Ultrasonic irradiation effect in the quenched benzene

Journal of Non-Crystalline Solids 357 (2011) 3080–3083

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Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l

Ultrasonic irradiation effect in the quenched benzene Hiroshi Abe ⁎, Toru Sakurai, Haruyo Yoshizaki Department of Materials Science and Engineering, National Defense Academy, 1-10-20 Hashirimizu, Yokosuka 239-8686, Japan

a r t i c l e

i n f o

Article history: Received 30 March 2011 Received in revised form 25 April 2011 Available online 27 May 2011 Keywords: Ultrasonic irradiation; Incubation time; Kinetics

a b s t r a c t On slow cooling, a precursor phenomenon in supercooled benzene was probed by longitudinal absorption. On quenching, in-situ observation of ultrasonic measurements was carried out at the fixed temperature. Sequence of the transmitted waves was multiple scattered in quenched benzene. The dynamic ultrasound scattering is sensitive to the local strain and dynamic inhomogeneous fluctuations. The quenched benzene shows the maximum value of longitudinal absorption at incubation time, tinc. Crystal domain growth/coarsening is promoted by the ultrasonic irradiation at tinc b t. In addition, tinc depends on the quenching temperature. Ultrasonic irradiation and quenching effects dominate the extraordinal nucleation and growth process of benzene in spite of simple and non-polar molecular liquid. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Benzene is known to be a prototype material with π electron. Nonpolarity and non-molecular association in benzene are well characterized by its perfectly symmetric molecular structure. The liquid structure of benzene was investigated by X-ray [1] and neutron [2] diffraction methods at room temperature. Also, the crystal structure of benzene was determined by X-ray [3] and neutron [4] diffraction methods below melting point, Tm (=5.4 °C). The feature of molecular aggregations in liquid and molecular orientational order in crystal is that the twodimensional circular π electron prefers not to form the parallel arrangement in liquid and to avoid superposition of π electron in solid. π −π repulsive interactions are not ignored both in liquid and solid. At the viewpoint from the amorphization, the deposition method on cold-metal substrates was performed to obtain the thin benzene film in an amorphous state [5]. This method can realize the ideal rapid cooling far from an equilibrium state. Thus, the continuous solidification from the liquid state occurs without crystallization. It was found that films deposited below 30 K were amorphous state by Raman spectroscopy and X-ray diffraction. Moreover, the crystallization temperature of films (TcF) was found to be 60 K for amorphous benzene films. In other films of simple organic molecules, TcF of amorphous naphthalene [6] is 105 K and that of amorphous anthracene [7] is 200 K. In a bulk state, the molecular reorientation of benzene molecules was observed at 90 K by the NMR experiments [8]. In contrast, glass transition temperature (Tg) of the bulk benzene was estimated to be around 130 K [9]. Relating to amorphous, it is reported that the complex relaxation process in the supercooled liquid of benzene is detected by time-resolved optical Kerr effect

⁎ Corresponding author. E-mail address: [email protected] (H. Abe). 0022-3093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2011.04.015

experiments [10]. The fast vibration and the slow relaxation are coupled extensively: vibrational dynamics is involved into the slow collective diffusive process. Ultrasonic irradiation effect was examined in the supercooled and quenched naphthalene [11]. Nucleation and growth process are influenced by ultrasonic irradiation. Particular in the quenched naphthalene, time evolution of longitudinal absorption was obtained by in-situ observation. After incubation time, tinc, crystallization occurred accompanying by increment of the longitudinal absorption. Here, incubation time is the waiting time until crystallization starts. After the ultrasonic irradiation, crystallization was verified by X-ray diffraction method. Moreover, tinc became longer with increasing holding temperature or applied transducer power. The ultrasonic irradiation modifies the nucleation and growth at the non-equilibrium state. In this study, we describe anomalous nucleation and growth process in the quenched benzene. The process at the non-equilibrium state is strongly coupled with external ultrasonic waves. Kinetics in the quenched benzene provides insight of a new type of dynamic collective fluctuations. 2. Experimental High quality liquid benzene for spectroscopy (Wako Pure Chemical Co.) was used in this experiment. Liquid benzene is set into the sample cell, whose diameter is 4 cm and thickness is 1 cm. To reduce ultrasonic absorption and reflection, we shield both transmission sides by poly vinylidene chloride films, whose thickness is 20 μm. A thermocouple (Chromel–Alumel) was set inside the sample cell as a temperature monitor. Temperature data were stored into digital multimeter (VOAC7412, Iwatsu Test Instruments Co.) By the conventional ultrasonic measurement (pulse echo method), the ultrasonic absorption and longitudinal wave velocity were measured. Sine waves are generated by

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a pulse function generator (8116A, Hewlett Packard Co.) The frequency of the pulse function generator was 1.0 MHz. Two piezoelectric transducers (PZT, Ultrasonic Engineering Co.) were used for generating longitudinal waves. The voltage applied to PZT is 10 V. The data collection for ultrasonic waves was performed using a digital storage oscilloscope (VC-6065, Hitachi Co.) The quenching method is that the liquid sample at the room temperature (20 °C) is put directly into the ethanol bath (Heto, Type CB7). The quenched temperature (Tq) is set to the range from 0 down to −20 °C. Temperature stability inside the ethanol bath is within 0.2 °C. Longitudinal absorption is given by [12], L = 20 log10

ξ0 ðZ + Z0 Þ2 −20 log10 ; 4ZZ0 ξ

ð1Þ

where ξ0 and ξ are amplitude of the ultrasonic wave without the sample (ethanol only) and with the sample, respectively. The second term of Eq. (1) reveals the correction of twice reflections on the organic films. Z (=ρv) and Z0 (=ρ0C) are acoustic impedance of the sample and ethanol, respectively. C is the velocity of liquid ethanol and v is the velocity of the sample. ρ and ρ0 are the densities of benzene and ethanol, whose data are listed in the handbook [13]. In previous study [14], the longitudinal absorption of liquid benzene was obtained by ultrasonic measurements. 3. Results Temperature dependence of longitudinal absorption on slow cooling and heating are shown in Fig. 1a. Here, the cooling rate is 0.08 (°C/min) and the heating one is 0.12 (°C/min). By monitoring abrupt increasing and decreasing of the longitudinal absorption, we can determine the phase transition temperatures. Even on the slow cooling, the crystallization of benzene occurred at −5.0 °C (Tc), which is below Tm. At Tc b T b Tm on cooling, supercooled liquid of benzene exists obviously. Close to Tc, the longitudinal absorption on slow cooling, LSC(T), increased a little within experimental errors. At least, longitudinal absorption is sensitive enough to detect the precursor phenomenon of solidification. In contrast to hysteresis of the longitudinal absorption, longitudinal wave velocity was almost constant both on the slow cooling and heating process (Fig. 1b). In addition to the slow thermal treatments, a quenching experiment is required to examine disorder at the non-equilibrium state. The quenched benzene was held at the fixed temperature. Fig. 2 displays the time dependence of the longitudinal absorption in the quenched benzene at 0 °C. We started to count time when the sample temperature achieved to the holding temperature (Tq). The peculiar features in quenching are summarized as follows; (i) pulse echo of ultrasonic waves becomes quite unstable in the quenched benzene, (ii) after quenching, the longitudinal absorption in the quenched benzene, LQ(Tq, t), is larger than LSC(T) and (iii) at all Tq, the longitudinal absorption has the maximum value at the incubation time, tinc. tinc depends on quenching temperature as shown in Fig. 3. It is figured out that longer tinc was observed with increasing the holding temperature. A picture of incubation time and nucleation process is interpreted in the classic nucleation model. We assume a nucleation barrier, ΔE(T), which is regarded as activation energy on the crystallization process. This is given approximately by [15,16], 2

ΔEðT Þ = α + βT + γT :

Fig. 1. Temperature dependence of (a) longitudinal absorption and (b) longitudinal wave velocity both on slow cooling and heating. Blue and red closed circles are corresponding to the points for cooling and heating process, respectively.

where kB is Boltzmann constant. As the result, α, β and γ in Eq. (2) are fitted by the least square method. The solid line in Fig. 3 shows the calculated tinc in this way. ΔE(T) contributes to nucleation process, that is, the waiting time until macroscopic transformation occurs. Finally, we show the elastic anomaly in the quenched benzene. Only in the quenched benzene, time dependence of longitudinal wave velocity, v, was observed, although v as a function of temperature was almost constant in the slow cooled benzene (Fig. 1b). Fig. 4a reveals v as a

ð2Þ

In addition, the nucleation probability, p, is proportional to the Boltzmann factor. Thus, tinc is calculated taking into account ergodic theorem as follows, tinc ∝1 = p∝expfΔEðT Þ = kB T g

ð3Þ

Fig. 2. Time development of longitudinal absorption in the quenched benzene, LQ, at 0 °C. LQ has the maximum value at the peculiar time, tinc.

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Fig. 3. Incubation time, tinc, as a function of quenching temperature. The waiting time is determined by the maximum values of longitudinal absorption (Fig. 2). tinc diverges close to Tm. The solid line shows the calculated incubation time regarded as the thermal activated process.

function of time at −20 °C. Indeed, v increased monotonically at the early stage of time region. At the late stage, longitudinal wave velocity remained constant. We define tv as the crossover time on the v plot. For a comparison, LQ(Tq, t) at −20 °C is shown in Fig. 4b. The discrepancy between tinc and tv still remains unclear. v slope in time varies extensively on Tq. Roughly, v at the early stage has a tendency of ta dependence, where a is constant parameter. By analyzing time dependence of v using the least square fit, a at Tq is obtained as shown in Fig. 5. Proportionally to quenching depth, a becomes larger. 4. Discussion In a conventional liquid circumstance, benzene as a simple molecule has no specific aggregation. In time resolved experiments at the fixed temperature, hidden information about dynamic fluctuations is extracted. In addition, ultrasonic irradiation at the non-equilibrium state is a useful technique to detect local diffusion, inhomogeneous fluctuations and other collective motions on the random media. Three anomalies relating to ultrasonic assisted nucleation were observed in the quenched benzene. The quenching feature (i) of random pulse echo suggests dynamic ‘embryo’ having some lifetime that appears inside the supercooled liquids. The ‘embryo’ possesses the molecular orientational order, which is similar to the crystal one. It

Fig. 4. Time dependence of (a) longitudinal wave velocity, v, and (b) longitudinal absorption, LQ, at − 20 °C on quenching process.

Fig. 5. The exponent, a, of time at the early stage in longitudinal wave velocity on quenching temperature, Tq.

is emphasized that the ‘embryo’ is different from the previously described cluster (weak molecular aggregation). The ‘embryo’ is randomly appeared or disappeared by the ultrasonic irradiation. The phenomenon is equivalent to acoustic speckle generated by ultrasonic waves traveling through random media [17]. The acoustic speckle is based on the idea that some propagating wave interferes with others through multiple scattering paths. Dynamics of inhomogeneous materials was evaluated by theoretical approach using diffusing acoustic wave spectroscopy. The feature (ii) provided by LQ(Tq, t) N LSC(T) means that dynamic ‘embryo’ provides larger influence to acoustic attenuation. If a distinct boundary is formed in the dynamic ‘embryo’, ultrasonic waves are scattered by the boundary. Compared with the cluster on slow cooling, longer lifetime of the dynamic ‘embryo’ also contributes to the additional longitudinal absorption. Hence, in quenching, external longitudinal waves (compressional waves) affect on local dynamic fluctuation differently and extensively. Here, we focus on the feature (iii) described by the peak of LQ(Tq, t). Due to externally injected vibrational energy, the dynamic ‘embryo’ might occur at the early stage (t b tinc). The ‘embryo’ is generated selectively at the high-density circumstance because of molecular packing efficiency. The increasing LQ(Tq, t) close to tinc implies that population of the ‘embryo’ becomes larger. Once the ‘embryo’ exceeds the critical size, a frozen nucleus occurs at around tinc. In the classic nucleation model, it is assumed that the crystal nucleus never disappears. At the late stage (tinc b t), the decreasing LQ(Tq, t) is caused by decreasing ‘embryo’ and growth/coarsening of crystal domain. A fully-grown crystal domain has small specific surface area of the domain. Less surface area leads to less ultrasonic scattering. Consequently, the maximum value of LQ(Tq, t) is explained by two regimes; ‘embryo’ strongly coupled with ultrasonic waves at the early stage and domain growth/coarsening promoted by ultrasonic irradiation at the late stage. Further, the peak on the LQ(Tq, t) in the quenched benzene is emphasized with comparing normal behavior in the quenched naphthalene [11]. LQ(Tq, t) in the quenched naphthalene became constant at the late stage. Thus, fine crystal particles in naphthalene cannot coarsen each other once crystallization occurred. On the other hand, longitudinal wave velocity provides information about the crystal domain growth accompanying defects. In general, sound velocity is connected with elastic constant. We confirm that, in the quenched sample, rapid growth/coarsening at the late stage is promoted by ultrasonic irradiations. We suppose that strain induced hardening contributes to increase of the longitudinal wave velocity, which is proportional to quenching depth, ΔTq. At the same time, we notice that, as a probe, the ultrasonic absorption is effective

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to mesoscopic flucutations such as nucleration process, and the sound velocity is sensitive to an eleastic anomarly on macroscopic scale. 5. Conclusions We have observed unexpected quenching effects in benzene. A significant finding is that the maximum value of longitudinal absorption appeared only in the quenched benzene. The anomaly is originated from dynamic heterogeneity as random media at the early stage and crystal domain growth/coarsening at the late stage. The incubation time until crystallization at the fixed temperature was observed in the quenched benzene. Since quenched temperature also governs time scale of the incubation time, the incubation time can explain the local fluctuation expressed by the classic nucleation model as thermally activated nucleation process. The above phenomena are one of examples that new type of fluctuations is enhanced by a strong coupling with external ultrasound fields even in simple molecular system. Acknowledgment We appreciate helpful discussions with Professor H. Matsumoto of National Defense Academy.

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