超聲探傷儀、超聲波探頭、測試塊和耦合劑等是超聲檢測系統的重要組成部分。超聲波檢測的主要設備是超聲波探傷儀,它可以快速、方便、無損傷地檢測、定位、評估和診斷工件中的各種缺陷。由于超聲波探頭可實現電聲轉換,所以超聲波探頭也叫超聲波換能器,其電聲轉換是可逆的,且轉換時間極短,可以忽略不計。根據超聲波的產生方式和電聲轉換的不同,超聲波換能器有很多種。這些電聲轉換方式有:利用某些金屬(鐵磁性材料)在交變磁場中的磁致伸縮,產生和接收超聲波;利用電磁感應原理產生電磁超聲以及利用機械振動、熱效應和靜電法等都能產生和接收超聲波,利用壓電效應原理制成的壓電材料是目前用得最多的超聲換能器。



1. 壓電效應


  有一種(zhong)晶體(ti),當(dang)(dang)受到擠壓或者拉(la)伸(shen)作用(yong)力的(de)時(shi)(shi)候(hou),產(chan)(chan)生(sheng)形(xing)變(bian),使(shi)得其中的(de)帶(dai)電(dian)(dian)質點發(fa)生(sheng)相對位移,因(yin)此(ci)(ci)大(da)小(xiao)相等極(ji)性相反的(de)正(zheng)(zheng)(zheng)電(dian)(dian)荷和負電(dian)(dian)荷會出現在晶體(ti)表面,然后(hou)在兩端(duan)產(chan)(chan)生(sheng)不(bu)同的(de)電(dian)(dian)荷,此(ci)(ci)時(shi)(shi)晶體(ti)將處于(yu)帶(dai)電(dian)(dian)狀態,并且由(you)(you)作用(yong)力產(chan)(chan)生(sheng)的(de)電(dian)(dian)荷量與作用(yong)力的(de)大(da)小(xiao)成正(zheng)(zheng)(zheng)比;當(dang)(dang)作用(yong)力撤去之后(hou),晶體(ti)恢復(fu)到它的(de)中性狀態,這(zhe)種(zhong)現象(xiang)被(bei)稱作正(zheng)(zheng)(zheng)壓電(dian)(dian)效(xiao)(xiao)應(ying)(ying)。當(dang)(dang)此(ci)(ci)類晶體(ti)處于(yu)電(dian)(dian)場中時(shi)(shi),晶體(ti)會沿一定的(de)方向(xiang)產(chan)(chan)生(sheng)機械形(xing)變(bian);電(dian)(dian)場撤去之后(hou)形(xing)變(bian)消(xiao)失,晶體(ti)恢復(fu)原狀,這(zhe)種(zhong)現象(xiang)被(bei)稱為(wei)逆壓電(dian)(dian)效(xiao)(xiao)應(ying)(ying)或電(dian)(dian)致(zhi)伸(shen)縮效(xiao)(xiao)應(ying)(ying)。正(zheng)(zheng)(zheng)壓電(dian)(dian)效(xiao)(xiao)應(ying)(ying)與逆壓電(dian)(dian)效(xiao)(xiao)應(ying)(ying)被(bei)統稱為(wei)壓電(dian)(dian)效(xiao)(xiao)應(ying)(ying),如圖3.1所(suo)示。這(zhe)種(zhong)物理現象(xiang)在1880年(nian)被(bei)居里(li)兄弟發(fa)現,正(zheng)(zheng)(zheng)是由(you)(you)于(yu)這(zhe)種(zhong)現象(xiang),壓電(dian)(dian)晶體(ti)被(bei)廣泛應(ying)(ying)用(yong)于(yu)產(chan)(chan)生(sheng)超(chao)聲波的(de)晶體(ti)振蕩器。


圖 1.jpg


  壓(ya)(ya)電(dian)(dian)(dian)(dian)效應(ying)(ying)的原理是,如果對壓(ya)(ya)電(dian)(dian)(dian)(dian)材(cai)料施(shi)加壓(ya)(ya)力(li),就會產生(sheng)電(dian)(dian)(dian)(dian)位差(稱為正壓(ya)(ya)電(dian)(dian)(dian)(dian)效應(ying)(ying)),反之施(shi)加電(dian)(dian)(dian)(dian)壓(ya)(ya)時,會產生(sheng)機(ji)械應(ying)(ying)力(li)(稱為逆壓(ya)(ya)電(dian)(dian)(dian)(dian)效應(ying)(ying))。如果壓(ya)(ya)力(li)是高(gao)(gao)頻振(zhen)動,就會產生(sheng)高(gao)(gao)頻電(dian)(dian)(dian)(dian)流。當高(gao)(gao)頻電(dian)(dian)(dian)(dian)信(xin)號應(ying)(ying)用于壓(ya)(ya)電(dian)(dian)(dian)(dian)陶瓷上(shang)時,會產生(sheng)高(gao)(gao)頻聲信(xin)號(機(ji)械振(zhen)動),通常稱為超聲信(xin)號。也(ye)就是說,壓(ya)(ya)電(dian)(dian)(dian)(dian)晶片可以因(yin)機(ji)械形變(bian)產生(sheng)電(dian)(dian)(dian)(dian)場(chang),也(ye)可以因(yin)電(dian)(dian)(dian)(dian)場(chang)的作用產生(sheng)機(ji)械形變(bian),實現(xian)機(ji)械能與電(dian)(dian)(dian)(dian)能之間的轉換和逆轉換,這種內在的機(ji)電(dian)(dian)(dian)(dian)耦合(he)效應(ying)(ying)使(shi)得壓(ya)(ya)電(dian)(dian)(dian)(dian)晶體在工(gong)程(cheng)中得到了廣(guang)泛(fan)的應(ying)(ying)用。



2. 壓(ya)電晶體


  在機械(xie)力的作(zuo)用(yong)下(xia),產生(sheng)形變,使(shi)帶電粒子具(ju)有(you)相對(dui)位移,使(shi)晶(jing)體(ti)(ti)表面具(ju)有(you)正負束縛電荷,這樣(yang)的晶(jing)體(ti)(ti)叫(jiao)作(zuo)壓(ya)電晶(jing)體(ti)(ti)。壓(ya)電晶(jing)體(ti)(ti)極(ji)軸(zhou)兩(liang)端產生(sheng)的電勢(shi)差(cha)的性質(zhi)稱為(wei)壓(ya)電特性。分為(wei)單(dan)晶(jing)體(ti)(ti)與多晶(jing)體(ti)(ti),其中多晶(jing)體(ti)(ti)材料(liao)又(you)稱作(zuo)壓(ya)電陶瓷。硫酸(suan)(suan)鋰、鈮酸(suan)(suan)鋰、石英等為(wei)常用(yong)的單(dan)晶(jing)材料(liao)。常用(yong)的多晶(jing)材料(liao)有(you)鈦酸(suan)(suan)鋇、鈦酸(suan)(suan)鉛等。多晶(jing)體(ti)(ti)材料(liao)又(you)稱為(wei)壓(ya)電陶瓷。其中單(dan)晶(jing)體(ti)(ti)材料(liao)對(dui)接收更(geng)靈(ling)敏(min),多晶(jing)材料(liao)的發射(she)靈(ling)敏(min)度(du)較高。


  超(chao)聲(sheng)(sheng)(sheng)波(bo)(bo)(bo)(bo)(bo)換(huan)能(neng)(neng)器(qi)中(zhong)的(de)(de)壓電(dian)(dian)(dian)(dian)(dian)(dian)晶(jing)片(pian)具有(you)壓電(dian)(dian)(dian)(dian)(dian)(dian)效(xiao)應,可(ke)(ke)利用超(chao)聲(sheng)(sheng)(sheng)換(huan)能(neng)(neng)器(qi)中(zhong)壓電(dian)(dian)(dian)(dian)(dian)(dian)芯片(pian)的(de)(de)壓電(dian)(dian)(dian)(dian)(dian)(dian)效(xiao)應實現(xian)超(chao)聲(sheng)(sheng)(sheng)波(bo)(bo)(bo)(bo)(bo)的(de)(de)產(chan)生和接收。在壓電(dian)(dian)(dian)(dian)(dian)(dian)晶(jing)體兩(liang)側的(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)極通(tong)交流電(dian)(dian)(dian)(dian)(dian)(dian),通(tong)過逆壓電(dian)(dian)(dian)(dian)(dian)(dian)效(xiao)應可(ke)(ke)知(zhi),晶(jing)片(pian)會在厚度方向產(chan)生伸縮的(de)(de)機(ji)械振(zhen)(zhen)動(dong),將電(dian)(dian)(dian)(dian)(dian)(dian)能(neng)(neng)轉換(huan)成(cheng)聲(sheng)(sheng)(sheng)能(neng)(neng)(機(ji)械能(neng)(neng)),此時(shi)探(tan)頭便(bian)發(fa)射出超(chao)聲(sheng)(sheng)(sheng)波(bo)(bo)(bo)(bo)(bo),再通(tong)過合(he)適的(de)(de)耦合(he)劑與待檢測工件連接,振(zhen)(zhen)動(dong)產(chan)生的(de)(de)超(chao)聲(sheng)(sheng)(sheng)波(bo)(bo)(bo)(bo)(bo)便(bian)進(jin)入(ru)了工件。當(dang)壓電(dian)(dian)(dian)(dian)(dian)(dian)晶(jing)片(pian)接收到(dao)超(chao)聲(sheng)(sheng)(sheng)波(bo)(bo)(bo)(bo)(bo)時(shi),受到(dao)聲(sheng)(sheng)(sheng)波(bo)(bo)(bo)(bo)(bo)能(neng)(neng)量(liang)的(de)(de)激發(fa)便(bian)會產(chan)生振(zhen)(zhen)動(dong)發(fa)生機(ji)械形(xing)變從而使晶(jing)體兩(liang)個表面(mian)產(chan)生大小相同極性相反(fan)的(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)荷,形(xing)成(cheng)超(chao)聲(sheng)(sheng)(sheng)波(bo)(bo)(bo)(bo)(bo)頻(pin)率的(de)(de)高頻(pin)電(dian)(dian)(dian)(dian)(dian)(dian)壓,超(chao)聲(sheng)(sheng)(sheng)波(bo)(bo)(bo)(bo)(bo)探(tan)傷儀的(de)(de)接收電(dian)(dian)(dian)(dian)(dian)(dian)路正是通(tong)過對返回的(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)信(xin)號進(jin)行一系列處理從而判斷(duan)工件是否(fou)有(you)傷。顯然,超(chao)聲(sheng)(sheng)(sheng)波(bo)(bo)(bo)(bo)(bo)換(huan)能(neng)(neng)器(qi)的(de)(de)作用是實現(xian)聲(sheng)(sheng)(sheng)能(neng)(neng)與電(dian)(dian)(dian)(dian)(dian)(dian)能(neng)(neng)的(de)(de)相互(hu)轉換(huan)。


 壓電(dian)晶(jing)體分為單壓電(dian)晶(jing)體與多晶(jing)壓電(dian)陶瓷(ci)。其中(zhong),壓電(dian)陶瓷(ci)占(zhan)有(you)相當大(da)的比重,是市場上應用最為廣泛(fan)的壓電(dian)材(cai)料(liao)。分述如下:


  a. 壓電單晶體:  石(shi)英(ying)、水溶性壓電晶體(酒石(shi)酸鉀鈉、酒石(shi)酸乙(yi)烯二銨、酒石(shi)酸二鉀、硫(liu)酸鉀等)。


  b. 多(duo)晶(jing)體壓(ya)(ya)(ya)電(dian)陶(tao)(tao)(tao)瓷(ci):   鈦酸鋇壓(ya)(ya)(ya)電(dian)陶(tao)(tao)(tao)瓷(ci)、鋯(gao)鈦酸鉛(qian)系(xi)(xi)壓(ya)(ya)(ya)電(dian)陶(tao)(tao)(tao)瓷(ci)、鈮酸鹽(yan)系(xi)(xi)壓(ya)(ya)(ya)電(dian)陶(tao)(tao)(tao)瓷(ci)和鈮鎂酸鉛(qian)壓(ya)(ya)(ya)電(dian)陶(tao)(tao)(tao)為(wei)代表(biao)性的壓(ya)(ya)(ya)電(dian)陶(tao)(tao)(tao)瓷(ci)。




3. 壓電單晶體


 石英(ying)晶體性(xing)(xing)能(neng)穩定(ding),機械強度高(gao)(gao),絕緣性(xing)(xing)能(neng)好,但價格(ge)昂貴,壓(ya)電(dian)(dian)系(xi)數遠低于壓(ya)電(dian)(dian)陶(tao)瓷,所以一般僅用(yong)于標準儀器(qi)或要求較(jiao)高(gao)(gao)的傳感器(qi)。石英(ying)晶體諧(xie)振器(qi)具有(you)很高(gao)(gao)的品質(zhi)因數和穩定(ding)性(xing)(xing),可用(yong)于對講(jiang)機、電(dian)(dian)子(zi)手表、電(dian)(dian)視機、電(dian)(dian)子(zi)儀器(qi)等(deng)產品的諧(xie)振腔,如圖(tu)3.2所示為石英(ying)晶體的壓(ya)電(dian)(dian)模型。


圖 2.jpg


 此外,酒(jiu)石酸(suan)鉀鈉、酒(jiu)石酸(suan)乙(yi)烯二銨、酒(jiu)石酸(suan)二鉀、硫酸(suan)鉀等水(shui)溶性壓(ya)電(dian)晶體是常見(jian)的(de)(de)單晶壓(ya)電(dian)材料。目前(qian),通過單晶化(hua)來提(ti)高多晶壓(ya)電(dian)陶瓷(如(ru)鈦酸(suan)鉛)的(de)(de)壓(ya)電(dian)性能是壓(ya)電(dian)材料的(de)(de)研(yan)究熱點之(zhi)一。



4. 多(duo)晶體壓電陶瓷(ci)


  多晶(jing)體壓電(dian)(dian)(dian)(dian)(dian)陶(tao)(tao)瓷(ci)(ci)是一種具有壓電(dian)(dian)(dian)(dian)(dian)效應的(de)(de)功(gong)能陶(tao)(tao)瓷(ci)(ci)材料,在高(gao)溫下(xia)將氧(yang)化(hua)物混合燒結(jie),可(ke)以實現機械能和電(dian)(dian)(dian)(dian)(dian)能的(de)(de)轉換。目前市場上常見的(de)(de)多晶(jing)體壓電(dian)(dian)(dian)(dian)(dian)陶(tao)(tao)瓷(ci)(ci)為鋯鈦酸鉛(PZT)系壓電(dian)(dian)(dian)(dian)(dian)材料。壓電(dian)(dian)(dian)(dian)(dian)材料的(de)(de)研(yan)究熱點主要有:①. 低溫燒結(jie)PZT陶(tao)(tao)瓷(ci)(ci);②. 大功(gong)率(lv)高(gao)轉換效率(lv)的(de)(de)PZT壓電(dian)(dian)(dian)(dian)(dian)陶(tao)(tao)瓷(ci)(ci);③. 壓電(dian)(dian)(dian)(dian)(dian)復合材料;④. 無鉛壓電(dian)(dian)(dian)(dian)(dian)陶(tao)(tao)瓷(ci)(ci);⑤. 單晶(jing)化(hua)。如圖3.3所(suo)示為壓電(dian)(dian)(dian)(dian)(dian)陶(tao)(tao)瓷(ci)(ci)的(de)(de)發展(zhan)歷史。


圖 3.jpg



5. 壓電晶體的主要性能參數


 a. 壓電應變常數 d33


 壓(ya)電(dian)(dian)應變常數表(biao)示單位電(dian)(dian)壓(ya)作用于(yu)壓(ya)電(dian)(dian)晶體時(shi)所產生的應變大小,其表(biao)達式為


  d33 = Δt/U


  式中 Δt-晶片在厚度方向的形變(bian)量(liang),單位(wei)為m(米);

 

         U--施加在壓電晶(jing)片兩面的應力,單位為V(伏特)。


 壓電應變常數d33是測量壓電晶體材料發射靈敏度的重要參數。d33值越大,發射性能越差,發射靈敏度越高。


b. 壓電電壓常數 g33


 壓(ya)電電壓(ya)常(chang)數表示施(shi)加在(zai)壓(ya)電晶片上(shang)的單位應力所產生(sheng)的壓(ya)電梯(ti)度(du)大小(xiao),其表達式為


  g33 = Up/P


 式(shi)中 P-施加(jia)在(zai)壓電晶片兩面的應力,單(dan)位為N(牛);


      Up-晶片表面產生的電壓梯度,Up = U/t,單位為V/m(伏特/米)。


c. 介電常(chang)數(shu) ε


介電常數是表(biao)示絕緣(yuan)能(neng)力特性(xing)的一個系數,其表(biao)達式為


 ε=C t/A


 式中 C-電(dian)容器(qi)電(dian)容;


       t-電(dian)容器極板距離;


       A-電容器極板面積。


 由介電(dian)(dian)(dian)常數(shu)表(biao)達式可知,當電(dian)(dian)(dian)容(rong)器(qi)極板距(ju)離和面積一(yi)(yi)定時,介電(dian)(dian)(dian)常數(shu)ε越(yue)大(da),電(dian)(dian)(dian)容(rong)C越(yue)大(da),即(ji)電(dian)(dian)(dian)容(rong)器(qi)存儲電(dian)(dian)(dian)量越(yue)多。壓(ya)電(dian)(dian)(dian)晶體的(de)ε應(ying)根據不同的(de)用(yong)途來選(xuan)取。超聲波檢測的(de)壓(ya)電(dian)(dian)(dian)晶體,頻率要求高時,應(ying)小(xiao)一(yi)(yi)些。由于(yu)ε小(xiao)、C小(xiao),電(dian)(dian)(dian)容(rong)器(qi)充放電(dian)(dian)(dian)時間短,頻率高。反之(zhi),應(ying)該(gai)大(da)一(yi)(yi)些。


d. 機電耦合系數(shu) K


 機電耦合系數K,表示(shi)壓(ya)電材(cai)料機械(xie)能(neng)(neng)(聲能(neng)(neng))與電能(neng)(neng)的轉換效率,即


 K= 轉換的能量(liang)/輸入的能量(liang)


 對(dui)于(yu)(yu)正(zheng)壓電效應,K=轉(zhuan)換的電能(neng)/輸入的機械能(neng)。對(dui)于(yu)(yu)負(fu)壓電效應,K=轉(zhuan)換的機械能(neng)/輸入的電能(neng)。


 探頭晶片振動時,會產生厚度和徑向兩個方向的伸縮變形,因此機電耦合系數分為厚度方向Kt和徑向Kp。Kt大,探測靈敏度高;Kp大,低頻諧振波增多,發射脈沖變寬,導致分辨力降低,盲區增大。


e. 機械品質因子 θm


 壓電晶片在諧振時儲存的機械能E與(yu)一個周期(qi)內損(sun)耗的能量E損(sun)之比稱為機械品(pin)質因子θm.


 壓電晶片振動損耗的能量主要是內摩擦引起的。θm值對分辨率有較大的影響:θm值越大,表示損耗越小,晶片持續震動時間長,脈沖寬度大,分辨率低。反之,θm值越小,表示損耗越大,脈沖寬度小,分辨率就高。


f. 頻率常數Ni


 由駐波理論可(ke)知,壓(ya)電(dian)晶片在(zai)高頻電(dian)脈沖激(ji)勵下產生(sheng)共振的條件是(shi)


1.jpg


 這意味著壓電晶片厚度與固有頻率的乘積是一個常數,稱為頻率常數,用Ni表示。厚度一定,頻率常數大的晶片材料,其固有頻率高。晶片材料一定,頻率越高,厚度越小。


g. 居里溫度Tc


 與磁性材料一樣,壓電材料的壓電效應與溫度有關。它只能在一定的溫度范圍內產生,超過這個溫度范圍,壓電效應就消失了。壓電材料的壓電效應消失的溫度稱為壓電材料的居里溫度,用Tc表示。例如,石英Tc=570℃,鐵酸鋇Tc=115℃.常見壓電材料性能參數見表3.1。


表 1.jpg



6. 壓電(dian)晶體的選用(yong)原則


 對于(yu)壓(ya)電超聲換能器采用的壓(ya)電晶片,其選用原(yuan)則(ze)可參考如下:


  a. 性(xing)能指標適(shi)當,以滿足具(ju)體使用要求為度量(liang),不宜(yi)過分追求各項性(xing)能的高指標;


  b. 工作性能要穩(wen)定、可(ke)靠;


  c. 價格低廉,加工方便。


超聲(sheng)波換能器對晶片的要求如(ru)下(xia):


  a. 機電耦(ou)合系數K較(jiao)(jiao)大,以(yi)便獲(huo)得較(jiao)(jiao)高的(de)轉換效率;


  b. 機械品質因子θm較大,以便獲得較高的轉換效率;


  c. 壓電應變常數d33和壓電電壓常數g33較大,以便獲得較高的發射和接收靈敏度;


  d. 頻率常數Ni較大,介電常數ε較小,以便獲得較高的頻率;


  f. 居里溫度Tc較高,聲阻抗 Z 適當。