Showing papers in "Journal of the American Ceramic Society in 2010"

Polymer-Derived Ceramics: 40 Years of Research and Innovation in Advanced Ceramics

Abstract: Preceramic polymers were proposed over 30 years ago as precursors for the fabrication of mainly Si-based advanced ceramics, generally denoted as polymer-derived ceramics (PDCs). The polymer to ceramic transformation process enabled significant technological breakthroughs in ceramic science and technology, such as the development of ceramic fibers, coatings, or ceramics stable at ultrahigh temperatures (up to 2000°C) with respect to decomposition, crystallization, phase separation, and creep. In recent years, several important advances have been achieved such as the discovery of a variety of functional properties associated with PDCs. Moreover, novel insights into their structure at the nanoscale level have contributed to the fundamental understanding of the various useful and unique features of PDCs related to their high chemical durability or high creep resistance or semiconducting behavior. From the processing point of view, preceramic polymers have been used as reactive binders to produce technical ceramics, they have been manipulated to allow for the formation of ordered pores in the meso-range, they have been tested for joining advanced ceramic components, and have been processed into bulk or macroporous components. Consequently, possible fields of applications of PDCs have been extended significantly by the recent research and development activities. Several key engineering fields suitable for application of PDCs include high-temperature-resistant materials (energy materials, automotive, aerospace, etc.), hard materials, chemical engineering (catalyst support, food- and biotechnology, etc.), or functional materials in electrical engineering as well as in micro/nanoelectronics. The science and technological development of PDCs are highly interdisciplinary, at the forefront of micro- and nanoscience and technology, with expertise provided by chemists, physicists, mineralogists, and materials scientists, and engineers. Moreover, several specialized industries have already commercialized components based on PDCs, and the production and availability of the precursors used has dramatically increased over the past few years. In this feature article, we highlight the following scientific issues related to advanced PDCs research: (1) General synthesis procedures to produce silicon-based preceramic polymers. (2) Special microstructural features of PDCs. (3) Unusual materials properties of PDCs, that are related to their unique nanosized microstructure that makes preceramic polymers of great and topical interest to researchers across a wide spectrum of disciplines. (4) Processing strategies to fabricate ceramic components from preceramic polymers. (5) Discussion and presentation of several examples of possible real-life applications that take advantage of the special characteristics of preceramic polymers. Note: In the past, a wide range of specialized international symposia have been devoted to PDCs, in particular organized by the American Ceramic Society, the European Materials Society, and the Materials Research Society. Most of the reviews available on PDCs are either not up to date or deal with only a subset of preceramic polymers and ceramics (e.g., silazanes to produce SiCN-based ceramics). Thus, this review is focused on a large number of novel data and developments, and contains materials from the literature but also from sources that are not widely available.

Flash Sintering of Nanograin Zirconia in <5 s at 850°C

Abstract: We show that yttrium-stabilized zirconia can be sintered in a few seconds at ∼850°C to full density, starting from a green density of 0.5, by the application of a dc electrical field (nominally, several hours at 1450°C are needed to complete the sintering process). This finding is explained by the local Joule heating at grain boundaries, which, on the one hand, promotes grain-boundary diffusion (a kinetic effect), while at the same time restricts grain growth (a thermodynamic effect). The smaller grain size and the higher temperature at grain boundaries can then act synergistically to enhance the rate of sintering. These results have a bearing in explaining the widespread spark plasma and microwave-assisted techniques for enhanced sintering.

Silicon Nitride for High‐Temperature Applications

Abstract: In this paper, a summary of the development of high-temperature silicon nitride (T>1200°C) is provided. The high-temperature capacity of various advanced commercial silicon nitrides and materials under development was analyzed in comparison with a silicon nitride without sintering additives produced by hot isostatic pressing. Based on this model Si 3 N 4 composed of only crystalline Si 3 N 4 grains and amorphous silica in the grain boundaries the influence of various sintering additive systems will be evaluated with focus on the high-temperature potential of the resulting materials. The specific design of the amorphous grain-boundary films is the key factor determining the properties at elevated temperatures. Advanced Si 3 N 4 with Lu 2 O 3 or Sc 2 O 3 as sintering additive are characterized by a superior elevated temperature resistance caused by effective crystallization of the grain-boundary phase. Nearly clean amorphous films between the Si 3 N 4 grains comparable to that of Si 3 N 4 without sintering additives were found to be the reason of this behavior. Benefit in the long-term stability of Si 3 N 4 at elevated temperatures was observed in composites with SiC and M o Si 2 caused by a modified oxidation mechanism. The insufficient corrosion stability in hot gas environments at elevated temperatures was found to be the main problem of Si 3 N 4 for application in advanced gas turbines. Progress has been achieved in the development of potential material systems for environmental barrier coatings (EBC) for Si 3 N 4 ; however, the long-term stability of the whole system EBC-base Si 3 N 4 has to be subject of comprehensive future studies. Besides the superior high-temperature properties, the whole application process from cost-effective industrial production, reliability and failure probability, industrial handling up to specific conditions during the application have to be focused in order to bring advanced Si 3 N 4 currently available to industrial application.

A Coupled Nanoindentation/SEM‐EDS Study on Low Water/Cement Ratio Portland Cement Paste: Evidence for C–S–H/Ca(OH)2 Nanocomposites

Abstract: A low water/cement ratio (w/c=0.20) hydrated Portland cement paste was analyzed by grid-indentation coupled with ex situ scanning electron microscope-energy-dispersive X-ray spectra (SEM-EDS) analysis at each indentation point. Because finite element and Monte-Carlo simulations showed that the microvolumes probed by each method are of comparable size (approximately 2 μm), the mechanical information provided by nanoindentation was directly comparable to the chemical information provided by SEM-EDS. This coupled approach provided the opportunity to determine whether the local indentation response was a result of a single- or a multiphase response--the latter being shown predominant in the highly concentrated w/c=0.20 hydrated cement paste. Results indicate that, in the selected microvolumes where C-S-H and nanoscale Ca(OH)2 (CH) are present, increasing fractions of CH increase the local indentation modulus (and hardness), yielding values above those reported for high-density (HD) C-S-H. Micromechanical analyses show that C-S-H and CH are associated, not merely as a simple biphase mixture, but as an intimate nanocomposite where nanoscale CH reinforces C-S-H by partially filling the latter's gel pores. The paper discusses the mechanism of forming the C-S-H/CH nanocomposite, as well as the impact of nanocomposites on various macroscopic properties of concrete (e.g., shrinkage, expansion). On a general level, this study illustrates how a coupled nanoindentation/X-ray microanalysis/micromechanics approach can provide otherwise inaccessible information on the nanomechanical properties of highly heterogeneous composites with intermixing at length scales smaller than the stress field in a nanoindentation experiment.

Synthesis and Microwave Dielectric Properties of Novel Temperature Stable High Q, Li2ATi3O8 (A=Mg, Zn) Ceramics

Abstract: Li2ATi3O8 (A=Mg, Zn) ceramics are prepared by the conventional solid-state ceramic route, and the phase purity, microstructure, and microwave dielectric properties are investigated. Li2MgTi3O8 ceramics show ɛr=27.20, Qu×f=42 000 GHz, and τf=(+) 3.2 ppm/°C, and Li2ZnTi3O8 ceramics have ɛr=25.6, Qu×f=72 000 GHz, and τf=(−) 11.2 ppm/°C, respectively, when sintered at 1075°C/4 h.

Temperature-Insensitive Large Strain of (Bi1/2Na1/2)TiO3–(Bi1/2K1/2)TiO3–(K0.5Na0.5)NbO3 Lead-Free Piezoceramics

Abstract: The effect of (K0.5Na0.5)NbO3 (KNN) addition on the ferroelectric behavior of (Bi1/2Na1/2)TiO3–(Bi1/2K1/2)TiO3 (BNT–BKT) lead-free piezoceramics was investigated. Polarization and strain hysteresis loops indicate that the ferroelectric order is disrupted significantly with the addition of KNN as a replacement for BNT and the destabilization of the ferroelectric order is accompanied by an enhancement of the unipolar strain, which peaks at a value of ∼0.48% (corresponding to a large signal d33 of ∼600 pm/V) at 1 mol% KNN content. This strain was analyzed as derived from an electrostrictive effect at lower electric fields and a converse piezoelectric effect at higher electric fields. By limiting the electric driving field to exclude the contribution from the converse piezoelectric effect, a temperature-insensitive large-field d33 of ∼250 pm/V up to 200°C was achieved.

Effect of Calcium Additions on N-A-S-H Cementitious Gels

Abstract: This paper reports a research on the properties of synthetic sodium aluminosilicate hydrate gels as models for the gels arising in alkali-activated aluminosilicate systems. The interaction between these N-A-S-H gels and aqueous calcium was studied to explore a potential consequence of mixing conventional Portland cements and alkali-activated systems. The properties of two N-A-S-H gels (with Si/Al = 1 and 2) synthesized in highly alkaline media, in the presence of calcium are reported. N-A-S-H gels were synthesized using aluminum nitrate, sodium hydroxide, and a sodium silicate. FTIR, NMR, and TEM/EDX techniques were used to characterize N-A-S-H gels before and after adding calcium. The results obtained show that calcium is uptaken in N-A-S-H gels resulting in a modification of the chemical composition due to the substitution of sodium per calcium.

Charge–Discharge Properties of an Antiferroelectric Ceramics Capacitor Under Different Electric Fields

Abstract: Charge–discharge properties of an La-modified Pb(Zr,Sn,Ti)O3 (PLZST) antiferroelectric (AFE) ceramics capacitor were investigated by directly measuring its hysteresis loops and pulse discharge current–time curves under different electric fields. Large increments in polarization and discharge current were observed when the electric field increases from 3 to 3.5 kV/mm. Under 4 kV/mm, the first current peak of this AFE ceramics capacitor (static capacitance around 4.4 nF) can surpass 1.1 kA, and more than 80% electric charge stored can be released in <65 ns. Moreover, this AFE ceramics capacitor can withstand 2000 times of charge–discharge cycling with no significant degradation of properties under 3.5 kV/mm.

From Three‐Dimensional Flower‐Like α‐Ni(OH)2 Nanostructures to Hierarchical Porous NiO Nanoflowers: Microwave‐Assisted Fabrication and Supercapacitor Properties

Abstract: A facile method is presented to prepare flower-like α-Ni(OH)2 involving only microwave heating of nickel nitrate hydrate in ethanol solvent without any hard/soft templates or precipitate-controlling agents. After calcination procedure, hierarchical and porous NiO nanoflowers with large BET-specific surface areas and controlled pore-size distributions are readily fabricated. A possible formation mechanism is proposed based on self-assembly under microwave heating. The structure, morphology, and crystalline phase of the products are characterized by transmission electron microscopy (TEM), X-ray diffraction, thermogravimetric analysis, Fourier transform infrared spectroscopy, and N2 adsorption/desorption isotherm analysis. TEM reveals that the obtained NiO nanoflowers have uniform size and are composed of sheet-like petals that are built up from nanocrystals. The resultant NiO nanostructures present a specific capacitance as high as 277 F/g and good cycling stability.

Antimony Tuned Rhombohedral-Orthorhombic Phase Transition and Enhanced Piezoelectric Properties in Sodium Potassium Niobate

Abstract: It has been reported that the rhombohedral–orthorhombic low-temperature polymorphic phase transition in (Na,K)NbO3 can be tuned close to room temperature by substituting Sb for Nb, such that enhanced piezoelectric properties are induced based on the theory of two-phase coexistence. A diagram of phase structures changing with Sb content and temperature has been generalized for (Na,K)(Nb,Sb)O3 (NKNS) compositions. The thermal stability of piezoelectric properties of NKNS ceramics was evaluated considering the existence of successive phase transition above room temperature. The microstructure, dielectric, and ferroelectric properties of NKNS ceramics were discussed from a crystallographic point of view.

Microwave Dielectric Ceramics in Li2O–Bi2O3–MoO3 System with Ultra-Low Sintering Temperatures

Abstract: A series of compounds in the Li2O–Bi2O3–MoO3 ternary system were investigated with regard to the preparation, phase composition, microwave dielectric properties, and chemical compatibility with silver (Ag) and aluminum (Al) electrodes. All the ceramics in this work have sintering temperatures lower than 750°C. The sintering behaviors and microwave dielectric properties of three single phases Li2MoO4, (Li0.5Bi0.5)MoO4, and Li8Bi2Mo7O28 bulk ceramics, were of particular focus in this investigation. The Li2MoO4 ceramic can be sintered to a high density at 540°C/2 h with a relative permittivity ∼5.5, a Q×f value of 46 000 GHz, and a temperature coefficient of resonant frequency (TCF) of ∼−160 ppm/°C. The (Li0.5Bi0.5)MoO4 ceramic has a scheelite structure and the largest relative permittivity of 44.4 among the ceramics studied in this work with a sintering temperature around 560°C, a Q×f value of 3200 GHz, and a large positive TCF of ∼+245 ppm/°C. The Li8Bi2Mo7O28 ceramic could be sintered at 540°C and has a relative permittivity of 13.6, a Q×f value of 8000 GHz, and a small negative TCF value of ∼−59 ppm/°C. From the X-ray diffraction analysis of cofired ceramics, the Li2MoO4 ceramic does not react with either Ag or Al powders. The Li8Bi2Mo7O28 ceramic reacts with Ag but not with Al at its densification temperature. The (Li0.5Bi0.5)MoO4 ceramic was found to strongly react with Ag powder and to a limited extent with Al powders. From this study, the Li2O–Bi2O3–MoO3 ternary system has a number of attractive new materials with low sintering temperatures, high-performing microwave dielectric properties, chemical compatibility with both Ag and Al metal electrodes, nontoxicity, and low-cost constituents. All these materials can be included in the new field of ultra-low-temperature cofiring dielectrics for multilayer applications.

In Situ Transmission Electron Microscopy of Electric Field-Triggered Reversible Domain Formation in Bi-Based Lead-Free Piezoceramics

Abstract: A lead-free piezoelectric 0.91(Bi1/2Na1/2)TiO3–0.06BaTiO3–0.03(K0.5Na0.5)NbO3 ceramic with high strain was examined in situ under an applied electric field using the transmission electron microscope. No domain structure is observed without an electric field, but an alternating electric field leads to the reversible formation of a lamellar domain structure. Correlations to polarization and strain hysteresis loop measurements indicate an electric field-induced phase transition from a nonpolar to a ferroelectric state and vice versa.

Piezoelectric and Dielectric Properties of (Ba1−xCax)(Ti0.95Zr0.05)O3 Lead-Free Ceramics

Abstract: Lead-free (Ba1−xCax)(Ti0.95Zr0.05)O3 (x=0.02–0.20) ceramics were prepared successfully using a solid-state reaction technique. The polymorphic phase transitions from orthorhombic to tetragonal phase around room temperature were identified in the composition range of 0.06

Synthesis and Luminescence Properties of BaMoO4:Sm3+ Phosphors

Abstract: BaMoO 4 :Sm 3+ phosphor system with a tetragonal structure was synthesized via a high-temperature solid-state method. The charge compensated behaviors, 2Ba 2+ →Sm 3+ +M + , where M + is a monovalent cation like Li + , Na + , and K + acting as a charge compensator, were investigated in this paper. It has been found that BaMoO 4 :Sm 3+ phosphor by doping with K + ions show greatly enhanced reddish orange emission compared with pure BaMoO 4 :Sm 3+ sample. Investigation on Sm 3+ and K + concentration-dependent emission spectra indicated that Ba 0.84 MoO 4 :0.08Sm 3+ ,0.12K + phosphor exhibited the strongest reddish orange emission with a CIE values of x = 0.55 and y = 0.43. After irradiation under the 402 nm ultraviolet (UV) excitation, three emission peaks centered at 561, 598, and 642 nm corresponding to the 4 G 5/2 to 6 H J (J = 5/2, 7/2, 9/2) emission lines of Sm 3 were obviously observed, and the lifetimes of three emissions ( 4 G 5/2 → 6 H J=5/2 , 7/2 , 9/2 ) have been calculated based on the measured decay curves.

Influence of TiO2 Nanoparticles on Early C3S Hydration

Abstract: The effect of nanoanatase titanium dioxide (TiO2) powder on early-age hydration kinetics of tricalcium silicate (C3S) was investigated. Isothermal calorimetry was performed on C3S pastes with 0%, 5%, 10%, and 15% of TiO2 addition by weight, and two mathematical models—the Avrami (or JMAK) model and the boundary nucleation model (BN model)—were fitted to the data. For all of the mixes, the addition of TiO2 increased the peak reaction rate, and increased the degree of hydration at 12 and 24 h. The rate of hydration of 10% and 15% TiO2 pastes were accelerated, while the 5% TiO2 paste was delayed, lengthening the induction period as compared with the control paste. The model fits demonstrate that the BN model captures the kinetics of the reaction better, particularly in the deceleration period, than the Avrami model. This is related to the ratio of rate parameters (kB/kG) of the BN model, that the increasing ratio gives a more unsymmetrical shape of a rate curve. The increase in kB/kG with TiO2 addition at 5%, 10%, and 15% suggests that hydration product is formed on or near the surfaces of TiO2 particles, as well as on the C3S surface. These results demonstrate that the addition of TiO2 nanoparticles accelerates the early hydration by providing additional nucleation sites, forming the foundation for future optimization of photocatalytic and other nanoparticle-containing cements.

Polymer-Derived SiOC/ZrO2 Ceramic Nanocomposites with Excellent High-Temperature Stability

Abstract: Polymer-derived SiOC/ZrO2 ceramic nanocomposites have been prepared using two synthetic approaches. A commercially available polymethylsilsesquioxane (MK Belsil PMS) was filled with nanocrystalline zirconia particles in the first approach. The second method involved the addition of zirconium tetra(n-propoxide), Zr(OnPr)4, as zirconia precursor to polysilsesquioxane. The prepared materials have been subsequently cross-linked and pyrolyzed at 1100°C in argon atmosphere to provide SiOC/ZrO2 ceramics. The obtained SiOC/ZrO2 materials were characterized by means of X-ray diffraction, elemental analysis, Raman spectroscopy as well as transmission electron microscopy. Furthermore, annealing experiments at temperatures from 1300° to 1600°C have been performed. The annealing experiments revealed that the incorporation of ZrO2 into the SiOC matrix remarkably increases the thermal stability of the composites with respect to crystallization and decomposition at temperatures exceeding 1300°C. The results obtained within this study emphasize the enormous potential of polymer-derived SiOC/ZrO2 composites for high-temperature applications.

Ferroelectric and Impedance Behavior of La- and Ti-Codoped BiFeO3 Thin Films

Abstract: To study the effects of La and Ti cosubstitutions on ferroelectric and impedance behavior of BiFeO3, (Bi0.90La0.10)FeO3, Bi(Fe0.95Ti0.05)O3, and (Bi0.90La0.10)(Fe0.95Ti0.05)O3 thin films were deposited on LaNiO3-buffered Pt/TiO2/SiO2/Si(100) substrates by off-axis radio frequency sputtering. The (Bi0.90La0.10)(Fe0.95Ti0.05)O3 thin film exhibits an Ohmic conduction behavior in the range of the electric field investigated, and its leakage current at high electric fields is greatly suppressed. The Curie temperature of (Bi0.90La0.10)(Fe0.95Ti0.05)O3 decreases to ∼690°C due to La and Ti cosubstitutions, and a direct band gap of 2.88 eV is identified for the (Bi0.90La0.10)(Fe0.95Ti0.05)O3 thin film, demonstrating the increase of a direct band gap with La and Ti codoping. A high remanent polarization (2Pr∼102.6 μc/cm2 and 2Ec∼538.0 kV/cm) as confirmed by positive up negative down was obtained for the (Bi0.90La0.10)(Fe0.95Ti0.05)O3 thin film at room temperature because of the great inhibition of the leakage current density at high electric fields. The La and Ti cosubstitutions also improve the fatigue behavior of BiFeO3 thin film. Impedance analyses at different temperatures and frequencies show that the La- and Ti-codoped BFO thin film exhibits rather different dielectric relaxation and conduction mechanism as compared with those of the undoped and La- or Ti-doped BFO thin films, where oxygen vacancies appear to be involved in the dielectric relaxation and conduction processes of these thin films.

The Effects of Temperature on the Local Structure of Metakaolin-Based Geopolymer Binder: A Neutron Pair Distribution Function Investigation

Abstract: Neutron pair distribution function (PDF) analysis is utilized to advance the understanding of the local atomic structural characteristics of geopolymer binders derived from metakaolin, specifically the nature and amount of the water associated with these materials. Samples were heated in air to temperatures up to 1200°C, then analyzed ex situ by high momentum transfer neutron total scattering and PDF analysis. Water contained in large pores, along with water associated with hydration of potassium cations in the geopolymer framework structure, comprise the majority of water in this material. The remaining water is situated in small pores and as terminal hydroxyl groups attached to the Si–Al framework. The Si–Al framework structure undergoes only subtle rearrangement upon heating, but maintains a tetrahedral aluminosilicate framework environment. After crystallization with heating beyond 1000°C, the geopolymer gel is predominantly converted to leucite, with small amounts of amorphous mullite and glassy silica, which have never before been observed in heated geopolymers. This demonstrates the value of neutron PDF analysis to probe the local structure of these important geopolymeric materials.

Enhanced Sintering Rate of Zirconia (3Y‐TZP) Through the Effect of a Weak dc Electric Field on Grain Growth

Abstract: We show, for the first time, that a dc electric field of 20 V/cm shifts the densification curve to a lower temperature in constant heating rate experiments with yttria-stabilized tetragonal zirconia powder (3Y-TZP). The enhanced sintering rate is ascribed, at least in part, to the reduced rate of grain growth under the applied field, consistent with earlier experiments on the influence of such fields on grain size in superplastic deformation and isothermal grain growth in zirconia polycrystals.

White Light Emission from NaCaPO4:Dy3+ Phosphor for Ultraviolet-Based White Light-Emitting Diodes

Abstract: Trivalent dysprosium (Dy 3+ )-doped NaCaPO 4 phosphors were synthesized by using a conventional solid-state reaction technique. The phase and the structure of the as-prepared powders were characterized by using X-ray diffraction analysis and revealed that the pure NaCaP0 4 formed with orthorhombic structure. The photoluminescence excitation and emission spectra were measured to characterize the luminescent properties of NaCaPO 4 :Dy 3+ phosphors. Sharp emission peaks were observed at 482 nm (blue) and 575 nm (yellow) upon 367 nm excitation, which are attributed to the characteristic 4 F 9/2 → 6 H J (J = 15/2 and 13/2) transitions of trivalent Dy 3+ ions, respectively. The suitable control of the blue/yellow intensity ratio is expected to realize a white luminescent system. The lifetime of 4 F 9/2 level was measured by exciting Dy 3+ ions at 355 nm excitation. The chromaticity coordinates were calculated from the emission spectra and analyzed with Commission International de I'Eclairage (CIE) programs and diagrams. The CIE color coordinates fall in the white light region under different ultraviolet (UV) excitations. These results indicate that NaCa-PO 4 :Dy 3+ phosphor could be a potential candidate for UV-based white light-emitting diodes.

Enhancement of Giant Dielectric Response in CaCu3Ti4O12 Ceramics by Zn Substitution

Abstract: Zn-substituted CaCu3Ti4O12 ceramics were synthesized by solid-state sintering. Their microstructures and dielectric properties were investigated. Ca(Cu1−xZnx)3Ti4O12 single-phase structures were obtained up to x=0.1, and the Cu+/Cu2+ and Ti3+/Ti4+ mixed-valent structure was enhanced with increasing Zn substitution. The giant dielectric response was significantly enhanced by Zn substitution. The dielectric constant increased with increasing x, and a giant dielectric constant plateau as high as ∼9 × 104 was achieved for x=0.1 at 10 kHz, while that for x=0 was ∼3 × 104. The enhanced giant dielectric response was profoundly concerned with the modified mixed-valent structure.

Electrical Properties of Gadolinium–Europium Zirconate Ceramics

Abstract: (Gd1−xEux)2Zr2O7 (0≤x≤1.0) ceramics are prepared via a solid-state reaction process at 1973 K for 10 h in air. (Gd1−xEux)2Zr2O7 (0.2≤x≤1.0) ceramics exhibit an ordered pyrochlore-type structure; however, Gd2Zr2O7 has a disordered defect fluorite-type structure. Raman spectroscopy analysis indicates that the degree of structural ordering increases with the increase of Eu content. The electrical properties of (Gd1−xEux)2Zr2O7 ceramics are investigated using complex impedance spectroscopy over a frequency range of 0.1 Hz to 20 MHz from 623 to 923 K. Electrical conductivity obeys the Arrhenius equation. Both the activation energy and the preexponential factor for grain conductivity decrease with increasing europium content from Gd2Zr2O7 (x=0) to Eu2Zr2O7 (x=1.0). The measured electrical conductivity of (Gd1−xEux)2Zr2O7 ceramics increases with increasing Eu content and temperature. Electrical conductivity of the pyrochlore-type materials is higher than that of the defect fluorite-type material in (Gd1−xEux)2Zr2O7 solid solution systems. The electrical conductivity of (Gd0.4Eu0.6)2Zr2O7 is almost independent of oxygen partial pressure from 1.0 × 10−4 to 1.0 atm. The high ionic transference number of (Gd0.4Eu0.6)2Zr2O7 ceramic at different temperatures proves that conduction is purely ionic with negligible electronic contribution.

The Rapid Formation of La(OH)3 from La2O3 Powders on Exposureto Water Vapor

Abstract: The reactivity of various lanthana powders in air was studied. The materials rapidly hydroxylate to form a stable hydroxide, La(OH)3, at room temperature. Smaller amounts of an oxycarbonate species (La2O2CO3) are also observed following air exposure. This oxycarbonate phase is stable to hydroxylation. All oxide materials synthesized here show rapid reactions so that 24 h of exposure to atmosphere is generally sufficient to cause complete hydroxylation at room temperature. The rate of reaction was related to the crystallite size as determined by XRD. The reaction was found to follow a two-stage kinetic process, a relatively slow surface reaction followed by a rapid bulk reaction. The relevance of the reactivity of these powders is discussed.

Fine-Grained Transparent Spinel Windows by the Processing of Different Nanopowders

Abstract: The study investigated the deagglomeration of spinel nanopowders (MgO·nAl2O3, n≈1) from different syntheses in correlation with their resulting performance on green processing and sintering. Transmission results are addressed as a means of evaluating the degree of last pore elimination. As a result, clear transparent components with a thickness of up to 20 mm and lateral dimensions up to about 240 mm were manufactured equally successfully from spinel powders with lower specific surface areas (13–15 m2/g↔120 nm particle size) or from nanopowders (25–35 m2/g↔55 nm particle size) if the different performance of these powders was balanced by differently accommodated processing and sintering. However, without additives only the 55 nm starting powders enabled the preparation of microstructures with a grain size of 270 nm and a maximum transmittance at sintering temperatures <1300°C. On the other hand, a better optical homogeneity was achieved for materials fabricated from the 120 nm powder.

Boron Speciation in Soda-Lime Borosilicate Glasses Containing Zirconium

Abstract: Boron speciation was investigated in soda-lime borosilicate glass containing zirconium. In such compositions, competition between charge compensators (here, sodium and calcium) can occur for the compensation of tetrahedral boron or octahedral zirconium units. 11B MAS NMR is particularly suitable for obtaining data on preferential compensation behavior that directly affects the boron coordination number. In addition to the classical proportions of tri- and tetrahedral boron, additional data can be obtained on the contributions involved in these two coordination numbers. An approach is described here based on simultaneous MAS spectrum analysis of borosilicate glass with variable Zr/Si and Ca/Na ratios at two magnetic field strengths (11.7 and 18.8 T), with constraints arising from MQMAS spectroscopy, detailed analysis of satellite transitions, and spin-echo experiments. New possibilities of 11B NMR were presented for improving the identification and quantification of the different contributions involved in tri- and tetrahedral boron coordination. Both NMR and Raman revealed a trend of decreased tetrahedral boron proportion with the increase of Ca/Na ratio or the Zr/Si ratio. This strongly suggests that zirconium compensation takes preference over boron compensation, and that zirconium and boron are both compensated mainly by sodium rather than calcium.

Metal-Organic Frameworks in Monolithic Structures

Abstract: For the first time, the manufacturing of metal-organic framework-based monoliths using a two-step process is reported In a first experiment, the in situ synthesis of Cu 3 (BTC) 2 (benzene tricarboxylate [BTC]) on cordierite monoliths was chosen to immobilize Cu 3 (BTC) 2 As this approach turned out to be of major disadvantages, the manufacturing of Cu 3 (BTC) 2 monolithic structures was chosen The two-step fabrication process included the manufacturing of a molding batch in a lab-scale kneader followed by extrusion in a ram-extruder As additives, methyl hydroxyl propyl cellulose and methoxy functionalized siloxane ether were chosen The resulting monolithic structures have a specific inner surface area of 370 m 2 /g and show a high mechanical stability of 320 N

Growth of One‐Dimensional Nanostructures in Porous Polymer‐Derived Ceramics by Catalyst‐Assisted Pyrolysis. Part I: Iron Catalyst

Abstract: Via catalyst-assisted pyrolysis, Si3N4 and SiC nanowires were produced on the cell walls of polymer-derived ceramic foams. The pyrolysis atmosphere and temperature were the main parameters affecting their development: silicon nitride single-crystal nanowires formed under nitrogen, while silicon carbide ones were produced under argon, and their amount increased with the increasing pyrolysis temperature. Brunauer–Emmett–Teller analysis showed that the presence of the nanowires afforded high specific surface area (SSA) values to the macroporous ceramic foams, ranging from 10 to 110 m2/g. Co-containing samples developed higher SSA values, especially after pyrolysis at 1400°C in N2, than samples containing Fe as a catalyst. The differences were explained in terms of morphology (diameter and assemblage), which depended on the processing conditions and the catalyst type (Co or Fe).

Synthesis and Photocatalytic Activity of Highly Ordered TiO2 and SrTiO3/TiO2 Nanotube Arrays on Ti Substrates

Abstract: Highly ordered TiO 2 nanotube arrays (TiO 2 -NTAs) are produced by electrochemical anodization of a Ti foil in ammonium fluoride (NH 4 F)-ethylene glycol solution. Photocatalytic (PC) investigations indicate that the length of the NTAs plays an important role in their photoactivity. The PC activity increases initially with the NT length and then decreases and the optimum length that yields the highest PC is 6.2 μm for TiO 2 -NTAs. The TiO 2 -NTAs are further converted to heterojunction strontium titanate (SrTiO 3 )/TiO 2 -NTAs by a hydrothermal reaction in Sr(OH) 2 solution. As the hydrothermal reaction proceeds, more TiO 2 is converted into SrTi0 3 and the thickness of the SrTi0 3 layer increases. The SrTiO 3 /TiO 2 -NTAs exhibit variable PC activities that depend on the hydrothermal reaction time, and the SrTiO 3 /TiO 2 -NTAs hydrothermally treated for 1 h or less have enhanced PC properties. The advantage of combining Ti0 2 and SrTi0 3 stems from the difference in the flatband potential, thereby improving the separation of the photogenerated electron-hole pairs and consequently the PC activity.

An Update on Nanomaterials-Based Textiles for Protection and Decontamination

Abstract: Protective clothing currently used against chemical and biological warfare (CBW) agents use activated charcoal impregnated with metal ions, which serve to physically adsorb nerve and blister agents thereby creating disposal hazards after its usage. Nanotechnology is booming in an unprecedented way in creating its impact in various applications such as in catalysis. Metal oxide nanoparticles (MONPs) such as TiO2 and MgO are currently used as potential catalysts for the decontamination of CBW agents. Various synthetic routes adopted for the preparation of MONPs are highlighted in this review. When compared with conventionally-prepared samples, aerogel-prepared samples are more reactive toward toxic chemicals and their ability to degrade CBW is presented here. TiO2 photocatalysts in the presence of UV light and mixed metal oxides are found to be efficient catalysts when compared with individual oxides. The recent trend of exploiting nanoparticles and the high aspect ratio ceramic oxide nanofibers for use in protective clothing, wipe materials, and textiles has been presented. Some of the issues concerning integration of metal oxides into fabrics for sensors are also reviewed in this article.

ZrB2 Powders Synthesis by Borothermal Reduction

Abstract: High-purity zirconium diboride (ZrB2) powders with submicrometer particle size were synthesized by borothermal reduction of nanometric ZrO2 powders in vacuum. The reaction process was experimentally and thermodynamically assessed. B2O3 was identified as a possible intermediate reaction product. ZrO2 completely converted to ZrB2 when thermally treated at 10001C for 2 h in a vacuum, but the removal of residual boron-related species required a temperature above 15001C. ZrB2 powders obtained at 10001–12001 Cs howed a faceted morphology, whereas those prepared above 15001 Ch ad a nearly spherical morphology. The particle size that was calculated from the measured surface area increased with the increasing synthesis temperature from 0.15 l ma t 10001 Ct o 0.66 l ma t 16501C. The oxygen content of the ZrB2 powders synthesized at 16501C was as low as 0.43 wt%.