Nasa Tech Briefs

Perovskite Superlattices as Tunable Microwave Devices

Interfacial interactions between paraelectric materials induce quasi-ferroelectric behavior.Experiments have shown that superlattices that comprise alternating epitaxial layers of dissimilar paraelectric perovskites can exhibit large changes in permittivity with the application of electric fields. The superlattices are potentially useful as electrically tunable dielectric components of such microwave devices as filters and phase shifters.

Posted in: Briefs, TSP, Materials, Architecture, Electrical systems, Radar


Reliability and Design Considerations for Long Life Using Mica Capacitors in High-Voltage Apps

Over the last 40 years, a series of misconceptions regarding mica capacitor applications has led novice users to consistently over- derate wound or rolled mica/ epoxy dielectric capacitors.Mica, K2A13(Si04)3, a complex aluminum silicate in dielectric form, has been successfully used for many years as an integral part of high- voltage (2KVDC to 50KVDC) capacitor manufacturing — particularly in the 50pF to 5μF value range. Mica has unrivaled physical and electrical properties in comparison to other capacitor dielectrics, especially ceramic. Mica is extremely stable. Capacitance will change only -2% at -54°C and to +3% at +125°C. Mica is an excellent insulator, and is resistant to high temperature, thermal shock, mechanical shock, and vibration.

Posted in: Briefs, Materials, Capacitors, High voltage systems, Education, Education and training, Reliability


Annealing Would Improve β" — Alumina Solid Electrolyte

The objective is to prevent a sudden reduction of ionic conductivity.A pre-operational annealing process is under investigation as a potential means of preventing a sudden reduction of ionic conductivity in a β"—alumina solid electrolyte (BASE) during use. On the basis of tests described below, the sudden reduction of ionic conductivity, followed by a slow recovery, has been found to occur during testing of the solid electrolyte and electrode components of an alkali metal thermal-to-electric converter (AMTEC) cell. This conductivity reduction may be observed quite infrequently; at lower operating temperatures, T<1,073 K, it is not usually observed at all, while at T=1,123–1,173 K, hundreds of hours may pass before conductivity reduction occurs. Only on tests running at higher operating temperatures for thousands of hours is this phenomenon regularly exhibited. The reduction of ionic conductivity would degrade the performance of an AMTEC cell. A pre-operational annealing process would help to sustain performance.

Posted in: Briefs, TSP, Materials, Heat treatment, Conductivity, Electrolytes, Performance tests


Low-Power RIE of SiO₂ in CHF₃ To Obtain Steep Sidewalls

Process parameters are chosen carefully to minimize deleterious effects.A reactive-ion etching (RIE) process has been developed to enable the formation of holes with steep sidewalls in a layer of silicon dioxide that covers a silicon substrate. The holes in question are through the thickness of the SiO2 and are used to define silicon substrate areas to be etched or to be built upon through epitaxial deposition of silicon. The sidewalls of these holes are required to be vertical in order to ensure that the sidewalls of the holes to be etched in the substrate or the sidewalls of the epitaxial deposits, respectively, also turn out to be vertical.

Posted in: Briefs, TSP, Materials, Machining processes, Chemicals, Silicon alloys


Process for Encapsulating Protein Crystals

Crystals can be grown in forms suitable for x-ray diffraction studies.A process for growing protein crystals encapsulated within membranes has been invented. This process begins with the encapsulation of a nearly saturated aqueous protein solution inside semipermeable membranes to form microcapsules. The encapsulation is effected by use of special formulations of a dissolved protein and a surfactant in an aqueous first liquid phase, which is placed into contact with a second, immiscible liquid phase that contains one or more polymers that are insoluble in the first phase. The second phase becomes formed into the semipermeable membranes that surround microglobules of the first phase, thereby forming the microcapsules. Once formed, the microcapsules are then dehydrated osmotically by exposure to a concentrated salt or polymer solution. The dehydration forms supersaturated solutions inside the microcapsules, thereby enabling nucleation and growth of protein crystals inside the microcapsules.

Posted in: Briefs, TSP, Materials, Forming, Biomaterials, Chemicals, Polymers


Mixed Conducting Electrodes for Better AMTEC Cells

These electrodes conduct both electrons and sodium cations.Electrode materials that exhibit mixed conductivity (that is, both electronic and ionic conductivity) have been investigated in a continuing effort to improve the performance of the alkali metal thermal-to-electric converter (AMTEC). These electrode materials are intended primarily for use on the cathode side of the sodium-ion-conducting solid electrolyte of a sodium-based AMTEC cell. They may also prove useful in sodium-sulfur batteries, which are under study for use in electric vehicles.

Posted in: Briefs, TSP, Materials, Battery cell chemistry, Heat exchangers, Conductivity


Advancements in Technology for Controlling Fiber Orientation in Composite Parts

The performance of a composite part is primarily determined by the orientation of fibers in the plies. Designers wishing to exploit the full potential of composite materials, while avoiding manufacturing problems and part failures, must define and control fiber orientation. Anticipating true fiber orientation for a single ply is seldom intuitive, and predicting the behavior of an entire laminate made of tens or hundreds of plies is nearly impossible.

Posted in: Briefs, Materials, Fabrication, Composite materials, Fibers


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