Microstructure,elastic, and inelastic properties of biomorphic carbons carbonized using a Fe-containing catalyst

The microstructure and amplitude dependences of the Young’s modulus E and internal friction (logarithmic decrement δ), and microplastic properties of biocarbon matrices BE-C(Fe) obtained by beech tree carbonization at temperatures T _carb = 850–1600°C in the presence of an iron-containing catalyst are studied. By X-ray diffraction analysis and transmission electron microscopy, it is shown that the use of Fe-catalyst during carbonization with T _carb ≥ 1000°C leads to the appearance of a bulk graphite phase in the form of nanoscale bulk graphite inclusions in a quasi-amorphous matrix, whose volume fraction and size increase with T _carb. The correlation of the obtained dependences E(Т _carb) and δ(T _carb) with microstructure evolution with increasing Т _carb is revealed. It is found that E is mainly defined by a crystalline phase fraction in the amorphous matrix, i.e., a nanocrystalline phase at Т _carb < 1150°C and a bulk graphite phase at T _carb > 1300°C. Maximum values E = 10–12 GPa are achieved for samples with Т _carb ≈ 1150 and 1600°C. It is shown that the microplasticity manifest itself only in biocarbons with T _carb ≥ 1300°C (upon reaching a significant volume of the graphite phase); in this case, the conditional microyield stress decreases with increasing total volume of introduced mesoporosity (free surface area).     

Thermal conductivity of the amorphous and nanocrystalline phases of the beech wood biocarbon nanocomposite

Natural composites (biocarbons) obtained by carbonization of beech wood at different carbonization temperatures T _carb in the range of 800–2400°C have been studied using X-ray diffraction. The composites consist of an amorphous matrix and nanocrystallites of graphite and graphene. The volume fractions of the amorphous and nanocrystalline phases as functions of T _carb have been determined. Temperature dependences of the phonon thermal conductivity κ(T) of the biocarbons with different temperatures T _carb (1000 and 2400°C) have been analyzed in the range of 5–300 K. It has been shown that the behavior of κ(T) of the biocarbon with T _carb = 1000°C is controlled by the amorphous phase in the range of 5–50 K and by the nanocrystalline phase in the range of 100–300 K. The character of κ(T) of the biocarbon with T _carb = 2400°C is determined by the heat transfer (scattering) in the nanocrystalline phase over the entire temperature range of 5–300 K.     

Thermal conductivity at the amorphous-nanocrystalline phase transition in beech wood biocarbon

High-porosity samples of beech wood biocarbon (BE-C) were prepared by pyrolysis at carbonization temperatures T _carb = 650, 1300, and 1600°C, and their resistivity ρ and thermal conductivity κ were studied in the 5–300 and 80–300 K temperature intervals. The experimental results obtained were evaluated by invoking X-ray diffraction data and information on the temperature dependences ρ(T) and κ(T) for BE-C samples prepared at T _carb = 800, 1000, and 2400°C, which were collected by the authors earlier. An analysis of the κ(T _carb) behavior led to the conclusion that the samples under study undergo an amorphous-nanocrystalline phase transition in the interval 800°C < T _carb < 1000°C. Evaluation of the electronic component of the thermal conductivity revealed that the Lorentz number of the sample prepared at T _carb = 2400°C exceeds by far the classical Sommerfeld value, which is characteristic of metals and highly degenerate semiconductors.     

Specific features of electrical properties of porous biocarbons prepared from beech wood and wood artificial fiberboards

This paper reports on comparative investigations of the structural and electrical properties of biomorphic carbons prepared from natural beech wood, as well as medium-density and high-density fiberboards, by means of carbonization at different temperatures T _carb in the range 650–1000°C. It has been demonstrated using X-ray diffraction analysis that biocarbons prepared from medium-density and high-density fiberboards at all temperatures T _carb contain a nanocrystalline graphite component, namely, three-dimensional crystallites 11–14 Å in size. An increase in the carbonization temperature T _carb to 1000°C leads to the appearance of a noticeable fraction of two-dimensional graphene particles with the same sizes. The temperature dependences of the electrical resistivity ρ of the biomorphic carbons have been measured and analyzed in the temperature range 1.8–300 K. For all types of carbons under investigation, an increase in the carbonization temperature T _carb from 600 to 900°C leads to a change in the electrical resistivity at T = 300 K by five or six orders of magnitude. The dependences ρ(T) for these materials are adequately described by the Mott law for the variable-range hopping conduction. It has been revealed that the temperature dependence of the electrical resistivity exhibits a hysteresis, which has been attributed to thermomechanical stresses in an inhomogeneous structure of the biocarbon prepared at a low carbonization temperature T _carb. The crossover to the conductivity characteristic of disordered metal systems is observed at T _carb ? 1000°C.     

Thermopower of beech wood biocarbon

This paper reports on measurements of the thermopower S of high-porosity samples of beech wood biocarbon with micron-sized sap pores aligned with the tree growth direction. The measurements have been performed in the temperature range 5–300 K. The samples have been fabricated by pyrolysis of beech wood in an argon flow at different carbonization temperatures (T _carb). The thermopower S has been measured both along and across the sap pores, thus offering a possibility of assessing its anisotropy. The curves S(T _carb) have revealed a noticeable increase of S for T _carb < 1000°C for all the measurement temperatures. This finding fits to the published data obtained for other physical parameters, including the electrical conductivity of these biocarbons, which suggests that for T _carb ∼ 1000°C they undergo a phase transition of the insulator-(at T _carb < 1000°C)-metal-(at T _carb > 1000°C) type. The existence of this transition is attested also by the character of the temperature dependences S(T) of beech wood biocarbon samples prepared at T _carb above and below 1000°C.     

Thermal conductivity of high-porosity biocarbon precursors of white pine wood

This paper reports on measurements of the thermal conductivity κ and the electrical conductivity σ of high-porosity (cellular pores) biocarbon precursors of white pine tree wood in the temperature range 5–300 K, which were prepared by pyrolysis of the wood at carbonization temperatures (T _carb) of 1000 and 2400°C. The x-ray structural analysis has permitted the determination of the sizes of the nanocrystallites contained in the carbon framework of the biocarbon precursors. The sizes of the nanocrystallites revealed in the samples prepared at T _carb = 1000 and 2400°C are within the ranges 12–35 and 25–70 Å, respectively. The dependences κ(T) and σ(T) are obtained for samples cut along the tree growth direction. As follows from σ(T) measurements, the biocarbon precursors studied are semiconducting. The values of κ and σ increase with increasing carbonization temperature of the samples. Thermal conductivity measurements have revealed that samples of both types exhibit a temperature dependence of the phonon thermal conductivity κ_ph, which is not typical of amorphous (and amorphous to x-rays) materials. As the temperature increases, κ_ph first varies proportional to T, to scale subsequently as ～T ^1.7. The results obtained are analyzed.     

Electrical and galvanomagnetic properties of biocarbon preforms of white pine wood

The electrical and galvanomagnetic properties of high-porosity biocarbon preforms prepared from white pine wood by pyrolysis at carbonization temperatures T _carb = 1000 and 2400°C have been studied. Measurements have been made of the behavior with temperature of the electrical resistivity, as well as of magnetoresistance and the Hall coefficient in the 1.8–300-K temperature interval and magnetic fields of up to 28 kOe. It has been shown that samples of both types (with T _carb = 1000 and 2400°C) are characterized by high carrier (hole) concentrations of 6.3 × 10²⁰ and 3.6 × 10²⁰ cm⁻³, respectively. While these figures approach the metallic concentration, the electrical resistivity of the biocarbon materials studied, unlike that of normal metals, grows with decreasing temperature. Increasing T _carb brings about a decrease in electrical resistivity by a factor 1.5–2 within the 1.8–300-K temperature range. The magnetoresistance also follows a qualitatively different pattern at low (1.8–4.2 K) temperatures: it is negative for T _carb = 2400°C and positive for T _carb = 1000°C. An analysis of experimental data has revealed that the specific features in the conductivity and magnetoresistance of these samples are described by quantum corrections associated inherently with structural characteristics of the biocarbon samples studied, more specifically with the difference between the fractions of the quasi-amorphous and nanocrystalline phases, as well as with the fine structure of the latter phase forming at the two different T _carb.     

Heat capacity of the white pine biocarbon preform and the related biocarbon/copper composite

This paper reports on measurements in the 80–300-K temperature interval of the heat capacity at constant pressure C _p (T) of high-porosity amorphous white pine carbon preforms (biocarbon) prepared by pyrolysis (carbonization) at T _carb = 1000 and 2400°C in an argon flow. The dependences C _p (T) for biocarbon/copper composites based on the carbon preforms obtained have also been determined. It is shown that the mixture rule holds for the composites, i.e., that C _p (T) of the composite is a sum of the heat capacities of the constituent materials taken in the corresponding ratios. Phonon mean free paths for the white pine carbon preforms prepared at T _carb = 1000 and 2400°C have been calculated and used to estimate the size of the nanocrystallites contributing to formation of the carbon frameworks of these preforms.     

Fast lithium-ion transport in composites containing lithium-bromide dihydrate

Composites containing lithium-bromide dihydrates and a second phase, either the monohydrate or dispersed Al₂O₃ particles, exhibit an abrupt increase in the electrical conductivity at thedihydrate melting temperature T_m = 43°C. Although the conductivity at T >62 T_m is characteristic of a good liquid electrolyte -σ ≈ 0.1 mho cm^-1 at 130°C with an activation energy E_A = 13 kJ/mol and a Li⁺-ion transport number 0.9 ± 0.2, the composite pellet may be handled as a solid; it is brittle at 100°C. The possible significance of this finding for the design of high-power battery electrodes is indicated.     

Features of electrical properties of BE-C(Fe) biocarbons carbonized in the presence of an Fe-containing catalyst

The effect of partial graphitization on electrical and galvanomagnetic properties of BE-C(Fe) biomorphic carbons produced by beech wood carbonization at temperatures of 850–1600°C in the presence of an iron-containing catalyst is studied. The use of an Fe catalyst at Т _carb ≥ 1000°C leads to the formation of nanoscale graphite-phase inclusions; its total volume and nanocrystallite sizes increase with Т _carb. The data on the carrier concentration and mobility are obtained. It was shown that partially graphitized BE-C(Fe) carbons with Т _carb ≥ 1000°C in the conductivity type and magnetoresistance features relate to highly disordered metal systems whose conductivity can be described taking into account the contribution of quantum corrections, mainly the correction caused by the electron–electron interaction. It is shown that nonmonotonic dependences of the Hall constant R on the magnetic field are characteristic of BE-C(Fe) samples with 1000 ≤ Т _carb < 1600°C, which is most probably caused by the contribution of various carrier groups, i.e., electrons and holes. In BE-C(Fe) samples with Т _carb = 1600°C, the Hall coefficient corresponds to the metal state, which is associated with conducting medium homogenization resulting from the formation of a significant graphite phase volume.     

Experimental proof of the existence of the A1 meson

In partial wave analyses of the (π^?π^?π⁺) system, substantial shape changes of the 1⁺S (?π) intensity as a function of t, and relative phase changes of ≈ 90°, provide compelling evidence for a resonant A₁ of mass ≈ 1280 MeV and width ≈ 300 MeV.     

Temperature dependence of Bi2O3 structural parameters close to the α–δ phase transition

This article presents the results of in situ X-ray powder diffraction, Raman spectroscopy, and electrical impedance spectroscopy of the α-phase of Bi₂O₃, at 0.1 MPa in the temperature range below and above the α–δ-phase transition. This work demonstrated subtle nonlinear temperature variations of the cell parameters, of the hard-mode Raman shift, and of the activation energy of electrical conductivity in the temperature range about 100–120°C below the α–δ phase transition temperature T _Tr ≈ 725°C in Bi₂O₃. At T < 600°C, the linear variation of the inverse dielectric susceptibility (χ ^?1) correlates well with the hard mode frequency shift Δ(ω ²) of Raman A_1g mode as Δ(χ ^?1)/Δ(ω ²) ≈ 5.5 × 10^?7 cm². A structural model describing the mechanism of O^2? anion distribution and electric dipole disordering in the vicinity of T _Tr is discussed.     

Thermal conductivity of partially graphitized biocarbon obtained by carbonization of medium-density fiberboard in the presence of a Ni-based catalyst

The thermal conductivity k and resistivity ρ of biocarbon matrices, prepared by carbonizing medium-density fiberboard at T _carb = 850 and 1500°C in the presence of a Ni-based catalyst (samples MDF-C( Ni)) and without a catalyst (samples MDF-C), have been measured for the first time in the temperature range of 5–300 K. X-ray diffraction analysis has revealed that the bulk graphite phase arises only at T _carb = 1500°C. It has been shown that the temperature dependences of the thermal conductivity of samples MDFC- 850 and MDF-C-850(Ni) in the range of 80–300 K are to each other and follow the law of k(T) ～ T ^1.65, but the use of the Ni-catalyst leads to an increase in the thermal conductivity by a factor of approximately 1.5, due to the formation of a greater fraction of the nanocrystalline phase in the presence of the Ni-catalyst at T _carb = 850°C. In biocarbon MDF-C-1500 prepared without a catalyst, the dependence is k(T) ～ T ^1.65, and it is controlled by the nanocrystalline phase. In MDF-C-1500(Ni), the bulk graphite phase formed increases the thermal conductivity by a factor of 1.5–2 compared to the thermal conductivity of MDF-C-1500 in the entire temperature range of 5–300 K; k(T = 300 K) reaches the values of ～10 W m^–1 K^–1, characteristic of biocarbon obtained without a catalyst only at high temperatures of T _carb = 2400°C. It has been shown that MDF-C-1500(Ni) in the temperature range of 40?300 K is characterized by the dependence, k(T) ～ T ^1.3, which can be described in terms of the model of partially graphitized biocarbon as a composite of an amorphous matrix with spherical inclusions of the graphite phase.     

Thermal conductivity of high-porosity biocarbon preforms of beech wood

This paper reports on measurements performed in the temperature range 5–300 K for the thermal conductivity κ and electrical resistivity ρ of high-porosity (cellular pores) biocarbon preforms prepared by pyrolysis (carbonization) of beech wood in an argon flow at carbonization temperatures of 1000 and 2400°C. X-ray structure analysis of the samples has been performed at 300 K. The samples have revealed the presence of nanocrystallites making up the carbon matrices of these biocarbon preforms. Their size has been determined. For samples prepared at T _carb = 1000 and 2400°C, the nanocrystallite sizes are found to be in the ranges 12–25 and 28–60 κ(T) are determined for the samples cut along and across the tree growth direction. The thermal conductivity κ increases with increasing carbonization temperature and nanocrystallite size in the carbon matrix of the sample. Thermal conductivity measurements conducted on samples of both types have revealed an unusual temperature dependence of the phonon thermal conductivity for amorphous materials. As the temperature increases from 5 to 300 K, it first increases in proportion to T, to transfer subsequently to ∼T ^1.5 scaling. The results obtained are analyzed.     

Covalent distribution of spin in V2O3:O17 nuclear resonance

O¹⁷ nuclear magnetic resonance has been observed in metallic V₂O₃ with frequency shifts from (?0.10 ± 0.02)-(?0.05 ± 0.02) per cent between 170 and 460°K respectively, a linewidth of 37 ± 5 oe and spin-lattice relaxation rate 1/T₁ ≈ 60 sec^?1 at 296°K. From these quantities, covalency parameters f_s/2S = ? 0.35 × 10^?3 and ?_π/2S ≈ ? 0.07 are calculated. One of the two vanadium 3d electrons in the antiferromagnetic state below the 170°K metal-insulator transition is inferred to lie in a non-magnetic state, while covalent charge transfer augments the spin moment of the other 3d electron to the observed 1.2 μ_B.     

Hopping conductivity of carbynes modified under high pressures and temperatures: Galvanomagnetic and thermoelectric properties

The conductivity, thermopower, and magnetoresistance of carbynes structurally modified by heating under a high pressure are investigated in the temperature range 1.8–300 K in a magnetic field up to 70 kOe. It is shown that an increase in the synthesis temperature under pressure leads to a transition from 1D hopping conductivity to 2D and then to 3D hopping conductivity. An analysis of transport data at T ≤ 40 K makes it possible to determine the localization radius a ～ (56?140) Å of the wave function and to estimate the density of localized states _g(E _F) for various dimensions d of space: _g(E _F) ≈ 5.8 × 10⁷ eV^?1 cm^?1 (d=1), _g(E _F) ≈5×10¹⁴ eV^?1 cm ^?2 (d=2), and _g(E _F)≈1.1×10²¹ eV^?1 cm_?3 (d=3). A model for hopping conductivity and structure of carbynes is proposed on the basis of clusterization of sp ² bonds in the carbyne matrix on the nanometer scale.     

Generalization of multiperipheral model with long range correlations; Van Der Waals approximation

Three enhancements are observed in the final state ω^°π⁺π⁺π⁺⁺π^?π^? selected from the channel $\text{p}\text{p → 3π}{}^{\mathrm{+}}\text{3π}{}^{\mathrm{?}}\text{π}{}^{\mathrm{°}}$ at 715 MeV/c: one in ω^°π^± at 1040 MeV (～ ≈ 55MeV) ω^°π⁺π^?, respectively near 1315 MeV (～ ≈ 100 MeV) and 1405 MeV (～ ≈ 40 MeV). The first two effects are strongly correlated and are interpreted in terms of a sequential decay A^°₂ through a new object, the B₁. The second (ωππ) enhancement seems to be an ?^°?^° effect below threshold and is attributed to a pionic decay of the $\text{K}\text{K}\text{)}{}_{\mathrm{I}}\text{=1}$ effect seen around the same mass in other reactions.     

Polymorphic phase transition and thermal stability in squaric acid (H2C4O4)

Phase transformations in squaric acid (H₂C₄O₄) have been investigated by thermogravimetry and differential scanning calorimetry with different heating rates β. The mass loss in TG apparently begins at onset temperatures T_di=245±5 °C (β=5 °C min^?1), 262±5 °C (β=10 °C min^?1), and 275±5 °C (β=20 °C min^?1). A polymorphic phase transition was recognized as a weak endothermic peak in DSC around 101 °C (T_c⁺). Further heating with β=10 °C min^?1 in DSC revealed deviation of the baseline around 310 °C (T_i), and a large unusual exothermic peak around 355 °C (T_p), which are interpreted as an onset and a peak temperature of thermal decomposition, respectively. The activation energy of the thermal decomposition was obtained by employing relevant models. Thermal decomposition was recognized as a carbonization process, resulting in amorphous carbon.     

Resistive sensor of weak magnetic fields on the basis of a thick HTSC film

The following parameters have been obtained for a thick (thickness t ～50 μm) film of high-temperature superconducting ceramics of the Bi-2223 system have been implemented: magnetosensitivity S _u ～ 29 V T^?1 and resolutions δB ～ 3 nT and δ? ～ 4?₀ in magnetic field and magnetic flux, respectively. It is shown that the film magnetosensitivity can be significantly increased due to the size effect. The expected characteristics, estimated with allowance for the size effect, are S _u ≥ 1000 V T^?1, δB ～ 0.01 nT, δ? ～ 0.01 ?₀, and the range of dynamic measurement ≥150 dB.