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Preliminary Investigation on Laser Ultrasonic NDE of Nano-meter Materials
Xiaorong Zhang, Changming Gan, Yuying Huang, Dingchang Xian
To cite this version:
Xiaorong Zhang, Changming Gan, Yuying Huang, Dingchang Xian. Preliminary Investigation on Laser Ultrasonic NDE of Nano-meter Materials. Journal de Physique III, EDP Sciences, 1995, 5 (6), pp.783-789. �10.1051/jp3:1995160�. �jpa-00249346�
Classification Physics Abstracts
42.60k 43.25 62.20
Preliminary Investigation on Laser Ultrasonic NDE of Nano-meter Materials
Xiaorong Zhang(I), Changming Gan(I), Yuying Huang(~) and Dingchang Xian(~)
(~) Laboratory of Modern Acoustics and Institute of Acoustics, Nanjing University, Nanjing 210008, China
(~) Synchrotron Radiation Lab., Institute of High Energy Physics, Chinese Academy of Science, Beijing 100039, China
(Received 25 July 1994, revised 31 January1995, accepted 15 March 1995)
Abstract. Some experimental results about laser ultrasonic NDE of nano-meter materials of silver and copper (hereafter nmAg and nmcu) are obtained. A pulsed laser is used as the
ultrasonic source, and a capacitive transducer as the receiver. The phenomenon related to the
particle size effect and the interface effect between particles of nano-mater material is observed.
The response signal resulting from artificial interface layer in nmcu is clearly seen.
1. Introduction
Nano-meter material is an advanced material developed in the mid-eighties. It is composed of super fine particles with nano-meter scale, and prepared by suppressing and sintering tech- niques. Because there are a lot of interfaces within the nano-materials, the volume fraction
occupation of interface is comparable with that of particles. They have particle size effect and
disordering effect of interface. They are referred to have "gaslike" structure. So, the nano- meter material has a number of advantages excelling to the traditional materials. Additionally,
the investigation on nano-meter scale material is situated between the research fields of macro- scopic medium and microscopic atom. Many new phenomena will be discovered from such
research. So, investigate its mechanical properties is very interesting. However, it is difficult to prepare a block of nano-meter material, so it is impossible to make NDE by traditional
piezoelectric ultrasonic method.
It is well known that the laser is a flexible ultrasonic source [1-3) with broad frequency bandwidth. Its beam size and duration of pulse depend on the optic lens system and the duration of the laser pulse itself. Usually, the laser beam can be focused on a spot with
a diameter of10 to 100 pm, and the duration of pulse ranges from 40 ns, 8 ns IQ-switch pulsed laser) to 35 or 40 ps (mode-locking pulsed laser). So the laser ultrasonic technique
can be used to measure the ultrasonic velocity of nano-meter materials with small area and
thin thickness and, therefore, to search the relation between the sound velocity and particle
@ Les Editions de Physique 1995
784 JOURNAL DE PHYSIQUE III N°6
Table I. The parameters for nmAg.
1# 2# 3# 4# 5# 6# 7#
23 20 ]5 28 18 19 27
(nm) 33 40 32 40 40 35 45
(nm) 12 5 4 8 8 8 10
(nm) 25 20 10 12 15 15 30
120 20 120 140 40 40 40
Table II. The technical condition and thickness for nmcu.
1# 2# 3# 4#*
Pressure(Gpa) 1.09 0.60 0.50 0.20
Thickness H (~m) I 18 125 150 300
This sample
prepared bysuppressing and sintcring again.
size. In this paper, the preliminary experimental results observed by pulsed laser-ultrasonic technique are presented. Especially, the unusual phenomena related to the particle size effect of nano-material in nmAg, and the influence of the artificial interface layer in nmcu on the
amplitude of response signal are reported.
2. Experiment
2.1. SAMPLES. A series of nmAg and four nmcu wafers with diameter of 6 mm are used in the experiment. They are made by inert gas coacervation and vacuum in site suppressing
methods and are prepared by the Institute of Solid Physics, Academy of Sciences, China. The parameters of the nmAg with thickness (H) of 0.04 0.14 mm and particle size of 10 40 nm
are shown in Table I, where D, Di, D2, and D3 are respectively the average size, maximum size, minimum size and the most size obtained by TEM. Six photographs for nmAg, No. 1#
-4#, 6# and 7# obtained by TEM are shown in Figure 1, in which the scale of the photograph
for nmAg No. I# is different from that of other's. The ratio of scale between No. 3# and No. I# is 2.5. The samples of nmcu are prepared under different technical conditions ii-e-,
different pressure). The wafers of nmcu have similar thickness H (150 pm < H < 300 pm)
and particle size is about 10 nm before the sample is suppressed and sintered. The technical conditions and thickness of samples of nmcu after suppressed and sintered are shown in Table II. It should be noted that the sample No. 4# of nmcu is prepared by stacking two thinner wafers together, at first, then suppressing and sintering again because the sample of nano-meter
materials can not be prepared too thick so far.
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786 JOURNAL DE PHYSIQUE III N°6
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diameter of 6 cm and thickness of 20 cm is used as the butler when the capacitive transducer is used. And a little of Si oil used as the coupling agent between the sample and the front
surface of the stainless steel block is quantified by micro-liter (pl) sample deliverer.
A slightly focused Laser beam impinges on the surface of sample. The ultrasonic pulse is
generated by thermal mechanism. This pulse signal received by capacitive transducer is am-
plified by an amplifier with band width of 20 MHz and fed into a digitized storage oscilloscope PM3315, then transferred to a computer for further processing. It is important that some of experimental conditions should be controlled carefully when the sample is changed every time and the experiment is carried on. For example, the energy of the laser is adjusted to keep the sample surface from damaging, the laser beam is focused slightly so that the diameter of laser spot is smaller than 6 mm and is larger than the thickness of nano-meter material. Thus the ultrasonic source can be considered as a plane wave source and the waves reflected from the side walls of sample arrive at the receiver much latter than that reflected from the bottom and top surfaces of sample.
The ultrasonic pulse waveforms received by capacitive transducer for the samples of nmAg
and nmcu are shown in Figure 3 and Figure 4, respectively. Figure 3 gives the waveforms for samples No. 5#, 6#, 7#, and 2# of nmAg. Figure 4 gives the waveforms for samples No. 1#, 2#, 3#, and 4# of nmcu. The first peak of each waveform corresponds to the straight arrived
longitudinal stress pulse.
3. Analysis
From Figure 3, we can see that the time of flight t of the first stress pulse for the different sample of nmAg is different. It should be pointed out that the time of flight for each waveform is compared, measured repeatedly at the screen of the digitized storage oscilloscope using the
expansive function of oscilloscope in the experiment. The time resolution is 8 ns. It can be
seen that the time of flight for the sample 7# with larger average particle size is shorter than that for the 5# and 6# with smaller average particle size. It is contrary to the phenomenon observed for traditional material. From Figure 4, two phenomena are observed:
5# 4#
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2#
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Fig. 3. Waveforms of stress pulse of nmAg.
Fig. 4. Waveforms of stress pulse of nmcu.
1) The time of flight t of first stress pulse for the different nmcu is different. For nmcu
samples No. 2# and 3#, the time of flight increases with thickness increasing, but it is not true for nmcu I#. The time of flight for I# is longer than that for the 2#, whereas the thickness of the sample I# is less than that of 2#. On the other hand, Table II shows that the pressure suffered by I#, during the preparing, is larger than that by the 2#. It is reasonable to suppose that the average particle size of the sample I# be smaller than
that of the others and the number of interface in the sample I# be more than that in others. Thus this phenomenon is consistent with that observed for nmAg.
2) From the waveform for nmcu No. 4#, it is surprising to find that the first peak (even the second peak) in the waveform is split into two peaks clearly. As mentioned above, this sample is prepared by stacking two wafers firstly and then by suppressing and sintering again. It can be thought that there is an artificial interface between the two wafers. This
interface causes the first peak to split.
Now we consider a simple model. Supposing a sample with "gaslike" structure have a volume fraction occupation of the interface comparable with that of particles. For this sample, the
Kapitza resistance may be existed at the interface between particles, where the eI§ect of soft mode may take place. So the elastic constants at the interfaces are lower than that within the
particles. It can be thought that the smaller of average particle size, the more the interfaces are within the sample. This fact results in that the velocity of ultrasonic wave for the sample with smaller average particle size is lower than that for the sample with larger average particle size.
Therefore, we can say that the phenomenon of larger average size having shorter time of flight
of the first arrived stress pulse is resulted from the particle size effect and the interface-effect of
nano-meter material. The phenomenon of peak split for nmcu No. 4# is due to the artificial
interface layer within the sample.
Additionally, in the above experiments, a ruby laser with pulse duration time 40 ns was used
as the ultrasonic source. Under this condition we can only get the information from the relative
788 JOURNAL DE PHYSIQUE III N°6
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lt ([i)
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I I
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0 20 40 60 80 100
tiiue us
Fig. 5. The waveforms for nmAg received by PVDF transducer a), and for resonance of PVDF
film excited by laser pulse b).
difference of the flight time of the first straight arrived pulses. In order to measure the absolute
flight time another experiment was carried out for nmAg No. 7#. In the experiment a Nd:YAG laser with duration time 8 ns and optic wavelength of 532 nm was used as the exciting source
and a PVDF transducer as the receiver (Fig. 2c). The ultrasonic pulse waveforms received by the PVDF transducer and the resonance response of the PVDF film itself are both recorded by digitized oscilloscope HP54510B with resolution of i ns shown in Figure 5. Figure 5a shows the echo pattern in the samples of nmAg. It is found that the time difference between two echoes
is 24.8 ns. This means that the longitudinal wave velocity for nmAg No. 7# is 3225m/s, which
is lower than that for crystal Ag (3650m Is). Figure 5b shows the resonance response of PVDF film excited by laser. The small ripples in the waveform shown in Figure 5a are corresponding
to the resonance peaks of PVDF film.
4. Conclusion
From the experimental observations, some conclusions can be drawn:
1) The laser ultrasonic technique is very useful in NDE of small and thin samples, such as nano-meter materials.
2) The velocity of longitudinal wave for nmAg with larger average particle size is faster than that with smaller average particle size. This phenomenon is related to particle size effect and interface effect between particles of nano-meter material.
3) The split-peak (or the split-blip) of laser ultrasonic pulse waveform observed for composed nmcu No. 4# is the response to the influence of the artificial interface layer in the sample.
The future work of this research will involve the ultrashort pulsed laser as the ultrasonic
source in the experiment. Thus, more information will be obtained.
Acknowledgments
This work is supported by the National Natural Science Foundation of China No. 19134040.
References
(1] White R-M-, Generation of elastic waves by transient surface heating, J. Appt. Phys. 34 (1963)
3559.
(2] Hutchin D.A., Ultrasonic generation by pulsed laser, in "Physical Acoustics", Vol. XVIII, Chapter 2, W.P. Mason et al. Eds. (Academic Press, 1988).
(3] Zhaug X-R- and Gan C-M-, Investigation of surface defect by laser ultrasonic surface acoustic
wave spectroscope, Appt. Laser China 7 (1987) 249-250.
(4] Zhang X-R- and Gan C-M-, Epicenter waveform of ultrasonic pulse generated by pulsed laser,
Proc. of1j1CA (Beijing, 1992) C9-4-5.
