation of state of the YM field always goes to -1, independent of the initial conditions. By solving the first order Einstein equations and the YM field equations, we find that in the YM field inflationary models, the scale-invariant primordial perturbation power spectrum cannot be generated. Therefore, only using this kind of YM field is not enough to account for inflationary sources. However, as a kind of candidate for dark energy, the YM field has the 'sound speed' cs2S = -1/3 < 0, which makes the perturbation oe have a damping behavior at large scales. This provides a way to distinguish the YM field dark energy models from other kinds of models. (research papers)
[en] Highlights: • Solubility of sodium 4-nitrotoluene-2-sulfonate in two aqueous organic solution were measured using dynamic method. • Solubility of disodium 4,4′-dinitrostilbene-2,2′-disulfonate in corresponding solution were measured using dynamic method. • The experimental data were correlated with electrolyte non-random two-liquid (E-NRTL) model. • A synergistic effect on disodium 4,4′-dinitrostilbene-2,2′-disulfonate solubility was observed. • A suitable solvent for oxidation of 4-nitrotoluene-2-sulfonic to 4,4′-dinitrostilbene-2,2′-disulfonic has been suggested. -- Abstract: Solid–liquid equilibrium (SLE) measurements for disodium 4,4′-dinitrostilbene-2,2′-disulfonate (DNSNa) and sodium 4-nitrotoluene-2-sulfonate (NTSNa) in aqueous ethylene glycol monoethyl ether solution and aqueous ethylene glycol monobutyl ether solution were conducted using a dynamic method over the temperature range from (280 to 335) K. A synergistic effect on DNSNa solubility was observed with the maximum solubility at solute-free mass fraction of ethylene glycol monoethyl ether w30=0.4000 and solute-free mass fraction of ethylene glycol monobutyl ether w40=0.5999, respectively. The solubility data were correlated using the thermodynamic electrolyte non-random two-liquid (E-NRTL) model and model parameters were determined simultaneously. Aqueous ethylene glycol monobutyl ether solution at solute-free mass fraction of ethylene glycol monobutyl ether w40=0.2000 was found to be suitable solvent medium for the oxidation of 4-nitrotoluene-2-sulfonic acid (NTS) to 4,4′-dinitrostilbene-2,2′-disulfonic acid (DNS) and the conclusion was confirmed using the static analytical method combined with UV–VIS spectrophotometer