Thermoelastic Characteristics Of The Photoacoustic Signal And Liquid Light Penetration Effect. Nano-silver Sol Acoustic Wave

Nanometer materiai is a new type of material which consists of a great deal ofsmall prtcles with nm order. Nowadas, the nanomeer material which is made withphysical method is very small and thin. It is difficult that we study the ultrasonicalcharacter of solid nanometer matrial and colloied suspension with nanmeter sizedprtcles by conventonal ultrasonical method. The tecboque of laser-ultraSoUnd hasW merits in some resPects, such as wide frequency band. high resolution of space,etc. so, it is very necessary that we study the microstructure and character of nanometermaterial by method of laser-ultrasound. When the laser beam conducts in the liquld, theoptical penetration depth|,the thermal diffusion length and electrostriction effect allhave influence on the photoacoustical resource as well as the characters of the laserbean such as the radius of laser beam,the distribution of space and time. But at thedifferent conditions, whether the ultrasonical waves induced by laser in the liquldinclude the same frequency components or not? Which factors do the frequencyspectrum of ultrasound have relation with ? These queshons will be studied in thisthesis.1. We use NaBH₄ solution (2nunol/l) and AgNO₃ solution(1mmol/l) to preparecolloidal suspensions of the nanometer sized Ag. The colloidal suspensions of thenanometer sized Ag is studied by the laser-ultrasound at the first time. Thephotoacoustical signal is received by 100KHz hydrophone. In the end, we use thefrequency spectum analyzer to analyze the photoacoustic signal. The photoacousticsignal is a signal of multifrequency. The frequency components of photoasoustic signalhave relation with the size of particles in the colloidal suspensions, and the intensty ofphotoacoustic signal is stronger. After the long-time radiation of laser the intensity ofphotoacoustic signal becomes lower, and the frequency spectrum of photoacoustic signalin the colloidal suspesions is different from the original spectrum.2. We assume that the laser beam radiates evenly the whole surface of the half-infinite liquid. The influences which the optical penetration effect,thermal diffusioneffect and electrostriction effect act on the sound resource are considered. And we buildthe one-dimension model. By Solving the wave equation,the thermal diffusionequation and using the boundary conditions, we can gain the formula of displacement ofparticle vibration:b/(U_(z,t)=[ik_aβD(l/(k₂^a+Ï„²)-I/(k₂^a+b²))The influence which the electrostriction effect act on displacement amplitude islower than ones which the thermal diffusion effect and optical penetration effect act ondisplacement amplitude, so we neglects electrostriction effect. We will analyze twokinds of conditions. .The thermal diffusion length is greater than the opticalpenetration depth. We only consider that the thermal diffusion length contributes to thesound resource, and the influence which the optical penetration length act on the soundresource is neglected. <2>. The other condition is that the optical penetration depth isgreater than the thermal ditheion length. With thes condition, the thennal ditheioneffect is neglectCd,. We only considered the contribution of the optical penetration depth.Whn ka/b=l, the diSPlacemen amPlitude of sound wave is the highest. Whnko/b>>l, the disp1acement amP1W dePends inversely on oPtical penetration dePth.When ka/b <