「様式３」Project No.： 2075

Laser-Induced Incandescence of Porous Carbon Materials［１］Organization
Project Leader：
Serge Zelensky, D.Sc., Prof. (Taras Shevchenko
National University of Kyiv, Ukraine)
Representative at RIE：
Toru Aoki, Ph.D., Prof. (Research Institute ofElectronics, Shizuoka University, Japan)
Participants：
Kateryna Zelenska, Ph.D., Junior Research
Fellow (Taras Shevchenko National University ofKyiv, Ukraine; Research Institute of Electronics,
Shizuoka University, Japan);Olga Tkach, MSc student (Taras Shevchenko
National University of Kyiv, Ukraine).
［２］Research Progress
According to the research plan of the project,
the following scientific activities were performed
at Taras Shevchenko National University of Kyiv
and at Research Institute of Electronics,
Shizuoka University.
In continuation of the previous projectNo.2006, pulsed laser heating of structured
surfaces was investigated both theoretically andexperimentally. The experiments revealed
unexpected similarity in the properties oflaser-induced thermal emission decay of different
samples of carbon materials. The appropriatecomputer simulations showed that laser-induced
thermal emission of rough surfaces is primarily
formed at peaks of the surface relief. Besides, theexperiments revealed that thermal diffusivity ofcarbon which forms the surface relief peaks isextremely low (the appropriate values are typicalfor heat-insulating materials, and this was also
an unexpected result).
Concerning the effect of surface roughness on
the decay of thermal emission of rough surface
layers, an important parameter is the ratio of thelaser penetration depth (∆) to the temperature

diffusion length (δ). In other words, it is importantwhether the temperature wave has time to travelout of the heat release region under the laser
irradiation. Here two different cases should beconsidered. If δ exceeds ∆, the presence of surface
roughness can significantly increase the local
surface temperature. However, in this case theemission decay time is relatively short and isdifficult to be measured with the use of
Q-switched laser pulses. In another case, when ∆exceeds δ, the emission decay curves contain slow
components with the emission decay timedependent on the ratio ∆/ δ. In this case, if thesurface relief height is of the order of the laser
radiation penetration depth, the surface
roughness can increase the emission decay timeby tens of percent.
Besides the above-mentioned, the activities
included discussions concerning the project’s
progress and results (via e-mail and on themeetings in Kyiv and Hamamatsu).
The participants of the project actively
collaborated in organizing the 20th Intern. Young
Scientists Conference “Optics & High Technology
Material Science” SPO 2019 held at the Faculty ofPhysics of Taras Shevchenko National Universityof Kyiv 26-29 Oct. 2019: Prof. S. Zelensky was themember of the Program Committee, Prof. T. Aoki
was the member of the International Committee,
Prof. T. Aoki and Dr. K. Zelenska were themembers of the Organizing Committee ofSPO-2019.
［３］Results(３－１) Research results
For excitation of thermal emission, aQ-switched YAG:Nd3+ laser was used similarly tothe previous project No.2006. Thermal emission
pulses were detected by a photomultiplier tube
H1949-51 (rise time 1.3 ns) through aband-selective glass filter SS-4 (transmittance1window 400–500 nm) with digital oscilloscopes
TDS-2022B(bandwidth200 MHz)
andMSO-4104 (1 GHz).
For laser-induced thermal emission of roughsurfaces, the computer simulations predict size
dependence of emission decay kinetics in themicrometer and sub-micrometer range of surface
roughness. That is why the experiments weremade with the samples which have surface
structural elements of various sizes and shapes.
The following carbon samples were used: (1)carbon electrode for spectroscopy applications; (2)porous carbon (charcoal) samples made of walnut
shells; (3) oak wood charcoal; (4) pharmaceuticalactivated carbon pellets; (5) dried China ink(carbon black); (6) dried black gouache paint.
Typical examples of SEM images of thestudied surfaces are given in Fig.1. As is seenfrom the figure, the samples are porous and havesophisticated surface structure with a lot of
micron- and submicron-sized structural elements.

AB-hM-h/2

C0r

H

Rr

1

Dr

2zFig.2. Surface layer element.

The procedure of calculations was describedin the previous report No.2006 (2018–2019).
In the experiments, the photomultiplier’s
photocurrent is proportional to the thermalradiation power which can be calculated as=∬ εd ,where σ is the area of the sample surface whichemits thermal radiation.
The total emitted energy of thermalradiation (within the interval ∆λ) was calculated:=d .

Typical examples of calculated surface distributionof maximal laser-induced temperature and exitance aregiven in Fig.3 for h=200 nm.

Tmax, K

3000
12500
2Fig.1. Typical SEM images of the sample
surfaces: (1) carbon electrode; (2) walnut charcoal;(3) oak wood charcoal; (4) activated carbon pellet;(5) China ink; (6) black gouache paint.
For simulations of laser heating of roughsurface layers, a micron-sized cylindrical elementwith a truncated-cone peak was used (see Fig.2).
Characters A, B, C, D, and M in Fig.2 denote
points on the irradiated surface where the surface
temperature and exitance are analyzed.

εmax, W/m2

2000
800600400

120020050100

150200250300r, nm

Fig.3. Maximal surface temperature Tmax and exitance εmax
as functions of coordinate r for ∆/δ=7.5 (1) and 0.5 (2).
As is seen from Fig.3, under the pulsed laserirradiation, peaks and valleys on rough surfaces can beheated to significantly different temperatures. As a result,2

complicated functions of the surface roughness andrequire thorough analysis.

Tmax, K

the integral characteristics of surface emission areA, B

3000

DCFig.4 demonstrates the effect of a peak height, h, on
2000

from peaks and valleys are denoted E1 and E2, respectively.
As is seen from Fig.4, E1 considerably exceeds E2 in awide range of h for both ∆>δ and ∆<δ. Hence we canTmax, K

the emitted pulse energy of laser-induced thermal radiationof rough surfaces. Thermal radiation energies emitted

(a)

2500

B

3000

A2500

(b)

D

2000
C

1500

0 100 200 300 400 500 600 700

conclude that the observed laser-induced thermal radiationh, nm

is mainly emitted by peaks of the surface relief.
Fig.5. Maximal temperature in points A, B, C, D as afunction of peak height h for ∆/δ=7.5 (a) and ∆/δ=0.5 (b).

4

Typical examples of calculated time history of10

exitance are given in Fig.6. Here the square laser pulse is3located between t=−10 ns and t=10 ns.
210.10100 200 300 400 500 600 700h, nm

Fig.4. Calculated ratios of emitted energy for peaks, E1,and valleys, E2, as functions of peak height: E2/(E1+E2)(curve 1), E1/(E1+E2) (2), and E1/E2 (3,4) for ∆/δ=0.5 (4)and ∆/δ=7.5 (curves 1–3).

1000
100
1010.10.01

ε, W/m2

1A (a)BCD0100 200 300 400 500 600

1000
100
1010.10.01

ε, W/m2

Ratio of energies

100(b)
D0CAB

20 t, ns 4060Fig.5 demonstrates the effect of h on the maximal
surface temperature Tmax under the laser irradiation.
Notation A–D in Fig.5 corresponds to Fig.2. As is seenfrom Fig.5(a), in case of ∆>δ the maximal temperature on
the top of the peak (points A and B) is almost independentof h; hence, the excess of E1 over E2 (curve 3 in Fig.4) iscaused by the decrease of temperature in points C and D(Fig.5(a)). Concerning the case of ∆<δ, as is seen fromFig.5(b), Tmax increases by approximately 250 K when h isclose to the value of δ=200 nm; hence, E1/E2 in Fig.4,curve 4, is much larger than unity. (Here it should be notedthat the surface exitance strongly depends on temperature:for example, ε increases by a factor of 2 when the surface
temperature increases from 3000 K to 3200 K.)

Fig.6. Calculated time history of ε in points A, B, C, D for∆/δ=7.5 (a) and 0.5 (b). h=300 nm.
As is seen from Fig.6(b), on condition ∆<δ the decay
time of laser-induced thermal emission is of the order often nanoseconds. As far as in the experiments the laser
generates bell-shaped pulses, the calculated decay time oflaser-induced thermal emission (Fig.6(b)) is of the order oftrailing edge of the laser pulse; hence, we do not analyze
the decay of thermal emission in case of ∆<δ.
In case of ∆>δ, as is seen from Fig.6(a), the decay
curves are much longer, and approximation of thesecurves with a double-exponent function
f=A1×exp(−t/τ1)+A2×exp(−t/τ2)
gives the values of time constants τ1 and τ2 of the order of310−8 s and 10−7 s respectively.

120

In this project, various carbon samples made of100

materials with different thermal characteristics were used.
estimated approximately as κ≈50 W/m K, whereas
charcoals typically have κ≈0.084 W/m K. However,despite the mentioned differences, all of the samples
demonstrated similar kinetics of decay of laser-induced

τ2, nsFor example, thermal conductivity of carbon rods was

28060

140
2000100 200 300 400 500 600 700

thermal emission. The measured values of τ2 of differenth, nm

carbon samples are given in Table 1. As is seen from theFig.7. Calculated values of thermal emission decay time τ2
as functions of h for ∆/δ=5 (curve 1) and 7.5 (2).

table, surface layers of different carbon materials haveclose values of τ2.

As is seen from Fig.7, if h is comparable with ∆, theTable 1. Thermal emission decay time τ2 of carbon
materials under irradiation by a sequence of ten laser
pulses.
τ2, nsMaterialemission decay time τ2 is increased by a factor ofapproximately 1.5 as compared with flat surfaces. This
circumstance should be taken into account in theestimation of material’s thermal diffusivity with the use ofCarbon electrode132±7

procedure described in the previous project No.2006.

Pharmaceutical activated carbon pellet

123±9

For example, for surface layers of carbon rods, theOak wood charcoal

167±12estimates of thermal diffusivity and thermal conductivity

Walnut shell charcoal

118±4should be increased up to the values of 2×10−8 m2/s andDried China ink

122±30.08 W/m K respectively.

Dried black gouache

132±4Concerning the above-given estimate of thermal
conductivity of carbon 0.08 W/m K, the following

As is seen from Fig.4, in a wide range of h,considerations should be taken into account. The

laser-induced thermal emission of peaks on rough surfaces

mentioned estimate is much smaller than typical values

exceeds the emission of valleys; hence, it can beaccepted in the literature. However, it is known that

concluded that the observed kinetics of thermal emission

thermal conductivity of carbon demonstrate significant

reflects the properties of the material which forms thevariations; sometimes thermal conductivity of graphite at

surface relief, and these properties can be different ashigh temperature is extremely small or tends to decrease

compared with the bulk material. That is why, with taking

significantly due to the loosening of graphite structure.

into account the data presented in the table, we conclude

Besides, it is known that thermal characteristics of carbon

that the investigated surface layers of different carbon

materials are strongly dependent on the material’s structurematerials have close values of thermal diffusivity.

and on the type and concentration of defects. With taking

Consider the effect of surface roughness on the value

into account the above mentioned we conclude that theof τ2. The total emitted power Iσ(t) was calculated as aobserved close values of τ2 of different carbon samples

function of time, and the calculated values of τ2 arecan be a consequence of similar thermal properties ofpresented in Fig.7 as functions of h for ∆=333 nm,

rough and structured surface layers which contain a lot of

δ=67 nm (curve 1) and δ=45 nm (curve 2).

structural imperfections and defects.
Concerning the effect of moisture on thekinetics of laser-induced thermal emission decay,4the experiments revealed significant shorteningof the decay time of slow component, as is shownin Fig.8.

1150

τ2, ns

2
100
3
50

00

2

46N

8

1012Fig.8. Laser-induced thermal emission lifetime asa function of laser irradiation dose for dry (1) andwet (2,3) activated carbon pellets for ∆m/m=0.15(2) and 0.95 (3), where m is the dry pellet mass,and ∆m is the mass of water absorbed by thepellet.
Thus we can conclude that the presence ofmoisture in porous carbon surface layer leads tothe significant decrease of laser-induced thermalemission (as is shown in the previous projectNo.2006), and also to the decrease of the emission
decay time (as is shown in Fig.8).
［４］Achievements (List of Publications)( １ ） S. Zelensky, T. Aoki, Decay Kinetics ofThermal Radiation Emitted by Surface Layers ofCarbon Materials under Pulsed Laser Excitation//Opt. Spectr. 127(5) 931–937 (2019). doi:
10.1134/S0030400X19110298.
（ ２ ） V. Karpovych, O. Tkach, K. Zelenska,
S. Zelensky, T. Aoki, Laser-Induced ThermalEmission of Rough Carbon Surfaces // Journal of
Laser Applications 32, 012010 (2020);
https://doi.org/10.2351/1.5131189.
（ 3 ） V. Karpovych, O. Tkach, K. Zelenska,
S. Zelensky, T. Aoki, Decay of Laser-Induced
Thermal Emission of Rough Carbon Surfaces //The 20th International Young Scientists
Conference Optics and High Technology Material
Science SPO-2019, Kyiv, Ukraine. p.169.

5Travelling Report（Mention each travel by CRP budget.）
Name:

Serge Zelensky

Affiliation:

Taras Shevchenko National University of Kyiv, Kyiv, Ukraine

Period of time:

02.10.2019 – 25.10.2019

Destination:

Research Institute of Electronics, Shizuoka University

Purpose:Study of kinetics of laser-induced incandescence of rough surfaces ofporous carbon materials containing liquid fillers.
Participation in scientific meetings at Research Institute of Electronics,
Shizuoka University during the period of stay.

Name of receiver:Prof. Toru Aoki

S. Zelensky
13.03.2020

6