Carbon Nanotube Field Emission Research Articles

SU-E-T-79: Fabrication of Miniature X-Ray Tube Using Carbon Nanotube Field Emitter for Electronic Brachytherapy

Purpose: A miniature x‐ray tube with the diameter of 10 mm has been fabricated using a carbon nanotube(CNT)field emitter, optimal design of electron optics and x‐ray target, and vacuum sealing of the x‐ray tube. The CNT emitter with nm∼μm scale and simpler cathode structure was used for miniaturization of x‐ray tube. A truncated conical shaped beryllium (Be) and thin tungsten (W) coating were adopted to emit 3‐dimensionally uniform x‐ray as an isotropic radiation source. Anode grounded high voltage (HV) connection and vacuum sealed x‐ray tube structure within 10 mm diameter are advantages for intra‐cavitary or skin surface electronic brachytherapy. Methods: The miniature x‐ray tube consisted with a tip‐shaped CNT emitter with the diameter of 0.25∼0.8 mm, a focusing electrode, a transmission‐type x‐ray target, getter film and a metalized HV isolation ceramic. The CNT emitter has been fabricated by coating and sintering of CNT and Silver nano particle mixture on the flat W tip. The miniature x‐ray tube is brazed in a 3×10−6 torr of vacuum furnace. The electron and x‐ray emission are calculated by EGN2 and MCNP5 program, and measure using a precision current meter (Fluke multimeter 189), a soft x‐ray chamber (PTW 34013), and x‐ray spectrometer (Amptek XR‐100t‐CdTe). Results: The CNT emitter emitted electron current over 1 mA at −9.8 kV/5 mm, and 0.1 mA was continued for over 3000 minutes with decrease ratio of −0.01 μA/min. The diode‐type miniature x‐ray tube is started to emit less than 5 Gy/min x‐ray at cm air depth from −20 kV to −50 kV following the gate to CNTcathode gap, and applying cathode voltage. Conclusions: The miniature x‐ray tube using CNTfield emitter has been demonstrated. The CNT based x‐ray tube can be more miniature, and is expected to apply various field of electronic brachytherapy, soon. This work was supported by the R&D Program of MKE/KEIT (10035553, Development of multi‐direction X‐ray emitting super‐miniature X‐ray tube for intra‐cavity imaging system) and the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2010‐0020387).

Optimization of plasma-enhanced chemical vapor deposition parameters for the growth of individual vertical carbon nanotubes as field emitters

In this article plasma enhanced growth of single vertical carbon nanotubes (CNTs) from individual nickel catalyst dots is studied, aiming at the fabrication of CNT field emitters. It is found that the growth of individual CNTs differs from that of CNT forests grown from unpatterned catalyst films, an effect that can be attributed to the difference in catalyst volumes. In the context of growth parameters the influence of temperature, growth time, catalyst volume, pressure and power is characterized. After determining the growth behavior, an individual CNT of desired geometry is fabricated on a conducting lead. The CNT is electrically characterized in terms of its field emission behavior and stable emission currents and its work function is determined to Φ = 5.4 ± 0.2 eV.

A carbon nanotube field emission multipixel x-ray array source for microradiotherapy application

The authors report a carbon nanotube (CNT) field emission multipixel x-ray array source for microradiotherapy for cancer research. The developed multipixel x-ray array source has 50 individually controllable pixels and it has several distinct advantages over other irradiation source including high-temporal resolution (millisecond level), the ability to electronically shape the form, and intensity distribution of the radiation fields. The x-ray array was generated by a CNT cathode array (5×10) chip with electron field emission. A dose rate on the order of >1.2 Gy∕min per x-ray pixel beam is achieved at the center of the irradiated volume. The measured dose rate is in good agreement with the Monte Carlo simulation result.

A vacuum sensor using field emitters made by multiwalled carbon nanotube yarns

A newly developed vacuum sensor using carbon nanotube (CNT) field-emission has been designed. The fabricated device is an ionization gauge with a silk-like CNT yarn cathode, and the vacuum is indicated by the ratio of the ion current to the electron-emission current. The metrological characteristics of the sensor were studied in a dynamic vacuum system. It showed good linearity ranged from 10−4 to 10−1 Pa. Taking advantage of the field-emission cathode, the power consumption is only about 5.5 mW. Moreover, comparing it to the conventional thermionic cathode, the CNT yarn cathode is more miniature and a cold cathode with no obvious thermal outgassing effect. Due to these features, the sensor described here could have potential applications in measuring vacuum inside sealed and miniaturized devices.

Growth of carbon nanotube field emitters on single strand carbon fiber: a linear electronsource

The multi-stage effect has been revisited through growing carbonnanotube field emitters on single strand carbon fiber with a thickness of11 µm. A prepared linear electron source exhibits a turn-on field as low as0.4 V µm − 1 and an extremely high field enhancement factor of 19 300, when compared withthose results from reference nanotube emitters grown on flat silicone wafer;3.0 V µm − 1 and 2500, respectively. In addition, we introduce a novel method to grow nanotubesuniformly around the circumference of carbon fibers by using direct resistive heating on thecontinuously feeding carbon threads. These results open up not only a new path forsynthesizing nanocomposites, but also offer an excellent linear electron source for specialapplications such as backlight units for liquid crystal displays and multi-array x-raysources.

Field emission of carbon nanotube under atmospheric pressure

Field emission of carbon nanotube in different gases (for example, Air, He, Ne) under atmospheric pressure has been studied. The distance between anode and cathode was 100—200 nm. Detailed comparison has been made to characterize the field emission currents and fluctuations in various gases with different anode-cathode distances.

Fabrication of carbon nanotubes field emitter arrays on the flexible substrate using reversal roller imprint lithography

Fabrication of carbon nanotubes (CNTs) field emitter arrays (FEAs) on flexible substrate was demonstrated with a novel microfabrication technique, namely, reversal roller imprint lithography (RRIL). In this process, CNTs paste was inkjetted coating onto a patterned roller mould and squeezed into the microstructures of the roller mould by an elasticity scraper, and then it was transferred to the flexible substrate with a suitable contact pressure. The field emission measurement showed that the turn-on field was about 2.3 V/µm at the emission current density of 10 µA/cm 2 . This process was suitable for the fabrication of flexible electronic devices with CNTs.

A carbon nanotube field emission multipixel x-ray array source for microradiotherapy application

A carbon nanotube field emission multipixel x-ray array source for microradiotherapy application

Room temperature mass production of carbon nanotube field emission micro-cathode arrays using electroplating of a CNT/Ni composite followed by micro-machining

A room temperature fabrication method for the mass production of carbon nanotube (CNT) field emission micro-cathode arrays is reported. The technique combines electroplating of a CNT/Ni composite and micro-machining. This method combines the advantages of direct growth and screen printing conventionally used to fabricate such structures and avoids their disadvantages. Due to its integration and room temperature processing, the technique is proven to be advantageous in mass production and low cost. Results of field emission testing show that the CNT micro-cathodes have excellent field-emission properties, such as high current density (15.7 mA/cm 2), field enhancement factor (2.4 × 10 6/cm), and good stability (109 h for 10% degradation of current density from 400 μA/cm 2).

The role of tunneling barrier modification for the saturation current of carbon nanotube field emission in strong electric field

Based on quantum mechanical tunneling of electrons in field emission, a new model has been described to explain the saturation of emission current for carbon nanotube (CNT) emitters. The formulation reveals the effect of geometrical aspect ratio of CNT and the effect of high electric field for non-Fowler–Nordheim (F–N) behavior in field emission characteristics. It is found that for a CNT of aspect ratio ∼10 3, the tunneling barrier converges to a constant square potential well of a small thickness, at applied electric field ⩾3 × 10 6 V/m, resulting in field independent emission current.

Design and feasibility studies of a stationary digital breast tomosynthesis system

Studies have shown that digital breast tomosynthesis (DBT) can improve breast cancer diagnosis by reconstructing 3D images. However, DBT scanners based on rotation gantry prolong the imaging time and reduce spatial resolution due to motion comparing with the regular two-view mammography. To obtain three dimension reconstruction images and maintain the high image quality of conventional mammography, we proposed a prototype stationary digital breast tomosynthesis system (s-DBT). The proposed s-DBT system acquires projection images without mechanical movement. The core component of the s-DBT system is a specially designed spatially distributed multi-beam X-ray tube based on the carbon nanotube field emission X-ray technology. The multi-beam X-ray source array enables collection of all projection images from different viewing angles without mechanical motion. Preliminary results show the s-DBT system can achieve a scan time comparable with the regular two-view mammography, and improve the spatial resolution comparing with rotating gantry DBT.

Carbon Nanotubes as Field Emitter

Carbon nanotubes (CNTs) have recently emerged as a promising material of electron field emitters. They exhibit extraordinary field emission properties because of their high electrical conductivity, high aspect ratio "needle like" shape for optimum geometrical field enhancement, and remarkable thermal stability. In this Review, we emphasize the estimation and influencing factors of CNTs' emission properties, and discuss in detail the emission properties of macroscopic CNT cathodes, especially fabricated by transplant methods, and describe recent progress on understanding of CNT field emitters and analyze issues related to applications of CNT based cold cathodes in field emission display (FED). We foresee that CNT-FED will take an important place in display technologies in the near future.

An improved planar-gate triode with CNTs field emitters by electrophoretic deposition

An improved planar-gate triode with carbon nanotubes (CNTs) field emitters has been successfully fabricated by conventional photolithography, screen printing and electrophoretic deposition (EPD). In this structure, cathode electrodes and ITO arrays linked with gate electrodes were interdigitated and paralleled on the same plane although the gate electrodes and cathode electrodes were isolated by dielectric layer, a so-called improved planar-gate triode structure. An electrophoretic process was developed to selectively deposit CNTs field emitters onto cathode electrodes in the CNTs suspension by an applied voltage between the gate electrodes and cathode electrodes. The optical microscopy and FESEM image showed that the CNTs emitters with the uniform packing density were selectively defined onto the cathode electrodes. In addition, field emission characteristics of an improved planar-gate triode with CNTs field emitters were investigated. The experiment results indicated that the turn-on voltage of this triode structure at current density of 1μA/cm2 was approximately 55V. The anode current and gate current came to 396μA and 325μA, at gate voltage and anode voltage of 100V and 4000V, respectively and at the anode–cathode spacing of 2000μm. The emission image became brighter and the luminous image with dot matrix on the anode plate obviously increased with the increase of the gate voltage. Moreover, the emission current fluctuation was smaller than 5% for 11h, which indicated that the improved planar-gate triode has a good field emission performance and long lifetime.

Fabrication and Optimization of Printed Carbon Nanotube Field Emitters for Flat-Panel Display Device

Screen-printing process was adopted to prepare the carbon nanotube cathode. For improving the field emission performance of cold-cathode material, the optimized carbon nanotube cathode fabrication structure was given, and the detailed manufacture process was also described. The photoetched cathode indium-tin-oxide (ITO) stripes including the narrow bar ITO connecting line was fabricated to form the cathode electrodes. The annular silver conducting layer was adopted between the cathode ITO stripes and the carbon nanotube field emitters. The whole display device was packaged by means of glass frit, and vacuum-sealed through the exhaust tube. The fabricated field emission display could display some dot matrix number and characters, which possessed better field emission performance, higher display brightness and good field emission uniformity.

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.

Field Emission Characteristics of FED with Carbon Nanotube Field Emitters Using Improved Cathode Electrode

Silver slurry was used as conducting material, and the improved silver cathode electrode was fabricated. The CNT paste formed as field emitters was screen-printed on the surface of silver cathode electrode. The diode FED panel with CNT field emitters was sealed, and the fabrication of cathode substrate and anode substrate was described. The screen-printing technology and the sintering process were employed in the course of device fabrication for the silver cathode electrode. The field emission current was measured and the emission image was presented. The packaged FED showed good field emission characteristics and better field emission uniformity, which the manufacture process was also low-cost, feasible and simple.

Preparation and Field Emission Properties of Ag-CNTs by Electroless Plating

Carbon nanotubes (CNTs) demonstrate attractive characteristics as cold cathode emitters for high aspect ratio, small radius of curvature, high mechanical strength and chemical stability. In this article, Ag-CNTs were prepared by a series of processes including acid-treating, sensitizing, activating and electroless plating. Then Ag-CNTs cathode were sucessfully fabricated by electrophoresis deposition (EPD) method. The morphologies of Ag-CNTs cathode and pristine CNTs cathode were observed by scanning electron microscopy (SEM). The results showed that the field emission properties of Ag-CNTs cathode were better than pristine CNTs cathode, for presenting lower turn-on field, larger emission current density, more stabilization and longer lifetime. Thus it can be seen that the Ag-CNTs cathode can improve the characteristics of carbon nanotubes field emission display (CNT-FED).

Enhancement of field emission of carbon nanotubes using a simple microwave plasma method

A microwave plasma-based method for the enhancement of electron field emission behaviour of carbon nanotubes is presented. This method is shown to be able to unzip and exfoliate carbon nanotubes, thereby significantly increasing the field emission current due to the formation of more emitting tips.

Prospective‐gated cardiac micro‐CT imaging of free‐breathing mice using carbon nanotube field emission x‐ray

Carbon nanotube (CNT) based field emission x-ray source technology has recently been investigated for diagnostic imaging applications because of its attractive characteristics including electronic programmability, fast switching, distributed source, and multiplexing. The purpose of this article is to demonstrate the potential of this technology for high-resolution prospective-gated cardiac micro-CT imaging. A dynamic cone-beam micro-CT scanner was constructed using a rotating gantry, a stationary mouse bed, a flat-panel detector, and a sealed CNT based microfocus x-ray source. The compact single-beam CNT x-ray source was operated at 50 KVp and 2 mA anode current with 100 microm x 100 microm effective focal spot size. Using an intravenously administered iodinated blood-pool contrast agent, prospective cardiac and respiratory-gated micro-CT images of beating mouse hearts were obtained from ten anesthetized free-breathing mice in their natural position. Four-dimensional cardiac images were also obtained by gating the image acquisition to different phases in the cardiac cycle. High-resolution CT images of beating mouse hearts were obtained at 15 ms temporal resolution and 6.2 lp/mm spatial resolution at 10% of system MTF. The images were reconstructed at 76 microm isotropic voxel size. The data acquisition time for two cardiac phases was 44 +/- 9 min. The CT values observed within the ventricles and the ventricle wall were 455 +/- 49 and 120 +/- 48 HU, respectively. The entrance dose for the acquisition of a single phase of the cardiac cycle was 0.10 Gy. A high-resolution dynamic micro-CT scanner was developed from a compact CNT microfocus x-ray source and its feasibility for prospective-gated cardiac micro-CT imaging of free-breathing mice under their natural position was demonstrated.

Nonlinear characteristics of the Fowler–Nordheim plots of carbon nanotube field emission

The barrier between substrate and field emitters can dominate the overall process of field emission. Carbon nanotube (CNT) films were synthesized by thermal chemical vapor deposition (CVD) on silicon substrate covered with a very thin SiO2 layer as the interface contact barrier. The current versus applied voltage curve shows a high turn-on voltage, and the Fowler–Nordheim (FN) plot exhibits nonlinearity characteristics and departures from the original line and exhibits current saturation in the high-voltage region. However, the FN plot of CNTs grown on Si substrates without SiO2 layers had no obvious critical voltage and approximately followed the FN law in our experimental voltage region. The reasons for the nonlinearity of FN plots of CNTs grown on a SiO2 layer were discussed in terms of circuit theory.