E-Journal of Advanced Maintenance Vol.4 No.2 (2012) 57-63
Japan Society of Maintenology

Fabrication of Micro Capsule Containing Fluorescent
Magnetic Particles for Advanced Inspection of Heat Exchanger Tubes
Fuyumi ITO 1,*, Akihiko NISHIMURA 1
1

Japan Atomic Energy Agency, Kansai Photon Science Institute 8-1 Umemidai, Kizugawa, Kyoto,
619-0215, Japan
ABSTRACT

Stress Corrosion Cracking (SCC) sometimes occurs in the heat exchanger tubes of aging nuclear
power plants. Magnetic particle Testing (MT) is used to facilitate visual observation on SCC. The MT
was improved by the micro capsules which were containing both fluorescent pigment and magnetic
particles. The magnetic particle micro capsules (MPMC) were fabricated by liquid mixture synthesis
in various magnetic particle weight concentrations. MPMCs with < 10wt% successfully maintained
the spherical shape. MPMCs with 2wt% and 6.7wt% MPMCs were drawn around the electromagnet
poles. Furthermore these capsules were easy to be confirmed by shining on side illumination. The
proposed MPMC will be useful for MT to inspect the SCC symptom.
KEYWORDS
magnetic particle inspection, heat exchanger tube, crack, micro capsule,emulsion, density matching, line of magnetic force, leakage magnetic flux,
a laser defect-processing head

Article history:
Received 3 Feb 2012
Accepted 4 June 2012

1. Introduction
Micro Capsule (MC) is used not only in pressure-sensitive copying paper, drug delivery systems,
adhesives, aromatic substances, and cleansers but also in nuclear fusion. MCs in nuclear fusion are
known as fusion targets [1]-[2], which are injected with deuterium and tritium fuel. The sphere of thefusion target and the uniformity of wall thickness require precision of 99.9% or higher. To
manufacture this high-precision fusion target, the development of processing technology [3]-[6] andthe quantitative evaluation technology is extremely important. This has led to the development of a
production process in which a catalyst is used. The membrane wall of this MC is produced frompolymers and the liquid can be kept inside the MC. By changing the polymers used in the MC and theliquid inside it, the application can be extended into various fields. Therefore, it is proposed that theinspection and the repair of heat exchanger tubes that move the heat caused by nuclear fusion to waterin nuclear power plants be adapted by combination both the polymer material of the MC itself and thematerial contained inside the MC.
Japanese nuclear power plants are now entering their 30th year. For the maintenance of nuclear
plants, nondestructive testing for cracks on the inside wall of the heat exchanger tubes is important. At
the present time, cracks are mainly detected by Eddy Current Testing (ECT) [7]-[9], which allows for
rapid inspection since the result is obtained by an electric signal. However, the results of ECT are
affected by small changes in the form of the wall material and by its magnetic characteristics. This
makes it difficult to measure welded zones, weld overlays and tiny cracks. In order to reduce theinfluence of the crack shape, Magnetic particle Testing (MT) [10]-[11] is used as a method ofinspecting cracks instead of ECT. MT makes it possible to detect complicated shapes and cracks only
a few micrometers wide. In the case of MT, dispersed liquid with magnetic particles is sprinkled onto
a magnetized sample. There are two kinds of magnetic particle, namely Fluorescent Magnetic
Particles (FMP), and ordinary magnetic particles. FMPs are used to confirm cracks in the heat*

Corresponding author, E-mail: ito.fuyumi@jaea.go.jp

ISSN-1883-9894/10 © 2010 – JSM and the authors. All rights
reserved.

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F. ITO, et al./Fabrication of Micro Capsule Containing
Fluorescent Magnetic Particles for Advanced Inspection of Heat Exchanger Tubes

exchanger tubes. The FMPs shine when illuminated with ultraviolet light, due to the fluorescentpigment coating on the magnetic particles. Cracks in dark locations can easily be found by using thedispersed liquid with FMPs. However, this dispersed liquid could enter inside the cracks and would
then be difficult to remove completely from cracks, since the particle diameter of FMPs is 0.2μm to
60μm. The cracks are difficult to repair, because the material of the magnetic particles (reduced orelectrolytic iron particles, γ-iron oxide particles (dyad), ferrous ferric oxide) is different from that usedin the heat exchanger tubes. In addition, the dispersed liquid with the FMPs separate into fluorescentpigment and magnetic particle when left to stand for one month. Fig. 1 a) shows the state immediately
after FMPs were dispersed in water. As shown in Fig.1 a), the decentralized FMP liquid had green
color. Fig.1 b) shows a state that Fig.1 a) allowed to stand for one month. The liquid turned
transparent and the FMPs were precipitating on the bottom. These FMPs were shaken and stirred inwater. Then this liquid color became yellow as shown in Fig.1 c). The expiration date of use could be
limited. It is because the fluorescent pigments of yellow color coated to the magnetic particle of blackcolor dissolved to water.

a)

b)

c)

Fig. 1. FMP dispersed in water a) immediately after dispersal, and b) one month later; c) fluid dispersion
shaken one month later.

Therefore, it is proposed that the magnetic particles be contained inside the MC (Magnetic
Particle Micro Capsule = MPMC) that can be changed easily. Because the MC can expand to a size
larger than the crack, this MPMC can prevent penetration into the crack. In addition, dispersed liquidwith FMPs can prevent the separation of the fluorescent pigment and magnetic particles, because thedispersed liquid is contained inside the MC. Thus, the present experiment involved the following
tests.
···

2.

A test to change the quantity of magnetic particles contained inside the MC.
A test to observe the change of movement by magnetomotive force.
A test to observe the gathering of MPMCs on the crack.

Experimental Procedure

All the chemicals were commercially available and used without further purification. 1,
2-dichloroethane (45mL, density: 1.235g/cm3; Nacalai Tesque) was mixed into benzene (55mL,
density: 0.8786g/cm3; Nacalai Tesque) until the density measured by a floating density meter(standard hydrometer) was 1.015g/cm3 at 25°C. Next, polystyrene (Nacalai Tesque, 7.0g) was added
to a solution of 1, 2-dichloroethane and benzene. This solution was stirred at 60°C for 1h. After this,the bottle was cooled in a water bath for 0.5h. 5wt% polyvinyl alcohol (PVA) solution was obtained asfollows: PVA (Nacalai Tesque, 25g) was dissolved in 475mL of pure water, and then this solution was
stirred at 60°C for 0.5h. 0.05wt% PVA solution was made by diluting 5wt% PVA solution with purewater, and the FMPs were dispersed into 0.05wt% PVA solution. Dispersion ratios are shown in Fig. 2
and Fig. 3.
A density-matched emulsion was produced according to the following procedure. (I) 7wt%
polystyrene solution (4.5mL) (Oil) was poured into a 10mL bottle, then the dispersed FMP solution(3mL) (Water 1) was added to this bottle. (II) Solution (I) was agitated by hand, and at this time a
Water 1/Oil emulsion was produced. (III) This emulsion was added to 500mL of 5wt% PVA solution(Water 2) rotated beforehand by 500rpm. At the same time, Water 2 containing the Water 1/Oil
emulsion was rotated at 500rpm for 1/350h, after which the rotational speed was slowed from 500rpm
to 200rpm, and the Water 1/Oil emulsion was stirred at 60°C for 1h. Finally, Water 2 containing the58

F. ITO, et al./Fabrication of Micro Capsule Containing
Fluorescent Magnetic Particles for Advanced Inspection of Heat Exchanger Tubes

Water 1/Oil emulsion was heated and was stirred for 2h at 80°C. After heating, Water 1/Oil emulsion
became MPMCs. (IV) Water 2 containing MPMCs was washed in pure water to remove the Water 2,
and the MPMCs were preserved in pure water. (V) The MPMCs were measured by microscope, thenthat moved to the crack by the influence of magnetomotive force, and the difference in the movementof these MPMCs was observed using different of concentrations of FMP decentralized liquid.

3.

Result and discussion

The MPMCs induced the FMP thus produced, and the FMP concentrations were 0.1wt%, 0.2wt%,
2wt%, 6.7wt%, 10wt%, 20wt% and 100wt%. In the case of FMPs at 20wt% and 100wt%, the shape ofthe dried MPMC was transformed from a globular shape to a resembling a “sand cluster”, as shown in
Fig. 2. The size of this “sand cluster” was about 1mm.
Quantityof FMPs

20wt%

100wt%

Shape of
MPMCs

1mm
1mm

1mm

Fig. 2. In both cases of 20wt% and 100wt%, MPMCs formed "sand-like" shape. It might be caused bycentrifugal force or propeller impact. So, it means that we should keep the density of MPMC lessthan 20wt%.

Comparing commercial polystyrene with polymeric polystyrene obtained by radical
polymerization of styrene using a radical initiator, commercial polystyrene is not subject to the
influence of monomers; specifically, its density does not increase. In this study, defects in the MPMC
such as distortion and bubbles did not readily occur, because the MPMCs were produced withcommercial polystyrene. The commercial polystyrene was used as a raw material in the Oil phase.
However, some of the MPMCs did change into the above “sand-like” shape. This was due to the
influence of gravity, because the density of the Water 1 phase, the Oil phase and the Water 2 phasevaried greatly. It was extremely important to prevent the effects of gravity while the MPMCs werebeing produced.
Thus, the density matching of the Water 1 phase, the Oil phase and the Water 2 phase wasimportant to produce highly spherical MPMCs [12]. Generally, the emulsion was affected bycentrifugal force and by collision with the propeller [12], as a result of which sufficiently spherical
MPMCs could be produced. However, when the FMPs were contained inside the emulsion, thedensity of the Oil phase, the Water 1 phase and the Water 2 phase was difficult to adjust. Because theWater 1 phase included FMPs, the FMPs precipitated easily at Water 1. When the Water 1 with 20wt%
and 100wt% FMPs were put inside the MCs, the FMPs quickly precipitated at the bottom of theemulsion. In addition, when 20wt% and 100wt% MPMCs with a “sand cluster” shape were put into
the water, the water became white and/or turbid yellow one month later. It is speculated that 20wt%
and 100wt% MPMCs were easily disintegrated by centrifugal force and by the impact of propeller
when 20wt% and 100wt% MPMCs were stirred. Then collided MPMCs became big one MPMCs at
the emulsion step. So the size of MPMCs is bigger than the emulsion. This made 20wt% and 100wt%
MPMCs form the “sand cluster” shape, and the FMPs were effused from 20wt% and 100wt%
MPMCs.
The dried 0.1wt%, 0.2wt%, 2wt%, 6.7wt% and 10wt% MPMCs are shown in Fig. 3. The 0.1wt%
and 0.2wt% MPMCs were smaller than the 2wt%, 6.7wt% and 10wt% MPMCs. The 0.1wt% and
0.2wt% MPMCs were transparent, because the quantity of the FMPs was small. Furthermore, in thecase of the 0.1wt%, 0.2wt%, 2wt% and 6.7wt% MPMCs, the FMPs containing inside the MCs
assembled under the inner parts of the MCs while the MPMCs were drying. However, the 10wt%

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F. ITO, et al./Fabrication of Micro Capsule Containing
Fluorescent Magnetic Particles for Advanced Inspection of Heat Exchanger Tubes

MPMCs were not only globular but also elliptical in shape, as shown in Fig. 3. The reason for thiselliptical shape was presumed to that the FMPs converged at the bottom of the MCs while the MPMC
were stirred at 80°C, because the quantity of FMPs contained inside the MCs was too large. Therefore,
the surface fraction of the MPMC wall collapsed, and the shape of the MPMCs was transformed.
However, the storage water of the 10wt% MPMCs did not become turbid one month latter. These
results indicate that 0.1wt%, 0.2wt%, 2wt%, 6.7wt% and 10wt% MPMCs can be used in MT.
Quantityof FMPs

0.1wt%

0.2wt%

2wt%

6.7wt%

10wt%
elliptical shape

Shape of
MPMCs

1mm

1mm

1mm

1mm

1mm

Fig. 3. The transformed MPMCs shape is changed gradually, when the quantity of the FMPs(0.1wt%,
0.2wt%, 2wt%, 6.7wt% and 10wt%) is varied.

These MPMCs including varied concentrations of FMPs (0.1wt%, 0.2wt%, 2wt%, 6.7wt% and10wt%) were put on an electromagnet (200AT, coil turns: 40, current: 5A) as shown in Fig. 4. Whenthe electric current was turned on, the 0.1wt% MPMCs were excluded, and all of 0.2wt%, 2wt%,
6.7wt% and 10wt% MPMCs assembled on the electromagnet. In addition, the movement speed ofthese MPMCs increased with the increase of FMPs contained in the MCs. Furthermore, the same
experiment using a weaker electromagnet (100AT, coil turns: 40, current: 2.5A) produced the same
result. In the case of the 0.1wt% MPMCs, the 0.1wt% MPMCs did not assemble on the electromagnet
because only a small quantity of FMPs was put into the MPMCs.
Quantityof FMPs

0.1wt%

2wt%

0.2wt%

6.7wt%

10wt%

Movingtransition of
MPMCs
Fig. 4. Difference in migration due to the difference in FMP quantities contained in MPMCs (0.1wt%,
0.2wt%, 2wt%, 6.7wt% and 10wt%), when the MPMCs were put on an electromagnet (200AT, coilturns: 40, current: 5A).

Fig. 5 shows MPMCs photographed by illuminating with UV light from the side and back. These
MPMCs were put in a laboratory dish that entered water, respectively. The quantity of FMPs
contained inside the MCs was 0.2wt%, 2wt%, 6.7wt% and 10wt%, respectively. These drying
MPMCs were kept on a laboratory dish, then illuminated by UV light. The fluorescent pigment
slightly radiated light when FMP were illuminated by UV light. As shown in Fig. 5, contour of these
MPMCs were observed. When MPMCs were illuminated with UV light from back, front side of
MPMCs was not illuminated. So the fluorescent pigment contained inside MC was not emitted.
Therefore shadow or contours of these MPMCs was seen because UV light was illuminated from backof MPMCs. When UV light illuminated MPMCs from the back respectively, a CCD camera recorded
the UV light reflected on the bottom and background color is pink. Because UV light penetrated water,and reflected with the glass of the laboratory dish. Meanwhile, the 0.2wt% MPMCs did not emit light,
because these MPMCs did not contain a sufficient quantity of FMPs.
On the other hand, when these 0.1wt%, 0.2wt%, 2wt%, 6.7wt% and 10wt% MPMCs were
illuminated with UV light from the side. In the case of 10wt%, each MPMC has its shadow or contour.

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F. ITO, et al./Fabrication of Micro Capsule Containing
Fluorescent Magnetic Particles for Advanced Inspection of Heat Exchanger Tubes

But, the fluorescent pigment of the MPMCs could not emit, because the MPMCs containing 10wt%
FMPs were not able to transmit the UV illumination. shadow or contours of MPMCs was seen,
because the UV light was shone on near the capsule. And the UV light reflected with the glass
laboratory. So background is pink color. Furthermore, the result was obtained that the fluorescentpigment emitted light even though the UV light was reflected on the MPMC wall, in the case of 2wt%,
and 6.7wt% MPMCs. Thus, when MPMCs were illuminated from the side, the 2wt% and 6.7wt%
MPMCs radiated the most light. However, some of the UV illumination was reflected on the surfaceof the MPMC wall, and the surface reflected light of a pink color.
Quantityof FMPs

0.2wt%

2wt%

6.7wt%

10wt%

Side
illumination

Backillumination

Fig. 5. Relationship between the quantity of magnetic particles contained in the capsule and the directionof illumination on the capsule.

A simulation was carried out to apply these 6.7wt% MPMCs to MT, with an iron plate used as atest sample. This plate was scratched artificially to create a crack (depth of crack: 50μm, length:
10mm, width: 0.6mm). A schematic design of the crack on the test sample is shown in Fig. 6. In thecase of the crack a) as shown in Fig.6 schematic design, the long axis direction of the crack is thesame as the direction of the magnetizer. On the other hand, the direction of magnetizer and the longaxis direction of the crack were perpendicular to each other in the case of the crack b) as shown in
Fig.6 schematic design. The 6.7wt% MPMCs put on the test sample were magnetized using the“Handy-Magna” magnetizer (magnetomotive force: 1099AT). The behavior of the 6.7wt% MPMCs
on the test sample was observed.
The simulation result is shown in Fig. 6. The crack a) and the crack b) as shown in the magnified
photograph is same as the crack a) and the crack b) as shown in the schematic design. The 6.7wt%
MPMCs assembled on the area surrounding the crack when the long axis of the crack wasperpendicular to the Handy-Magna. However, the 6.7wt% MPMCs did not assemble on the areasurrounding the crack when the long axis direction of the crack was the same as the Handy-Magna.
When the Handy-Magna was put on ferromagnetic material, a line of magnetic flux arose. This line ofmagnetic flux was generally parallel to the Handy-Magna. In addition, the line of magnetic flux
leaked from the crack when the crack existed on the ferromagnetic material.
In the simulation, the leakage magnetic flux occurred at crack b) as seen on the magnified
photographs. The 6.7wt% MPMCs did not assemble on the area of the surrounding crack a), as shownin magnified photograph. On the other hand, the 6.7wt% MPMCs assembled on the area of thesurrounding crack b) as shown in enlarged photograph. The reason for this was that the long axis of
the crack was perpendicular to the Handy-Magna. The magnet function had strong force, because therespective boundaries that created S and N poles were near to each other. Therefore, the crack
perpendicular to the Handy-Magna was observed like the line formed by MT and MPMCs. The
magnetization of the test sample was stopped after the observation of the crack ended, and water waspoured on the test sample. MPMCs on the test sample were washed away by the water, and no61

F. ITO, et al./Fabrication of Micro Capsule Containing
Fluorescent Magnetic Particles for Advanced Inspection of Heat Exchanger Tubes

MPMCs could be seen inside the crack, as shown in Fig. 6.
Before the test sample
was magnetized.

During the test sample
was became magnetized.

Handy-Magna(magnetizer)

Crack b)Crack a)Crack b)After the test sample
was magnetized.
Crack a)Test sample(iron plate)A schematic design
Magnified photographFig. 6. When MPMCs containing FMPs were put on the magnetized iron plate, MPMCs moved to thecrack on the magnetized iron plate. The right is a schematic design of the crack on the test sample
and a direction of Handy-Magna (magnetizer). The crack a) and the crack b) as shown in theschematic design is same as the crack a) and the crack b) as shown in the magnified photograph.

It is thought that the 6.7wt% MPMCs did not enter the crack because the 6.7wt% MPMCs were
bigger than the crack. From these results, it is thought easy to solve the problems of MT by using
MPMCs instead of magnetic particles. However, a problem with this observation method arises
because the heat exchanger tubes are long and narrow, and therefore white light and UV light do notshine into the deepest part of the heat exchanger tubes. Therefore, the employment of a laser
defect-processing head [13]-[14] is suggested for observation in MT.
In the present study, a laser defect-processing head was specially designed to access the innerwall of heat exchanger tubes as in other studies. It was introduced in a movable sleeve that is driven
by two pulse motors. In addition, the tip of a composite-type optical fiber [15] was inserted into thelaser processing head, and the ECT sensor unit was installed at the side of this composite-type opticalfiber. This composite-type optical fiber was made of synthetic quartz with three cylindrical structures,
each with its own function. A center fiber with 0.2mm delivered the processing high-energy laser
beam. Image fibers for endoscopes surrounded the center fiber, and were able to deliver images. Light
guide fibers were located on the outer side of the imaging fibers to supply high-intensity illumination.
Using the laser, cracks can be welded and detected using the ECT sensor unit.
The inside wall of heat exchanger tube can be observed easily using the composite-type fiber ofthe laser defect-processing head, because the laser defect-processing head can move freely to theobservation point. When the light from the light guide fiber is changed from white light to UV light,
the fluorescent pigment emits light. In other words, MPMCs used in MT are able to observe cracks on
the inside wall of heat exchanger tubes. Using the laser defect-processing head is a useful way ofobserving MT. Inspection of heat exchanger tubes is made more efficient by the use of ECT and MT.
In the case of ECT, the inspection of welding produces inaccurate results. This is because theevaluation (variety, shape, size) of cracks is difficult, while similar signals and noise also occur.
Therefore, microscopic cracks and welding are inspected by MT using MPMCs, while straight tubes
are inspected by ECT.

4. Conclusion
The MT was improved by the micro capsules which were containing both fluorescent pigment
and magnetic particles. The magnetic particle micro capsules (MPMC) were fabricated by liquid
mixture synthesis in various magnetic particle weight concentrations. MPMCs contained 0.1wt%,
0.2wt%, 2wt%, 6.7wt%, 10wt%, 20wt% and 100wt% FMPs into MCs was manufactured. As a result,the shape of MPMCs was transformed according to different concentrations of the decentralized FMP
liquid. 0.1wt%, 0.2wt%, 2wt%, 6.7wt% and 10wt% MPMCs created fine shapes. However, MPMCs

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F. ITO, et al./Fabrication of Micro Capsule Containing
Fluorescent Magnetic Particles for Advanced Inspection of Heat Exchanger Tubes

with more than 10wt% displayed a “sand-like” shape. In addition, 0.2wt%, 2wt%, 6.7wt% and 10wt%
MPMCs were displaced toward the electromagnet, and to observe 0.1wt%, 0.2wt%, 2wt%, 6.7wt%
and 10wt% MPMCs in dark location, these MPMCs were illuminated with UV light from the side.
As a result, 2wt% and 6.7wt% MPMCs were found to radiate the most light. However, some of the
UV illumination was reflected on the surface of the MPMC wall. A simulation test was carried out toapply 6.7wt% MPMCs to MT of heat exchanger tubes. As a result, cracks perpendicular to the
Handy-Magna were observed, similar to the line detected by MT and MPMCs. The magnetization of
the test sample was stopped after the observation of the crack ended. Water was poured on the test
sample, and MPMCs on the test sample were washed away by water, after which it was not possible
to confirm the presence of MPMCs inside the crack. MPMCs can be prevented from entering cracks
by changing the MPMC size, and by washing after magnetizing the test sample.

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