This paper suggests and demonstrates a novel flow measurement technique, tunable AC mode hotwire anemometry that allows simple integration, robust measurement, and extremely high accuracy. The principle and simple theoretical analysis of... more
This paper suggests and demonstrates a novel flow measurement technique, tunable AC mode hotwire anemometry that allows simple integration, robust measurement, and extremely high accuracy. The principle and simple theoretical analysis of the technique are shown. To find the optimal frequency at which the phase lag becomes most sensitive to flow speed change, the phase lag was measured scanning the heating frequency from 1 to 100 Hz, while the flow speed of ethanol was increased stepwise from 0 to 10 mm/s. To optimize the sensitivity of technique, the periodic thermal characteristic of the hotwire should be understood and is currently under study. 기호설명 cp 비열 [J/kg·K] h 대류열전달계수 [W/m·K] I 전류 [A] l 열선의 길이 [mm] q̇ 단위부피당 열량 [W/m] R 저항 [Ω] r 열선의 반지름 [μm] T 온도 [K] t 시간 [s] U 속도 [m/s] V 전압 [V] 그리스문자 α 저항온도계수 [K] φ 위상 뒤짐 [°] γ 비저항 [Ω·m] ∆ 두 값의 차이 θ 온도차 [K] ρ 밀도 [kg/m] τ 열시간상수 [s] ω 주파수 [rad/s] 하첨자 AC 교류성분 h 열선 opt 최적점 S 부가저항 ∞ 유동 0 진폭
ABSTRACT
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Manipulation of the chemical vapor deposition graphene synthesis conditions, such as operating P, T, heating/cooling time intervals, and precursor gas concentration ratios (CH4/H2), allowed for synthesis of polycrystalline single-layered... more
Manipulation of the chemical vapor deposition graphene synthesis conditions, such as operating P, T, heating/cooling time intervals, and precursor gas concentration ratios (CH4/H2), allowed for synthesis of polycrystalline single-layered graphene with controlled grain sizes. The graphene samples were then suspended on 8 μm diameter patterned holes on a silicon-nitride (Si3N4) substrate, and the in-plane thermal conductivities k(T) for 320 K < T < 510 K were measured to be 2660-1230, 1890-1020, and 680-340 W/m·K for average grain sizes of 4.1, 2.2, and 0.5 μm, respectively, using an opto-thermal Raman technique. Fitting of these data by a simple linear chain model of polycrystalline thermal transport determined k = 5500-1980 W/m·K for single-crystal graphene for the same temperature range above; thus, significant reduction of k was achieved when the grain size was decreased from infinite down to 0.5 μm. Furthermore, detailed elaborations were performed to assess the measurement...
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We developed a measurement technique that can quantitatively map out the dopant density profile of a silicon integrated-circuit device. This method obtains the quantitative doping density profile by simultaneously carrying out local... more
We developed a measurement technique that can quantitatively map out the dopant density profile of a silicon integrated-circuit device. This method obtains the quantitative doping density profile by simultaneously carrying out local heating, temperature sensing, and thermoelectric voltage measurement at the tip of a diamond thermocouple probe. This probe, which is the key component of the proposed scheme, is fabricated through a nano-fabrication technique that makes use of boron-doped diamond film that can resist stress up to 10 Gpa, which is necessary for stable electric contact with silicon samples. The tip and cantilever of the probe are made of B-doped diamond by means of the silicon lost-mold technique that guarantees a sharper tip apex than that of a diamond-coated probe. A gold-chromium thermocouple junction is integrated at the tip apex for simultaneous heating and sensing. The size of the thermocouple is about 500 nm and the radius of the tip apex is less than 50 nm. The me...
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Thermopower (S) profiling with nanometer resolution is essential for enhancing the thermoelectric figure of merit, ZT, through the nanostructuring of materials and for carrier density profiling in nanoelectronic devices. However, only... more
Thermopower (S) profiling with nanometer resolution is essential for enhancing the thermoelectric figure of merit, ZT, through the nanostructuring of materials and for carrier density profiling in nanoelectronic devices. However, only qualitative and impractical methods or techniques with low resolutions have been reported thus far. Herein, we develop a quantitative S profiling method with nanometer resolution, scanning Seebeck microscopy (SSM), and batch-fabricate diamond thermocouple probes to apply SSM to silicon, which requires a contact stress higher than 10 GPa for stable electrical contact. The distance between the positive and negative peaks of the S profile across the silicon p-n junction measured by SSM is 4 nm, while the theoretical distance is 2 nm. Because of its extremely high spatial resolution, quantitative measurement, and ease of use, SSM could be a crucial tool not only for the characterization of nano-thermoelectric materials and nanoelectronic devices but also f...
Research Interests: Biochemistry, Genetics, Biophysics, Materials Science, Inorganic Chemistry, and 15 moreMolecular Biology, Computational Biology, Biotechnology, Cancer, Cell Biology, Nanotechnology, Medicine, Multidisciplinary, Method, Material, Nanometre, Contact, Electrodes, Materials Testing, and Nanometer Resolution
This paper presents a technique, scanning thermal wave microscopy (STWM), which can image the phase lag and amplitude of thermal waves with sub-micrometer resolution by scanning a temperature-sensing nanoscale tip across a sample surface.... more
This paper presents a technique, scanning thermal wave microscopy (STWM), which can image the phase lag and amplitude of thermal waves with sub-micrometer resolution by scanning a temperature-sensing nanoscale tip across a sample surface. Phase lag measurements during tip-sample contact showed enhancement of tip-sample heat transfer due to the presence of a liquid film. The measurement accuracy of STWM is proved by a benchmark experiment and comparison to theoretical prediction. The application of STWM for sub-surface imaging of buried structures is demonstrated by measuring the phase lag and amplitude distributions of an interconnect via sample. The measurement showed excellent agreement with a finite element analysis offering the promising prospects of three-dimensional thermal probing of micro and nanostructures. Finally, it was shown that the resolving power of thermal waves for subsurface structures improves as the wavelengths of the thermal waves become shorter at higher modul...
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An ac type thermopower measurement technique was suggested and demonstrated with a simple experimental setup. The thermopower distribution across a silicon p-n junction was measured point by point at every 10nm, so that it was free from... more
An ac type thermopower measurement technique was suggested and demonstrated with a simple experimental setup. The thermopower distribution across a silicon p-n junction was measured point by point at every 10nm, so that it was free from the noise due to the built-in potential and photoionization effects, and it was compared with the theoretical result. Although this ac type thermopower measurement technique could not follow the sharp variation of the theoretical thermopower near the p-n junction, it could identify a smooth peak of the thermopower distribution in the depletion layer of the p-n junction.
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Although scanning thermal microscope has shown the highest spatial resolution in local temperature and thermophysical property measurement, its usefulness has been severely limited due to difficulties in quantitative measurement. We... more
Although scanning thermal microscope has shown the highest spatial resolution in local temperature and thermophysical property measurement, its usefulness has been severely limited due to difficulties in quantitative measurement. We propose a double scan technique that measures temperature only from the heat transfer through the tip-sample contact by the subtraction of the signal due to the heat transfer through the air. A rigorous theoretical model for this technique is derived. The effectiveness of the double scan technique in quantitative temperature measurement is demonstrated experimentally.
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Abstract Null-point scanning thermal microscopy (NP SThM) quantitatively measures undisturbed temperature without the influence of changes in physical properties and surface topography of the specimen. Simultaneously NP SThM measures the... more
Abstract Null-point scanning thermal microscopy (NP SThM) quantitatively measures undisturbed temperature without the influence of changes in physical properties and surface topography of the specimen. Simultaneously NP SThM measures the ratio of the sum of the tip-specimen contact thermal resistance and the spreading thermal resistance of the specimen to the effective thermal resistance of the SThM probe. Hence, arguably, NP SThM is an ideal SThM that meets all the requirements of SThM. However, in practice, the use of NP SThM has been limited to one-dimensional profiling only, and two-dimensional extension of NP SThM has been virtually impossible so far. This is because NP SThM is very difficult to implement and ensure a sufficient measurement sensitivity. In this study, we enable two-dimensional extension of NP SThM with almost a 20-fold improvement in measurement sensitivity even under mild vacuum conditions (
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Using null-point scanning thermal microscopy (NP SThM), we have measured and analyzed the size dependence of the thermal conductivity of graphene. To do so, we rigorously re-derived the principal equation of NP SThM in terms of thermal... more
Using null-point scanning thermal microscopy (NP SThM), we have measured and analyzed the size dependence of the thermal conductivity of graphene. To do so, we rigorously re-derived the principal equation of NP SThM in terms of thermal property measurements so as to explain how this technique can be effectively used to quantitatively measure the local thermal resistance with nanoscale spatial resolution. This technique has already been proven to resolve the major problems of conventional SThM, and to quantitatively measure the temperature profile. Using NP SThM, we measured the variation in the thermal resistance of suspended chemical vapor deposition (CVD)-grown graphene disks with radii of 50-3680 nm from the center to the edge with respect to the size. By thoroughly analyzing the size dependence of the thermal resistance, we show that, with increasing graphene size, the ballistic resistance becomes more dominant in the thermal resistance experienced by a heat source of finite size and that the thermal conductivity experienced by such a heat source can even decrease. The results of this study reveal that the thermal conductivity of graphene detected by a heat source depends on the size of the heat source relative to that of the suspended graphene and on how the heat source and graphene are connected. As demonstrated in this study, NP SThM will be very useful for quantitative thermal characterization of not only CVD-grown graphene but also various other nanomaterials and nanodevices.
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Understanding of heat generation in semiconductor devices is important in the thermal management of integrated circuits and in the analysis of the device physics. Scanning thermal microscopy was used to measure the temperature... more
Understanding of heat generation in semiconductor devices is important in the thermal management of integrated circuits and in the analysis of the device physics. Scanning thermal microscopy was used to measure the temperature distribution of the cross-section of an operating metal-oxide-semiconductor field-effect transistor (MOSFET). The temperature distributions were measured both in DC and AC modes in order to take account of the leakage current. The location of the maximum temperature was observed to approach the drain as the drain bias was increased.
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Various characteristics of a thin liquid film in its vapor-phase are investigated using the molecular dynamics technique. Local distributions of the temperature, density, normal and tangential pressure components, and stress are... more
Various characteristics of a thin liquid film in its vapor-phase are investigated using the molecular dynamics technique. Local distributions of the temperature, density, normal and tangential pressure components, and stress are calculated for various film thicknesses and temperature levels. Distributions of local stresses change considerably with respect to film thicknesses, and interfacial regions on both sides of the film start to overlap with each other as the film becomes thinner. Integration of the local stresses, i.e., the surface tension, however, does not vary much regardless of the interfacial overlap. The minimum thickness of a liquid film before rupturing is estimated with respect to the calculation domain sizes and is compared with a simple theoretical relation.
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In the development of graphene-based electronic devices, it is crucial to characterize the thermal contact resistance between the graphene and the substrate precisely. In this study, we demonstrate that the thermal contact resistance... more
In the development of graphene-based electronic devices, it is crucial to characterize the thermal contact resistance between the graphene and the substrate precisely. In this study, we demonstrate that the thermal contact resistance between CVD-grown graphene and SiO2 substrate can be obtained by measuring the temperature drop occurring at the graphene/SiO2 interface with null point scanning thermal microscopy (NP SThM), which profiles the temperature distribution quantitatively with nanoscale spatial resolution (-50 nm) without the shortcomings of the conventional SThM. The thermal contact resistance between the CVD-grown graphene and SiO2 substrate is measured as (1.7 ± 0.27) x 10(-6) M2K/W. This abnormally large thermal contact resistance seems to be caused by extrinsic factors such as ripples and metal-based contamination, which inevitably form in CVD-grown graphene during the production and transfer processes.
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Scanning Thermal Microscope (STU) has been known for its superior resolution for local temperature and thermal property measurement. However, commercially available STU probe which is the key component of SThM does not provide resolution... more
Scanning Thermal Microscope (STU) has been known for its superior resolution for local temperature and thermal property measurement. However, commercially available STU probe which is the key component of SThM does not provide resolution enough to explore nanoscale thermal phenomena. Here, we developed a SThM probe fabrication process that can achieve spatial resolution around 50 m. The batch-fabricated probe has a thermocouple junction located at the end of the tip. The size of the thermocouple junction is around 200 m and the distance of the junction from the very end of the tip is 150 m. The probe is currently being used for nanoscale thermal probing of nano-material and nano device.
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Despite the high spatial resolution of scanning thermal microscope, its usefulness has been limited because of its lack of quantitative measurement. In this study, utilizing the principle of double scan technique, we developed the... more
Despite the high spatial resolution of scanning thermal microscope, its usefulness has been limited because of its lack of quantitative measurement. In this study, utilizing the principle of double scan technique, we developed the null-point method by which one can measure the temperature of a nanoscale sample quantitatively without the disturbances due to the heat transfer through the air and
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Thermopower profiling with nanometer resolution has important applications in the development of nano-structured high ZT thermoelectric materials and in dopant density profiling of nanoelectronic devices. The authors suggested a new AC... more
Thermopower profiling with nanometer resolution has important applications in the development of nano-structured high ZT thermoelectric materials and in dopant density profiling of nanoelectronic devices. The authors suggested a new AC type thermopower measurement technique and demonstrated it with a simple experimental setup. Thermopower distribution across a silicon p-n junction was measured point-by-point at every 10 nm free from the noises due to built-in potential and photo-ionization effect and compared with theoretical result. Although this new AC type thermopower measurement technique could not follow the sharp variation of the theoretical thermopower near the p-n junction, it could identify a smooth peak of thermopower distribution in the depletion layer of the p-n junction
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ABSTRACT
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The proton exchange membrane plays a critical role as an electrolyte for proton transports in the PEMFC. Generally, the membrane, such as Nafion 117, consists of a polytetrafluoro-ethylene (PTFE) backbone and side-chains terminated with a... more
The proton exchange membrane plays a critical role as an electrolyte for proton transports in the PEMFC. Generally, the membrane, such as Nafion 117, consists of a polytetrafluoro-ethylene (PTFE) backbone and side-chains terminated with a sulfonate group (SO3−). Operating the fuel cell, the membrane preferentially becomes hydrated by absorbing water. Then, the hydrogen atom on the SO3− part of the side-chain can detach from its own position and hop to the next SO3− site. The water management is the key to the efficient operation of the fuel cell, since the water content is the one of decisive factors for membrane’s lifetime and efficient operations of fuel cells as well. In this report, we set up the molecular model for hydrated Nafion 117 and simulate the molecular movements for various temperatures and monomer numbers. Here, we obtain the mean square displacements of water molecules and estimate the self-diffusion coefficients of water in the Nafion 117.
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The need for a subsurface imaging technique to locate and characterize subsurface defects in multidimensional micro- and nanoengineered devices has been growing rapidly. We show that a subsurface heater can be located accurately using the... more
The need for a subsurface imaging technique to locate and characterize subsurface defects in multidimensional micro- and nanoengineered devices has been growing rapidly. We show that a subsurface heater can be located accurately using the phase lag of a thermal wave. We deduce that the absolute phase lag is composed of four components. Among the four components, we isolate the component directly related to the position and the structure of the periodic heat source. We demonstrate that the position of the heater can be estimated accurately from the isolated phase lag component.
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In this and the following companion articles, the authors present the 2ω method, a novel ac mode local thermal property imaging technique with nanoscale spatial resolution. To demonstrate the use of the thermoelectric probe as an active... more
In this and the following companion articles, the authors present the 2ω method, a novel ac mode local thermal property imaging technique with nanoscale spatial resolution. To demonstrate the use of the thermoelectric probe as an active one that can function as both a heater and a temperature sensor, the authors develop and implement the 2ω signal measurement technique, which can extract thermoelectric signals from a thermocouple junction while electrically heating it simultaneously. The principle of the 2ω signal measurement technique is explained by a steady periodic electrothermal analysis. The authors use a specially designed test pattern to experimentally verify that the 2ω signal is caused by the temperature oscillation induced by Joule heating. In addition, based on the results from an experiment using a cross-shaped pattern, the measurement accuracy of the 2ω method depends on the junction size of the thermoelectric probe. The 2ω method is implemented and compared with other...
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Nanofluid is a mixture of nanoscale particles of metal, metal oxide or carbon nanotube and heat transfer fluids such as water and ethylene glycol. This work presents the application of the 3-omega (3omega) method for measuring the... more
Nanofluid is a mixture of nanoscale particles of metal, metal oxide or carbon nanotube and heat transfer fluids such as water and ethylene glycol. This work presents the application of the 3-omega (3omega) method for measuring the colloidal stability and the transient thermal conductivity of multi-wall carbon nanotube (MWCNT), Al2O3 and TiO2 nanoparticles suspended in water or ethylene glycol. The microfabricated 3omega device is verified by comparing the measured thermal conductivities of pure fluids with the table values. After the validation, the transient thermal responses of the nanofluids are measured to evaluate the colloidal stability. All of Al2O3 nanofluid samples show a clear sign of sedimentation while the acid-treated MWCNT (tMWCNT) nanofluid and a couple of TiO2 nanofluids with pH control or surfactant addition are found to have excellent colloidal stability. The thermal conductivities of tMWCNT nanofluids in the de-ionized water and ethylene glycol are measured, which are found to be in good agreement with previous data.
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Most of organic devices showed poor thermal stability and short lifetime due to Joule heating by current injection during operation. To increase the lifetime of the devices, thermal management must be considered. We demonstrated the... more
Most of organic devices showed poor thermal stability and short lifetime due to Joule heating by current injection during operation. To increase the lifetime of the devices, thermal management must be considered. We demonstrated the polymer light-emitting diodes with thermally conductive substrate and Al/Cu double cathode to enhance the thermal stability of the device. Also, we proposed the correlation between lifetime (Δt) and device heat sink (ΔT). The heat sink of all organic devices is required to enhance device durability.
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This paper suggests and demonstrates a novel flow measurement technique: tunable AC thermal anemometry, that allows simple integration, robust measurement and extremely high accuracy. The principle and simple theoretical analysis of the... more
This paper suggests and demonstrates a novel flow measurement technique: tunable AC thermal anemometry, that allows simple integration, robust measurement and extremely high accuracy. The principle and simple theoretical analysis of the technique are presented. To find the optimal condition at which the phase lag becomes most sensitive to the flow speed change, the phase lag was measured scanning the heating frequency from 1 to 100 Hz, while the flow speed of ethanol was increased stepwise from 0 to 40 mm/s. The sensitivity of the phase lag depended on the heating frequency and the flow speed. It was possible to measure the average flow speed of 0.7 mm/s with the resolution of 0.1 mm/s at 4 Hz.