Electrical Conductivity of SiOCN Ceramics by the Powder‐Solution‐Composite Technique

SiOCN ceramics have been prepared by the polymer pyrolysis method. The preceramic polymers were synthesized from a polysiloxane cross‐linked with two different N‐containing compounds: a silazane or a ternary amine. The corresponding SiOCN ceramics were obtained by pyrolysis in nitrogen atmosphere at five different temperatures from 1000°C to 1400°C. The electrical conductivity of the powdered SiOCN ceramic samples was determined by the powder‐solution‐composite technique. The results show an increase in room temperature AC conductivity of three orders of magnitude, from ≈10^?5 (S/cm) to ≈10^?2 (S/cm), with increasing pyrolysis temperature from 1000°C to 1400°C. Furthermore, the electrical conductivity of the amine‐derived SiOCN is three to five times higher than that of the silazane‐derived ceramic at each pyrolysis temperature. The combined structural study by Raman spectroscopy and chemical analysis suggests that the increase of electrical conductivity with the pyrolysis temperature is due to the sp³‐to‐sp² transition of the amorphous carbon phase. The higher conductivity of the amine‐derived SiOCN is also discussed considering features like the volume% of the free‐carbon phase and its possible N‐doping.     

Effect of Precursor on Speciation and Nanostructure of SiBCN Polymer‐Derived Ceramics

A comparative structural study of silicon borocarbonitride polymer‐derived ceramics synthesized using polyborosilylcarbodiimide and polyborosilazane precursors is carried out using high‐resolution, multinuclear, one‐ and two‐ dimensional NMR spectroscopy. The polyborosilylcarbodiimide‐derived ceramics contain relatively pure Si₃N₄ and C nanodomains with the BN domains being present predominantly at the interface such that the bonding at the interface consists of Si–N–B, Si–N–C, and B–N–C linkages. In contrast, the structure of the polyborosilazane‐derived ceramics consists of significant amount of mixed bonding in the nearest‐neighbor coordination environments of Si and B atoms leading to the formation of SiC_xN_4?x (0 ≤ x ≤ 4) tetrahedral units and BCN₂ triangular units. The interfacial region between the SiCN and C nanodomains is occupied by the BCN phase. These results demonstrate that the chemistry of the polymeric precursors exerts major influence on the microstructure and bonding in their derived ceramics even when the final chemical compositions of the latter are similar.     

Effect of Pyrolysis Temperature on the Structure and Conduction of Polymer‐Derived SiC

We studied the electric conductivity and structure of polymer‐derived carbon‐rich amorphous SiC pyrolyzed at different temperatures. The conductivity of the material increased drastically with pyrolysis temperature followed an Arrhenius relationship with the activation energy of ~3.4 eV. Raman and X‐ray photoelectron spectroscopy analysis revealed that the order of free carbon phase increased with pyrolysis temperature, accompanied by sp³→sp² transition. The activation energy for such a structure change was 3.1–3.8 eV, which is close to that for the conductivity change. We thus believe that the increase in the conductivity was mainly due to the increase in conductivity of the free carbon phase.     

Sm‐Doped LaSi3N5: Synthesis,Computed Electronic Structure,and Band Gaps

LaSi₃N₅‐based phosphor doped with Sm was prepared by the nitridation of LaSi‐Si‐Si₃N₄‐Sm₂O₃ powder mixture. The emission spectrum shows two main bands with maxima at 595 nm in the orange region and at ~650 nm in the red region. The excitation spectrum of Sm‐doped LaSi₃N₅ shows a maxima at 585, 570, and 405 nm. First‐principles density‐functional theory calculations were performed using Vienna ab initio simulation package to enhance the understanding of the electronic structure of the stoichiometric LaSi₃N₅ and Sm‐doped LaSi₃N₅. The electronic structure and band gaps were calculated in 2 × 1 × 2 supercell with 144 atoms using the more precise screened Coulomb hybrid functional HSE06. Both La³⁺/Sm³⁺ and La³⁺/Sm²⁺ substitutions were calculated. The calculated band gap of Sm(III)‐doped LaSi₃N₅ is 2.01 eV, in reasonable agreement with the experimental value of 2.12 eV, but corresponds to the unrealistic transition between the N, Si p states, and unoccupied Sm 4f states. The band gap of 1.43 eV calculated for Sm(II)‐doped LaSi₃N₅ is smaller than the available experimental value, but corresponds to the correct transition between nonbonding Sm 4f states and empty La 5d states. Optical properties are found to be governed by f electrons of the Sm(II) dopant.     

Effect of Niobium Oxide on the Structure and Properties of Melt‐Derived Bioactive Glasses

The effects of adding Nb₂O₅ on the physical properties and glass structure of two glass series derived from the 45S5 Bioglass^® have been studied. The multinuclear ²⁹Si, ³¹P, and ²³Na solid‐state MAS NMR spectra of the glasses, Raman spectroscopy and the determination of some physical properties have generated insight into the structure of the glasses. The ²⁹Si MAS NMR spectra suggest that Nb⁵⁺ ions create cross‐links between several oxygen sites, breaking Si–O–Si bonds to form a range of polyhedra [Nb(OM)_6?y(OSi)_y], where 1 ≤ y ≤ 5 and M = Na, Ca, or P. The Raman spectra show that the Nb–O–P bonds would occur in the terminal sites. Adding Nb₂O₅ significantly increases the density and the stability against devitrification, as indicated by ΔT(T_x ? T_g). Bioglass particle dispersions prepared by incorporating up to 1.3 mol% Nb₂O₅ by replacing P₂O₅ or up to 1.0 mol% Nb₂O₅ by replacing SiO₂ in 45S5 Bioglass^® using deionized water or solutions buffered with HEPES showed a significant increase in the pH during the early steps of the reaction, compared using the rate and magnitude during the earliest stages of BG45S5 dissolution.     

Effect of Impurity and Carrier Concentrations on Electrical Resistivity and Thermal Conductivity of Sic Ceramics Containing BeO

Silicon carbide ceramics with BeO as an additive exhibit unusually high electrical resistivity and thermal conductivity compared to conventional SiC ceramics. Studies concerning the effects of carrier concentration in the SiC grains on electrical properties and thermal conductivity are described. The low carrier concentration in this ceramic is responsible for the high electrical resistivity. The thermal conductivity, however, decreases gradually with increasing impurity concentration.     

Semiconductive Behavior of Polymer‐Derived SiCN Ceramics for Hydrogen Sensing

A low‐cost sensing mechanism of hydrogen gas is developed using polymer‐derived ceramic, a liquid organic precursor, polysilazane with the addition of 5 wt% of photoinitiator, 2,2 Dimethoxy‐2‐phenyl acephenone. UV photopolymerization is utilized to partially cross‐link the H‐shaped free standing specimen, and then pyrolyzed at 1400°C in hot isostatic press under nitrogen gas to convert the partially cross‐linked polymer into conducting and amorphous ceramic, silicon carbonitride. This work presents the preparation of free standing silicon carbonitride specimens as the sensor body for sensing hydrogen gas, depending on the semiconductive behavior of polymer‐derived ceramics in high‐temperature environments. The band gap of silicon carbonitride would be varied from adsorbing hydrogen molecules on the surface of the H‐shaped free standing specimen with two different thicknesses. An amenable specimen‐geometry for the four‐point test of measuring resistance is developed in a furnace filled with pure hydrogen and vacuumed environments.     

Effect of Powder Calcination Temperature and Ceramic Post‐Treatment on the Optical Properties of Nd:YAG Ceramics

Using the raw powders synthesized via a modified coprecipitation method with the ethanol–water precipitant solvent and combined with various powder calcination temperatures, the ceramics were fabricated by dry pressing, vacuum sintering, and air annealing. The results showed that higher powder calcination temperature could promote powder crystallinity and green‐compact density, and the corresponding ceramic transmittance increased. Meanwhile, more micrometer‐sized pores emerged in final ceramic. Fortunately, these large‐sized pores did not cut the ceramic transmittance sharply. To eliminate these pores, all ceramics were post‐treated with hot isostatic pressing and re‐annealing. All post‐treated ceramics showed increasing transmittances.     

Thermal Conductivity of Very Porous Kaolin‐Based Ceramics

A clay‐based material exhibiting high pore volume fraction and low thermal conductivity suitable for thermal insulation is described. Starting with a commercial clay containing >75% kaolinite, foams were made by mixing in water and methyl cellulose as a surfactant then beating. After drying at 70°C, the pore volume fraction >94% remains almost constant for treatments up to 1150°C. In contrast, the phases constituting the solid skeleton evolve strongly with removal of surfactant, dehydroxylation of kaolinite, and formation of mullite. The latter leads to greater mechanical strength but also an increase in thermal conductivity. Thermal treatment of the kaolin foam at 1100°C yields a suitable compromise between low thermal conductivity of 0.054 W.(m.K)^?1 at room temperature with a compressive yield stress of 0.04 MPa. The radiation component in the effective thermal conductivity is <10% at 20°C increasing to >50% at 500°C.     

Thermal Properties of Hf‐Doped ZrB2 Ceramics

The effect of Hf additions on the thermal properties of ZrB₂ ceramics was studied. Reactive hot pressing of ZrH₂, B, and HfB₂ powders was used to synthesize (Zr_1?x,Hf_x)B₂ ceramics with Hf contents ranging from x = 0.0001 (0.01 at.%) to 0.0033 (0.33 at.%). Room‐temperature heat capacity values decreased from 495 J·(kg·K)^?1 for a Hf content of 0.01 at.% to 423 J·(kg·K)^?1 for a Hf content of 0.28 at.%. Thermal conductivity values decreased from 141 to 100 W·(m·K)^?1 as Hf content increased from 0.01 to 0.33 at.%. This study revealed, for the first time, that small Hf contents decreased the thermal conductivity of ZrB₂ ceramics. Furthermore, the results indicated that reported thermal properties of ZrB₂ ceramics are affected by the presence of impurities and do not represent intrinsic behavior.     

BaSO4‐, CaF2‐, and Al‐Maleate‐Derived Nanocomposite Coatings with Excellent Mechanical,Thermal, and Optical Properties

Acrylate‐based nanocomposite coatings prepared from uniformly sized, nanoscaled inorganic, i.e., BaSO₄‐ and CaF₂‐ as well as organometallic, i.e., Al‐maleate‐derived nanoparticles were prepared applying photochemical curing. Excellent mechanical and thermal stability as well as high optical transparency was achieved as compared to standard SiO₂‐based coatings. The performance of CaF₂‐based nanocomposites could be further enhanced by addition of nanocorundum. A comprehensive data set on surface and Martens hardness, the penetration depths, glass transition temperatures, and UV–Vis transparency of the final coatings is presented.

     

A Unique Approach to Characterization of Sol‐Gel‐Derived Rare‐Earth‐Doped Oxyfluoride Glass‐Ceramics

Using the sol‐gel route Nd³⁺‐doped oxyfluoride glass‐ceramics were prepared. LiYF₄ and YF₃ crystals were deposited in the glass‐ceramics and their size, distribution, and amount ratio were varied by changing the compositions and heating temperatures. The incorporation of Nd³⁺ ions into both the fluoride crystals was confirmed by the high‐resolution elemental mapping of the glass‐ceramics. The incorporated Nd³⁺ ions showed up and down conversion photoluminescence whose properties were obviously different among the samples. The preliminary site analysis for Nd³⁺ ions was carried out using a unique approach associated with the Prony series approximation. Finally, the approach was found to be useful for the analysis of materials that are structurally complicating.     

Effect of Grain Thermal Expansion Mismatch on Thermal Conductivity of Porous Ceramics

Many ceramics contain microcracks, which are often situated between sintered grains. These microcracks constitute thermal resistances, which may affect heat transfer through the material and its effective thermophysical properties. The thicknesses and the contact areas of the microcracks change with temperature as a result of the thermal expansion mismatch between the grains on opposite sides of the microcracks. This physical mechanism affects changes of the material's thermal conductivity, k , with temperature. The above mechanism usually plays a minor role at atmospheric pressure, where heat may flow via the gas filling the cracks. Hence, the temperature-induced changes of the crack geometry have little effect on heat transfer. However, at low gas pressures, where the heat flow between the grains occurs mainly via the contact areas, the grains' thermal expansion mismatch causes unusual temperature behavior of the material's thermal conductivity observed for several industrial refractories. In this paper, the influence of the above physical mechanism is discussed relative to other heat transfer mechanisms described in the literature. A simple physical model of the thermal expansion of grains bonded by an agent, having different thermal expansion coefficients, is developed. This model allows calculation of the contact area and the average microcrack opening between the grains as functions of the temperature, the characteristic grains sizes and their thermal expansion coefficients, and the permanent crack area. These parameters are evaluated and used to calculate the effective thermal conductivity of ceramic materials containing microcracks that appear as a result of thermal contraction of grains. The calculated thermal conductivity satisfactorily correlates with the experimental data collected for several chrome-magnesite refractories over a wide range of temperatures and gas pressures.     

The Effect of Composition on the Optical Properties and Hardness of Transparent Al‐rich MgO·nAl2O3 Spinel Ceramics

The study investigates the transmittance and hardness of Al‐rich spinel ceramics (MgO·nAl₂O₃, 1 ≤ n ≤ 2.5) prepared by reaction air sintering (up to closed porosity) of different ratios of fine and coarse‐grained commercial Al₂O₃ and MgO raw powders completed by subsequent hot isostatic pressing (HiP). Different compositions give rise to a wide range of presintering temperatures. With starting compositions 1 ≤ n ≤ 1.5, presintering results in a formation of single‐phase spinel, in which the excess of Al is solved. With higher Al contents (n > 1.5), however, a biphasic ceramic of stoichiometric MgAl₂O₄ and residual alumina is formed first. This excess alumina is incorporated into the spinel lattice during the final HiP at a temperature of 1750°C. Single‐phase, highly transparent spinel is obtained by increasing the Al‐content up to n = 2.5, which gives about 85% in‐line transmittance in the visible range of light and about 63% at a UV wavelength of 200 nm. Whereas the optical properties can be improved, the hardness (HV1) slightly decreases with increasing Al content. Depending on the raw powders, the hardness of samples prepared by finer powders tend to higher values enabled by the development of a bimodal microstructure with a finer grain fraction (≤2 μm) between coarser grains (≤156 μm). In contrast, samples made of coarser powders need higher sintering temperatures and exhibit, then, a monomodal microstructure of very large grains (≤622 μm) only.     

Effect of Ammonia Treatment on the Crystallization of Amorphous Silicon–Carbon–Nitrogen Ceramics Derived from Polymer Precursor Pyrolysis

An effort was made toward modifying the Si₃N₄-SiC phase ratio in bulk nanocomposites obtained from polymer precursors. While pyrolyzing the polymer, flowing ammonia was introduced, to facilitate a chemical exchange, resulting in a different C/N ratio in the ceramic pyrolysis product. A pre-pyrolysis/binding/pyrolysis approach was used for sample consolidation. Comparison was made between the crystallization behavior of the pyrolysis-derived ceramic powders and consolidated bulk samples. A profound enhancement in crystallization tendency was observed in the consolidated samples whose nitrogen content was increased by ammonia treatment. A mechanism based on the particle/binder interface energy was proposed to account for this observation.     

Design and Fabrication of Transparent and Gas‐Tight Optical Windows in Low‐Temperature Co‐Fired Ceramics

Low‐temperature co‐fired ceramics (LTCC) enable the fabrication of microfluidic elements such as channels and embedded cavities in electrical devices. Hence, LTCC facilitate the realization of complex and integrated microfluidic devices. Examples can be applied in many areas like reaction chambers for synthesis of chemical compounds. However, for many applications it is necessary to have an optically transparent interface to the surroundings. The integration of optical windows in LTCC opens up a wide field of new and innovative applications such as the observation of chemiluminescent reactions. These chemical reactions emit electromagnetic radiation and thus offer a method for noninvasive detection. Thin glasses (≤500 μm) were bonded by thermocompression onto a LTCC substrate. As the bonding agent, a glass frit paste was used. Borosilicate glasses, fused silica as well as silicon were successfully bonded onto LTCC. To join materials with a large coefficient of thermal expansion mismatch (i.e., fused silica and LTCC), it is necessary to limit the heat input to the bond interface. Therefore, a heating structure was integrated into the LTCC substrate beneath the bond interface. This bonding process provides a gas‐tight optical port with a high bond strength.     

Thermal and Electrical Conductivity of Amorphous and Graphitized Carbide‐Derived Carbon Monoliths

The influence of graphitization and composition of carbide‐derived carbon (CDC) monoliths on the electrical and thermal conductivity was investigated. Carbon monoliths with varying porosities were synthesized employing biomorphous macroporous TiC and SiC as precursors. Graphitization was carried out in situ during high‐temperature chlorination with and without addition of nickel, iron, and cobalt chloride to the carbide. The graphitized monoliths showed improved properties. The results demonstrate that despite graphitic carbon also glass‐like carbon, stemming from the carbide synthesis, increases the thermal and electrical conductivity significantly.     

Wet‐Route Synthesis and Characterization of Yb:CaF2 Optical Ceramics

This paper reports a new strainless fabrication method for ytterbium‐doped CaF₂ laser ceramics involving no drying step before green body casting. The nanoparticles were kept in aqueous solution until green body shaping. Centrifugation was used to obtain correct compactness of the green body before sintering. Characterizations were conducted at different steps of the fabrication process. No grain boundaries oxidation was observed in the sintered ceramics although the nanoparticles were permanently maintained in water until they were sintered. Finally, these ceramics are more homogeneous and have less light scattering defects (no porosity), and present improved optical properties when compared to ceramics obtained from dried nanopowders.     

Effect of Interface Structure on the Microstructural Evolution of Ceramics

The interface atomic structure was proposed to have a critical effect on microstructure evolution during sintering of ceramic materials. In liquid-phase sintering, spherical grains show normal grain growth behavior without exception, while angular grains often grow abnormally. The coarsening process of spherical grains with a disordered or rough interface atomic structure is diffusion-controlled, because there is little energy barrier for atomic attachments. On the other hand, kink-generating sources such as screw dislocations or two-dimensional (2-D) nuclei are required for angular grains having an ordered or singular interface structure. Coarsening of angular grains based on a 2-D nucleation mechanism could explain the abnormal grain growth behavior. It was also proposed that a densification process is closely related to the interface atomic structure. Enhanced densification by carefully chosen additives during solid state sintering was explained in terms of the grain-boundary structural transition from an ordered to a disordered open structure.