Non-thermal E ffect of M icrowave on the Conduction C haracteristics of Na C l Solution

Shaoquan Hou Deli Li (Instructor) Mengxiao Li

Shanghai High School

Non-thermal E ffect of M icrowave on the Conduction C haracteristics of Na C l Solution

Shaoquan Hou Deli Li(Instructor) Mengxiao Li

Shanghai High School

A bstract:
Using the household microwave oven, we studied the effects that microwave has on the conductivity of NaCl solution. We compared the currents of NaCl solution under the microwave irradiation with the one under non-microwave irradiation under the same temperature. To our surprise, the values are different. We consider this is one of the non-thermal effects of microwave on the electrolyte solution. Further experiments indicate that the current change caused by this non-thermal effect is closely related to the concentration and temperature of the solution. Through real-time observations on the current change, we eliminated that the non-thermal effect is caused by the redistribution of the positive and negative ions. Thus the experiment results suggest that the non-thermal effect of microwave is caused by the change of the surrounding environment of the ions. K eywords : Microwave, non-thermal effect of microwave, electrolyte solution

The affects the microwave has on substances or organisms which cannot be explained by thermal effect is called non-thermal effect of microwave [1][2]. In chemical reactions, the non-thermal effect of microwave has been proven to be able to accelerate reaction rate[3]-[6]. Similarly, besides heating the biological tissues, microwave radiation also has other effects on organisms[7]. Scientists come up with various theories to explain the non-thermal effects such as dissipative structure, tunnel effect model and so on[8]. However, the mature explanation to the non-thermal effect of microwave is still not formed. In addition, whether the electric conduction of the electrolyte solution has non-thermal effect of microwave or not is an interesting question. Scientists have already conducted a large amount of studies on the electric conduction of the electrolyte solution. Electrolyte solution has conduction characteristics different from traditional conduction characteristics of metals and semiconductors, of which the main manifestation is its dependency on temperature and electric field. Explanations to these conduction characteristics can be found from the ionic atmosphere theory of Debye-Huckel-Onsager and the theory of ion pair resolution of Onsager and so forth [9]. According to these theories, the heat generated by electrolyte solution through absorbing microwaves can influence the activity of the ion cloud and dissociation rate of the ion pairs, thus enhancing the electrical conductivity of the solution. However, the fact that whether the electric conduction of electrolyte solution has non-thermal effect of microwave or not has not been proven yet through systematic experimental demonstration[10]. In the present article, control experiments are designed to study the effects the separately the currents that flow through the NaCl solution while heating the microwave oven and when the heating is stopped, and compared their current values under the circumstance of the same temperature. Our studies eliminate the factors of chemical reactions, biological structures, etc, and the effects the microwave has on the physical characteristics of the electric conduction of electrolyte solution are studied. -thermal effect of microwave in the electrolyte solution and preliminarily measured the effects temperature has on the non-thermal effect.

a

M icrowave oven Digital multimeter

b
Resistance

Computer

Signal generator

C ur rent detector

Fig. 1.We used an ordinary microwave oven to provide the microwave source. We first study the impact of microwave radiation on the electrical properties of the NaCl solution under DC voltage, this effect was reflected by the value of Numerical Ammeter.

During the experiment, we used an ordinary microwave oven to provide the microwave source. During the experiment, we put the beaker containing NaCl solution into the microwave oven and used two thin conductors to connect the solution with a voltage stabilized power supply and a numerical ampere-meter, forming a loop circuit. The conductors with insulating layers could avoid electric discharge to the wall inside the microwave oven. We chose the tepefaction grade for heating. After switching on the power, the microwave oven radiated a certain amount of microwaves every 20s. We separately measured the values on the numerical ampere-meter during microwave heating and the stable values in the 20s interval without microwave. We consider that the amount of microwave radiated each time is constant. We ignored the influence of the lost heat due to evaporation on temperature. So the temperature of water will increase lineally with the increase of the heating times. Under certain heating times, we can measure the initial temperature and the ultimate temperature so as to calculate corresponding temperatures of each heating point. Finally, we can monitor real-time current changes using AC power supply and the electric current transducer to observe the effects the microwave has on the conduction characteristics of NaCl solution.

FIG. 2. Currents flow through the prepared NaCl solution with and without microwave irradiation under the tempera

Under DC voltage, we could not determine the change of the solution s space charge under microwave irradiation. However, solution s space charge lead to the spread of the space charge current. In order to exclude the influence of the space charge, we designed a device shown in Figure 1 b. We use a signal generator and a resistive voltage divider circuit box to make a bleeder circuit. Signal generator can provide an AC voltage from 0 to 10 volts. Then we connected the generator with the current sensors and the electrodes. Computer displayed the current measured by the sensor signal. The frequency of the AC voltage output is 10Hz, in this case, we could not only avoid the effects of space charge but also had chance to make real-time analysis of the current change caused by the microwave. Figure 2 shows the currents flow through the prepared NaCl solution with and without microwave irradiation under the temperatures from 34 degree to 80 degree. The square-dots black line in the figure 2 represent the currents flowing through the NaCl solution under microwave irradiation of different temperatures. The round-dots red line represent corresponding currents flowing through the NaCl solution of different temperatures without microwave irradiation. The experimental results show that the conductivity of the solution increases with the rising of temperature. This corresponds to the traditional conductive theories of electrolyte. However, our experimental results show that besides temperature, the existence of the microwave also affects the currents flowing through the NaCl solution. The blue line in the figure 2 represents the differences of the currents flowing through the solution with and without microwave irradiation at the same temperature. This difference is not the thermal effect caused by the temperature rise of the water through heating with the microwave. We call it non-thermal effect current. The experiments show that NaCl solution has this effect at the temperatures from 34 degree to 80 degree.

FIG. 3. The differences of the currents flowing through the solution with and without microwave irradiation at the same temperature.

Although the NaCl solution exerts non-thermal effect at the temperatures from 34 degree to 80 degree, the non-thermal effect currents are different. For a further study on the relations between non-thermal effect current and temperature, we measured the relations between the change of non-thermal effect current of NaCl solutions (different concentrations) with the temperature. As shown on figure 3, the concentration of the NaCl solution rises successively from the bottom to the top. sing of the NaCl solution concentration, the non-thermal effect currents increase within the scope of the measuring temperatures. We consider that the fluctuations in the curves are caused by the errors in the readings. The non-thermal effect current increases with the enhanced concentration of the NaCl solution, which indicates that the non-thermal effect is the result of the interaction between microwave and ions in water. Relevant theories indicate that in the NaCl solution, the sodion and the chloridion respectively associate with the hydrone, forming baroque hydrations: Na+(H2O)n (n=4, 5, 6, 14) and Cl-(H2O)n (n=4, 5, 6, 7, 8, 14). If the non-thermal effect current is caused by the microwave effect on the mobility of the ions, the current change is independent on temperature. But our results also show that temperature is related closely with the non-thermal effect current. Within the scope of measurement, when the temperature is lower than 50 degree, the non-thermal effect current is only related with the solution concentration. However when the temperature is higher than 50 degree, the non-thermal effect current increases with the rise of the temperature. The theory of the interaction between the microwave and the hydrated ions seems to be insufficient to explain the temperature dependent effect. However, relevant theories indicate that the structures of the hydrations of Na+(H2O)n (n=4, 5, 6, 14) and Cl-(H2O)n (n=4, 5, 6, 7, 8, 14) are very complicated. In the electrolyte solution conductivity theory of Lee-Wheaton, they are regarded as spheres with structure (Gurney ball). The Gurney ball, which takes ion as its center, has 3 sections. The solvent molecules in the outmost section are not influenced by the central ion electric field, maintaining the state of body connection. In the middle section, polar hydrones are partially oriented by the ion electric field. In the innermost section, the hydrones are directionally aligned to form the solvation layer under the influences of the high-field of the central ions. We think that when the temperature is lower than 50 degree, microwave only influences the solvent molecules in the outmost section, leading to the non-thermal effect current that does not change with the change of temperature. With the temperature rising, the directional

distribution of the hydrones in the middle section is gradually spoiled. The enhancing of the non-thermal effect leads to the increase of the non-thermal effect current along with the rising of the temperature.

FIG. 4. The current sensor measured current that flowing through the solution under the 10HZ AC voltage

In order to illustrate that the non-thermal effect current is caused by the microwave induced changes of the properties of the surrounding solvent molecules, we need to eliminate the current changes caused by the effects the microwave has on the redistribution of ions in the solution. Under DC voltage, the positive pole of the solution assembles more chloridion while the negative pole assembles more sodion. The microwave electric field may change the current values through the redistributions of the positive and negative ions. Afterwards, we changed the DC voltage to AC voltage of 10HZ, and measured the current changes over time using the electric current sensor. Figure 4 shows the relation of changes (measured by the electric current sensor) between the current flowing through the solution and time span in the first second under the 10HZ AC voltage. It can be easily found that the current is still the sinusoidal alternating current. Therefore under the AC voltage, distributions of the positive and negative ions in the solution are maintained basically even. Figure 5 shows the relation of changes (when discontinuously transmit the microwave radiation at low temperature grade) between the current and time. As shown in the figure 5 , the time spans in which the current suddenly increases are those under microwave radiation. Therefore, non-thermal effect current is not caused by changing the distribution of ionic concentrations by the microwave, but is closely related to the interaction of the surrounding solvent environment of the ions.

FIG. 5. The relation of the current changes (when discontinuously transmit the microwave radiation at low temperature grade) between the current and time.

In conclusion, we studied the conduction characteristics of NaCl solution under the microwave irradiation using ordinary microwave oven and electrical measuring instruments. We discovered that under same temperature, the current with microwave irradiation is higher than that without microwave irradiation, which is an embodiment of the microwave non-thermal effect in the conduction characteristics of the electrolyte solution. Further studies show that the non-thermal effect current of the microwave is closely related to temperature. Our experiments have preliminarily revealed that the non-thermal effect current of the microwave is closely related to the interactions between microwave and the surrounding solvent environment of the ions. A more precise study needs the microwave transmitter of which the power is constant and adjustable. We expect to reveal more physical phenomena in future.

T hanks
The authors would like to acknowledge Zhou Tian for his support. To acknowledge East China University of Science and Technology for its apparatus and laboratory support. To acknowledge Shanghai High School for its support.

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