Chapter 11 Eperimen~dlechniques.. .

EXPERIMENTAL TECHNIQUES

The experimental techniques employed in the present study to determine the

ultrasonic veloc~ty, absorption, density and viscosity are described in this chapter All

the chemicals used in this present work are of AR/BDH quality and are used as such

without further purification.

2.1. ULTRASONIC LIQUID CELL:

The ultrasonic velocity and absorption measurements are done with a ultrasonic

liquid cell and the cell is designed and fabricated in this laboratory.

The cell fabricated in this laboratory is a double walled stainless steel cell with a

central uniform bore of 24.5mm diameter and a depth of 23.89mm with a perfectly

plane, highly polished bottom surface(f1oor). A stainless steel circular d ~ s c of 65mm

diameter with inner hole of 22mm diameter and 5mm length is designed to fix the PEZ

crystal. It has two 8mm Allen bolts and springs to moves up and down the PEZ crystal

in the bore. Through the fine adjustment of the Allen bolts with an Allen key,

parallelism can be achieved between the floor of the cell, which acts as a reflector and

the PEZ crystal. The cell has an outer shell through which water can be circulated. It

has an inlet and outlet pipes through which the water is circulated in order to malntain

the temperature of the solution constant. The cell diagram is shown in figure 2.1. The

temperature of the circulating water is maintained at a fixed temperature by a

thermostatically controlled water bath. The temperature is maintained constant with in

M.01K. A two terminal digital thermometer is designed and fabricated in this

laboratory for measuring the temperature of the measuring liquid and the water

circulated through the outer shell of the cell. Two small provisions are provided in the

cell for this purpose. The thermometer has an accuracy of 10.01K.

Figure 2.1 Ultrasonic Liquid Cell Diagram

Chapter I I ~ ~ e r i m e n ~ d T e c h n i ~ u e s . . .

The ultrasonic velocity and absorption are measured using pulse echo overlap

and pulse echo technique respectively. The experimental setup is shown in figure 2.2 &

figure 2.3 and the echo pattern is shown in figure 2.4.

2.2, ULTRASONIC ABSORPTION MEASUREMENT:

2.2.1. PULSED POWER OSCILLATOR:

A sharp RF electrical pulse with variable pulse duration from 2ps to 20ps is

applied to the crystal with a suitable repetition frequency. The reflected wave returns to

the transducer and a part of its energy is converted into electrical signal. This signal is

amplified and displayed on the oscilloscope. The trace of the oscilloscope is a series of

echoes with decreasing amplitude. The decrease in the amplitude is a measure of

absorption.

The schematic block diagram of the PULSED POWER OSCILLATOR (EL - 550)

is shown in the figure 2.5. It is basically a 100 watts pulsed RF power source designed to

generate low distortion fast rising / falling pulsed RF signals. It generates RF pulses

from 1 to 50 MHz with use of six plug - in coils. The pulse repetition rate of the pulsed

power oscillator is variable from 50 to 2500 Hz. The accurate RF signal output from PPO

is measured using a frequency counter (APLAB - 1116 UNIVERSAL TIME AND

FREQUENCY COUNTER). In present the study, the ultrasonic absorption

measurements are carried out using t h s pulsed power oscillator in the frequency range

3 - 11MHz.

The RF signal is fed to the cell through a matchng network (Pre Amplifier)

designed in this laboratory. The matching net work (Pre Amplifier) circuit has been

incorporated from the ULTRASONIC PULSE ECHO INTERFEROMETER

(SD UI - 003). The diagram of the matching network is shown in the figure 2.6. It is

acting as a transmitter and receiver for the cell through 50 Ohm coaxial cable, i.e., it

transmits the RF signal from the PPO to the cell and receives the output signal from the

Figure 2.2 Experimental setup for Pulsed Power Oscillator and Ultrasonic Time Intervalormeter

Figure 2.3 Experimental setup for MATEC 7700 model Ultrasonic System

Figure 2.4 Echo pattern

Matching Network

Trig in 1 Trig Out

Figure 2.5 Pulsed Power Oscillator Block Diagram

1 2 . 6 ~ AC

Power Supplies

+300 v DC

Prf & Trig Int - Ext

v v V

1 2 . 6 ~ AC

+700v DC

_, Pulse Width . -

+70v DC I

Power Oscillator

Buffer Amplifer

Pufse CW / Pulse Amplifer

Pulsed Power Oscillator

Figure 2.6 Matching Network (Pre - Amplifier) Circuit

Chapter 11 EperimentdTechniques.. .

cell. Then this matching network output is fed to the triggered oscilloscope

[BPL - INDIA DTO 31500 OSCILLOSCOPE]. All the power supplies of the system are

electrically regulated.

The echo heights are fitted to an exponentially decaying ~7ave of the form Ae-'"

and the value of a is evaluated and expressed in nepersjm. The distance between crystal

and floor is measured and the absorption per unit length is calculated and expressed in

nepers/m. It is assumed that the floor reflects the entire sound energy incident upon it.

The accuracy in the measurement of absorption is 3%.

2.2.2. MATEC 7700 MODEL ULTRASONIC SYSTEM:

The experimental setup used for making the ultrasonic absorption measurements

in the frequency 25 - 89 MHz consists of mainly the PEO system, the temperature

controlling system, the cryostat for low temperature measurements and the water bath

for h g h temperature measurements.

The PEO system consists of MATEC model 7700 pulse modulator and receiver

together with model 760 V rf plug-in, model 110 high resolution frequency source,

model 1128 decade divider and dual delay generator, model 2470B attenuation

recorder, model 70 impedance matching network etc. The frequency counter used as

HIL (Inch) model 2722 and the oscilloscope was a 100 MHz one with z-axis input

(HIL model 5022).

The block diagram of the experimental setup used is shown in figure 2.7. The

tunable cw soruce (model 110) has a highly stable internal high frequency oscillator

(12-50 MHz) from whch the required low frequency cw signal for PEO is generated by

selectable frequency division. This signal is available at terminal 2, while the high

frequency is available at terminal 1 for accurate counting by the frequency counter

(model 2722). The dual delay and divider unit (model 122B) has dividers selectable as

10, 100 and 100. The division factor 100 is quite acceptable for most measurements,

Frequency Tunable CW Counter Source ( I 10)

1 CW out -- -

1 2 Trigger Pulse (DirectIDivided - SW2)

3 Intensifying Pulse

I

Dual Delay & Divider ( I 22B)

Divided

Trig.

Attenuation Recorder Out -

(2470B)

Trig. In Video In

Intense. -mG--j, SWI*.

Trig. In

(7700) Receiver Out yt Recert- 100 MHz

Oscilloscope (5022)

Pulse Modulator & video+ CH2 Ext. Trig. Receiver 10-90 MHz Out

1 I

-

Impedance

.+

I ultrasonic I 7

Figure 2.7 Block Diagram of the MATEC 7700 Model

I Liquid Cell

Ultrasonic System

1 Digital Thermometer I

Chapter I I 'E,perimenid%chniques. . .

which means that the next rf pulse is sent to the sample only after a time interval

corresponding to 100 number of echoes. The terminal 2 of this unit gives trigger pulses

for the CIIO. The CRO is always operated in the external sweep trlgger mode. By using

switch SW2 in 1228 the trigger at terminal 2 can be made direct trigger or dilrided

trigger for observing the overlapped echoes or the full echo pattern respectively. The

dual delay generators in 122B can be adjusted for delay and width of the intensifying

pulses for selecting the two echoes of interest for overlap. These pulses are available at

terminal 3 and are connected to the z-input of the CRO through the selector switch

SW1.

The divided trigger from 1228 goes to the pulse modulator and receiver unit

(model 7700 with rf-plug-in in model 760 V). This is the most important unit in the

setup. An rf pulse packet of peak power 1 kW is obtained at the output when the unitis

triggered at the input. The rf frequency of the triggered power oscillator can be adjusted

in the range of 10 to 90 MHz. the width and amplitude of the pulse are adjustable. For

good pulse shape the unit is usually operated at full amplitude and the amplitude

reduction is achieved by using an rf attenuator (Alan attenuator) at the output as shown

in the block diagram. The unit has a sensitive tunable superhetrodyne receiver with a

maximum gain of 100 dB for amplifying the echoes. The amplified echoes are available

through the receiver out terminal whch is connected to the CRO channel 1 input. The

amplified echoes are also detected and the detected output (envelope of the echoes) is

available at the video out terminal which is connected to the attenuation recorder and

CRO channel 2 input. For optimum signal to noise ratio in the amplification of weak

echoes, an impedance matching network (model 70) is connected before the PEZ

transducer.

The attenuation can be most conveniently measured by an automatic procedure

for whch commercial equipment is available (Matec. hc . (USA) Model 2470). In the

automatic system, two gates with variable delay are set on the two echoes of interest to

Chapter I I EperirnentdTechniques. . .

sample them. The amplitude of the first echo 1s held constant by AVC circuitry, and the

amplitude of the second echo is sampled at its peak. A calibrated logarithmic amplifier

converts the sampled amplitude to decibels relative to the constant amplitude of the

first echo. The decibel level is recorded on a built-in strip chart that has several

calibrated scales and a variable baseline, so that small changes in attenuation can be

measured at various total loss levels. In this equipment the attenuation can also be

noted from panel mater calibrated in dB. The swritch SWl can be toggled to pole 2 for

connecting the intensifying pulse output from the attenuation recorder to the Z-input of

the CRO. By this way the two echoes of interest can be selected for attenuation

measurement. One important advantage of the PEO technique is the capability to

measure the velocity and attenuation at the same time. In present the study this

MATEC model 7700 ultrasonic system is used only for measuring attenuation in pulse

echo mode in the frequency range 25 - 90 MHz.

2.3. MEASUREMENT OF ULTRASONIC VELOCITY:

The ultrasonic velocity is measured using the pulse echo overlap technique. The

pulse echo overlap (PEO) method is very versatile and hghly accurate technique for

measuring the velocity of the ultrasonic waves in materials and structures. A high

absolute accuracy is capable of accurately measuring from any cycle of one echo to the

corresponding cycle of the next echo. The pulse-echo-overlap method is able to handle

diffraction (beam spreading) and phase corrections, properly, so that absolute accuracy

of the PEO method may exceed the accuracy of most of the other methods.

The block diagram of pulse echo overlap measurements with broad pulses is

shown in figure 2.8. The principle of measurement is to make the two signals of interest

overlap on the oscilloscope by driving the X-axis with a frequency whose period is the

travel time between the signals of interest. Then one signal appears on one sweep of the

oscilloscope, and the other signal appears on the next sweep. The X-axis sweep

r

CW - Pulse Pulse

oscillator . 5 Pulser - Limiter

- 'I

Counter Cell

Figure 2.8 Block diagram of the Pulse Echo Overlap method

Chapter 11 E~erimentdlec/migues . . .

frequency is supplied by a Cn7 oscillator. For jitter-free overlap, the signals of interest

must be synchronized with the phase of the CW voltage. This condition is achielred by

the repetition rate of the input pulse from the phase of the CW voltage by a

frequency divider. Division by a large number (e.g. 1000) allows all the echoes from one

pulse to be attenuated before the next pulse is applied. The output of the frequency

divider is a trigger signal synchronous, with the phase of the CW voltage. The trigger

signal triggers the main pulser, which pulse the transducer. A diode limiter circuit

keeps the input pulse from overloading the amplifier. The main pulser also triggers two

intensifying pulses, which are applied to the cathode ray tube to intensify the trace. This

feature is necessary to distinguish the two signals of interest from the rest of the echoes

in the trace.

The ULTRASONIC TIME INTERVALOMETER (UTI-101) is intended for

precise measurement of ultrasonic velocity in solids and liquids using pulse-echo-

overlap technique. The absolute accuracy of velocity measurement using this technique

may be as high as 2 parts in lo4, while the relative sensitivity (AV) can be as high as 10

parts in lo6. The instrument uses a broadband pulse to excite the transducer. All the

circuitry required for pulse-echo-overlap such as high voltage pulse to excite the

transducer, a continuous wave oscillator with high resolution and low phase jitter,

delayed strobe pulse generators to aid intensification of the trace, eight digit frequency

counter are built into this compact instrument. One of the important features of this

instrument is provision of extra facility, which enables one to achieve overlap with

scopes having no intensity modulation facility. The accuracy in the measurement of the

velocity is kO.OOlrn~-~.

Chapter I I E~enmrrtallechniques. . .

2.4. MEASUREMENT OF DENSITY:

The density of the solution is measured using a specific gravity bottle of capacity

10ml. It is filled with the solution carefully up to the fixed marks. The mass of the

solution is iound using a single pan balance [SARTORIUS] with an accuracy of

0.001gm. The density p is computed using the relation

where m is the mass of the solution and V is the volume of the specific gravity bottle.

The accuracy of the measurement is 0.001x103kg/rn3.

2.5. MEASUREMENT OF VISCOSIT\I':

The shear viscosity of solutions is determined using an OSTWALD'S

VISCOMETER. Using the values of viscosity of water at 303K from literature, and by

measuring the time of flow of the solution from the viscometer, the shear viscosity is

where qs, pl, ti, are the viscosity, density, and t ~ m e of flow of solution in the viscometer

respectively and qo,po,to are the corresponding values for the distilled water. The

accuracy in the measurement of the measurement is M.ls.

2.6. JEOL - JES - TElOO ELECTRON SPIN RESONANCE SPECTROMETER:

The schematic diagram of the instrumentation of JEOL - JES - TElOO EPR

spectrometer is shown in figure 2.9. The main principal components of the instrument

comprises of Source, Resonator, Magnetic field and Detector.

Recorder

r - - - - - - - - -

- - . copel

I

I

1 I

- A

I I

L d

Figure 2.9 Block diagram of a typical X-band EPR spectrometer

Chapter I I E~erirrzentdlechnigues.. .

2.6.1. SOURCE:

The microwave source is ~~sual ly the klystron. It is a vacuum tube that is

characteristic for its low noise property. The valve of this klystron consists of a heater

filament, a cathode, a reflector filament and a resonant cavity in which the electrical

oscillations are maintained. The field is generated by oscillations within its own tunable

cavity.

2.6.2. RESONATOR:

This is a resonant cavity that allows the microwave through the hole called iris.

The frequency of the source is tuned to the appropriate resonate frequency of the cavity.

The corresponding w7avelength of the resonant are related to the cavity dimensions.

2.6.3. MAGNETIC FIELD:

The static magnetic field for the EPIi must be stable. An electromagnet of field

strength of about 10000 gauss is connected with the sweep generator and the subsidiary

coils to provide variable field strength. The sample is placed in the magnetic field of a

cavity resonator. The sample placed inside the cavity is such that the static magnetic

field is perpendicular to the microu7ave magnetic field.

2.6.4. DETECTOR:

All modern EPR spectrometers use the semiconducting crystal diode rectifiers as

the basis for the detecting system. The essential requirements for the detector is that the

signal leaving the crystal with the frequency and amplitude corresponds to the

modulation frequency applied the sample. The subsequent amplification of the signal

with or without phase selection leads the ultimate display of the spectrum either by an

oscilloscope or pen recorder.

Chapter 11 E ~ p e ~ m ~ n t d l e c h n q u e s . . .

The work carried out in this thesis is done on a jEOL JES-TE100 ESIt

spectrometer operating at X-band frequenc~es, ha~ring a 100 kHz field modulatior~ to

obtain a first derivative EIJR spectrum. DPIJH, with a g value of 2.0036, has been used

for g-factor calculations.

2.6.5. INTERPRETATION OF EPR SPECTRA:

As one can measure EIJR spectra for solution, powder and single crystal samples,

the procedure to obtain spin Hamiltonian parameters from these spectra must be

known. In order to calculate g and A values, the follomring expression is used,

g = ( g u r ~ B~IJI 'H )/ B

where,

B is the magnetic field position at the EPR peak

BDPP~: is the field position corresponding to DPPH

g~~1~1.i is the g-value of DI'PH (g = 2.0036)

One can as well calculate the g-value d~rectiy using the spectrometer frequency

at wluch resonance occurs. The expression is as follows,

g = (hv / PB)

where, v is the resonance frequency.

The hyperfine (hf) coupling constant 'A' is given by the field separation between

the hyperfine components. If the spacing is unequal, an average of them is taken to be

the value for A. For n number of hyperfine lines, the average hyperfine value is,

A = ( B n - B I ) / ( ~ - 1 )

where,

Bn is the field position for the ntk hyperfine line

BI is the first hyperfine line field position.

Chapter I I E;i-pe~trzentailech~ziques.. .

2.7. COMPUTATION:

From the measured parameters viz., density, viscosity, velocity and attenuation,

the other parameters viz., adiabatic compressibillty, free length, observed absorption,

classical absorption and excess absorption are computed using the standard relations.

The absorption coefficient (a) is computed from the measured attenuation values using

the straight line fitting method. The computation of parameters is done with the help of

a "C" program written for this purpose and be given In Appendix B.