Yoshikazu Kuga

Size distribution measurement of nanometer-sized aerosol particles using dma under low-pressure conditions

Abstract A system for measuring nm-sized aerosol particles under low pressure conditions using a differential mobility analyzer (low pressure DMA, LPDMA) with a Faraday cup electrometer (FCE) as the detector is developed. The performance of the LPDMA is investigated experimentally using a tandem DMA technique. It is shown that LPDMA can be used to measure nm-sized particles (4–10 nm) under low-pressure conditions (60–760 Torr). The effect of Brownian diffusion on the performance of LPDMA is qualitatively discussed by comparing the results obtained using LPDMA with those of conventional and Vienna DMAs with dimensionless parameters. The equilibrium charge distribution of aerosol particles reached under low-pressure conditions is also evaluated by solving population balance equations for particle-ion collision. The LPDMA/FCE system is also applied to the measurement of nm-sized silver particles produced by physical vapor deposition (PVD) and TiO 2 particles produced by low pressure chemical vapor deposition (LPCVD)

Shape Estimation of Anisometric Particles Using Size Measurement Techniques

This study was conducted to establish a simple method for evaluating the morphology of fine anisometric particles using size measurement techniques. The size distributions of mica particles and carbon fibers classified into narrower size ranges were measured by gravitational sedimentation and laser diffraction techniques. The ratio of mean diameters determined for flaky particles strongly depended on the aspect ratio, i.e. flatness. The relationship between particle shape and diameter is discussed theoretically. The experimental results were similar to those predicted. The flatness of fine particles can be evaluated by the ratio of the median diameter determined by laser diffraction to that determined by sedimentation.

Production of finely ground natural graphite particles with high electrical conductivity by controlling the grinding atmosphere

Abstract Natural graphite particles with high crystallinity sieved to obtain a particle size range of under 63 μm were ground with a ball mill, under various well-controlled grinding atmospheres such as N 2 , O 2 , He, H 2 , and vacuum. The ratio, X dif50 / X st50 , i.e. between the 50 wt.% Stokes diameter and the 50 wt.% laser diffraction diameter, of the ground particles, was used as an index of the flakiness of the particles. The specific resistance of films composed of the ground graphite particles was systematically measured. The rate of reduction in the size of the particles by grinding was slow under an O 2 -rich atmosphere such as 100% O 2 and dry air. On the other hand, it was relatively fast in vacuum, or under an N 2 or He atmosphere, and a gas mixture of 99% N 2 and 1% O 2 . The rate of size reduction by grinding under a H 2 atmosphere was intermediate. In our experimental conditions, the flakiness of the ground particles increased with the decrease in the particles’ sizes. The electrical conductivity of the ground particles, however, tended to decrease with the decrease in their sizes. Under the condition that the Stokes diameter of the ground particles remains constant, the electrical conductivity of films made from the ground particles increases with the increase in the flakiness of the particles. It was finally determined from our systematic grinding experiments that small flaky particles, which had a size, X st of ∼1 μm, with a high electrical conductivity can be produced by grinding in a gas mixture of 99% N 2 and 1% O 2 . In this case, the flaky shape of the ground particles was visually confirmed by scanning electron microscopy.

Classification Performance of a Low Pressure Differential Mobility Analyzer for Nanometer-sized Particles

Size distribution measurements and classification tests by a low pressure differential mobility analyzer (LPDMA) for nanometer-sized silver particles and cesium iodide particles under low pressure conditions (123–300 Torr) were performed using a transmission electron microscope (TEM) and the tandem DMA technique. When the ratio of the sheath gas flow rate and the aerosol gas flow rate was set at 5 : 1, the targeted sizes calculated from the classification voltage applied to the LPDMA at 160 Torr are found to be in good correlation with the count mean Feret diameter obtained from the TEM observation of the particles collected after a classification ranging from 6 to 25 nm, although the targeted sizes set by the LPDMA were approximately 15% greater than the count mean Feret diameter measured by the TEM after classification. The geometric standard deviations of DMA-classified particles measured by TEM and those obtained from the tandem DMA method ranged from 1.08 to 1.17 and from 1.05 to 1.13, respectively. They were slightly greater than the ideal geometric standard deviations (1.05) of the particles classified with the LPDMA, which was calculated by neglecting the broadening effects due to Brownian diffusion. We experimentally demonstrated the validity of our LPDMA system for size measurements and classification of the nanometer-sized particles under low pressure conditions.

Laser-assisted exfoliation of potassium-ammonia-graphite intercalation compounds

Potassium-ammonia-graphite intercalation compounds (K-NH3-GIC) synthesized from small natural graphite particles from China were rapidly heated by a cw-Nd:YAG laser beam (1.056 μm) in an Ar purge environment to yield exfoliated small graphite particles as sources for electrical conductive films. The laser heating method was demonstrated to be effective for exfoliation of K-NH3-GIC prepared from natural graphites in sizes of under 37 μm, 37 ~ 63 μm and 63 ~ 88 μm, whereas no exfoliation of these graphites occurred by either gas burner heating or electric furnace heating. The results also show that K-NH3-GIC prepared from natural graphite 350 ~ 500 μm in size was exfoliated to 32-fold of the volume of feed natural graphite. This exfoliation ratio is 2.6 times larger than the ratio obtained by the gas burner heating method.

Characteristics of Aerosol Charge Distribution by Surface-Discharge Microplasma Aerosol Charger (SMAC)

The surface discharge on a dielectric barrier induced by dc pulses was successfully utilized as a stable bipolar ion source for neutralizing submicron aerosol particles where the concentration of positive and negative ions could be adjusted independently (a surface-discharge microplasma aerosol charger: SMAC; Kwon et al., 2005). The aim of this study was to determine the charge distribution obtained by the SMAC, which has been qualitatively presented in our previous study, and to investigate the effect of unequal bipolar ion concentration on the charge distribution. For this purpose, we performed quantitative analysis of the charge distribution of monodisperse particles in the size range of 30–200 nm acquired by the SMAC and compared the charge distributions with calculated charge fractions obtained from the diffusion charging theory. The ion parameters were calculated by measuring the ion mobility of positive and negative ions and they were used to obtain the analytic solutions of charge distribution. The charge distributions obtained by the equal or unequal concentration of bipolar ions adjusted by the SMAC showed general agreement with the diffusion charging theory.

Unipolar ion charging and coagulation during aerosol formation by chemical reaction

Simultaneous coagulation and charging of newly formed titania aerosol by thermal decomposition of titanium tetraisopropoxide (TTIP) is investigated using a discrete-sectional population balance model in two dimensions: particle size and charge. The time evolution of particle size distribution and charge distribution is presented for various initial unipolar ion concentrations with and without electrostatic dispersion. Accounting for the electrostatic dispersion, unipolar ions do not affect the average particle size and geometric standard deviation since ions are dissipated so quickly that particles have little opportunity to be charged. On the other hand, when the electrostatic dispersion is suppressed, product titania average diameter, geometric standard deviation and concentration are affected greatly.

Effect of exfoliation ratio on the flakiness of fine graphite particles obtained by grinding of exfoliated graphite.

In this study, we ground exfoliated graphite and investigated the flakiness of the ground products using the previously reported particle size measurement techniques

Grinding characteristics of bromine-exfoliated graphite and natural graphite

The grinding characteristics of natural graphite are discussed and a new graphite grinding method is proposed for obtaining fine particles. Natural graphite was well ground in dry air after vacuum drying at 10−3 Torr and 80°C and its 50% diameter of cumulative underside of products ground for 12 h was approximately 1.4 μm. The new grinding method was performed in dry air after treatments designed to increase the distance between layers and weaken the bonding strength by use of an intercalation reaction. Bromine gas was used as an intercalate and bromine graphite intercalation compounds were exfoliated by vacuum heating. The product size distribution for the exfoliated bromine graphite ground in dry air after vacuum drying was slightly different from that of ground natural graphite.

Improvement of multi jet low pressure impactor for high collection efficiency of UF5 in the molecular laser isotope separation of uranium

A numerical and experimental study for the collection of photo-produced UF5 particles was performed for the low pressure impactors which have different design factors at typical flow conditions (upstream pressure of the impactor = 10–15 Torr, pressure ratio of downstream to upstream of the impactor, PdownPup = 0.2–0.5) in the molecular laser isotope separation of uranium at RIKEN (RIMLIS). Smaller HW ratios (the distance between the impactor orifice exit and the impaction plate, H, divided by the orifice diameter of the impactor, W) and the smaller PdownPup were found to be preferable to obtain a higher collection efficiency from both numerical and experimental investigations. In addition it was experimentally demonstrated that the use of a 16 μm laser system for the selective reaction of 235UF6 to form 235UF5 was not relevant for the study of the collection of UF5 particles. So, we used an ultraviolet laser system (fourth harmonic YAG laser (266 nm) and an excimer laser (KrF, 248 nm)) which was more convenient to cope with various operating conditions. The collection efficiency was found to increase with the initial concentration of UF5 molecules produced. Applying the improved impactor stage, we obtained a collection efficiency which was approximately 10 times higher than that of our previous work. Higher collection efficiencies of photo-produced UF5 particles enriched in 235U reduce the enrichment cost.