1、Research Article研究報告Microstructure and Current-voltage Characteristics of (ZnO - CuO) Varistor System in the Presence of Additive Oxides, Cr2O3, Bi2O3 and NiO 在壓敏電阻氧化鋅-氧化銅內中添加氧化鉻, 氧化鉍, 氧化鎳對顯微結構與電流-電壓特性的改變S.E. Mansour, O.A. Desouky, S.M. Negim and W.A. Kamil作者：馬來西亞聖英大學(University Sains Malaysia )翻譯：晟
2、銘電子 技術長 Accepted: November 14, 2010; Published: April 22, 2011INTRODUCTION簡介Varistors are materials with nonlinear current-voltage characteristics. They are used both as protecting devices against over voltages in electronic and industrial equipments and as surge arrestors (Souza et al., 2003). Comm
3、ercial varistors used in protection systems are based on SiC (silicon carbide) or ZnO (zinc oxide). SiC-based varistors have low nonlinearity coefficients (= 5) where is the nonlinearity constant defined by the relation: I = KV, where I is current, V is voltage and K is a proportionality constant (S
4、kuratovsky et al., 2004). The ZnO-based varistors have very high nonlinearity coefficients (= 50) and their major phase contains (besides ZnO) small amounts of Bi2O3, Sb2O3, CoO, MnO2 and Cr2O3 (Tong et al., 2009; Wu et al., 2010). The reaction between the ZnO and the additives at high temperatures
5、leads to the formation of several phases at the ZnO grain boundaries (Peiteado et al., 2005). Thus, despite their chemical composition and phases, the processing method as well as the sintering temperature, heating and cooling rates influence the electrical properties of these ceramics fundamentally
6、 (Lin et al., 2007). In view of this fact, the literature contains extensive reports describing the influence of processing variables on the properties and mechanisms that govern these system properties (Bernik and Daneu, 2007; Ott et al., 2001; Cong et al., 2007; So and Park, 2002; Nahm, 2004, 2007
7、). 壓敏電阻是一種具有非線性的電流-電壓的特性之功能陶瓷，被用來作為電器裝置的過電壓保護電路以及工業上的避雷器的兩種應用。商業化的壓敏電阻有兩大系列材料-以碳化矽(SiC)和氧化鋅(ZnO)，因為碳化矽的低非線性係數 (= 5)， 這個非線性係數決定於 I = KV, I為電流(單位為安培), V為電壓, K為比例常數。氧化鋅的比例常數就非常高(= 50)，其中以氧化鋅為主並添加有微量的氧化鉍，氧化銻，氧化鈷，氧化錳和氧化鉻的材料系統為主。複雜的添加物造成高溫燒結過程會在氧化鋅晶粒邊界析出形成複雜的相。如此，利用這些化學成份和相，可以影響壓敏電阻的燒結溫度、加熱過程與冷卻速率，使整個功能陶瓷
8、的電子特性改變。鑑於這一事實，文獻中描述的處理變量、屬性和管理機制，這些影響壓敏電阻系統特性已經有廣泛的研究和報告。Other varistor systems based on SrTiO3 (Skuratovsky et al., 2006) or TiO2 (Glot and Skuratovsky, 2006; Antunes et al., 2002; Leach, 2005) have been described in the literature, but the nonlinearity of these systems is around (212), which is l
9、ower compared to that of the multicomponent ZnO varistors. Rare earth elements can improve Eb of ZnO based varistor ceramics (Bernik et al., 2004) and other properties of various varistors (Santos et al., 2001; Wang et al., 2005). The ZnO varistor ceramics with Y203 added exhibited improved Eb (Glot
10、, 2006a; Wang et al., 2000) reported that addition of rare earth oxides RE,O, (RE = Er, Y, Dy) to ZnO-Pr6OI1-based varistors improved both Eb and nonlinearity. However, they all used traditional ball milling or low-energy ball milling and high-temperature sintering methods, which bore no advantage i
11、n both technology and economy.其他壓敏電阻材料還有如鈦酸鍶或是氧化鈦也被發表過一些研究、但它們的非線性係數較低(212)，同時沒有辦法與多重添加物之氧化鋅系統相比。當然還有利用稀土元素來改善氧化鋅的Eb值(擊穿電場強度)。例如加入氧化釔可以提升 Eb, 還有添加一些稀土元素氧化物(稀土 RE= 鉺, 釔, 鏑)到氧化鋅-氧化鐠的壓敏電阻材料中同時改善擊穿電場和非線性係數。無論如何，這些添加物都是透過傳統的球磨或是低壓撞擊球磨後加入材料系統，再經過高溫燒結方法，在經濟價值和技術性並沒有太多改善。(譯者：意思就是以氧化鋅系統為主流，添加賤金屬氧化就好不要用稀土，貴且沒
12、有提升性能太多)MATERIALS AND METHODS材料準備與研究方法Experimental procedures實驗程序 Reagent grade ZnO and one or more additive oxides of type CuO, Bi2O3, Cr2O3 and NiO were mixed in the desired proportion and ball Milled for 2 h. Five series of mixes were suggested to study the effect of copper oxide alone in a binar
13、y system ZnO plus CuO and in the presence of additive oxides, Cr2O3, Bi2O3 and NiO according to Table 1. The mixture was then pressed into discs and sintered at temperatures of 900 to 1200C in air. The final dimensions of the samples were 1.2 cm diameter and 0.2 cm thickness. The sintered samples we
14、re lapped to remove surface flaws and an ohmic contact was provided on both surfaces by coating with silver paint. The samples thus prepared could be used for measurement of electrical properties such as V-I characteristics. The V-I measurements of the samples were made by using a dc power supply in
15、 current range up to 2 mA and pulse technique for higher current ranges. The electric circuit of the varistor sample is shown in Fig. 1.所有的添加物如氧化銅, 氧化鉍, 氧化鉻和氧化鎳等與氧化鋅的混合都是使用球磨添加法兩小時滾動。表 2顯示出五種成份含量作為研究氧化銅加入的氧化鋅基礎系統。混合好的粉末壓成圓片餅在900C到 1200C的空氣中燒結，燒成的尺寸為直徑 1.2cm厚度為 0.2cm，並且經過研磨去除表面的裂縫使兩端面上銀電極有良好的接觸。試片的準備
16、測試電流電壓(V-I)特性，使用標準的直流電可加到 2mA的高電流測試。電路佈置如圖 1。Fig. 1: The position of varistor sample in electric circuit 圖 1壓敏電阻於測試電路的擺放方式。The non-linearity index () was calculated in a current range of 1 mA to 1 A using the following equation: 非線性係數()的獲得來自一個電流範圍由 1mA1A使用下列方程式where, I1 and I2 are the currents at th
17、e voltages V1 and V2. I1和 I2是不同電壓 V1和 V2輸下的電流值。The phase composition of the samples was analyzed by X-ray diffraction (XRD) using Cu K radiation on a power diffractometer (Philips apparatus type 170). Samples for microstructure analysis were polished on the plane surface with carborundum paper of di
18、fferent grades 300, 600 and 1000 followed grades of diamond paste 7, 2.5 and 1 micron. The samples were then thoroughly washed in an ultrasonic bath for half an hour. The polished samples were etched in 5% dilute HCl solution for 5 sec. Microstructure developed was examined under SEM type Philips XL
19、 30 provided with EDAX, after sputtering with gold.試片中的組成相由 X光繞射分析儀使用銅靶繞射分析儀分析。顯微結構部份則以碳化矽砂紙按照 300#, 600#與 1000#磨平後，以鑽石膏 7um, 2.5um和 1um依序拋亮。並且以超音波清洗半小時，然後以鹽酸溶液腐蝕 5秒鐘所獲得。顯微結構的觀察以帶有成份分析的電子掃描式顯微鏡觀察，試片表面需要先鍍金。RESULT結果Sinterability of the suggested mixes is deuced from the results of physical properties
20、 in terms of water absorption, apparent porosity, bulk density and firing shrinkage. Results of firing shrinkage and bulk density showed an increase with rise in temperature. Maximum values of firing shrinkage were attained in specimens fired at 1200C for 2 h. Firing the specimens more than 1100C fo
21、r 2 h caused the start of deformation of the different mixes. Therefore, it is not recommended to increase the temperature more than 1100C as the increase in bulk density displayed there did not show remarkable change with increasing temperature.這樣混合試片的燒結性表現以吸水率, 孔隙率, 試體密度和燒結收縮率等物理性質的量測結果來評估。觀察燒結收縮率
22、和試體密度的結果表明：燒結溫度上升會增加這兩個數值，燒結收縮達到最大值是在 1200C , 2小時持溫的時候。試片超過 1200C , 2小時之後，不同的混合料開始發生燒結收縮引起的變形。因此，不推薦的燒結溫度超過1100C來改善密度，因為增加燒結溫度密度沒有太顯著的提升。(譯者：觀察圖 2的孔隙率變化，孔隙率下降代表密度提升，但是不能表示變形度。液相燒結最擔心就是產品的變形度增加)Table 1: Composition of different mixes in mol % 表 1不同的試片成份組成(莫耳比)Fig. 2: (a-e) Apparent porosity of differ
23、ent groups圖 2不同的添加物造成的孔隙率變化(添加氧化物種類越多，試片孔隙度就隨燒結溫度提高越發降低，但變形相對會嚴重起來，這是液相燒結的通病)Addition of 0.5 mol % Bi2O3 with increase the content of CuO lead better densification. CuO improves densification by minimizing the present of closed pores. Water absorption of group (V) is less than of group (III) and gro
24、up IV, i.e., addition of 0.5 mol % of each oxide (Cr2O3+ Bi2O3+NiO) with increase the content of CuO lead better densification as shown in Fig. 2. 添加0.5mol%的氧化鉍會使得氧化銅-氧化鋅系統快速緻密。氧化銅可以改善緻密化過程的封閉型孔洞最小化。由吸水性的測定可以知道第 5組(V)低於第 3,4組及其他，同時也可以發現第 5組將每種添加物超過(Cr 2O3+ Bi2O3+NiO)0.5mol%，具有最好的緻密度。X-ray diffractio
25、n analysis of sintered samples reveals no formation of new phases, but lattice constants of the phases were changed after sintering. Lattice constants of each sample were changed in different extents depending on the phases affinity to a particular ion and on overall varistor composition. XRD patter
26、ns of the respective mixes fired at 1100C for 2 h, showed a shift in the d-spacing equivalent to (0.02-0.04 )A in the main ZnO peak indicating a kind of limited solid solution of CuO in the oxide . A maximum shift of 0.04A was recorded in mix containing 6 mol % CuO, XRD results of group (II) which c
27、ontaining 0.5 mol % Cr2O3, showed that no binary compound was formed. The electronic defects are introduced by adding minor dopants such as the oxides of Bi, Co, Mn and Cr. X-光繞射分析主要用來偵測有無新的相成形，但只發現試驗的燒結後試片晶格常數有所改變卻沒有出現新的相。每一測試組都因為不同添加物的離子大小改變了氧化鋅的晶格常數。各組的混合物之 X光繞射分析圖採用燒結到 1100C, 2 小時持溫的試片，同時在 d-間隙等
28、同於中的氧化锌主峰值一個移位(0.02-0.04)A 找出氧化銅固溶的證據，最大的偏移移 0.04A則表示氧化鋅中含有6mol%的氧化銅，X 光繞射對第 2組氧化铜加入 0.5 mol% 氧化鉻那組，發現没有二元化合物形成的混合。通过添加少量的氧化物掺雜劑形成電子缺陷，如氧化铋、氧化钴、氧化錳、氧化铬的。The effects of dopants are classified into three categories. The first are additives forming the basic microstructure (Glot, 2006b; Ramirez et al.,
29、 2005; Simoes et al., 2003; Yongjun et al., 2000) of the sintered body, such as Bi2O3 and BaO. The second are additives used for the improvement of non-ohmic properties, such as CoO and MgO. The third are additives used for the improvement of reliability ( Oliveira et al., 2003; Takemura et al., 198
30、6) such as NiO and glass frit. The role of metal oxide can be predicted to be a source of oxygen which will be chemisorbed at ZnO grain boundaries. This chemisorbed oxygen will generate the electronic interface states at the grain boundaries (Parra et al., 2005). 掺雜劑的作用被分為三類，第一類用來作為添加的是讓壓敏電阻的燒結體有基本結
31、構成形，例如氧化鉍和氧化鋇(譯者：兩種都是形成玻璃態液相的主要添加物，類似傳統陶瓷的低溫釉料也是這些成份)；第二種添加物則是用來改善非歐姆電阻特性，例如氧化鈣和氧化鎂；第三種添加物則是用來改善可靠度，如氧化鎳和玻璃料。這些不同程度的氧化物的角色扮演都發生在氧化鋅的晶粒邊界，也因氧化價數不同而具有不同的電子特性。The role of monovalent ions is of particular interest. Although, such ions should formally behave as acceptor in ZnO, all attempts to produce P-
32、type behaviour by doping in single crystal and films (Fayat and Castro, 2003) have failed. Thus, it is unlikely that they can act as acceptors in varistors. Some insight as to how they may substitute into ZnO comes from recent atomistic simulations (Li et al., 2002). These indicated that the lowest
33、solution energy is associated with their being amphoteric dopants, creating both substitutional and interstitial ions. 一價離子的作用是特別讓人感興趣，雖然這些離子應該在氧化鋅內產生 P型摻雜單晶和薄膜以正式作為授體，但所有過去的嘗試都失敗了。因此一價離子的加入氧化鋅是不可能在壓敏電阻中當作受體，一些有識之士，希望用可能代替氧化鋅來自最接近的原子模擬。這些能量最低的解決方案，便是加入兩性摻雜劑同時建立置換及插入的離子。Thus, for Na2O in corporation:
34、 如此，氧化鈉的作用如化學式De la Rubia et al. (2004), reported that, monovalent ions (Li+) tend to increase the resistivity of doped sintered poly crystalline ZnO by creating electron traps and trivalent ions (i.e., Al3+ and Cr3+) to decrease it by supplying extra conduction electron. 根據研究，一價離子(鋰 1+)添加到多晶化鋅中經燒結後
35、可以創造出電洞和三價離子(如鋁 3+和鉻 3+)，來供應額外傳導的電子。The deep or shallow trap levels commonly found in bulk ZnO are as interstitial Zinc, oxygen vacancy and impurity induced donor or acceptor levels (Wang et al., 2003, 2005). It is known that the manganese oxide in ZnO varistors drastically improve the nonlinear (V-
36、I) characteristics (Batzill and Diebold, 2005). 從巨觀氧化锌來看，深或浅的陷阱能階普遍存在於作锌原子附近，氧的空位和雜質誘引起的電子的施放或接受能階。在氧化锌壓敏電阻中添加的氧化锰，可大幅度提高的非線性電流-電壓特性(V-I)，便是眾所周知的方式。The equilibrium between di- and trivalent manganese oxide in oxygen atmosphere is expressed by the equation (Bueno et al., 2005).下列化學式便是一個二價轉三價錳氧化物的氧化過程
37、受溫度變化的轉變(先經過 537C結合然後又於 959C分解)Mn3O4 doping on ZnO is expected to cause interesting phenomena in relation to the point defect formation in poly crystalline ZnO. 這便是四氧化三錳添加到氧化鋅中會在多晶氧化鋅晶體周圍發生有趣的現象的原因。Metz et al. (2007) reported that, the varistors without Al content exhibited poor non-linear character
38、istics as only 19.8 in non linear coefficient. However, the non-linear characteristics were greatly enhanced in nonlinear coefficient above 30 by incorporation of Al(NO3)3. 9H2O. The Al atoms would act as donors. Al2O3 doping of ZnO is likely to involve accommodation of Al ions on host lattice sites
39、. In this case, a trivalent Al3+ ion will replace a divalent Zn2+ ion allowing an electron to move the conduction band and the reaction can be written as follows: 報告指出，沒有存在鋁離子的氧化鋅有很差的非線性特性，僅有非線性指數僅 19.8。但是加入了硝酸鋁所得到的鋁離子使壓敏電阻的非線性指數可以增加到 30。鋁原子扮演的是電子施體，氧化鋁添加在氧化鋅中可能是铝離子佔據的晶格位置。在这种情况下，三價的鋁離子(Al 3+)取代的二價鋅
40、離子(Zn 2+)使得其中一電子一到導電帶上，反應式如下列：The similar behaviour was observed in the present study with CuO and Cr2O3 and NiO i.e.,相似的行為也發生在添加氧化銅/氧化鎳/氧化鉻那組試片This gives a net increase in the concentration of electrons, Thereby increasing the electrical conductivity of the grains, i.e., decreasing the resistance o
41、f ZnO grains, which cause an increase in non linear coefficient and leakage current. 這樣的添加給出了一個電子的濃度淨增加，從而提高了氧化鋅晶體導電性，即降低氧化鋅晶體的阻抗，導致非線性係數和漏電流增加。The (V-I) Characteristics were measured at voltage between (0-5) KV and current between (010) mA. The effect of CuO alone is demonstrated in mixes of group
42、(I). (V-I) characteristics of group (I) all mixes exhibit non-ohmic relation demonstrated by the values calculated for () in Table 2.電流電壓特性的測量在電壓由(05)千伏及(010)毫安培之間的特性。單獨的 CuO的效果表現在混合物第 1組(I) 其()計算出的值在表 2中表示。其他組混合物試片的電流電壓特性表現出非歐姆性的關係，也列在其後。Mixes of group (II) representing ZnO + CuO and 0.5 mol % Cr2O
43、3 additives show very good plateau in (V-I) relation with increase in the percentage of CuO beginning at 10 A mG2, which present in Fig. 3. Similar curves were obtained for group (III, IV and V) representing ZnO + CuO and 0.5 mol % Bi2O3, ZnO+CuO and 0.5 mol % NiO and ZnO + CuO and 0.5 mol % Cr2O3+0
44、.5 mol % Bi2O3 + 0.5 mol % NiO, respectively. The characteristic curves of different groups are greatly divided into two regions, that is, pre-breakdown at low voltage region and an off-state and nonlinear properties as an on-state high voltage region, the sharper 試片 Z1(第 3組，在氧化鋅-氧化銅 0.3mol%中加入0.5mo
45、l%氧化鉍), between the two regions, the better the non-linearity. Table 26, show the variation of nonlinear exponent as function of CuO content of different mixes for all groups at different temperatures. Table 2 illustrated that, the calculated () in ZnO ceramic with CuO only in group (I) was from 10-
46、65, achieving maximum 65 in CuO content of 4 mol %, which fired at 1100C for 2 h. 第 2組 (II)混合物試片為氧化鋅-氧化銅中添加 0.5mol%氧化鉻，有非常好的 V-I特性增加，比起原來氧化鋅-氧化銅多出了約 10uAmG2, 請看圖 3。相似的行為也發生在第 3, 4, 5組(III, VI, and V)那些多重添加試片中，即不同組的電流電壓特性曲線可以觀察到，一個是預擊穿前關閉狀態的非線性低電壓區域和已擊穿的線性高電壓區域，兩者之間的曲線的有一個明顯的屈膝(拐點)變化區域，當摻雜氧化銅的比例增多時。表
47、 26顯示不同氧化銅含量在的不同組合下，在不同溫度下的所有表現的非線性指數的變化。Table 2: The non linearity of group I, (ZnO - CuO) 表 2第 1組的非線性係數Table 3: The non linearity of group II, (ZnO CuO - Cr2O3) 表 3第 2組的非線性係數Table 4: The non linearity of group III, (ZnO CuO - Bi2O3) 表 4第 3組的非線性係數Table 5: The non linearity of group IV, (ZnO CuO - N
48、iO) 表 5第 4組的非線性係數Table 6: The non linearity of group V, (ZnO CuO - Cr2O3 - Bi2O3 - NiO) 表 6第 5組的非線性係數Fig. 3: (I-V) Characteristics of group II ( ZnO-CuO-Cr2O3) at 1100C圖 3電流電壓特性在第二組燒結於 1100C的表現 (譯者：添加氧化物越多，拐點越往高電流耐受度移動)The same behaviour was observed in Table 36, the calculated () in ZnO ceramic wit
49、h different additives from 10-80, 10-75, 10-110 and 60-120 for group II, III, IV and V respectively. The value of () was increased with increasing CuO content until 4 mol% for all different groups, which fired at 1100C 2 h. It can be forecasted that the ZnO-ceramics doped with 4 mol % CuO and other additives should exhibit the best nonlinear properties because it has the sharpest knee. Adding of more CuO, the knee gradually becomes less pronounced and the non-linear properties reduce.同樣的行為被觀察在表 36，這個非線性係數()在氧化鋅含有不同氧化物，從1080(第 2組,
