Sam-Yong Woo

Application of carbon nanotube field emission effect to an ionization gauge

Using the field emission effect of a carbon nanotube (CNT), we characterized a new type of technology for detecting low pressure. The fabricated low pressure sensor is of a triode type, consisting of a cathode (carbon nanotubes field emitter arrays), a grid, and a collector. The gauge described here has a triode configuration similar to that of a conventional hot cathode ionization gauge but also has a cold emission source. Due to the excellent field emission characteristics of CNT, it is possible to make a cost effective cold cathode type ionization gauge. For an effective CNT cathode, we used the screen-printing method and also, we controlled the collector and the grid potentials in order to obtain a high ionization current. We found that the ratio of the ionization current to the CNT cathode current changes according to the pressure in the chamber. In short, we elaborate the various metrological characteristics of a home-made pressure sensor that uses CNTs.

Comparison of the effects of 40% oxygen and two atmospheric absolute air pressure conditions on stress-induced premature senescence of normal human diploid fibroblasts.

The pressure during hyperbaric oxygen treatment may increase oxygen toxicity via an augmented oxygen pressure in the gas. Nevertheless, only a few reports have been published on the effect of cells grown under 2 atmospheric absolute (ATA) pressure. To evaluate the effect of pressure on oxygen toxicity and to study effects in addition to oxygen toxicity, we designed an experiment to compare the effects of normobaric mild hyperoxia (NMH, 40% oxygen) and hyperbaric air condition (HA, air with 2 ATA) on human diploid fibroblasts (HDF) in a hyperbaric incubator. HDFs in both the NMH and the HA condition had a similar oxidative stress response and exhibited premature senescence. To investigate differences in gene profiling in cells grown in the NMH and HA conditions, samples from cells exposed to each condition were applied to microarrays. We found no expression difference in genes related to aging and deoxyribonucleic acid damage, but the expression of genes including cell adhesion, stress response, and transcription were significantly increased in fibroblasts that were responsive to pressure. Among 26 statistically reliable genes, the expression of apoptosis related genes such as ADAM22, Bax, BCL2L14, and UBD, as well as tumor suppressor-related genes like Axin2 and ATF, and also mitogen-activated protein kinase-related genes like mitogen-activated protein kinase kinase kinase 1, histamine receptor, and RAB24, were significantly changed in cells responsive to pressure-induced oxidative stress.

Improved metrological characteristics of a carbon-nanotube-based ionization gauge

A low-pressure detection technology utilizing the field emission of carbon nanotubes (CNTs) is introduced and its performance is characterized. To overcome the limitations of conventional ionization gauges, the CNTs had been proposed as an electron source. However, the simple triode structure of previous CNT pressure sensor not only has suffered from a limited measurement range but is also subject to a serious CNT array degradation problem. In this letter, the authors report the characterization results of an improved CNT-based pressure gauge, which is implemented by a screen-printing method, a modified mesh structure, and a pulse width modulation signal.

Parallelism error analysis and compensation for micro-force measurement

The null balance method for micro- and nano-force measurement is widely used in various precision industries. A parallel spring and a lever mechanism are designed and analysed for an electrostatic force balance by the null balance method. Various design parameters are required to obtain high resolution, a large measurement range, and fast response. The corner loading error is one of the most dominant error sources that should be removed. A parallel spring known as the Roberval mechanism, used in the micro-weighing device, can be used for one-axis force measurement since it is very sensitive to the vertical direction force, but insensitive to other force components. However, precise control of parallelism to below a micrometre is required not only to compensate for but also to remove the corner loading error. In this paper, the effects of the parallelism error have been analysed using the Lagrange method and verified by the FEA method. It was found that parallelism control by the proposed precise compensation mechanism enables the simultaneous elimination of and compensation for the corner loading error.

Hot cathode ionization gauge calibration with the KRISS ultra-high vacuum standards

The calibration system for ultra-high vacuum standards at the Korea Research Institute of Standards and Science (KRISS) uses a restriction in the range 5 × 10-7 Pa to 2 × 10-3 Pa. Using this system, we have calibrated for argon two commercially available gauges, an extractor gauge and a stabil-ion gauge. The correction factor of the extractor gauge in the lower pressure range was about 1.07 but it rose by 4%, reaching 1.11 at 2 × 10-5 Pa, while that of the stabil-ion gauge slowly decreased from 1.04 to 1.02 over four decades of the pressure range.

Development of low pressure sensor based on carbon nanotube field emission

A newly developed low pressure detection technology using carbon nanotube (CNT) field emission effect has been designed and manufactured. The fabricated pressure sensor is of a triode type, consisting of a cathode (CNTs field emitter arrays), a grid and a collector. The principle described here is that, for a constant number of electrons available for ionization emitted from CNT arrays by a grid potential, a constant fraction of gas molecules will be ionized and the number of ions collected in a collector will be proportional to the number of gas molecules in the chamber traversed by the electrons. Due to the excellent field emission characteristics of CNT, it is possible to make a cost-effective cold cathode type ion gauge. A screen-printing method has been used to make the CNT cathode. A glass grid with Cr deposited by an e-beam has been put on the cathode with a gap of 200 µm between two electrodes. Due to the voltage applied to the grid, the electrons emitted from the CNTs ionize gas molecules in the chamber and the ionized molecules are gathered to the collector. Then, the collector voltage is made lower than the grid voltage to obtain a large ionization ratio. The current detected in the collector is proportional to the pressure in the chamber. The ionization characteristics are dependent on the gas and the voltage applied to the grid and the collector. In this paper we will show the various metrological characteristics of the home-made pressure sensor utilizing CNTs.

Vacuum measurement by carbon nanotube field emissiona)

A vacuum measurement technology utilizing the carbon nanotube (CNT) field-emission effect has been developed and characterized. The fabricated pressure sensor is a triode type similar to a conventional ionization gauge, but has a planar structure similar to a field-emission display. Owing to the excellent field-emission characteristics of CNTs, it is possible to make a cost effective cold cathode-type ion gauge. The triode-type CNT sensor has been manufactured by a screen-printing method and by a thermal chemical-vapor deposition growth method. A modified structure with an electron filter has also been introduced and characterized so as to improve the performances, such as sensitivity and stability. A glass grid with Cr deposited by e-beam is placed on the cathode. By the voltage applied to the grid, electrons are emitted from the CNTs and they ionize gas molecules in the chamber. Two modes are available to detect the gas density in the chamber, an electron emission mode and an ionization mode. In the ele...

New apparatus for calibrations in the range of 2?kPa absolute pressure

Capacitance diaphragm gauges (CDGs) are precise electromechanical pressure sensors in which the displacement of a stretched thin metal diaphragm is detected by the measurement of a capacitance. These are very accurate gauges, and are frequently used as transfer gauges. To calibrate such accurate low-pressure gauges, precise mercury manometers have been used. However, complexity, concern about mercury vapour, and cost of mercury manometers have made it difficult to use these manometers in many industrial calibration laboratories. As a substitute, gas-operated piston gauges can be used for the calibration of such low-pressure gauges. However, the minimum pressure that is necessary to balance the tare weight, which generally corresponds to a pressure of several kilopascals, is a major obstacle. To reduce this minimum operating pressure, we adopted a variable bell-jar pressure method. To realize this method effectively, we developed a new mass-handling device that makes it possible to add or remove weights up to 200 g easily, with a resolution of 10 g, without breaking the vacuum during the calibration. This calibration system can be used to measure pressures from 100 Pa to 2 kPa in the absolute mode. In this paper, we also present the calibration results for two types of CDGs with full-scale ranges of 1330 Pa and 1000 Pa, respectively.

Evaluation of high temperature pressure sensors

It is becoming more important to measure the pressure in high temperature environments in many industrial fields. However, there is no appropriate evaluation system and compensation method for high temperature pressure sensors since most pressure standards have been established at room temperature. In order to evaluate the high temperature pressure sensors used in harsh environments, such as high temperatures above 250 °C, a specialized system has been constructed and evaluated in this study. The pressure standard established at room temperature is connected to a high temperature pressure sensor through a chiller. The sensor can be evaluated in conditions of changing standard pressures at constant temperatures and of changing temperatures at constant pressures. According to the evaluation conditions, two compensation methods are proposed to eliminate deviation due to sensitivity changes and nonlinear behaviors except thermal hysteresis.

New weight-loading device for calibration of precise barometers

In order to calibrate highly accurate barometers, laser/ultrasonic mercury manometers have been used. However, the complexity and cost of mercury manometers have gradually taken them out of use in most calibration laboratories. As a substitute, a gas-operated pressure balance is used to calibrate precise barometers. In such a case, many commercially available pressure balances are not desirable because the consequent exposure of the piston, cylinder and masses to the atmosphere causes the problem of contamination and the ingress of dust particles to the gap between the piston and cylinder. In this paper, a novel weight-loading device for changing the masses in situ without breaking the vacuum is described. This device makes it possible to add or remove weights easily in the absolute mode, thereby greatly reducing the time between observations. Using this device we could easily calibrate a precise quartz resonance barometer (Paroscientific, Model 760-16B) from 940?hPa to 1050?hPa.