Synthesis,microstructure and electrical conductivity of carbon nanotube–alumina nanocomposites

Carbon nanotube–alumina (CNT–Al₂O₃) nanocomposites have been synthesized by direct growth of carbon nanotubes on alumina by chemical vapor deposition (CVD) and the as-grown nanocomposites were densified by spark plasma sintering (SPS). Surface morphology analysis shows that the CNTs and CNT bundles are very well distributed between the matrix grains creating a web of CNTs as a consequence of their in situ synthesis. Even after the SPS treatment, the CNTs in the composite material are still intact. Experimental result shows that the electrical conductivity of the composites increases with the CNT content and falls in the range of the conductivity of semiconductors. The nanocomposite with highest CNT content has electrical conductivity of 3336 S/m at near room temperature, which is about 13 orders of magnitude increase over that of pure alumina.     

Influence of CeO2 addition on crystal growth behavior of CeO2–Y2O3–ZrO2 solid solution

Nanopowders with cubic fluorite-type structure as well as uniform distribution in particle size were synthesized by hydrothermal method in the ternary oxide zirconia–yttria–ceria system with ceria content of 0–25 mol%. X-ray diffraction (XRD), thermogravimetric analysis/differential scanning calorimeter (TG/DSC), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy (Raman), specific surface area (S_BET) and high resolution transmission electron microscopy (HRTEM) were applied to characterize the structure, thermal decomposition, morphological characteristic and crystal growth of the produced powders. Qualitative analyses indicate that the as-synthesized nanoparticles are single-phase crystallites with an average particle size of 4–9 nm. The specific surface area, lattice parameter and microstrain are closely related to Ce⁴⁺ concentration. Moreover, activation energy of crystal growth is significantly dependent on the dopant (CeO₂) concentration. It firstly increased and then decreased with increasing dopant concentration, and the maximum value was observed at the dopant concentration of 5 mol%.     

Effect of sintering temperature and dopant concentration on microstructure and electrical conductivity of ultra-fine-grained ZrO2–Sc2O3 ceramics

Phase transformations in ZrO₂ + xSc₂O₃ solid solutions (6.5 < x < 11 mol%) at sintering of ceramics obtained from nanopowders produced by laser evaporation of the ceramic targets have been studied. The Sc₂O₃ concentration increasing from 6.5 to 11 mol% is accompanied by the sintering temperature decreasing and the average grain size growth from 130 nm to 760 nm. At concentration of about 7 mol% Sc₂O₃ an abrupt increase of the average grain size and electric conductivity is observed. The sinterability of the ZrO₂ − хSc₂O₃ ceramics is affected by the prehistory of nanopowders preparation. The characteristics of ceramics obtained from nanopowders evaporated from the targets based on (ZrO₂ + xmol% Sc₂O₃) mixture and on the (ZrO₂ − 11mol% Sc₂O₃) solid solution significantly differ, namely, in the latter the sintering temperature is markedly lower and the shrinkage rate is higher. Besides, its average grain size is substantially lower and the conductivity is higher.     

Investigation of structure and conductivity properties of polyaniline synthesized by solid–solid reaction

Conductive polyaniline has been prepared by solid–solid reaction using ammonium peroxydisulfate as an oxidant. The obtained polymer was examined by X-ray diffraction, UV visible, FTIR spectroscopy, thermogravimetric analysis and impedance spectroscopy. The effect of oxidant/monomer molar ratio (R) on the structure and electrical properties of polymer has been examined. The analyses of X-ray diffraction patterns demonstrated that polyaniline prepared by this method is more crystalline than that obtained by conventional solution method. The FTIR spectroscopy showed that the emeraldine salt has been formed. The electrical properties were measured at different temperatures in the range of 296–523 K. The ac conduction shows a regime of constant dc conductivity at low frequencies and a crossover to a frequency-dependent regime of the type A ω^S at high frequencies.     

Nanocrystaline solid solution CeO2–Bi2O3

Nanocrystalline powders of solid solution CeO₂–Bi₂O₃ were synthesized by self-propagating room temperature reaction (SPRT) procedure with composition (Ce_1?xBi_xO_2?δ where the x = 0.1–0.5). X-ray diffraction analyses show that for x < 0.50 a solid solution with fluorite structure is formed. Rietveld's structure refinement method was applied to characterize prepared powders and its microstructure (size–strain). The lattice parameters increase according to Vegard's rule with increasing of Bi concentration. The average crystallite size is about 2–3 nm. Spectroscopic ellipsometry and Raman scattering measurements were used to characterize the samples at room temperature. The Raman measurements demonstrated electron molecular vibrational coupling and increase of oxygen vacancy concentration whereas increase of Bi content provokes a small decrease of optical absorption edge in comparison with pure ceria. Specific surface area of obtained powders was measured by Brunauer–Emmet–Teller (BET) method.     

Enhancement of thermal shock and slag corrosion resistance of MgO–ZrO2 ceramics by doping CeO2

The purpose of this paper was to develop ceramics materials with high thermal shock resistance and corrosion resistance for preparing gas blowing components. In this paper, MgO-rich MgO–ZrO₂ ceramics were obtained by using MgO powder and ZrO₂ powder as starting materials and CeO₂ as an additive. Changes in the properties in terms of thermal shock resistance, mechanical properties, and slag corrosion-resistance with chemical compositions were examined correlated to microstructure and phase changes. Especially, the effect of doping CeO₂ on phase transition of zirconia in MgO-rich system was discussed. The results showed that doping amount of CeO₂ significantly improved properties of MgO–ZrO₂ ceramics. Especially when doping amount of CeO₂ was 2 wt%, residual strength ratio was enhanced over 100% after thermal shock testing. In samples doped with CeO₂, ZrO₂ was stable in cubic or tetragonal form due to complete solution of CeO₂, which was important reason for the improvement of various properties of MgO–ZrO₂ ceramics.     

Structural,magnetic properties,and electronic structure of Cr1−xMnxO2 solid solution

Bulk Cr_1-xMn_xO₂ samples are prepared by high pressure synthesis technology. The crystal structure, magnetic properties and electronic structure of the samples are investigated by experiments and theoretical calculation. The crystal structure of the samples are indexed to a rutile structure with space group P4₂/mnm. The lattice parameter a of the samples remains basically unchanged in accordance with Vegard's law, but the lattice parameter c decreases due to increasing Mn dopant content (x) as well as strong Metal–Metal bonding along the c-axis. The saturation magnetization of the Cr_1-xMn_xO₂ samples decreases with an increase in x. According to XPS analysis, there is electron transfer between Mn and Cr in Cr_1-xMn_xO₂. Mn exists as Mn²⁺ and Mn³⁺ions, and part of Cr is oxidized to Cr⁶⁺. Based on the XPS analysis, the magnetic moment of Cr_1-xMn_xO₂ is calculated and its value is in accordance with the experimental data.     

Tailored thermal expansion and electrical properties of α-Cu2V2O7/Al

α-Cu₂V₂O₇/Al composites (with 5–80 wt% of Al) were prepared by a solid state method. Their structural stability, thermal expansion, hardness and electrical properties were studied in detail. The coefficient of thermal expansion (CTE) and hardness of α-Cu₂V₂O₇/Al composites sample can be tailored with the content of Al. The CTE is only 0.49×10⁻⁶ K⁻¹ (RT–780 K) when the Al content is 10 wt%, which is near-zero thermal expansion. The electrical conductivity of α-Cu₂V₂O₇/Al composites increases with increasing the content of Al. When the content of Al is larger than 40 wt%, the α-Cu₂V₂O₇/Al composites exhibit excellent electrical conductivity, which can be mainly attributed to the conductive percolation phenomena of Al in the α-Cu₂V₂O₇/Al composites.     

An investigation of conductivity,microstructure and stability of HfO2–ZrO2–Y2O3–Al2O3 electrolyte compositions for high-temperature oxygen measurement

In search of better ionically conducting ceramics for high temperature oxygen fuel cells and sensors, the conductivity and microstructure of the HfO₂–ZrO₂–Y₂O₃ system with 15 mol% of Y₂O₃ and the HfO₂–ZrO₂–Y₂O₃–Al₂O₃ system with 50 mol% of Al₂O₃ have been investigated with X-ray diffractometry (XRD), scanning electron microscopy (SEM) and conductivity measurements as a function of temperature. The stability of electrolyte compositions was studied by continuously monitoring conductivity as a function of time at 1000°C. A majority of the investigated samples exhibited linear Arrhenius plots of the lattice conductivity as a function of temperature. In the HfO₂–ZrO₂–Y₂O₃–Al₂O₃ electrolyte systems the parameter pe′ was measured at a temperature range of 1000–1400°C. The HfO₂–ZrO₂–Y₂O₃–Al₂O₃ electrolyte systems have also showed better thermal shock resistance than the ZrO₂–Y₂O₃ systems. A comparison between the ageing of ZrO₂- and HfO₂-based electrolyte systems, as a result of long time annealing at a temperature of 1000°C, indicated that the degradation of the HfO₂-based system at a temperature of 1000°C and above is 1.5 times lower than the degradation of the ZrO₂-based systems.     

Microwave hybrid sintering of Al2O3 and Al2O3–ZrO2 composites,and effects of ZrO2 crystal structure on mechanical properties,thermal properties,and sintering kinetics

Structural ceramics such as Al₂O₃ and Al₂O₃–ZrO₂ composites are widely used in harsh environment applications. The conventional sintering process for fabrication of these ceramics is time-consuming method that requires large amount of energy. Microwave sintering is a novel way to resolve this problem. However, to date, very limited research has been carried out to study the effects of different ZrO₂ crystal structures on Al₂O₃–ZrO₂ composites, especially on the sintering kinetics, when fabricated by microwave sintering.The microwave hybrid sintering of Al₂O₃ and Al₂O₃–ZrO₂ composites was performed in this study. Tetragonal zirconia and cubic zirconia were used as two different reinforcements for an α–alumina matrix, and the mechanical and thermal properties were studied. It was found that Al₂O₃ experienced a remarkable increase in fracture toughness of up to 42% when t-ZrO₂ was added. Al₂O₃–c-ZrO₂ also showed increased fracture toughness. The sintering kinetics were also thoroughly investigated, and the average activation energy values for the intermediate stage of sintering were estimated to be 246 ± 11 kJ/mol for pure Al₂O₃, 319 ± 71 kJ/mol for Al₂O₃–c-ZrO₂, and 342 ± 77 kJ/mol for Al₂O₃–t-ZrO₂. These values indicated that the activation energy was increased by the addition of either type of ZrO₂, with the highest value shown by Al₂O₃–t-ZrO₂.     

Crystal structure,thermal expansion,and electrical properties of layered oxides LnBa(Fe,Co, Cu)2O5 + δ (Ln = Nd,Sm, Gd)

LnBaFe_0.5Co_0.5CuO_{5 + δ} and LnBaFeCo_0.5Cu_0.5O_{5 + δ} solid solutions (Ln = Nd, Sm, Gd) are synthesized by the solid-phase method; their structural parameters and oxygen nonstoichiometry are determined; and the thermal expansion, conductivity, and thermal emf of these crystals are investigated. The thermal expansion coefficient of sintered ceramics and its electric transport parameters are calculated. The influence of the nature of rare-earth elements (REE) and 3d metal on the crystal structure, oxygen nonstoichiometry, and physicochemical properties of layered oxides LnBa(Fe, Co, Cu)₂O_{5 + δ} are analyzed.     

Structure, thermal stability and electrical conductivity of CaMoO4+δ

Single-phase tetragonal scheelite CaMoO₄ (space group, Pmmm) was prepared via the conventional solid state reaction method. The oxidation state of the transition metal species for the as-sintered CaMoO₄ was analyzed by X-ray photoelectron spectroscopy (XPS). The average thermal expansion coefficient was measured as about 11 × 10⁻⁶ K⁻¹ over the temperature range of 303-1373 K. From the thermodynamic point of view, the phase diagram for CaMoO₄ was constructed by computing the equilibrium phase boundaries. Finally, the electrical conductivity of CaMoO₄ was also investigated by an AC impedance analyzer. The activation energy of bulk conductivity for CaMoO₄ was 2.1 eV.     

Structural parameters and oxygen ion conductivity of Y2O3–ZrO2 and MgO–ZrO2 at high temperature

The oxygen ion conductivity of zirconia-based solid electrolytes doped with 8 mol% Y₂O₃–ZrO₂ (YSZ) and 9 mol% MgO–ZrO₂ (Mg-PSZ) at high temperature was investigated in terms of their thermal behavior and structural changes. At room temperature, YSZ showed a single phase with a fluorite cubic structure, whereas Mg-PSZ had a mixture of cubic, tetragonal and some monoclinic phases. YSZ exhibited higher ionic conductivity than Mg-PSZ at temperatures from 600 °C to 1250 °C because of the existence of the single cubic structure and low activation energy. A considerable increase in the conductivity with increasing temperature was observed in Mg-PSZ, which showed higher ionic conductivity than YSZ within the higher temperature range of 1300–1500 °C. A monoclinic-to-tetragonal phase transformation was found in Mg-PSZ and the lattice parameter of the cubic phase increased at 1200 °C. The phase transformation and the large lattice free volume contributed to the significant enhancement of the ionic conductivity of Mg-PSZ at high temperatures.     

Low–thermal–conductivity and high–toughness CeO2–Gd2O3 co–stabilized zirconia ceramic for potential thermal barrier coating applications

Yttria partially stabilized zirconia (～4.0?mol% Y₂O₃–ZrO₂, 4YSZ) has been widely employed as thermal barrier coatings (TBCs) to protect the high–temperature components of gas–turbine engines. The phase stability problem existing in the conventional 4YSZ has limited it to application below 1200?°C. Here we report an excellent zirconia system co–doped with 16?mol% CeO₂ and 4?mol% Gd₂O₃ (16Ce–4Gd) presenting nontransformable feature up to 1500?°C, in which no detrimental monoclinic (m) ZrO₂ phase formed on partitioning. It also exhibits a high fracture toughness of ～46?J m^?2 and shows high sintering resistance. Besides, the thermal conductivity and thermal expansion coefficient of 16Ce–4Gd are more competent for TBCs applications as compared to the 4YSZ. The combination of properties suggests that the 16Ce–4Gd system could be of potential use as a thermal barrier coating at 1500?°C.     

Microstructure and thermal expansion of Al2TiO5–MgTi2O5 solid solutions obtained by reaction sintering

Solid solutions Mg_0.1Al_1.8Ti_1.1O₅ and Mg_0.5AlTi_1.5O₅ were obtained by reaction sintering of mixtures of the binary oxides at 1350–1600 °C using different precursor powders. For the composition Mg_0.1Al_1.8Ti_1.1O₅, ceramics sintered at 1400–1500 °C have high relative density (⩾90%), reduced grain size (2–6 μm), low thermal expansion (−0.8 to 0.3×10⁻⁶ K⁻¹ in the range 200–1000 °C) and reproducible expansion behaviour. At higher temperature, grain size rapidly increases owing to anisotropic and exaggerated grain growth (EGG) resulting in severe microcracking. Microstructure evolution is affected by the nature of the starting oxides, in particular for what concerns the onset temperature of EGG, the size and the fraction of abnormal grains. For the composition Mg_0.5AlTi_1.5O₅, EGG already takes place at 1350 °C and materials with grain size < 5 μm are difficult to obtain by conventional reaction sintering. Large grained samples (>10 μm) of both compositions show a reduced hysteresis and complex thermal expansion behaviour. In particular, heating to 1000 °C results in a significant increase in specimen size on return to room temperature. Repeated thermal cycling leads to an increase of the hysteresis.     

R-curve behavior,mechanical properties and microstructure of sintered ZrB2–SiCp–ZrO2f ceramics

In this paper, zirconium diboride based ceramics added with 20 vol.% silicon carbide particle and 15 vol.% zirconia fiber (Z20S_p15Z_f) were prepared by hot-pressing at 1850 °C for 60 min under a uniaxial load of 30 MPa in Ar atmosphere. R-curves for Z20S_p15Z_f ceramics were studied using the indentation-strength in bending technique and the envelope method. The results indicated that these two testing methods were consistent and viable for estimating R-curve. Z20S_p15Z_f ceramics had high resistance to crack growth and damage tolerance with the 6.8 MPa m^1/2 of steady-state toughness. The toughening mechanism was fiber debonding, fiber pull-out, crack bridging, crack branching, crack deflection and transformation toughening.     

Effects of La doping on electrical conductivity,thermal expansion and electrochemical performance in LaxSr1–xCo0.9Sb0.1O3–δ cathodes for IT–SOFCs

Perovskite oxides La_xSr_1–xCo_0.9Sb_0.1O_3–δ (LSCSbx, x=0.0–0.8) are investigated as IT–SOFC cathodes supported with La_0.9Sr_0.1Ga_0.8Mg_0.2O_3–δ (LSGM) electrolyte. All LSCSbx oxides have a tetragonal distorted perovskite structure with s.g. P4/mmm, while a La₂Co₂O₅ impurity phase was observed within La doping levels at x=0.6–0.8. The LSCSb0.4 has a good chemical compatibility with LSGM electrolyte for temperatures up to 1050 °C. XPS examinations indicate the existence of Co³⁺/Co⁴⁺ mixed valence states in LSCSbx. The conductivity increases with La doping and the LSCSbx with x=0.4 exhibits the highest electrical conductivity (e.g., 673–1637 S cm⁻¹ at 300–850 °C). The thermal expansion coefficient (TEC) decreases from 25.89×10^–6 K^–1 for x=0.0 to 18.5×10^–6 K^–1 for x=0.6 at 30–900 °C. Among the LSCSbx compositions, the LSCSb0.2 exhibits the lowest polarization resistance (R_p), which is merely 0.069 Ω cm² at 700 °C. The maximum power density of the cell with LSCSb0.2 cathode on 300 µm thick LSGM electrolyte attains 564 mW cm^–2 at 850 °C, which is higher than that of SrCo_0.9Sb_0.1O_3–δ (SCSb) cathode. All of the results indicate that LSCSb0.2 is a promising material for application in IT–SOFCs cathodes.     

A new ternary phase in the system CaO–Al2O3–Cr2O3: Crystal structure and thermal expansion of CaAl2Cr2O7

Polycrystalline material of a novel phase in the system CaO–Al₂O₃–Cr₂O₃ has been obtained by solid-state reactions. Chemical analysis indicated the composition CaAl₂Cr₂O₇. Single-crystal growth of the new compound using borax as a mineralizer was successful. Diffraction experiments at ambient conditions on a crystal with composition CaAl_2.13Cr_1.87O₇ yielded the following basic crystallographic data: space group P 3, a = 7.7690(5) Å, c = 7.6463(5) Å, V = 399.68(6) ų, Z = 3. Structure determination and subsequent least-squares refinements resulted in a residual of R(|F|) = 2.3% for 1440 independent observed reflections and 113 parameters. To the best of our knowledge, the structure of CaAl_2.13Cr_1.87O₇ or CaAl₂Cr₂O₇ represents a new structure type. It belongs to the group of double layer structures where individual double layers contain octahedrally and tetrahedrally coordinated cation positions. Linkage between neighboring sheet packages is provided by additional calcium cations. Furthermore, thermal expansion has been studied in the interval between 29 and 790°C using in situ high-temperature single-crystal diffraction. No indications for a structural phase transition were observed. From the evolution of the lattice parameters the thermal expansion tensor has been obtained. A pronounced anisotropy is evident. The response of structural building units to variable temperature has been discussed.     

Structural studies of a ZrO2–CeO2 doped system

The local structure around Zr, Ce and dopant atoms (Fe and Ni) in the ZrO₂–CeO₂ system investigated by X-ray absorption spectroscopy (XAS) is reported to better understand the tetragonal phase stabilization process of zirconia. The first coordination shell around Zr atoms is not sensitive to the introduction of dopants or to an increase in the ceria content (from 12 to 20 mol%). Ce ions maintain the eight-fold coordination as in CeO₂, but with an altered bond distance. The formation of vacancies resulting from reduction of Ce atoms can be discarded, because XANES spectra clearly show that Ce ions are preferentially in a tetravalent state. XANES and EXAFS experiments at the Fe K-edge evidence that the local order around Fe is quite different from that of the Fe₂O₃ oxide. On the one hand, ab initio EXAFS calculations show that iron atoms form a solid solution with tetragonal ZrO₂. The EXAFS simulation of the first coordination shell around iron evidences that the substitution of zirconium atoms by iron ones generates oxygen vacancies into the tetragonal network. This is a driven force for the tetragonal phase stabilization process. For Ni doped samples, EXAFS results show that Ni–O mean bond length is similar to the distance found in the oxide material, i.e., NiO compound. Besides this result, no evidence of similar solid solution formation for Ni-doped systems has emerged from the EXAFS analysis.     

Effect of N–Ni coordination bond on the electrical and thermal conductivity of epoxy resin/nickel-coated graphite

The agglomeration of nickel-coated graphite (NCG) in epoxy resin (EP) composites leads to low electrical conductivity of EP composites, which limits their development in electronic devices and multilayer circuits. In order to improve the electrical and thermal conductivity of NCG/EP composites, ethylenediamine (EDA) was used to modify NCG and compared with pure NCG-filled EP composites. It was found that the conductive effect of modified composites with 20 wt% filler is better than that of unmodified composites with 40 wt% filler. The results of Fourier transform infrared spectroscopy and thermogravimetric analysis of EDA-modified NCG (ENCG) showed that a coordination adsorption reaction occurred between EDA and NCG, forming N–Ni coordination bonds. When the filling amount of ENCG was 40 wt%, the conductivity and thermal conductivity of the composite are improved most significantly. The volume resistivity was reduced from 2.636 to 0.109 Ω cm, a decrease of 95.85%, and the thermal conductivity was improved from 0.517 to 0.968 W/(m K), an increase of 87.23%, respectively. Meanwhile, ENCG has better dispersion in the EP matrix than NCG.