「様式３」Project No.： 2006

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., Engineer (TarasShevchenko National University of Kyiv,Ukraine)
Valeria Karpovych, MSc student (TarasShevchenko National University of Kyiv,
Ukraine).
［２］Research Progress
According to the research plan of the project,
the following scientific activities were performedat Taras Shevchenko National University of Kyiv
and at Research Institute of Electronics,
Shizuoka University.
- for pulsed laser heating of structured surfaces,computer simulations of thermal emission decay
were performed for different objects (flat andrough surfaces, undersurface cavities, andsurface-located particles); the simulations wereintended to clarify the role of surface structure in
the formation of LII decay curves; the simulations
revealed an unexpected result that LII decay
curves demonstrate “slow” components which canbe observed even on flat surfaces (withoutroughness of structural elements); thiscircumstance led to the changes of accents offurther activities; as a consequence, thetheoretical and experimental investigations of LII
decay curves of flat and rough surfaces wereadditionally performed;- a procedure is developed for preparation ofporous carbon samples by pyrolysis ofbio-materials;- for LII decay measurements with the use of

Hamamatsu Photonics H1949-51 PMT, anexperimental setup was assembled and tested;
the setup was installed at TSNUK inprof. Zelensky’s lab with YAG:Nd laser;- with the prepared porous carbon samples,
measurements of LII integral signals and of LII
decay curves were performed under irradiation bya sequence of laser pulses, including themeasurements with dry and wet samples.
Besides the above-mentioned, the activities
included discussions concerning the project’s
progress and results (via e-mail and on meetingsin Kyiv and Hamamatsu).
The participants of the project actively
collaborated in organizing the Nineteenth Intern.
Young Scientists Conference “Optics & HighTechnology Material Science” SPO 2018 held at
the Faculty of Physics of Taras Shevchenko
National University of Kyiv 25-28 Oct. 2018. Prof.
S. Zelensky was the member of the Program
Committee, Prof. T. Aoki was the member of theInternational Committee. Prof. T. Aoki, Dr. K.
Zelenska, and V. Karpovych were the members ofthe Organizing Committee of SPO-2018.
［３］Results(３－１) Research results
The project is related to the investigations ofphysical mechanisms of formation of transient
thermal emission of light-absorbing materials
under powerful pulsed laser irradiation. Under
the nanosecond-timescale laser irradiation, thinsurface layers of such materials can be heated toincandescent temperatures, hence thermalemission is observed. This thermal emission(laser-induced incandescence, LII) is typically ashort pulse of white light with a specific pulse
shape. Consider the decay behavior of LII ofcarbon materials under irradiation bynanosecond-scale laser pulses.
For excitation of LII of carbon surface layers,a Q-switched YAG:Nd laser was used(wavelength 1064 nm, pulse length 20 ns). The

following double-exponent function was used

I LII = I1e −t τ1 + I 2 e −t τ21.0
A
5, 40

1ILII, a.u.

laser was operated in a single-pulse regime, andits intensity was adjusted for carbon samples
within the margins 15–30 MW/cm2.
The samples for LII measurements weremade of carbon electrodes intended forspectroscopy applications. Two kinds of samples
were prepared (hereinafter referred to as A and Bsamples). For A-samples, the carbon electrodesurface was ground with an abrasive tool andthen polished on a flat surface covered with asheet of paper until high luster is observed
visually. For B-samples, electrode rods werebroken into parts, and the cleaved surfaces wereused without further treatments. SEM images ofthe surfaces of A-samples and B-samples are
given in Fig.1-A and B respectively.

0.5B
150.0

400

100200300t, nsFig.2. Typical oscillograms of LII of polished (A)and cleavage (B) surfaces. Numbers near thecurves represent the number N of laser pulse in
the sequence.
LII decay time τ1 and τ2 as functions ofnumber N of laser pulses in the sequence are
given in Fig.3.

τ2

τ1, τ2, ns100(a)τ110110N100

5I1/ I24Fig.1. SEM images of polished (A) and cleavage(B) surfaces of carbon samples.

3(b)
2
1

Typical oscillograms of LII of carbon surfaces Aand B are given in Fig.2. As is seen from Fig.2,
under irradiation by a sequence of laser pulses,
LII oscillograms are changing with the increase
laser irradiation dose.
For approximation of LII decay curves, the0110N100Fig. 3 Fitting parameters of LII decay curves asfunctions of laser irradiation dose. Open circles –B-surfaces, filled triangles – A-surfaces.

1ILII, a.u.

For surface layers of carbon materials, under
excitation by a Q-switched YAG-Nd laser, it isrevealed that the decay curves of LII can be fit bya sum of two exponential components with thedecay times of the order of 10-8 and 10-7 s. Under
irradiation by a sequence of laser pulses, theobserved transformations of the emission decay
curves can be interpreted as redistribution ofintensityoftheabove-mentioned
twocomponents.

(for estimation of thermal diffusivity of thinsurface layers at incandescent temperatures).

0.1

107

2.35

Laser heating of surface layers oflight-absorbing materials can be described withthe following classical equation of heat conduction

∂T
∂t ,

and the equation of transport of laser power:dF F = −α dz ,where κ is the thermal conductivity, α is theabsorption coefficient at the laser wavelength; F
is the intensity of laser radiation; cp and ρ are
specific heat and density of the material.
Surface luminosity within the narrow spectralinterval ∆λ around the wavelength λLII can becalculated with Planck’s blackbody emission law
as follows

2πhc 2 λ −LII5 ∆λε=exp ( hc λ LII kT ) − 1
For the analysis, an important parameter isthe length of thermal diffusion at the timeinterval equal to the laser pulse length δ=(κτi/cp)1/2
and the laser absorption depth ∆=1/α.
Typical calculated oscillograms of LII aregiven in Fig.4.
Calculated values of LII decay time asfunctions of the ratio of laser absorption lengthand thermal diffusion length are given in Fig.5. Itshould be noted, the calculated values are close tothe measured ones (Fig.3).
The main conclusion is that the value of LII
decay time is determined by the ratio ∆/δ, but notby the value of thermal diffusivity of the material.
The results of computer simulations show that
LII decay time can be used for sensing thethermal characteristics of the surface’s material

13.5

0100

200t, ns300

Fig. 4 Calculated oscillograms of LII excited bysquare laser pulses. Numbers near the curves
represent the values of ∆/δ.

120100τ280

τ1, τ2, nsdiv ( κ gradT ) + αF = c p0.01

6040

τ1200024∆/δ6810Fig. 5 Calculated values of LII decay time asfunctions of ∆/δ for square (circles) and Gauss(triangles) laser pulses.
For measurements of LII of porous carbon
surfaces, the samples were made of natural
bio-materials (nut shells) by pyrolysis. Besides,
samples of pharmaceutical activated carbon werealso used.
It is revealed that LII of porous carbon
materials is sensitive to the content of moisture inthe material. For example, the parameters of LIIof porous carbon are changed significantly afteraddition of water (see Fig.6. Arrow indicates theaddition of 5 drops of water to the sample).

100

Physical processes involved in laser heating ofwet porous surface layers are complicated,
including phase transformations of both carbonand water, with possible mechanical damage ofthe surface’s structure. The observed sensitivity of
LII to the presence of moisture in the pores ofcarbon samples requires theoretical explanation.

τ2101

I20.1
0.01
02

4681012NFig.6. Parameters of LII of a tablet ofpharmaceutical activated carbon as functions oflaser irradiation dose.
It is also revealed, that LII lifetime becomessensitive to the presence of moisture in thesample at larger values of laser irradiation dose(see Fig.7).

LII decay time, ns120

dry sample

10080
6040

wet sample

2000

1020304050

Number of laser pulses

Fig.7. LII lifetime τ2 of a tablet of pharmaceuticalactivated carbon as a function of laser irradiation
dose.
Besides, LII demonstrates different evolution
under irradiation by a sequence of laser pulses fordry and wet samples of porous carbon. (see Fig.8).

LII intensity, a.u.

1.00.5dry sample

0.00

wet sample

50 100 150
0 50 100 150
Number of laser pulses

Fig.8. LII intensity as functions of number of laser
irradiation pulses.

(３－２) Ripple effects and further developments
The project was based on two suppositions:(1) the intensity of LII and the shape of LII pulses
depend on the shape and size of pores and surface
structure elements, and (2) LII is sensitive to thecontent of pores.
Both of these suppositions were confirmed during
the implementation of the project’s plan. However,it was also revealed that the decay stage of LII
pulse is complicated and contains at least twocomponents with significantly different lifetimes(Fig.3). This finding was important for both flatand structured surface layers (including porouslayers), hence the investigations of LII of porous
materials became more complex, and d additional
investigations of flat surfaces were performed. As
a result, the effect of pore content on the decay of
LII is principally confirmed (Figs. 6–8) butrequires further investigations.
［４］Achievements (List of Publications)（１）S. Zelensky, T. Aoki, Decay of Laser-Induced
Incandescence of Surface Layers of Carbon
Materials// Prepared for publication（２）O. Tkach, V. Karpovych, K. Zelenska,
S. Zelensky, T. Aoki, Kinetics of Laser-Induced
Incandescence of Rough Surfaces // Prepared forpublication（3）V. Karpovych, K. Zelenska, S. Zelensky, S.
Yablochkov, T. Aoki, Thermal Emission of Porous
Carbon Materials under Pulsed Laser Excitation//Proceedings of Young Scientists Conference on
Semiconductor Physics “Lashkaryov’s readings”,2018, Kyiv, Ukraine.
（4）V. Karpovych, K. Zelenska, S. Zelensky, S.
Yablochkov, T. Aoki, Influence of humidity on
laser-induced incandescence of porous carbon
materials// Proceedings of the 19th International
Young Scientists Conference Optics and HighTechnology Material Science SPO, 2018, Kyiv,Ukraine. p.159-160.

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

Serge Zelensky

Affiliation:

Taras Shevchenko National University of Kyiv, Kyiv, Ukraine

Period of time:

18.10.2018 – 16.11.2018

Destination:

Research Institute of Electronics, Shizuoka University

Purpose:Study of kinetics of laser-induced incandescence of flat and rough surfacesof carbon materials irradiated by nanosecond laser pulses. Study ofmorphology of porous carbon samples. Participation in scientific meetings
at Research Institute of Electronics, Shizuoka University during theperiod of stay.

Name of receiver:Prof. Toru Aoki

S.Zelensky
Professor
Taras Shevchenko National University of Kyiv
Kyiv, Ukraine