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Enhanced TcO 4 − reduction by metallic Fe 0 in the presence of particulate and structural Si. Rhythmical precipitation of dissolved iron leads to formation of layered structures related to geological phenomena such as orbicular rocks and Liesegang rings.
A proton-transfer reaction between squaric acid (H 2 sq) and 2,3-dimethylpyrazine (2,3-Me 2 pyz) results in crystallization of a new organic antiferroelectric (AFE), (2,3-Me 2 pyzH + )(Hsq – )·H 2 O ( 1 ), which possesses a layered structure. The structure of each layer can be described as partitioned into strips lined with methyl groups of the Me 2 pyzH + cations and strips featuring extensive hydrogen bonding between the Hsq – anions and water molecules. Variable-temperature dielectric measurements and crystal structures determined through a combination of single-crystal X-ray and neutron diffraction reveal an AFE ordering at 104 K. The phase transition is driven by ordering of protons within the hydrogen-bonded strips. Considering the extent of proton transfer, the paraelectric (PE) state can be formulated as (2,3-Me 2 pyzH + ) 2 (Hsq 2 3– )(H 5 O 2 + ), whereas the AFE phase can be described as (2,3-Me 2 pyzH + )(Hsq – )(H 2 O). The structural transition caused by the localization of protons results in the change in color from yellow in the PE state to colorless in the AFE state. The occurrence and mechanism of the AFE phase transition have been also confirmed by heat capacity measurements and variable-temperature infrared and Raman spectroscopy. This work demonstrates a potentially promising approach to the design of new electrically ordered materials by engineering molecule-based crystal structures in which hydrogen-bonding interactions are intentionally partitioned into quasi-one-dimensional regions.
Smectic liquid crystals are phases of rod shaped molecules organized into one dimensionally (1 D) periodic arrays of layers, each layer being between one and two molecular lengths thick. In the least ordered smectic phases, the smectics A and C, each layer is a two dimensional (2D) liquid. Additionally there are a variety of more ordered smectic phases having hexatic short range translational order or 2D crystalline or quasi long range translational order within the layers. The inherent fluid-layer structure and low vapor pressure of smectic liquid crystals enables the long term stabilization of freely suspended, single component, layered fluid films as thin as 30A, a single molecular layer. The layering forces the films to be an integral number of smectic layers thick, quantizing their thickness in layer units and forcing a film of a particular number of layers to be physically homogeneous with respect to its layer structure over its entire area. Optical reflectivity enables the precise determination of the number of layers. These ultrathin freely suspended liquid crystal films are structures of fundamental interest in condensed matter and fluid physics. They are the thinnest known stable fluid structures and have the largest surface-to-volume ratio of any stable fluid preparation, making them ideal for the study of the effects of reduced dimensionality on phase behavior and on fluctuation and interface phenomena. Their low vapor pressure and quantized thickness enable the effective use of microgravity to extend the study of basic capillary phenomena to ultrathin fluid films. Freely suspended films have been a wellspring of new LC physics. They have been used to provide unique experimental conditions for the study of condensed phase transitions in two dimensions. They are the only system in which the hexatic has been unambiguously identified as a phase of matter, and the only physical system in which fluctuations of a 2D XY system and Kosterlitz Thouless phase transition has been observed and 2D XY quasi long range order verified. Smectic films have enabled the precise determination of smectic layer electron density and positional fluctuation profiles and have been used to show that the interlayer interactions in antiferroelectric tilted smectics do not extend significantly beyond nearest neighbors. Freely suspended films played a pivotal role in the recent discovery of macroscopic chiral-polar ordering in fluids of achiral molecules. The interactions which are operative in liquid crystals are generally weak in comparison to those in crystalline phases, leading to the facile manipulation of the order in liquid crystals by external agents such as applied fields and surfaces. Effects arising from weak ordering are significantly enhanced in ultrathin free films and filaments, in which the intermolecular coupling is effectively further reduced by loss of neighbors. Over the past four years this research, which we now detail, has produced a host of exciting new discoveries and unexpected results, maintaining the study of freely suspended liquid crystal structures as one of most exciting and fruitful areas of complex fluid physics. In addition, a class of experiments on the behavior of 1D interfaces in 2D films have been pursued with results that point to potentially quite interesting effects in microgravity.
A new class of stationary double layer structure, with three or more distinct steps, is demonstrated in the laboratory. A large monotonic potential increase results from a series of smaller double layers. In many respects, these double layer structures resemble those inferred from satellite measurements of auroral double layers. This new class of double layer appears to depend on turbulence for its existence and to be a hybrid structure, intermediate between anomalous resistivity and BGK double layers.
Smectic liquid crystals are phases of rod shaped molecules organized into one dimensionally (1D) periodic arrays of layers, each layer being between one and two molecular lengths thick. In the least ordered smectic phases, the smectics A and C, each layer is a two dimensional (2D) liquid. Additionally there are a variety of more ordered smectic phases having hexatic short range translational order or 2D crystalline quasi long range translational order within the layers. The inherent fluid-layer structure and low vapor pressure of smectic liquid crystals enable the long term stabilization of freely suspended, single component, layered fluid films as thin as 30A, a single molecular layer. The layering forces the films to be an integral number of smectic layers thick, quantizing their thickness in layer units and forcing a film of a particular number of layers to be physically homogeneous with respect to its layer structure over its entire area. Optical reflectivity enables the precise determination of the number of layers. These ultrathin freely suspended liquid crystal films are structures of fundamental interest in condensed matter and fluid physics. They are the thinnest known stable condensed phase fluid structures and have the largest surface-to-volume ratio of any stable fluid preparation, making them ideal for the study of the effects of reduced dimensionality on phase behavior and on fluctuation and interface phenomena. Their low vapor pressure and quantized thickness enable the effective use of microgravity to extend the study of basic capillary phenomena to ultrathin fluid films. Freely suspended films have been a wellspring of new liquid crystal physics. They have been used to provide unique experimental conditions for the study of condensed phase transitions in two dimensions. They are the only system in which the hexatic has been unambiguously identified as a phase of matter, and the only physical system in which fluctuations of a 2D XY system and Kosterlitz Thouless phase transition has been observed and 2D XY quasi long range order verified. Smectic films have enabled the precise determination of smectic layer electron density and positional fluctuation profile and have been used to show that the interlayer interactions in anti-ferroelectric tilted smectics do not extend significantly beyond nearest neighbors. The interactions which are operative in liquid crystals are generally weak in comparison to those in crystalline phases, leading to the facile manipulation of the order in liquid crystals by external agents such as applied fields and surfaces. Effects arising from weak ordering are significantly enhanced in ultrathin free films and filaments wherein the intermolecular coupling is effectively reduced by loss of neighbors. Over the past four years this research, which we now detail, has produced a host of exciting new discoveries and unexpected results, maintaining the position of the study of freely suspended liquid crystal structures as one of most exciting and fruitful areas of complex fluid physics. In addition, several potentially interesting microgravity free film experiments have been identified.
Oxygen-redox of layer-structured metal-oxide cathodes has drawn great attention as an effective approach to break through the bottleneck of their capacity limit. However, reversible oxygen-redox can only be obtained in the high-voltage region (usually over 3.5 V) in current metal-oxide cathodes. Here, we realize reversible oxygen-redox in a wide voltage range of 1.5-4.5 V in a P2-layered Na 0.7 Mg 0.2 [Fe 0.2 Mn 0.6 $\square$0.2]O 2 cathode material, where intrinsic vacancies are located in transition-metal (TM) sites and Mg-ions are located in Na sites. Mg-ions in the Na layer serve as "pillars" to stabilize the layered structure during electrochemical cycling, especially in the high-voltage region. Intrinsic vacancies in the TM layer create the local configurations of "$\square$-O-$\square$", "Na-O-$\square$" and "Mg-O-$\square$" to trigger oxygen-redox in the whole voltage range of charge-discharge. Additionally, time-resolved techniques demonstrate that the P2 phase is well maintained in a wide potential window range of 1.5-4.5 V even at 10 C. It is revealed that charge compensation from Mn- and O-ions contributes to the whole voltage range of 1.5-4.5 V, while the redox of Fe-ions only contributes to the high-voltage region of 3.0-4.5 V. The orphaned electrons in the nonbonding 2p orbitals of O that point toward TM-vacancy sites are responsible for reversible oxygen-redox, and Mg-ions in Na sites suppress oxygen release effectively.
This is a biography of Michael Naguib, winner of the Workforce Development Award at the EFRC 10@10 Awards Ceremony in 2019. Michael Naguib was born in Egypt and completed his undergraduate and master’s degrees at Cairo University. He wanted to study advanced materials, but there was no place to do so in Egypt. Instead he enrolled in a PhD program at Drexel University in Philadelphia. There he was assigned to explore whether certain materials that have a layered structure (with three layers, each having a different chemical composition) could be used to make better electrodes for lithium ion batteries. Michael found that the materials were not useful as electrodes because lithium ions (the charge carriers in modern batteries) could not penetrate the material. To address this, he tried to modify the material by removing the middle layer with acid. The unexpected result was the discovery of a completely new nano-material: a titanium-carbon compound in the form of a two-dimensional sheet less than a nanometer thick. He and his advisors at Drexel immediately recognized the new material’s potential for energy storage. Publication of that discovery—of what soon became clear was a whole family of two-dimensional materials now called MXenes—triggered a world-wide surge of research. To explore MXenes and related phenomena in more detail, the U.S. Department of Energy’s Office of Science funded an Energy Frontier Research Center (EFRC) on Fluid Interface Reactions, Structures and Transport. The research team included computational modelers, university experts in materials synthesis and thermodynamic analysis, and scientists at DOE’s Oak Ridge National Laboratory (ORNL) who could probe the material’s structure with neutrons and powerful microscopes.
One of the most important methods of reducing the noise and vibration level is the damping of the secondary sources, such as metal plates, often used in vehicle structures, by means of covering materials with high internal viscosity. Damping layers are chosen at an optimum thickness corresponding to the frequency and temperature range in which a certain structure works. The structure's response corresponding to various real situations is analyzed by means of a measuring chain including electroacoustical or electromechanical transducers. The experimental results provide the dependence of the loss factor and damping transmission coefficient as a function of the damping layer thickness or of the frequency for various viscoelastic covering materials.
Here, the novel α-BaZn 2 P 2 structural polymorph has been synthesized and structurally characterized for the first time. Its structure, elu-cidated from single crystal X-ray diffraction, indicates that the compound crystallizes in the orthorhombic α-BaCu 2 S 2 structure type, with unit cell parameters a = 9.7567(14) Å, b = 4.1266(6) Å, and c = 10.6000(15) Å. With β-BaZn 2 P 2 being previously iden-tified as belonging to the ThCr 2 Si 2 family, and with the precedent of structural phase transitions between the α-BaCu 2 S 2 type and the ThCr 2 Si 2 type, the potential for the pattern to be extended to the two different structural forms of BaZn 2 P 2 was explored. Thermal analysis suggest that a first order phase transition occurs at ~1123 K, whereby the low-temperature orthorhombic α-phase transforms to a high-temperature tetragonal β-BaZn 2 P 2 ; the structure of which was also studied and confirmed by single-crystal X-ray diffraction. Preliminary transport properties and band structure calculations indicate that α-BaZn 2 P 2 is a p-type, narrow-gap semiconductor with a direct bandgap (Eg) of 0.5 eV. The Seebeck coefficient, S(T), for the material increases steadily from the room temperature value of 119 µV/K to 184 µV/K at 600 K. The electrical resistivity of α-BaZn 2 P 2 is relatively high, on the order of 40 mΩ·cm, and the ρ(T) dependence shows gradual decrease upon heating. Such behavior is comparable to those of the typical semimetals or degenerate semiconductors. Although the observed values of the as-synthesized samples are not optimal, the carrier concentration can apparently be tuned to fall between the values of metals and semiconductors, thus providing an open window for optimizing this phase towards achieving an enhanced thermoelectric figure of merit zT. The calculated indirect band gap for the β-BaZn 2 P 2 phase is Eg = 0.03 eV which is about an order of magnitude lower than that of α-BaZn 2 P 2 . It is expected that the cage-like structural motif and layered structure possessed by α-BaZn 2 P 2 and β-BaZn 2 P 2 phases, respectively, would promote the realiza-tion of a low thermal conductivity in both compounds.
It is clear from simple skin depth considerations that steady state electromagnetic inspection of thick multi-layered conductors requires low frequency excitation. Conventional pickup sensors, however, lose sensitivity at lower frequencies. Giant magneto resistive materials offer a unique alternative for very low frequency electromagnetic NDE due to their high sensitivity to low frequency fields, small size, ease of use, and low cost. This paper outlines the development and testing of a Very Low Frequency Self-Nulling Probe incorporating a GMR sensor. The initial test results show flaw detectability at depths up to 1 cm in aluminum 2024. Optimization of the probe design based upon finite element modeling and GMR sensor characteristics (including hysteresis, linearity and saturation) is under way.
Film structures in the Fe–Bi system have been studied experimentally. The magnetic state of the two-layer structures is shown by electron magnetic resonance to be dependent on the order of depositing magnetic and nonmagnetic layers. The three-layer structures demonstrate the effect of the exchange bias, the value of which is dependent on the bismuth interlayer thickness.
Some types of precise polyethylenes, or linear polyethylenes containing precisely periodic functional groups, exhibit significant strain hardening under tensile deformation. In this work we perform in situ X-ray scattering during tensile deformation of two precise polyethylenes: one with pendant carboxylic acid groups every 21st carbon (p21AA) and another with pendant imidazolium bromide groups every 15th carbon (p15ImBr). p21AA exhibits significant strain hardening and its layered structure orients with the layer normal along the tensile axis, while p15ImBr does not exhibit strain hardening and its layer normal orients perpendicular to the tensile axis. Quantitative evaluation of the 2D fiber patterns shows that p21AA undergoes a morphological transition, from a semicrystalline structure to a nanofibrillar structure, while the morphology of p15ImBr reorients and the chain folding structure is preserved. This suggests that fibrillization plays an important role in the strain hardening mechanism.
Molybdenum disulfide (MoS 2 ) films are attractive materials for electronic and optoelectronic devices, but the temperatures used in the chemical vapor deposition (CVD) of these materials are too high for device integration. Recently, a low-temperature atomic layer deposition (ALD) process was demonstrated for growth of MoS 2 films at 200 °C using MoF 6 and H 2 S. However, the as-deposited films were amorphous and required annealing to obtain the desired layered structure. The MoS 2 films were sulfur-deficient; however, after annealing the crystallinity improved. To study the structure of these films and the process by which they crystallize, we performed in this work X-ray absorption spectroscopy and high-energy X-ray scattering experiments on both as-deposited and annealed MoS 2 films. Analysis indicated that molybdenum atoms in the as-deposited films were well coordinated with sulfur but not well coordinated with other molybdenum atoms when compared to a crystalline reference. Further analysis revealed clusters of the sulfur-rich phase [Mo 3 S(S 6 ) 2 ] 2– , which decomposed after annealing in H 2 and H 2 S at 400 and 600 °C. When compared to the sulfur-deficient films reported previously for this ALD process, the sulfur-rich phase found here indicates that nucleation on the substrate plays an important role in the resulting film stoichiometry, which could be tuned to produce higher quality films for microelectronic applications.
A silicon alignment pin is used to align successive layers of components made in semiconductor chips and/or metallic components to make easier the assembly of devices having a layered structure. The pin is made as a compressible structure which can be squeezed to reduce its outer diameter, have one end fit into a corresponding alignment pocket or cavity defined in a layer of material to be assembled into a layered structure, and then allowed to expand to produce an interference fit with the cavity. The other end can then be inserted into a corresponding cavity defined in a surface of a second layer of material that mates with the first layer. The two layers are in registry when the pin is mated to both. Multiple layers can be assembled to create a multilayer structure. Examples of such devices are presented.
A silicon alignment pin is used to align successive layer of component made in semiconductor chips and/or metallic components to make easier the assembly of devices having a layered structure. The pin is made as a compressible structure which can be squeezed to reduce its outer diameter, have one end fit into a corresponding alignment pocket or cavity defined in a layer of material to be assembled into a layered structure, and then allowed to expand to produce an interference fit with the cavity. The other end can then be inserted into a corresponding cavity defined in a surface of a second layer of material that mates with the first layer. The two layers are in registry when the pin is mated to both. Multiple layers can be assembled to create a multilayer structure. Examples of such devices are presented.
RULCHK computer program aids in structuring layers of information pertaining to part or assembly designed with software described in article "Software for Drawing Design Details Concurrently" (MFS-28444). Checks and optionally updates structure of layers for part. Enables designer to construct model and annotate its documentation without burden of manually layering part to conform to standards at design time.
Figure 1 shows a single-stage monolithic microwave integrated circuit (MMIC) power amplifier in which the gain element is a double-heterojunction bipolar transistor (DHBT) connected in common-base configuration. This amplifier, which has been demonstrated to function well at a frequency of 172 GHz, is part of a continuing effort to develop compact, efficient amplifiers for scientific instrumentation, wide-band communication systems, and radar systems that will operate at frequencies up to and beyond 180 GHz. The transistor is fabricated from a layered structure formed by molecular beam epitaxy in the InP/InGaAs material system. A highly doped InGaAs base layer and a collector layer are fabricated from the layered structure in a triple mesa process. The transistor includes two separate emitter fingers, each having dimensions of 0.8 by 12 m. The common-base configuration was chosen for its high maximum stable gain in the frequency band of interest. The input-matching network is designed for high bandwidth. The output of the transistor is matched to a load line for maximum saturated output power under large-signal conditions, rather than being matched for maximum gain under small-signal conditions. In a test at a frequency of 172 GHz, the amplifier was found to generate an output power of 7.5 mW, with approximately 5 dB of large-signal gain (see Figure 2). Moreover, the amplifier exhibited a peak small-signal gain of 7 dB at a frequency of 176 GHz. This performance of this MMIC single-stage amplifier containing only a single transistor represents a significant advance in the state of the art, in that it rivals the 170-GHz performance of a prior MMIC three-stage, four-transistor amplifier. [The prior amplifier was reported in "MMIC HEMT Power Amplifier for 140 to 170 GHz" (NPO-30127), NASA Tech Briefs, Vol. 27, No. 11 (November 2003), page 49.] This amplifier is the first heterojunction- bipolar-transistor (HBT) amplifier built for medium power operation in this frequency band. The performance of the amplifier as measured in the aforementioned tests suggests that InP/InGaAs HBTs may be superior to high-electron-mobility (HEMT) transistors in that the HBTs may offer more gain per stage and more output power per transistor.