Processing of mesocarbon microbeads to high-performance materials: Part I. Studies towards the sintering mechanism

Owing to their exceptional properties, carbon-carbon composites have a variety of important applications. One attractive approach for the efficient and low-cost production of such materials is to utilize the unique sintering ability of mesocarbon microbeads (MCMB). However, the mechanism of MCMB sintering is not fully understood. In this work, detailed studies are made towards this goal, using dilatometric, thermogravimetric, and mass spectrometry techniques along with microstructural analysis. It is shown by independent measurements that significant changes in pycnometric density, mass loss, and shrinkage all occur in the same temperature range (800-1200 K). Based on the obtained results, a new explanation is suggested for the high sinterability of the investigated material, which includes two main stages: (i) neck formation between particles by a viscous phase non-densifying sintering mechanism (<800 K); (ii) rapid sample shrinkage due to crystallographic transformations leading to changes of theoretical particle density in the temperature range 800-1200 K.     

Processing of mesocarbon microbeads to high-performance materials: Part II. Reaction bonding by in situ silicon carbide and nitride formation

The processing of carbon-ceramic composites by utilizing the unique sintering ability of mesocarbon microbeads (MCMB) is reported. The ceramic constituents (silicon nitride and silicon carbide) are formed in situ by reactions between MCMB and silicon in different atmospheres. In comparison with direct addition of ceramic (SiC, Si₃N₄) phases, in situ formation shows several appealing features. By inducing the reaction of silicon with MCMB, the sintering ability of the composite is enhanced via reaction bonding mechanisms. Similarly, it is demonstrated that composite porosity is limited owing to silicon reaction with nitrogen. The reactive formation of nanoscale ceramic reinforcements via decomposition of the silicon-containing polymer (e.g. poly-carbomethyl-silane) is also reported. This approach results in formation of uniform nanosized (>100 nm) SiC layers strongly bonded to the surface of the carbon particles. The presented results contribute to the development of carbon-ceramic materials with high-operational properties.     

Activation of mesocarbon microbeads with different textures and their application for supercapacitor

Three kinds of mesocarbon microbeads (MCMBs) with different textures were activated by potassium hydroxide at 900 °C and used as electrode materials for supercapacitor. The effects of textures of precursors on electrochemical performances of activated MCMBs were investigated. Nitrogen sorption measurements (at 77 K) showed that three kinds of activated MCMBs possess high specific surface areas (> 2000 m²/g) and different porosity characteristics. MCMB prepared by emulsion method from bulk mesophase pitch (MCMB-e) has an irregular and distorted lamellar structure of oriented aromatic hydrocarbons. The unique texture of MCMB-e leads to the largest specific surface area (2542.8 m²/g) and the highest micropore volume (0.8236 cm³/g) after activation. Galvanostatic charge-discharge results showed that the activated MCMB-e has the highest specific capacitance of 326 F/g at the current density of 20 mA/g and better rate capability in 6 M KOH electrolyte. The good capacitive behavior of the activated MCMB-e may be attributed to the high-surface area, abundant micropores, closed-packed mesopores and macropores, as well as moderate crystal structures.     

Expansion of mesocarbon microbeads

Expanded mesocarbon microbeads (EMCMB) were prepared from graphitized mesocarbon microbeads by a chemical method. The expanded volume of EMCMB was significantly influenced by the reaction time, temperature, weight ratio of sulfuric acid and nitric acid, the maximum value of which was 4.1 ml/g. The morphology and structure characters of MCMB after chemical reaction and expansion were also investigated by SEM, BET and XRD measurements. The results show that the spherical, layered structure of MCMB could be verified convincingly by SEM micrographs of EMCMB, and the values of crystallite parameters of intercalated MCMB and EMCMB such as Lc and La decreased significantly compared with that of pristine MCMB, indicating that the stack height and stack width of carbon layers in a crystallite decreased after intercalation reaction and expansion. Meanwhile, EMCMB with appropriate porosity using as anode material for lithium-ion batteries exhibited an excellent high-rate discharge capacity and good cycle stability at the large current density.     

Processing of an aqueous tape casting of mesocarbon microbeads for high-performance carbonaceous laminations

Tape casting is a traditional method for the manufacture of ceramic laminations. In this study, aqueous tape casting was adopted to obtain high-performance carbonaceous laminations with homogeneous density and high strength. For the preparation of a stable and homogeneous slurry of mesocarbon microbeads, the research focuses on the rheological behavior of slurries consisting of a solvent and additives such as a binder, plasticizer and dispersant. After three to five slips of casting tape laminated together under 40 MPa at 85 °C and sinter heated to 1400 °C for 1 h at a given heating ratio, carbonized laminations are obtained with an average density of 1.66 g/cm³, a bending strength of 82.76 MPa and an electrical conductivity of 169.2 S/cm. During sintering of green laminations, the additives are pyrolyzed at 500 °C to form amorphous carbon, which reduces the electrical conductivity and the mechanical strength of the carbonized laminations. However, by controlling the total additive content of the slurries, the influence of the additives can be reduced to a minimum.     

Densification of fine-grained alumina ceramics doped by magnesia,yttria and zirconia evaluated by two different sintering models

The influence of various dopants (500 ppm MgO and Y₂O_3; 250 ppm ZrO₂) on sintering of fine-grained alumina ceramics was evaluated by high-temperature dilatometry. The apparent activation energy of sintering was estimated with the help of Master Sintering Curve and a model proposed by Wang and Raj. The densification kinetics was controlled by at least two mechanisms operating at low (higher activation energy) and high (lower activation energy) densities. Good agreement between the activation energies calculated with both models was observed for low as well as for high densities. The lowest value of activation energy exhibited undoped alumina; the addition of MgO resulted in slight increase of the activation energy. Y₂O₃ and ZrO₂ significantly inhibited the densification, which was reflected in the higher sintering activation energies. The low activation energies in the final sintering step indicates the importance of proper choice of sintering temperature, namely in the two-step sintering process.     

Effect of sintering on structural and physical properties of nickel and lithium co-substituted barium titanate ceramics

In this work, polycrystalline Ba_0.90Li_0.1Ni_0.05TiO₃ ceramics were prepared by conventional solid state reaction technique after microwave calcination of starting materials. The effect of sintering on structural, dielectric and activation energy of Ba_0.90Li_0.1Ni_0.05TiO₃ ceramics was investigated. Phase structure was confirmed by XRD and Rietveld refinement of the XRD patterns was carried out to estimate the lattice parameters and atomic positions. It was observed that distortion of the structure (c/a) increased as the sintering temperature increased and this leads to a shift in phase transition towards higher temperature. The average size of grains was affected by the sintering process. The Curie temperatures were observed to vary within an interval of 150–160 °C with a degree of diffusivity γ>1. The dielectric constant of the different sintered samples increased from 500 to 1340 at ambient and from 1200 to 2512 at Curie temperature, while the dielectric loss decreased from 0.024 to 0.008 at RT and from 0.04 to 0.022 at Curie temperature as the sintering temperature is increased. The activation energies of all sintered samples were investigated using Arrhenius plots of the temperature dependence of AC conductivity at different frequencies. The result shows that the activation energy decreased with an increase of sintering temperature. The decrease of activation energy with an increase of sintering temperature is attributed to the reduction in a number of grains due to densification which leads to the individual grains come closer and the effective area of the grain to grain contact increases while the grains themselves begin to mimic single crystals.     

Dynamic-mechanical behavior of polyethylenes and ethene-/α-olefin-copolymers. Part I. α′-Relaxation

Polyethylenes and polyethylene/α-olefin-copolymers covering a range in crystallinity between 12 and 85% were investigated by means of dynamic-mechanical measurements between −145 °C and their melting point. From the temperature and frequency dependence of the complex modulus α′-, α-, β- and γ-relaxations were analyzed. The α′-relaxation was discovered in all HDPE-, LDPE- and LLDPE-samples but not in plastomer- and elastomer-samples. The activation energies (30-140 kJ/mol) of this relaxation were found to decrease with increasing crystallinity. The α′-transition temperature at a fixed frequency rises with increasing degree of crystallinity and tends to reach the melting point when approaching the fully crystalline state. Thus, it is concluded that the α′-relaxation originates from the interface between crystal lamellae and amorphous interlamellar regions. By extrapolation of the storage modulus to the amorphous state the entanglement molar mass was calculated as 2300 g/mol for a completely amorphous polyethylene/α-olefin-copolymer.     

Orthorhombic YBCO-123 ceramic oxide superconductor: Structural,resistive and thermal properties

Two YBa₂Cu₃O_7-x materials (YBCO-I sintered at 1000?°C, and YBCO-II sintered at 960?°C and following special cooling programs) belonging to the orthorhombic system were obtained. YBCO-I has a complete volume texture, while for YBCO-II it is only on sample surface. On heating in air (with 10?K?min^?1 in the temperature range 20–1000?°C), two endothermic processes are recorded on DSC curves (of YBCO-II) and they are due to oxygen diffusion from O1 to O5 sites, and then to the release into the environment; second endothermic effect is attributed to the decomposition reaction. Different behaviour is encountered on heating in argon: an endothermic process in two steps; continuous mass decrease (oxygen loss) during argon cooling is recorded on the thermogravimetric curve. In argon atmosphere, the obtained material shall correspond to the chemical composition: YBa₂Cu₃O_6.77, while in air atmosphere, the maximal composition than may be obtained is: YBa₂Cu₃O_6.88. The barrier energies from Arrhenius plots of resistivity vs. temperature (91–93?K) show 2D (YBCO-I) and 3D (YBCO-II) behaviour for the thermally activated flow of melted vortex lattice, below the critical temperature.     

Influence of different bonding and fluxing agents on the sintering behavior and dielectric properties of steatite ceramic materials

The focus of the study was on providing insights into interconnections between sintering and development of the crystalline microstructure, and consequently variations in dielectric behavior of four steatites fabricated from a low-cost raw material, i.e. talc. The changes, induced by the alternations of the binders (bentonite, kaolin clay) and fluxing agents (BaCO₃, feldspar), were monitored in the temperature range 1000° to 1250 °C in which complete densification and re-crystallization of the investigated structures were accomplished. The critical points in the synthesis of steatite materials were assessed by instrumental analyses. Crystallinity changes and mineral phase transition during sintering were monitored by X-ray diffraction technique. Microstructural visualization of the samples and the spatial arrangements of individual chemical elements were achieved via scanning electron microscopy accompanied with EDS mapping. The thermal stability was observed on the green mixtures using differential thermal and thermo gravimetric analyses. Electrical measurements recorded variations of the dielectric constant (ε_r) and loss tangent (tan δ) as a function of the sintering temperature. The investigation highlighted critical design points, as well as the optimal combinations of the raw materials for production of the steatite ceramics for advanced electrical engineering applications.     

Preparation and mechanical performance of ductile Csf/Al2O3-BN composites,Part 1: Effects of fiber length and sintering temperature

Since its invention, alumina ceramics have been extensively investigated for potential various applications. However, their intrinsic brittle nature is still an insurmountable obstacle when they are applied as structural components. This paper provides a simple routs to prepare ductile alumina based composites with the addition of chopped carbon fiber (C_sf/Al₂O₃-BN). Effects of fiber length and sintering temperature on the microstructure, phase composition, mechanical properties together with fracture behavior were systematically investigated. The results showed that composites with mixed fiber lengths of 12 mm and 1 mm exhibited homogeneous microstructure and striking enhancement in mechanical performances compared with composites with other fiber length. With the increase in sintering temperature from 1500 °C to 1650 °C, interfacial bonding strength increased and interface state converted from mechanical interlocking at 1500 °C into chemical bonding at 1650 °C. Chemical reaction in the composites degraded carbon fiber properties, which resulted in the decrease in mechanical performance of the composites.     

Effect of ball milling on the sintering performance of indium-gallium-zinc oxide ceramics: The diffusion mechanism and lattice distortion of milled powders

The mechanisms inducing lattice distortion and grain boundary diffusion in mixed In₂O₃–Ga₂O₃-ZnO (IGZO) powders were investigated during ball-milling treatment at different rotation speeds. After sintering, the highest relative densities for the sintered IGZO compacts were 92.87%, 98.72%, and 99.12% at rotation speeds of 100 rpm, 200 rpm, and 300 rpm, respectively, and the corresponding grain growth activation energies were 78.78, 67.17, and 49.52 kJ/mol. The sintering process of InGaZnO₄ ceramics was shown to be dominated by grain boundary diffusion. X-ray diffraction line profile analysis indicated that the highest average lattice distortion of IGZO powders milled at 200 rpm and 300 rpm was 2.85 and 10.27 times higher, respectively, than that of IGZO powder milled at 100 rpm. The enhancements of the sintering properties of the IGZO powders induced by the faster milling speeds were attributed to the higher lattice distortion energies and shorter diffusion distances of the elements. This elucidated the mechanism behind the high-energy ball-milling process for achieving enhanced sintering performances in ceramic powders.     

Liquid reagent CVD of carbon. I. Processing and microstructure

The processing and microstructure of carbon coatings deposited using liquid reagent CVD were studied. High density pyrolytic carbon coatings were successfully deposited on graphite and molybdenum substrates from benzene and cyclohexane precursors. Very high deposition rates were obtained. Examination via transmission electron microscopy showed that the deposits were of the desired turbostratic nodular structure with low texture.     

Spectroscopic properties and conduction mechanism of KAl(SO4)2:xSm: A multifunctional materials for optical and electrochemical applications

Anhydrous α-alum materials doped with the trivalent samarium oxide Sm₂O₃ and denoted as KAl(SO₄)₂:xSm (x = 0; 0.5; 1; 1.5; 2; 2.5% mol.) are prepared by the solid-state reaction method at 350 °C. The resulting phases are crystallized in a simple hexagonal structure with space group P321. Powder X-ray diffraction (XRD), Infrared (IR), and Raman spectroscopies confirmed a high purity of phases with variation in lattice parameters according to the amount of doping.Optical measurements through absorption and fluorescence spectroscopies in the ultra-violet and visible regions prove the different electronic transitions between excited levels and ⁶H_5/2 ground state of Sm³⁺, the incorporation of samarium in the crystal structure, and suggest the quenching phenomenon.The materials presented in the study showed an ionic semiconductor behavior with an increase in their conductivity as a function of the doping level. A 1D conduction is made according to the Correlated Barrier Hopping CBH model by cations mobility in crystalline sites under the effect of thermal agitation in the [170–250 °C] region. KAl(SO₄)₂: xSm (x = 1.5% mol.) with its lower activation energy value, is suggested as a suitable cathode material for aluminum-based batteries.     

Chemical structure of cement aged at normal and elevated temperatures and pressures, Part II: Low permeability class G oilwell cement

Recently, Low Permeability Cement formulation has been developed for oilwell cementing. Therefore, it is important to understand the physical and chemical processes causing cement degradation in the downhole environment. In this study, we have characterised a Low Permeability Class G oilwell Cement immersed for one year in brine at T = 293 K, p = 10⁵ Pa and T = 353 K, p = 7 × 10⁶ Pa using ²⁹Si, ²⁷Al NMR and XRD techniques. Elevated temperature and pressure conditions increase the rate of the pozzolanic reaction and have significant effects on the polymerisation of C-S-H and on the incorporation of Al in the C-S-H structure. Leaching resulted in the formation of calcite and a more polymerised C-S-H with the appearance of tobermorite in the sample cured at elevated temperature and pressure.     

Viscoelastic and Processing Effects on the Fiber-Matrix Interphase Strength. Part III. The Effects of Postcure

A novel method for tailoring the interphase of carbon fiber-polymer composites by resistive electric heating is presented. The single-fiber fragmentation test is used to investigate the adhesion and fracture properties of the interphase. Electric resistive heating is shown to increase adhesion and toughness at the interphase region. In analyzing the results, the strength and fracture energy of the interphase are related to the thermal postcure conditions created by resistive electric heating. For this purpose, a difference analysis method is used to obtain a numerical solution for the heat conduction problem in the single-fiber test specimen and the temperature distributions are determined. Improvements obtained by using resistive electric heating of the carbon fiber are compared with those obtained by postcuring of the whole sample via convective thermal postcuring. The results obtained using these two different postcure methods seem to be similar.     

Viscoelastic and Processing Effects on the Fiber-Matrix Interphase Strength. Part III. The Effects of Postcure

A novel method for tailoring the interphase of carbon fiber-polymer composites by resistive electric heating is presented. The single-fiber fragmentation test is used to investigate the adhesion and fracture properties of the interphase. Electric resistive heating is shown to increase adhesion and toughness at the interphase region. In analyzing the results, the strength and fracture energy of the interphase are related to the thermal postcure conditions created by resistive electric heating. For this purpose, a difference analysis method is used to obtain a numerical solution for the heat conduction problem in the single-fiber test specimen and the temperature distributions are determined. Improvements obtained by using resistive electric heating of the carbon fiber are compared with those obtained by postcuring of the whole sample via convective thermal postcuring. The results obtained using these two different postcure methods seem to be similar.     

Real-time X-ray powder diffraction investigations on cocoa butter. III. Direct β-crystallization of cocoa butter: Occurrence of a memory effect

Direct β-crystallization of different samples of cocoa butter has been investigated. The influence of the thermal history of cocoa butter on its phase behavior is defined as a memory effect. The chemical composition of cocoa butter has been related to the occurrence of the short-term β-memory effectvia statistical analysis of the results. We explain how this effect can be attributed mainly to stearic acid and its related triacylglycerols. The total phase behavior of cocoa butter is discussed on the basis of the results obtained from the series of three papers of which this is the last.     

Gelcasting and sintering of hydroxyapatite materials: Effect of particle size and Ca/P ratio on microstructural,mechanical and biological properties

The sintering behaviour and microstructural evolution of two batches of a commercial calcium-deficient hydroxyapatite powder were investigated. First, the sintered density as a function of the starting particle size distribution was studied, and the minimum particle size to get the desired target density was determined. Then, as the two batches were characterized by a slight difference in Ca/P ratio, the role of such ratio on phase and microstructural evolutions during sintering, as well as on mechanical and biological properties was investigated.It was observed that the powder with lower Ca/P ratio underwent significant hydroxyapatite (HA) to β-tricalcium phosphate (β-TCP) decomposition, with a simultaneous formation of tetracalcium phosphate (TTCP). The microstructure of sintered gelcast samples evolved during isothermal sintering at 1300 °C, moving from a starting homogeneous and narrow grain size distribution to a bimodal distribution after 3 h sintering. In fact, over time, large grains decomposed into smaller ones, finally providing a microstructure composed of coarse grains surrounded by plenty of ultra-fine grains. On the contrary, the powder with the higher Ca/P ratio provided a limited HA to β-TCP transformation, and normal grain growth by increasing the sintering time. Such differences lead to different mechanical properties for gelcast samples produced by the two powder batches, as the material with the lower Ca/P ratio affected by lower mechanical strength. Finally, sintered samples from both powders showed in-vitro bioactivity, with a larger surface coverage observed for the lower Ca/P ratio material. The morphology of the apatite layer seemed to be affected by the material composition, too, showing flake-like and needle-like morphologies depending on the Ca/P ratio of the starting powder.