| This thesis is consisted of three parts.The first part is about exploring the characteristics and regularities of thermo-acoustic emission(TA)from the low-dimensional materials.In recent years,with the development of nanomaterials and nanotechnology,low-dimensional materials such as carbon nanotubes(CNTs)and graphene were successively used as TA materials.Their higher acoustic pressure emission and the advantages of larger frequency bandwidth and lower distortion of TA,induced by their ultra-low heat capacity per unit area,has made this ancient TA technology rejuvenated,and has made it be worldwide hotpot,known as Nanothermophones.However,experimental studies were much more abundant than theoretical ones.Theory investigations on TA is far more behind its experimental study.In order to have a more comprehensive and systematic understanding to TA emission,the exploration start with the research of TA emission from basic point source,and then from the point to line,and then to surface systematic study the TA emission from low-dimension materials.The specific technique route is as follows:firstly,establish the thermal-mechanical coupling model of TA emission from homogenous sphere.Secondly,while the radius of the sphere is assumed to be zero and the model is further simplified,the acoustic expression of TA from point source can be derived based on the analytical solution for the formula of TA acoustic pressure in the homogenous sphere.And then with point superposition,namely line integral or face integral,the equations of acoustic pressure of TA emission from the line source and surface source are derived,which are fit for the near and far filed.In those equations of acoustic pressure,the factors of heating film,backing material,viscous dissipation,the thermal diffusion and the heat loss of thermal convection were considered.According to those theoretical formulas,the characters of TA emission from point,line and surface source,of which the material used is low-dimensional carbon nanotubes,and the main influencing factors were analyzed in detail.The transformation rules of the sound pressure of TA emission from low-dimensional material along with frequency,measured distance and measured angle et al are presented.The main features of near and far field are also revealed.It is found that:1)acoustic pressure of TA emission from different shapes of TA sources present a linear growth along with the logarithmic frequency under low frequency,and attenuate dramatically at high frequency.The curve of sound pressure frequency response for point source shows clearly two sections:linear increasing and dramatically attenuating segments,while for the line and surface source,the curves present slowly fluctuating increase or decrease segment,with the increasing of measured distance,the line and surface source tend to be regarded as point ones,the fluctuating part of acoustic pressure curve decreases gradually and disappears finally.2)The maximum frequency response frequency of the point source is inversely proportional to the 1/2 power of the measured distance.According to this law,the criterion of the surface source can be regarded as point source or not is revealed.3)The larger the scale of the TA transducer is,the smaller the frequency value when the attenuation begins will be.And the characters of near filed is more obvious.4)For the reason that the property of acoustic pressure in far field is similar with that when the source is a point one,its value only depends on the total heat exerted on the emitter,and it has nothing to do with the scale of the source.5)For line and face source,there is a big difference on the characteristic of sound filed between on-axis and off-axis.When the measured point has an angle between the vertical of the sound source and the line between the center of the source and the measured point,in a certain angle direction,the periodically fluctuation of the sound pressure frequency response will not change with the varying distance larger than a fixed measured distance.And the period of frequency has nothing to do with TA material.6)Neither near field nor far field,the heat capacity of TA emitter has significant influence on TA sound pressure level(SPL).TA SPL falls rapidly with the increase in heat capacity of TA emitter from the very beginning,and the higher the frequency,the faster the decline of TA SPL.So,TA source with a small heat capacity is the key for TA emitter to be applicable.This is also the reason why TA emission only receives great attention recently until the emergence of nanothermphone,while it's being found out one hundred year ago.7)For nanothermphone,heat convection is effective only when frequency is smaller than 1KHz,while the influence of attenuation due to thermal diffusion and viscosity dissipation on TA waves is significant only when frequency is larger than 100KHz,thus the best working frequency range can be obtained,that is 1 KHz-100KHz.The second part is to study the characteristics and regularities of phased array TA emission.Phased array acoustic emission is a combined emission of multiple faces in different phases.Based on the interference effect,its sound waves emission can be directional and the direction can be changed without rotating the surface,thus fast focusing and scanning of sound waves can be realized.Over the past twenty years,phased array technology has developed rapidly.It has been widely used in medical ultrasound imaging systems and industrial nondestructive testing.For the reason that TA ultrasound shows a greater superiority than electro-acoustic ultrasound,it has been paid much more attention.In this paper,the calculating formula of phased array TA emission is proposed,which can be used to explore the feasibility of directional emission and scanning of acoustic waves.The influences of the area of the elements,inter-elements spacing,frequency,the phase difference distribution on the direction and the scanning regularities of TA acoustic waves from TA phased array is discussed by simulating study on linear and rectangular array.The results show that:1)The main lobe can achieve ideal scanning from 0° to 180° by rational arrangement of array elements and regularly distribution and orderly change of phase.2)Along with the increase of frequency,the directivity of array TA emission will be stronger,while the number of side lobe will increase.So the frequency should be moderate,higher frequency will narrower the range of scanning,lower frequency will weaker the directivity of acoustic waves.3)The larger inter-elements spacing is,the lower sound pressure of main lobe,the higher sound pressure of side lobe,and the weaker direction of acoustic waves will be.Oversized inter-elements spacing will bring more main lobes and the directivity will be influenced.4)Keeping frequency and spacing distance being constant,the effect of increasing the area of the element is equivalent with the increases of frequency.To increase the area of surface elements can bring about a stronger directivity of TA emission from phased array and a more concentrated energy.But it also leads to a narrower range of scanning and less number of side lobes.5)To increase the number of surface elements will also make the directivity of TA acoustic waves better,but will increase the number of side lobe.6)The antisymmetric phase distribution can achieve the scanning of acoustic waves,while the symmetric phase distribution cannot.The third part is to study the characteristic and regulation of TA emission from a spherical cavity surface.So far,the investigations of TA emission are mainly in an open space,with the relatively little effort devoted to TA emission in an enclosure.In this work,through modelling studies about TA focused emission from a spherical cavity surface,we hold the TA acoustic field distribution of spherical cavity,frequency response pattern and variation regularity,The results are as follows:1)there exists a new resonant frequency response pattern similar with the standing wave caused by the interference of acoustic wave in the cavity;2)For the interference of emitted wave and its reflected wave,sound pressure in the cavity fluctuate dramatically along with the frequency and location.The wideband flat frequency response,treated as an important feature in the open space,is extremely destructed;3)by the dual actions of spherical focusing and standing wavelike resonance,a much higher SPL can be achieved than out of cavity.4)The SPL would be further improved by increasing the gas filling pressure in cavity.5)The effect of changing the wall thickness of TA sample and type of gas on acoustic pressure field are basically insignificant.The study mentioned above will play an important role in guiding the development new type of TA devices,such as the loudspeaker without magnetic or moving parts,TA tomography detector and non-contact TA actuator and so on. |
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