Carbon Nanotube Field Emission Cathode Research Articles

Proof-of-Concept Vacuum Microelectronic NOR Gate Fabricated Using Microelectromechanical Systems and Carbon Nanotube Field Emitters.

This paper demonstrates a fully integrated vacuum microelectronic NOR logic gate fabricated using microfabricated polysilicon panels oriented perpendicular to the device substrate with integrated carbon nanotube (CNT) field emission cathodes. The vacuum microelectronic NOR logic gate consists of two parallel vacuum tetrodes fabricated using the polysilicon Multi-User MEMS Processes (polyMUMPs). Each tetrode of the vacuum microelectronic NOR gate demonstrated transistor-like performance but with a low transconductance of 7.6 × 10-9 S as current saturation was not achieved due to a coupling effect between the anode voltage and cathode current. With both tetrodes working in parallel, the NOR logic capabilities were demonstrated. However, the device exhibited asymmetric performance due to differences in the CNT emitter performance in each tetrode. Because vacuum microelectronic devices are attractive for use in high radiation environments, to test the radiation survivability of this device platform, we demonstrated the function of a simplified diode device structure during exposure to gamma radiation at a rate of 45.6 rad(Si)/second. These devices represent a proof-of-concept for a platform that can be used to build intricate vacuum microelectronic logic devices for use in high-radiation environments.

Development of an electron gun based on CNT-cathode for traveling wave tube application

In this work, an electron gun with carbon nanotube (CNT) field emission cathode is demonstrated for traveling wave tube (TWT) application with laminar flow beam features. A maximum emission current of 9.76 mA at a bias of −11 kV is achieved by such electron gun, with current density up to 1.24 A cm−2. The output current of the electron gun can reach 8.08 mA, with the grid interception rate less than 22%. Furthermore, a Quasi-TWT with a beam tunnel of 1.6 mm is realized, which has a maximum collector current of 7.52 mA with the corresponding transmittance of 93.07%. The simulation of the beam-wave interaction implies further that our CNT field electron gun is able to meet the demands of TWT applications.

Position and Attitude Tolerances of Carbon Nanotube Field Emission Cathode as a Neutralizer in an Ion Engine System

Position and Attitude Tolerances of Carbon Nanotube Field Emission Cathode as a Neutralizer in an Ion Engine System

Review on development of carbon nanotube field emission cathode for space propulsion systems

In recent years, small-size, low-weight aerospace propulsion systems have developed rapidly for space exploration, on-orbit scientific instruments and extra space missions, while additional electron emission devices are commonly required in those propulsion systems. Carbon nanotube (CNT) based field emission cathodes exhibit extraordinary field emission properties and are regarded to be an ideal alternative of conventional thermionic or hollow cathodes. In this study, the authors give an overview of present status of researches on CNT-based electron emission cathodes for utilising as neutralisers or electron sources particularly in space electric propulsion systems, the theory and characteristics of CNT are also illustrated. Furthermore, challenges, problems and possible solutions before actual applications of CNT in a space mission are discussed accordingly.

Field Emission Cathodes to Form an Electron Beam Prepared from Carbon Nanotube Suspensions.

In the first decade of our century, carbon nanotubes (CNTs) became a wonderful emitting material for field-emission (FE) of electrons. The carbon nanotube field-emission (CNT-FE) cathodes showed the possibility of low threshold voltage, therefore low power operation, together with a long lifetime, high brightness, and coherent beams of electrons. Thanks to this, CNT-FE cathodes have come ahead of increasing demand for novel self-sustaining and miniaturized devices performing as X-ray tubes, X-ray spectrometers, and electron microscopes, which possess low weight and might work without the need of the specialized equipped room, e.g., in a harsh environment and inaccessible-so-far areas. In this review, the author discusses the current state of CNT-FE cathode research using CNT suspensions. Included in this review are the basics of cathode operation, an evaluation, and fabrication techniques. The cathodes are compared based on performance and correlated issues. The author includes the advancement in field-emission enhancement by postprocess treatments, incorporation of fillers, and the use of film coatings with lower work functions than that of CNTs. Each approach is discussed in the context of the CNT-FE cathode operating factors. Finally, we discuss the issues and perspectives of the CNT-FE cathode research and development.

Operation of a carbon nanotube field-emission cathode in low Earth orbit

A carbon nanotube-based field-emission cathode (FEC) has been researched and developed by the Japan Aerospace Exploration Agency mainly for space-debris removal systems with electrodynamic tether (EDT) propulsion. An in-orbit experiment to demonstrate the EDT technologies including the FEC was planned and conducted in early 2017. The FEC operated well without any critical trouble during the one-week mission period. The data obtained include its electron emission capability to space plasma and its durability in low Earth orbit, which suggest the prospect of using the FEC in space.

Low-cost, additively manufactured electron impact gas ionizer with carbon nanotube field emission cathode for compact mass spectrometry

We report the design, fabrication, and experimental characterization of the first additively manufactured electron impact gas ionizer for compact mass spectrometry in the literature. The device occupies a total volume equal to 2.3 cm3 and is composed of a carbon nanotube (CNT) field emission electron source and a multi-material, 3D-printed multi-electrode ion-generating structure. The ionizer’s electron source is a silicon chip coated with a forest of plasma-enhanced chemical vapour deposited CNTs as emitting substrate and a 100 µm thick stainless-steel stencil with thousands of 100 µm diameter apertures at 150 µm aperture pitch as extraction gate. The device’s ion-generating structure is a set of 3D-printed, finely featured dielectric and metallic parts that kindle interaction between electrons and neutrals and shepherd ions outside the structure; its dielectric components were printed in polymer via high-resolution digital light projection stereolithography (25 µm pixels), while its metallic pieces were binder inkjet-printed in SS 316L with features as small as 340 µm (thickness of the wires of the ion cage). The CNT field emission electron sources were experimentally characterized in high vacuum (2.0 × 10−7 Torr), emitting up to 1.4 mA @ 700 V with 31.5% gate transmission. The ionizers were characterized in air at pressures as high as 5 mTorr, generating up 38 µA ion current with 8.5% ionization efficiency. The ion current is linear with pressure, in agreement with the electron impact ionization model.

Geometric calibration of a stationary digital breast tomosynthesis system based on distributed carbon nanotube X-ray source arrays.

Stationary digital breast tomosynthesis (sDBT) with distributed X-ray sources based on carbon nanotube (CNT) field emission cathodes has been recently proposed as an approach that can prevent motion blur produced by traditional DBT systems. In this paper, we simulate a geometric calibration method based on a proposed multi-source CNT X-ray sDBT system. This method is a projection matrix-based approach with seven geometric parameters, all of which can be obtained from only one projection datum of the phantom. To our knowledge, this study reports the first application of this approach in a CNT-based multi-beam X-ray sDBT system. The simulation results showed that the extracted geometric parameters from the calculated projection matrix are extremely close to the input values and that the proposed method is effective and reliable for a square sDBT system. In addition, a traditional cone-beam computed tomography (CT) system was also simulated, and the uncalibrated and calibrated geometric parameters were used in image reconstruction based on the filtered back-projection (FBP) method. The results indicated that the images reconstructed with calibrated geometric parameters have fewer artifacts and are closer to the reference image. All the simulation tests showed that this geometric calibration method is optimized for sDBT systems but can also be applied to other application-specific CT imaging systems.

A miniature electron ionization source fabricated using microelectromechanical systems (MEMS) with integrated carbon nanotube (CNT) field emission cathodes and low-temperature co-fired ceramics (LTCC)

The use of microelectromechanical systems (MEMS) components in miniature mass spectrometers is particularly attractive due to their small size and scalable manufacturing capability. Our group has pioneered the development of miniature electron ionization sources combining MEMS fabricated structures with integrated carbon nanotube (CNT) cold-cathode field emitters. However, until now they have been of limited use due to the limited ability to direct the ions into a mass analyzer. In this work, we design a miniature ion source using a microfabricated MEMS device and a low temperature co-fired ceramic (LTCC) carrier that includes electrical connections, ion optics for directing ions out of the device, and a sample inlet. We present the design and fabrication of the ion source; simulate the energy and angular dispersion; and experimentally determine the energy and angular dispersion.

Fabrication of Carbon Nanotube Field-Emission Cathodes by Laser-Induced Transfer of Carbon Nanotubes and Silver Paste

A new laser material-transfer scheme for transfer printing both the silver-paste and the carbon-nanotube (CNT) thin film in pattern simultaneously is reported. The transfer mechanism is discussed and demonstrated to be unlike to those conventional laser-transfer approaches which only the material on the laser-irradiated region is transferable, the present scheme is capable of transferring the materials on the unexposed locations as well. The transferred CNTs are directly on the top of the silver paste which is a suitable configuration for application to the CNT field-emission cathodes. Measurements demonstrate that the fabricated CNT field-emission cathodes exhibit a low turn-on field of 2.5 V/μm at current density of 10 μA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . A lifetime test also demonstrates that the device is stable. By a single laser shot, this study successfully demonstrates that the transferred CNTs are preserved on the silver paste's surface that requires no subsequent post-treatment processes. Moreover, all the fabrication steps are executable at room temperature and in the ambient environment.

Field Emission Behavior of Carbon Nanotube Yarn for Micro-Resolution X-Ray Tube Cathode

Carbon nanotube (CNT) has excellent electrical and thermal conductivity and high aspect ratio for X-ray tube cathode. However, CNT field emission cathode has been shown unstable field emission and short life time due to field evaporation by high current density and detachment by electrostatic force. An alternative approach in this direction is the introduction of CNT yarn, which is a one dimensional assembly of individual carbon nanotubes bonded by the Van der Waals force. Because CNT yarn is composed with many CNTs, CNT yarns are expected to increase current density and life time for X-ray tube applications. In this research, CNT yarn was fabricated by spinning of a super-aligned CNT forest and was characterized for application to an X-ray tube cathode. CNT yarn showed a high field emission current density and a long lifetime of over 450 hours. Applying the CNT yarn field emitter to the X-ray tube cathode, it was possible to obtain micro-scale resolution images. The relationship between the field emission properties and the microstructure evolution was investigated and the unraveling effect of the CNT yarn was discussed.

TU‐E‐108‐09: Development of An Einzel Electron Focusing Lens for Microbeam Radiation Therapy Device

Purpose: The purpose of this work is to develop a novel Einzel electron focusing lens for carbon nanotube (CNT) field emission based microbeam radiation therapy (MRT). The CNT cathode used for the proposed microbeam radiation source has a unique linear cathode profile so that the focusing structure needs to be optimized in order to achieve the desired focal spot size for efficient generation of microbeam radiation. Methods: The proposed design of the MRT device has a square system configuration with four independent linear x‐ray tube modules. Each tube module includes a linear carbon nanotube field emission cathode array, which is the core component of the MRT device. The CNT cathode is comprised of 5 individually controlled CNT cathode segments, each of which has a dimension of 2.5mm×30mm. A commercial software package (Opera 3D, Cobham plc) was used to simulate and design the optimal focusing structure. A full scale computer model covering the entire electron beam path was constructed for the electron optical simulation. Results: Based on the simulation, the two‐stage electrostatic focusing lens is capable of providing the 102um focal spot size required for the proposed microbeam radiation therapy treatment with 100um beam width. Based on our simulation results, we are able to achieve over 65% electron beam transmission rate. Both results were verified experimentally using a small scale testing x‐ray chamber. Conclusion: We have designed a novel Einzel electron focusing lens to achieve the desired focal spot in order to improve the efficiency of microbeam generation, thus the dose rate. The simulation and experiment show the two‐stage electrostatic focusing lens is capable of providing a 102um effective focal spot size. Meanwhile we have also demonstrated reasonable electron beam transmission rate (&gt;65%), which is critical in terms of achieving high x‐ray output.

Microscopic analysis of performance variations in carbon nanotube field emission cathodes: Implications for device optimization

AbstractCarbon nanotubes (CNTs) are considered very promising for the realization of low‐cost field emission electron sources. However, despite intensive research and development efforts, the fabrication of reliable CNT cathodes for high current density (&gt;100 mA/cm2) applications remains a formidable challenge. In this study we use scanning anode field emission microscopy (SAFEM) to investigate the microscopic origins of macroscopic emission performance variations in chemical vapor deposition (CVD) grown CNT planar field emission cathodes. The field enhancement distributions are determined and the field emission properties of individual emission sites on the cathodes are probed. Contact I(V) measurements are carried out to estimate the resistance of individual emitters. The degradation behavior of individual sites is also studied and can be related with the macroscopic cathode performances. Scanning (SEM) and transmission electron microscopy (TEM) provide additional information on the contact and structural properties of the cathodes. Our results indicate that the sample macroscopic performances depend strongly on the individual emitter field emission properties in terms of maximum current before degradation and contact resistance.

Long-term stability of a horizontally-aligned carbon nanotube field emission cathode coated with a metallic glass thin film

A horizontally-aligned carbon nanotube (HACNT) field emission cathode was coated with a metallic glass thin film (MGTF) to improve the stability of the field emission properties. HACNT field emission cathodes have previously been fabricated on glass substrates using composite plating and crack-formation techniques. A carbon nanotubes/nickel (CNTs/Ni) composite film is deposited onto a glass substrate at 80 °C by the composite plating technique alone. Cracks are then formed in the CNT/Ni composite film during 30 min heating at 300 °C, and HACNTs are exposed in the cracks. The field emission properties of the HACNT field emission cathode show a low turn-on electric field E on of about 2.3 V/μm, a low threshold electric field E th of about 4.7 V/μm at an emission current density of 1 mA/cm 2, and a stability time of 78 h. The degradation of the HACNT field emission cathode is prevented by using a MGTF-coating technique and superior long-term stability (i.e. >125 h, with 5 nm MGTF; >270 h, with 10 nm MGTF) for the MGTF/HACNT field emission cathode is achieved.

Lifetime and Failure Mechanisms of an Arrayed Carbon Nanotube Field Emission Cathode

There is interest in the use of carbon nanotubes (CNTs) to create a field emission (FE) cathode for the neutralization of exhaust plumes of low-power (<; 500 W) electric propulsion devices since FE cathodes do not require a gas flow to operate. To incorporate CNT emitters into propulsion systems, the current emission output over the lifetime of the cathode must be understood. Multiple FE cathodes that consist of multiwalled CNT arrays have been fabricated. Seven cathodes are characterized at pressures below 10 5 T at constant voltage between the CNTs, and the gate until failure occurs. The maximum current density observed is 9.08 mA/cm2 with average current densities up to 2.52 mA/cm2, and the maximum life span is 368 h. The behavior of the cathode current emission is highly unstable and consists of oscillations and sudden shifts. Resistive heating is believed to be the primary cause for failure in two thermally assisted modes: 1) oxidative ablation at the root of the nanotube and 2) field evaporation at the tip.

Fabrication of Carbon Nanotubes Field Emission Cathode by Composite Plating

Carbon nanotubes (CNTs) have high aspect ratio and have great potential to be applied as the field emission cathode because of its large field enhancement factor. In this work, a high performance carbon nanotube field emission cathode (CNTFC) was fabricated by using a composite plating method. The CNTs were purified by acid solutions and then dispersed in electrobath with nickel ions at temperatures of 60, 70, or 80 degrees C for the electroless plating process on glass substrate. The resulting CNT-Ni composite film has strong adhesion on the glass substrate. The degree of graphitization and the microstructure of the CNTFCs were studied by Raman spectroscopy and scanning electron microscopy. The field emission properties of the CNTFCs show a low turn-on electric field E(on) of about 1.2 V/microm, and a low threshold electric field E(th) of about 1.9 V/microm. Such a composite plating method could be applied to the fabrication of large area CNT field-emission displays.

Field emission properties of carbon nanotube cathodes produced using composite plating

Field emission properties of carbon nanotube field emission cathodes (CNT-FECs) produced using composite plating are studied. The experiment uses a CNT suspension and electroless Ni plating bath to carry out composite plating. The CNTs were first purified by an acid solution, dispersed in a Ni electrobath, and finally co-deposited with Ni on glass substrates to synthesize electrically conductive films. Field emission scanning electron microscopy and Raman spectroscopy results show that the field emission characteristics and graphitic properties of CNT-FECs depend on the pH value of the electrobath. Experiments show that the optimum electrobath pH value is 5.4, achieving a field emission current density of 1.0 mA/cm 2 at an applied electric field of 1.5 V/μm. The proposed CNT-FECs possess good field emission characteristics and have potential for backlight unit application in liquid crystal displays.

SU-DD-A4-05: Development of a Tetrahedron Beam Computed Tomography Benchtop System with a 75 Pixel Field Emission X-Ray Tube: Collimator Design and Image Reconstruction

Purpose: We are developing a novel Tetrahedron Beam Computed Tomography (TBCT) benchtop system using a multiple pixel x‐ray tube with 75 carbon nanotubefield emissioncathodes and an in‐house curved CTdetector array. In this paper, we describe the development of multi‐slot collimators and image reconstruction algorithms.Material and Methods: The 75 x‐ray sources are arranged in a linear array with 4 mm cathode spacing. The 5‐row detector array is curved with a 145 cm radius. The linear scanning x‐ray source array and a curved detector array are aligned perpendicular to and within the rotation plane, respectively, with the source array positioned precisely on the central axis of the detector array. A group of multi‐slot collimators were designed to collimate the beam from each source exactly to the detector array. An FDK TBCT reconstruction algorithm was developed. Image reconstruction was performed using numerical and actual projection data. Results: The shapes of each slot opening are partial ellipses. The slot geometries were derived and drawn using CAD software. Actual collimators were made using a chemical milling technique. The FDK TBCT image reconstruction algorithm was adapted from the cylindrical, equiangular detector cone beam reconstruction algorithms. The TBCT reconstruction was evaluated using numerical Shepp‐Logan phantom as well as real phantom scans.Conclusions: Multi‐slot collimators were designed and built. The TBCT image reconstruction algorithm was able to accurately reconstruct 3D volume with both numerical and real projection data. This work is partially supported by NIH grant 1R21CA130330‐01A1 and DOD Prostate Cancer Research Program W81XWH‐07‐0083.

Carbon nanotube field emission cathodes fabricated with trivalent chromium conversion coated substrates

The properties of carbon nanotube (CNT) field emission cathodes fabricated by a dip coating method with trivalent chromium conversion coated substrates are studied. Two kinds of substrates with different morphologies, one with a rough crackled surface and the other with a smooth surface, were used for making the CNT cathodes, and their I– V curves and emission patterns were evaluated. The results show that, as compared to the smooth substrate surface, the rough surface with self-assembled sub-micro-cracks on the substrate can dramatically enhance the uniformity of the emission pattern and the emission efficiency. The cathode fabricated with the crackled substrate shows good field emission properties such as high brightness, good uniformity, a low turn-on field (0.86 V/μm) and a high current density of 10 mA/cm 2 at 2.5 V/μm.

A carbon nanotube field emission cathode with high current density and long-termstability

Carbon nanotube (CNT) field emitters are now being evaluated for a wide range of vacuumelectronic applications. However, problems including short lifetime at high current density,instability under high voltage, poor emission uniformity, and pixel-to-pixel inconsistencyare still major obstacles for device applications. We developed an electrophoretic process tofabricate composite CNT films with controlled nanotube orientation and surface density,and enhanced adhesion. The cathodes have significantly enhanced macroscopic fieldemission current density and long-term stability under high operating voltages. Theapplication of this CNT electron source for high-resolution x-ray imaging is demonstrated.