Boron Nitride Insulator Research Articles

Thermal Design and Experimental Verification of a Three-Dimensional-Printed Resistojet

A high-efficiency concentric tubular-type resistojet with potential application to short-term orbit-raising maneuvers has been fabricated by 3-D printing and demonstrated. The propellant flows through multiple layers of cylindrical shells, with this structure also functioning as a single-piece heater. A 6-cm-high cylinder was realized with a wall thickness of 0.2 mm, using Inconel 718. A nodal thermal analysis was performed to identify the upper-limit current at a temperature limit of the wall material, and it was revealed that an outlet gas temperature of 871 K can be achieved with 77 A of current at 0.2 g/s of mass flow rate. The designed heater was combined with a boron nitride insulator and a stainless-steel housing, and thrust was measured in a vacuum chamber with nitrogen as the propellant. At a mass flow rate of and 75 A of current, an outlet temperature of 747 K, a specific impulse of 108 s, and a heater efficiency of 72% were achieved. These results with nitrogen propellant were used to predict the performance of a tungsten-made resistojet with a hydrogen propellant, and a specific impulse of over 700 s can be expected at a heater temperature of 2000 K.

Characterization and manipulation of individual defects in insulating hexagonal boron nitride using scanning tunnelling microscopy.

Defects play a key role in determining the properties and technological applications of nanoscale materials and, because they tend to be highly localized, characterizing them at the single-defect level is of particular importance. Scanning tunnelling microscopy has long been used to image the electronic structure of individual point defects in conductors, semiconductors and ultrathin films, but such single-defect electronic characterization remains an elusive goal for intrinsic bulk insulators. Here, we show that individual native defects in an intrinsic bulk hexagonal boron nitride insulator can be characterized and manipulated using a scanning tunnelling microscope. This would typically be impossible due to the lack of a conducting drain path for electrical current. We overcome this problem by using a graphene/boron nitride heterostructure, which exploits the atomically thin nature of graphene to allow the visualization of defect phenomena in the underlying bulk boron nitride. We observe three different defect structures that we attribute to defects within the bulk insulating boron nitride. Using scanning tunnelling spectroscopy we obtain charge and energy-level information for these boron nitride defect structures. We also show that it is possible to manipulate the defects through voltage pulses applied to the scanning tunnelling microscope tip.

Erosion Measurements in a Low-Power Cusped-Field Plasma Thruster

Results from a long duration test of a cusped-field plasma thruster operating at an anode power of 165 W are presented and discussed. Profile measurements of the boron nitride insulator were performed before and after the 204 h test, enabling the quantification of average erosion rates over a large portion of the interior. Unlike most Hall thrusters, the lifetime is not limited by erosion near the exit plane, due to the positioning of magnetic circuit elements. Additionally, the maximum erosion rate is found to be lower than rates measured in low-power Hall thrusters. The lifetime of this laboratory prototype is estimated at 920–1220 h based on the time needed to erode through the insulator in one of the ring cusps. However, as an indicator of the potential for cusped-field thruster designs to obtain longer lifetimes, the low erosion rate measurements are more significant. For example, fortifying cusps with additional or more durable material could increase lifetime. A summary of long duration tests and erosion measurements in Hall thrusters is also provided. During the tests discussed here, the thruster operated in the “high-current” mode, characterized by strongly oscillatory anode currents. Peak erosion rates are located upstream of the exit in this mode of operation. Reliable performance measurements were not acquired during this test, but anode current values suggest the thruster did not operate with the desired anode efficiency.

Resonant and nondissipative tunneling in independently contacted graphene structures

The tunneling current between independently contacted graphene sheets separated by boron nitride insulator is calculated. Both dissipative tunneling transitions, with momentum transfer due to disorder scattering, and non-dissipative regime of tunneling, which appears due to intersection of electron and hole branches of energy spectrum, are described. Dependencies of tunneling current on concentrations in top and bottom graphene layers, which are governed by the voltages applied through independent contacts and gates, are considered for the back- and double-gated structures. The current-voltage characteristics of the back-gated structure are in agreement with the recent experiment [Science 335, 947 (2012)]. For the double-gated structures, the resonant dissipative tunneling causes a ten times enhancement of response which is important for transistor applications.

Near Exit Plane Velocity Field of a 200-Watt Hall Thruster

This work presents the near exit plane velocity field of a 200-W laboratory xenon Hall thruster at a single operating condition with a 250-V anode potential. The ionized propellant velocities were measured using laser-induced fluorescence of the 5d[4] 7/2 -6p[3] 5/2 excited state xenon ionic transition at 834.72 nm. Ion velocities were interrogated from the acceleration channel exit plane to a distance 107 mm from the exit plane (3.3 exit plane diameters). Both axial and radial velocities were measured. A nearly uniform axial velocity profile of approximately 13,800 ± 500 m/s (130 ± 10 eV) was measured at the thruster exit plane. The maximum axial velocity, measured 107 mm from the exit plane, was 16,800 m/s (192 eV). The ion flow exiting the thruster acceleration channel mixes downstream due to both the coaxial thruster geometry and a possible ion-acoustic shock. This behavior appears in regions where multiple, or broadly distributed, radial and axial velocity components occur. These regions also exhibit broadened fluorescence line shapes, likely indicative of collisions between the various velocity populations as well as possible ionization of background neutral, and correspond to the brighter, more visible portions of the plume. This region has been previously identified as a possible ion-acoustic shock. This hypothesis appears consistent with the low radial velocity ion populations measured in this more luminous portion of the plume. In addition, a limited study at five off-nominal conditions near a region of high insulator erosion indicates that the impinging ion energy on the protruding center pole boron nitride insulator is predictably changed by flow rates and anode potentials; however, it also appears to vary significantly with applied magnetic field strength.

Development of a rotating ring-disc electrode for high temperature studies in cryolite-based melts

A compact rotating ring-disc electrode incorporating a molybdenum disc, gold ring and boron nitride insulator has been designed, constructed and evaluated in molten cryolite-based electrolytes at temperatures up to 1000°C and rotation rates between 0 and 2000 rpm. The electrode design is extremely versatile and relatively maintenance free, with no visible evidence of melt leakage at the ring-insulator and disc-insulator interfaces. The operating performance of the gold-molybdenum rotating ring-disc electrode was evaluated from collection efficiency measurements based on the dissolution of the disc surface and subsequent detection of soluble species transported to the ring. The observed collection efficiency was less than the theoretical value determined from the geometry of the electrode, primarily because of noncoplanarity of the electrode surface at the working temperature. The results confirm that the electrode should be useful for mechanistic studies in high temperature molten fluoride electrolytes.

Measurement of MPD thruster erosion using surface layer activation

An in-situ method for measuring MPD thruster component erosion has been developed and tested on two multimegawatt thruster configurations. The technique involves activation of selected areas on the components to be studied by precisely controlled high-energy ion beam bombardment. Monitoring the decrease in activity during thruster operation provides a precise, quantitative measure of the amount of material removed from the surface. In preliminary tests, erosion of the tungsten cathode occurred at all operating conditions, but a number of factors contributed to the lack of detectable erosion of the copper anode or the boron nitride insulator. These results indicate that cathode erosion is linearly related to the total charge transfer, but scatter in the individual test sequence erosion rate data prevents a more definitive conclusion. Nevertheless, a comparison of erosion data obtained at two different pulse lengths indicates that cathode material loss is generally independent of the number of pulse cycles. Work is continuing in a broad effort to refine the surface layer activation technique and to obtain a data base for additional thrusters.

Blunt-Nosed Bodies in a High-Temperature Gas Jet

RECENTLY, the heating of nose cones on re-entry vehicles has become a challenging technical problem. To provide thermal protection of an object in the core of the nose cone for short duration, two major kinds of shielding materials may be used, namely, metals and plastics. The former is favourable for a heat sink while the latter is favourable for ablation cooling. To demonstrate these features, some simple experiments were performed using a variety of those common materials. The materials were machined into ½-in. diameter hemispheres. A thermocouple (Fig. 1A) was attached to the centre of the base of the hemispheric sample (B) next to a boron nitride insulator (C). The whole assembly was mounted on an arm (D) and swung into a jet of argon 3/16 in. in diameter emitted from a plasma generator (E). The velocity of the jet impinging on the testing sample was subsonic. The temperature of the jet, T f, as estimated from ref. 1, was approximately 15,500° R. The measured temperature of the thermocouple, T 0, increased with time, t, and was recorded on a Sanborn strip-chart recorder. Figs. 2 and 3 show these results respectively for metal and plastic samples. For convenience, both T 0 and the non-dimensional temperature, θ0 = T f − T 0/T f − T i, are used for the ordinate and t is chosen for the abscissa in the figures (here T i is the room temperature).