Patent Publication Number: US-2010117271-A1

Title: Process for producing zinc oxide varistor

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a process for producing zinc oxide (ZnO) varistors, more particularly to a novel method of making zinc oxide (ZnO) varistors through two independent procedures to perform the doping of zinc oxide and the sintering of zinc oxide grains with a high-impedance sintering material respectively. 
     2. Description of Prior Art 
     Traditionally, a ZnO varistor is made by sintering zinc oxide, together with other oxides, such as bismuth oxide, antimony oxide, silicon oxide, cobalt oxide, manganese oxide and chrome oxide, at a temperature higher than 1000° C. During sintering, semi-conductivity of the ZnO grains increases due to the doping of Bi, Sb, Si, Co, Mn and Cr while a high-impedance grain boundary layer of crystalline phase is deposited among the ZnO grains. 
     Therefore, the conventional process for producing ZnO varistors is to utilize a single sintering procedure to accomplish two purposes. One involves growth of ZnO grains and doping ZnO with ions to enhance semi-conductivity of the ZnO grains while the other involves depositing the high-impedance grain boundary layer that encapsulates the ZnO grains to endow the resultant ZnO varistors with non-ohmic characteristics. 
     In other words, the conventional ZnO varistor principally depends on the semi-conductivity of ZnO grains and the high-impedance grain boundary layer among the ZnO grains to present its surge-absorbing ability, thus possessing superior non-ohmic characteristics and better current impact resistance. 
     The above-mentioned conventional process that resorts to the single sintering procedure for grain doping and high-impedance grain boundary layer forming nevertheless has its defects. That is, formation of the high-impedance grain boundary layer in the above-mentioned conventional process requires a relatively high sintering temperature. On the other hand, properties of the resultant ZnO varistor are less adjustable. For example, in the sintering procedure, the applicable species and quantity of ions for doping ZnO grains are relatively restricted. Consequently, properties of the resultant ZnO varistor, including breakdown voltage, nonlinear coefficient, C value, leakage current, surge-absorbing ability, and ESD-absorbing ability, are restricted. Similarly, in the sintering procedure, formation of the high-impedance grain boundary layer of crystalline phase among the ZnO grains also faces restriction. Hence, because selectiveness of composition and quantity of the high-impedance grain boundary layer is limited, improvement in technical conditions of the resultant ZnO varistors is unachievable and properties of the resultant ZnO varistors are rather inflexible. 
     SUMMARY OF THE INVENTION 
     In view of the shortcomings of the prior art, one primary objective of the present invention is to provide a process for producing zinc oxide varistors through two independent procedures to perform the doping of zinc oxide and the sintering of zinc oxide grains with a high-impedance sintering material respectively. The process for producing zinc oxide varistors comprises:
     a) preparing doped ZnO grains that possess sufficient semi-conductivity;   b) preparing a high-impedance sintering material (or glass powder) separately;   c) mixing the doped ZnO grains and the high-impedance sintering material in a predetermined ratio to form a mixture, and   d) using the mixture to make zinc oxide varistors through the known conventional technology.   

     By implementing the process of the present invention, species as well as quantity of the doping ions of the doped ZnO grains, and composition as well as preparation conditions of the high-impedance sintering material (or glass powder) can be independently designed by according to desired properties and processing requirements of the resultant zinc oxide varistors, such as breakdown voltage, nonlinear coefficient, C value, leakage current, surge-absorbing ability, ESD-absorbing ability, and permeability, or by according to preparation conditions of low-temperature sintering to realize zinc oxide varistors with various desired properties. 
     Hence, the process of the present invention allows enhanced adjustability to properties of the resultant zinc oxide varistors, thereby meeting diverse practical needs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention as well as a preferred mode of use, further objectives and advantages thereof will be best understood by reference to the following detailed description of illustrative embodiments when acquire in conjunction with the accompanying drawings, wherein: 
         FIG. 1  shows the X-ray diffraction pattern of ZnO; 
         FIG. 2  shows the X-ray diffraction pattern of ZnO doped with 2 mol % of Si; 
         FIG. 3  shows the X-ray diffraction pattern of ZnO doped with 2 mol % of W; 
         FIG. 4  shows the X-ray diffraction pattern of ZnO doped with 2 mol % of V; 
         FIG. 5  shows the X-ray diffraction pattern of ZnO doped with 2 mol % of Fe; 
         FIG. 6  shows the X-ray diffraction pattern of ZnO doped with 2 mol % of Sb; 
         FIG. 7  shows the X-ray diffraction pattern of ZnO doped with 2 mol % of Sn; 
         FIG. 8  shows the X-ray diffraction pattern of ZnO doped with 2 mol % of In; 
         FIG. 9  shows the X-ray diffraction pattern of ZnO doped with 2 mol % of Y; 
         FIG. 10  is a resistance-temperature graph of Si-doped Zn—X144 sintered with 5% of G1-08 sintering material; 
         FIG. 11  is a resistance-temperature graph of Ag-doped Zn—X141 sintered with 5% of G1-38 sintering material; and 
         FIG. 12  is a schematic drawing showing a dual-function element made from materials of Formula A and Formula B. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     According to the present invention, a process for producing zinc oxide varistors comprises the following steps: a) preparing doped ZnO grains that are doped with doping ions; b) preparing a high-impedance sintering material or glass powder; c) mixing the ZnO grains of a) and the high-impedance sintering material of b) to from a mixture; and d) processing the mixture of c) to produce the resultant ZnO varistors, which steps will be expounded hereinafter. 
     a. Preparing ZnO Grains Doped with Doping Ions 
     A solution containing zinc ions and another solution containing doping ions are prepared based on the principles of crystallography. Then nanotechnology, such as the coprecipitation method or the sol-gel process, is applied to obtain a precipitate. The precipitate then undergoes thermal decomposition so that ZnO grains doped with the doping ions are obtained. 
     The ZnO grains may be doped with one or more species of ions. Therein, quantity of the doping ion(s) is preferably less than 15 mol % of ZnO, more preferably less than 10 mol % of Zn, and most preferably less than 2 mol % of Zn. 
     The doping ion(s) is one or more selected from the group consisting of Ag, Li, Cu, Al, Ce, Co, Cr, In, Ga, La, Y, Nb, Ni, Pr, Sb, Se, Ti, V, W, Zr, Si, B, Fe, and Sn. 
     The solution containing zinc ions may be zinc acetate or zinc nitrate. The solution containing doping ions may be made by dissolving one or more species of said doping ions in acetate or nitrate. 
     Then the solution containing zinc ions and the solution containing doping ions are mixed and stirred to form a blended solution containing zinc ions and doping ions by means of the chemical coprecipitation method. While mixing, a surfactant or a high polymer may be added according to practical needs. Then a precipitant is added into the blended solution during stir in a co-current or counter-current manner. Through proper adjustment of the pH value of the solution, a co-precipitate is obtained. After repeatedly washed and then dried, the co-precipitate is calcined at proper temperature so that ZnO grains doped with the doping ions are obtained. 
     The aforementioned precipitant may be selected from the group consisting of oxalic acid, carbamide, ammonium carbonate, ammonium hydrogen carbonate, ammonia, or other alkaline solutions. 
     Another approach to making doped ZnO grains involves immersing fine ZnO powder into a solution containing the doping ions. After dried, the precipitate is calcined in air, or in an inter gas, such as argon gas, or in a reducing gas containing hydrogen or carbon monoxide, to form ZnO grains doped with the doping ions. 
     ZnO grains doped with 2 mol % of Si made by any of the foregoing approaches. The X-ray diffraction pattern thereof obtained by an X-ray diffractometer is shown in  FIG. 2 . As compared with  FIG. 1  that shows the X-ray diffraction pattern of pure ZnO grains,  FIG. 2  suggests that Si ions are fully dissolved into the lattices of the ZnO grains. 
     ZnO grains doped with 2 mol % of W or V or Fe ions can be obtained similarly. The X-ray diffraction patterns of the ZnO grains doped with 2 mol % of W ions, the ZnO grains doped with 2 mol % of V ions, and the ZnO grains doped with 2 mol % of Fe ions are shown in  FIG. 3 ,  FIG. 4  and  FIG. 5 , respectively. As compared with  FIG. 1  that shows the X-ray diffraction pattern of pure ZnO grains,  FIGS. 3 through 5  prove that W, V, and Fe ions are fully dissolved into the lattices of the ZnO grains. 
     ZnO grains doped with 2 mol % of Sb, Sn, In, and Y ions, respectively, may be obtained in the same manner.  FIG. 6  shows the X-ray diffraction pattern of ZnO doped with 2 mol % of Sb,  FIG. 7  shows the X-ray diffraction pattern of ZnO doped with 2 mol % of Sn,  FIG. 8  shows the X-ray diffraction pattern of ZnO doped with 2 mol % of In, and  FIG. 9  shows the X-ray diffraction pattern of ZnO doped with 2 mol % of Y, it is indicated that Sb, Sn, In or Y ions are partially dissolved into the lattices of the ZnO grains, according to comparison between the diffraction patterns of  FIGS. 6 through 9  with  FIG. 1  that shows the X-ray diffraction pattern of pure ZnO grains. 
     Thus, in the step of preparing ZnO grains doped with doping ions, the species and quantity of the doping ions can be selected from an enlarged scope. Consequently, properties of the resultant ZnO varistors, including breakdown voltage, nonlinear coefficient, C value, leakage current, surge-absorbing ability, and ESD-absorbing ability, can be effectively modulated. 
     b. Preparing High-Impedance Sintering Material or Glass Powder 
     Raw material of a sintering material or glass powder having the composition determined by the desired properties of the resultant ZnO varistor is used. The material includes one or more selected from the group consisting of oxide, hydroxide, carbonated, and oxalate. The selected raw material after undergoing a series of processing procedures, including mixing, grinding and calcination, is turned into the sintering material. The sintering material is then ground into powder of desired fineness. Therein, the oxide is a mixture of two or more selected from the group consisting of Bi 2 O 3 , B 2 O 3 , Sb 2 O 3 , Co 2 O 3 , MnO 2 , Cr 2 O 3 , V 2 O 5 , ZnO, NiO and SiO 2 . 
     Alternatively, pastes prepared with different compositions are mixed, melted in high temperature, water-quenched, oven-dries, and ground into fine glass powder. Alternatively, nanotechnology is implemented to turn raw materials with different compositions into a sintering material in the form of nanosized powder or into nanosized glass powder. 
     In the step of preparing the sintering material or glass powder, the sintering material or glass powder with different compositions may be made to endow the ZnO varistors with thermistor properties, inductor properties, capacitor properties, etc., in addition to varistor properties. 
     For example, when the resultant ZnO varistor is desired to have additional thermistor properties, the sintering material or glass powder may be barium titanate oxide or nickel manganese cobalt oxide. When the resultant ZnO varistor is desired to have additional inductor properties, the sintering material or glass powder may be soft ferrite. When the resultant ZnO varistor is desired to have additional capacitor properties, the sintering material or glass powder may be titanate of high dielectric constant. 
     c. Mixing ZnO Grains and High-Impedance Sintering Material 
     The ZnO grains of Step a) mentioned above and the high-impedance sintering material or glass powder of Step b) mentioned above are properly made according to the desired properties of the resultant ZnO varistors. Then the ZnO grains and the sintering material or glass powder are well mixed in a weight ratio the preferably ranging between 100:2 and 100:30, and more preferably ranging between 100:5 and 100:15. 
     d. Processing Mixture to Produce ZnO Varistors 
     At last, the mixture as the product of Step c) mentioned above is processed with high-temperature calcination, grinding, binder adding, tape pressing, sintering, and silver electrode coating to produce the resultant ZnO varistors. Therein, the calcination temperature is desirably ranging between 950° C.±10° C. and 1100° C.±10° C. 
     Some embodiments will be later explained for proving the process for producing zinc oxide varistors of the present invention possesses the following features:
     1. The varistor properties of the resultant ZnO varistors, including breakdown voltage, nonlinear coefficient, C value, leakage current, surge-absorbing ability, and ESD-absorbing ability, can be changed or adjusted by selecting the species of the ions doping the ZnO grains or by modulating the weight ratio between the ZnO grains and the high-impedance sintering material.   2. The varistor properties of the resultant ZnO varistors can be changed or adjusted by changing the quantity of the ions doping the ZnO grains.   3. The varistor properties of the resultant ZnO varistors can be changed or adjusted by doping the ZnO grains with at least two species of doping ions or by controlling the sintering temperature.   4. The varistor properties of the resultant ZnO varistors can be changed or adjusted by modifying the composition of the sintering material or glass powder.   5. By using ZnO grains doped with appropriate doping ions and by modifying the composition of the sintering material, it is possible to have pure silver made as inner electrode and produce ZnO varistors possessing excellent varistor properties through low-temperature sintering.   6. By using sintering materials of different formulas, it is possible to produce a dual-function element having varistor properties and thermistor properties. For instance, the resultant ZnO varistor may possess varistor properties and thermistor properties at the same time, or may possess varistor properties and inductor properties at the same time, or may possess varistor properties and capacitor properties at the same time.   

     EXAMPLE 1 
     The chemical coprecipitation method was used to prepare sample ZnO grains doped with 1 mol % of different single species of ions and a sintering material numbered G1-00, which has the composition as provided below. 
     
       
         
           
               
               
            
               
                   
               
               
                 Sintering 
                 Composition (wt %) 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Material 
                 ZnO 
                 SiO 2   
                 B 2 O 3   
                 Bi 2 O 3   
                 Co 2 O 3   
                 MnO 2   
                 Cr 2 O 3   
               
               
                   
               
               
                 G1-00 
                 8 
                 23 
                 19 
                 27 
                 8 
                 8 
                 7 
               
               
                   
               
            
           
         
       
     
     The sample ZnO grains and G1-00 sintering materials were well mixed in a weight ratio of 100:10 or 100:15 or 100:30, and then pressed into sinter cakes under 1000 kg/cm 2 . The sinter cakes were sintered at 1065° C. for two hours, and got silver electrode formed thereon at 800° C. At last, the sintered product with silver electrode was made into round ZnO varistors. The varistors were tested on their varistor properties and the results are listed in Table 1. 
     From Table 1, it is learned that when the same sintering material is used, the varistors have their varistor properties varying with the species of the doping ions doped in the ZnO grains. For example, the breakdown voltage, abbreviated as “BDV”, may range from 230 to 1729V/mm. Similarly, when the ZnO grains doped with the same doping ions, the varistors have their varistor properties varying with the mix ratio between the ZnO grains and the high-impedance sintering material. 
     Thus, the varistor properties of the ZnO varistor can be modified or adjusted by changing the species of the doping ions doped in ZnO grains or the mix ratio between the ZnO grains and the high-impedance sintering material. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Properties of ZnO Varistors Made of ZnO Grains Doped with Different Single 
               
               
                 Species of Doping Ions and the Same Sintering Material in Different Ratios 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Silver/ 
                   
                   
                   
                   
                   
                   
               
               
                   
                   
                 Reduction 
                 Green Size 
                 Grog Size 
                 BDV 
                   
                 I L   
                 Cp 
               
               
                 No. 
                 Composition 
                 (° C.) 
                 (mm) 
                 (mm) 
                 (V/mm) 
                 α 
                 (μA) 
                 (pF) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                  1 
                 Zn—Ce + 10% G1-00 
                 7472/845° C. 
                 8.4 × 1.08 
                 7.12 × 0.90 
                 392 
                 21 
                 25 
                 253 
               
               
                  2 
                 Zn—Ce + 15% G1-00 
                 7472/845° C. 
                 8.4 × 1.09 
                 7.14 × 0.87 
                 386 
                 22 
                 27 
                 228 
               
               
                  3 
                 Zn—Co + 10% G1-00 
                 7472/845° C. 
                 8.4 × 1.12 
                 7.21 × 0.93 
                 441 
                 22 
                 20 
                 205 
               
               
                  4 
                 Zn—Co + 15% G1-00 
                 7472/845° C. 
                 8.4 × 1.12 
                 7.25 × 0.95 
                 435 
                 22 
                 28 
                 193 
               
               
                  5 
                 Zn—Ni + 10% G1-00 
                 7472/845° C. 
                 8.4 × 1.18 
                 7.17 × 0.98 
                 451 
                 20 
                 29 
                 208 
               
               
                  6 
                 Zn—Ni + 15% G1-00 
                 7472/845° C. 
                 8.4 × 1.18 
                 7.21 × 0.97 
                 437 
                 21 
                 32 
                 178 
               
               
                  7 
                 Zn—Al + 10% G1-00 
                 7472/845° C. 
                 8.4 × 1.18 
                 7.06 × 0.96 
                 395 
                 7 
                 187 
                 293 
               
               
                  8 
                 Zn—Al + 15% G1-00 
                 7472/845° C. 
                 8.4 × 1.18 
                 7.10 × 0.97 
                 348 
                 8 
                 157 
                 283 
               
               
                  8a 
                 Zn—Al + 30% G1-00 
                 7472/845° C. 
                 8.4 × 1.18 
                 7.10 × 0.97 
                 320 
                 14 
                 65 
                 31 
               
               
                  9 
                 Zn—Sb + 10% G1-00 
                 7472/845° C. 
                 8.4 × 1.18 
                 7.01 × 0.93 
                 809 
                 29 
                 7.2 
                 127 
               
               
                 10 
                 Zn—Sb + 15% G1-00 
                 7472/845° C. 
                 8.4 × 1.18 
                 7.08 × 0.92 
                 807 
                 31 
                 10 
                 105 
               
               
                 11 
                 Zn—Cu + 10% G1-00 
                 7472/845° C. 
                 8.4 × 1.17 
                 7.13 × 1.03 
                 447 
                 11 
                 84 
                 270 
               
               
                 12 
                 Zn—Cu + 15% G1-00 
                 7472/845° C. 
                 8.4 × 1.17 
                 7.17 × 0.96 
                 470 
                 13 
                 72 
                 238 
               
               
                 13 
                 Zn—Pr + 10% G1-00 
                 7472/845° C. 
                 8.4 × 1.19 
                 7.03 × 0.95 
                 356 
                 20 
                 24 
                 259 
               
               
                 14 
                 Zn—Pr + 15% G1-00 
                 7472/845° C. 
                 8.4 × 1.19 
                 7.09 × 0.98 
                 311 
                 23 
                 19 
                 237 
               
               
                 15 
                 Zn—Se + 10% G1-00 
                 7472/845° C. 
                 8.4 × 1.12 
                 7.17 × 0.93 
                 399 
                 20 
                 34 
                 284 
               
               
                 16 
                 Zn—Se + 15% G1-00 
                 7472/845° C. 
                 8.4 × 1.12 
                 7.19 × 0.92 
                 372 
                 21 
                 33 
                 243 
               
               
                 17 
                 Zn—Fe + 10% G1-00 
                 7472/845° C. 
                 8.4 × 1.14 
                 7.22 × 0.94 
                 230 
                 10 
                 87 
                 557 
               
               
                 18 
                 Zn—Fe + 15% G1-00 
                 7472/845° C. 
                 8.4 × 1.14 
                 7.18 × 0.91 
                 251 
                 13 
                 55 
                 386 
               
               
                 19 
                 Zn—Cr + 10% G1-00 
                 7472/845° C. 
                 8.4 × 1.08 
                 7.22 × 0.88 
                 566 
                 20 
                 28 
                 185 
               
               
                 20 
                 Zn—Cr + 15% G1-00 
                 7472/845° C. 
                 8.4 × 1.08 
                 7.21 × 0.90 
                 526 
                 22 
                 28 
                 152 
               
               
                 21 
                 Zn—Nb + 10% G1-00 
                 7472/845° C. 
                 8.4 × 1.10 
                 7.14 × 0.89 
                 392 
                 12 
                 77 
                 319 
               
               
                 22 
                 Zn—Nb + 15% G1-00 
                 7472/845° C. 
                 8.4 × 1.10 
                 7.17 × 0.92 
                 399 
                 15 
                 60 
                 265 
               
               
                 23 
                 Zn—V + 10% G1-00 
                 7472/845° C. 
                 8.4 × 1.07 
                 7.59 × 0.91 
                 445 
                 17 
                 46 
                 236 
               
               
                 24 
                 Zn—V + 15% G1-00 
                 7472/845° C. 
                 8.4 × 1.07 
                 7.53 × 0.90 
                 417 
                 18 
                 45 
                 215 
               
               
                 25 
                 Zn—La + 10% G1-00 
                 7472/845° C. 
                 8.4 × 1.13 
                 7.09 × 0.94 
                 431 
                 14 
                 46 
                 230 
               
               
                 26 
                 Zn—La + 15% G1-00 
                 7472/845° C. 
                 8.4 × 1.13 
                 7.11 × 0.95 
                 424 
                 15 
                 46 
                 213 
               
               
                 27 
                 Zn—Ti + 10% G1-00 
                 7472/845° C. 
                 8.4 × 1.16 
                 7.06 × 0.98 
                 424 
                 10 
                 100 
                 239 
               
               
                 28 
                 Zn—Ti + 15% G1-00 
                 7472/845° C. 
                 8.4 × 1.16 
                 7.10 × 0.96 
                 421 
                 14 
                 64 
                 200 
               
               
                 29 
                 Zn—Sn + 10% G1-00 
                 7472/845° C. 
                 8.4 × 1.19 
                 6.96 × 0.99 
                 775 
                 28 
                 6.6 
                 99 
               
               
                 30 
                 Zn—Sn + 15% G1-00 
                 7472/845° C. 
                 8.4 × 1.19 
                 7.02 × 0.93 
                 773 
                 27 
                 11 
                 98 
               
               
                 30a 
                 Zn—Sn + 30% G1-00 
                 7472/845° C. 
                 8.4 × 1.19 
                 7.02 × 0.93 
                 758 
                 25 
                 14 
                 103 
               
               
                 31 
                 Zn—Li + 10% G1-00 
                 7472/845° C. 
                 8.4 × 1.15 
                 7.21 × 0.94 
                 434 
                 18 
                 38 
                 237 
               
               
                 32 
                 Zn—Li + 15% G1-00 
                 7472/845° C. 
                 8.4 × 1.15 
                 7.22 × 0.90 
                 414 
                 20 
                 33 
                 196 
               
               
                 33 
                 Zn—Ag—W + 10% G1-00 
                 7472/845° C. 
                 8.4 × 1.11 
                 7.40 × 0.92 
                 380 
                 17 
                 41 
                 280 
               
               
                 34 
                 Zn—Ag—W + 15% G1-00 
                 7472/845° C. 
                 8.4 × 1.11 
                 7.38 × 0.92 
                 354 
                 17 
                 42 
                 234 
               
               
                 35 
                 Zn—Zr + 10% G1-00 
                 7472/845° C. 
                 8.4 × 1.17 
                 7.09 × 0.97 
                 457 
                 13 
                 68 
                 237 
               
               
                 36 
                 Zn—Zr + 15% G1-00 
                 7472/845° C. 
                 8.4 × 1.17 
                 7.13 × 0.94 
                 440 
                 15 
                 59 
                 205 
               
               
                 37 
                 Zn—W + 10% G1-00 
                 7472/845° C. 
                 8.4 × 1.07 
                 7.28 × 0.91 
                 465 
                 14 
                 60 
                 277 
               
               
                 38 
                 Zn—W + 15% G1-00 
                 7472/845° C. 
                 8.4 × 1.07 
                 7.28 × 0.91 
                 445 
                 15 
                 55 
                 210 
               
               
                 39 
                 Zn—Si + 10% G1-00 
                 7472/845° C. 
                 8.4 × 1.17 
                 7.11 × 0.95 
                 282 
                 22 
                 16 
                 316 
               
               
                 40 
                 Zn—Si + 15% G1-00 
                 7472/845° C. 
                 8.4 × 1.17 
                 7.14 × 0.93 
                 272 
                 22 
                 14 
                 248 
               
               
                 41 
                 Zn—In + 10% G1-00 
                 7472/845° C. 
                 8.4 × 1.23 
                 6.85 × 0.97 
                 1729 
                 10 
                 54 
                 36 
               
               
                 42 
                 Zn—In + 15% G1-00 
                 7472/845° C. 
                 8.4 × 1.23 
                 6.91 × 1.00 
                 1409 
                 9 
                 100 
                 43 
               
               
                 43 
                 Zn—Ag + 10% G1-00 
                 7472/845° C. 
                 8.4 × 1.13 
                 7.22 × 0.94 
                 386 
                 21 
                 28 
                 276 
               
               
                 44 
                 Zn—Ag + 15% G1-00 
                 7472/845° C. 
                 8.4 × 1.13 
                 7.25 × 0.94 
                 356 
                 22 
                 28 
                 237 
               
               
                   
               
            
           
         
       
     
     EXAMPLE 2 
     The chemical coprecipitation method was used to prepare sample ZnO grains doped with different quantity of the same single species of doping ions. The sintering material G1-00 of Example 1 was also used. 
     The sample ZnO grains and the sintering material G1-00 were well mixed in a weight ratio of 100:10 and then the mixture was used to make round ZnO varistors under the same conditions as provided in Example 1. The varistors were tested on their varistor properties and the results are listed in Table 2. 
     From Table 2, it is learned that when the ZnO grains is doped with the same doping ions and then mixed with the same sintering material, the varistors have their varistor properties varying with the quantitative variation of the doping ions doped in ZnO grains. 
     Thus, the varistor properties of the ZnO varistor can be adjusted by controlling the quantity of the doping ions doped in ZnO grains. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Properties of ZnO Varistors Made of ZnO Grains Doped with the Same Single 
               
               
                 Species of Doping Ions in Different Quantity and the Same Sintering Material 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Sinter 
                 Silver/ 
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                   
                 Temp. 
                 Reduction 
                 Green Size 
                 Grog Size 
                 BDV 
                   
                 I L   
                 Cp 
               
               
                 No. 
                 Composition 
                 (° C.) 
                 (° C.) 
                 (mm) 
                 (mm) 
                 (V/mm) 
                 α 
                 (μA) 
                 (pF) 
                 Clamp 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 45 
                 Zn—0.5% Ni + 
                 1065 
                 7472/845° C. 
                 8.4 × 1.13 
                 7.07 × 0.90 
                 298 
                 24 
                 8.2 
                 325 
                 1.81 
               
               
                   
                 10% G1-00 
               
               
                 46 
                 Zn—1.0% Ni + 
                 1065 
                 7472/845° C. 
                 8.4 × 1.15 
                 6.99 × 0.93 
                 291 
                 24 
                 9.2 
                 304 
                 1.92 
               
               
                   
                 10% G1-00 
               
               
                 47 
                 Zn—1.5% Ni + 
                 1065 
                 7472/845° C. 
                 8.4 × 1.14 
                 7.03 × 0.91 
                 326 
                 24 
                 9.7 
                 304 
                 1.84 
               
               
                   
                 10% G1-00 
               
               
                 48 
                 Zn—0.5% Sn + 
                 1065 
                 7472/845° C. 
                 8.4 × 1.27 
                 6.72 × 0.89 
                 683 
                 31 
                 3.6 
                 145 
                 1.66 
               
               
                   
                 10% G1-00 
               
               
                 49 
                 Zn—1.0% Sn + 
                 1065 
                 7472/845° C. 
                 8.4 × 1.27 
                 6.67 × 1.02 
                 669 
                 30 
                 10 
                 125 
                 1.70 
               
               
                   
                 10% G1-00 
               
               
                 50 
                 Zn—1.5% Sn + 
                 1065 
                 7472/845° C. 
                 8.4 × 1.26 
                 6.75 × 0.98 
                 661 
                 33 
                 4 
                 111 
                 1.65 
               
               
                   
                 10% G1-00 
               
               
                 51 
                 Zn—0.5% Li + 
                 1065 
                 7472/845° C. 
                 8.4 × 1.14 
                 7.02 × 0.92 
                 258 
                 24 
                 7.7 
                 292 
                 1.83 
               
               
                   
                 10% G1-00 
               
               
                 52 
                 Zn—1.0% Li + 
                 1065 
                 7472/845° C. 
                 8.4 × 1.14 
                 7.00 × 0.93 
                 251 
                 24 
                 6.8 
                 255 
                 1.87 
               
               
                   
                 10% G1-00 
               
               
                 53 
                 Zn—1.5% Li + 
                 1065 
                 7472/845° C. 
                 8.4 × 1.14 
                 7.03 × 0.93 
                 265 
                 24 
                 6.6 
                 273 
                 1.87 
               
               
                   
                 10% G1-00 
               
               
                 54 
                 Zn—0.5% Sb + 
                 1065 
                 7472/845° C. 
                 8.4 × 1.17 
                 6.91 × 0.95 
                 575 
                 29 
                 3.7 
                 130 
                 1.70 
               
               
                   
                 10% G1-00 
               
               
                 55 
                 Zn—1.0% Sb + 
                 1065 
                 7472/845° C. 
                 8.4 × 1.17 
                 6.76 × 0.97 
                 659 
                 31 
                 3.3 
                 97 
                 1.62 
               
               
                   
                 10% G1-00 
               
               
                 56 
                 Zn—1.5% Sb + 
                 1065 
                 7472/845° C. 
                 8.4 × 1.20 
                 6.81 × 0.96 
                 596 
                 32 
                 2.6 
                 94 
                 1.57 
               
               
                   
                 10% G1-00 
               
               
                 57 
                 Zn—0.5% Pr + 
                 1065 
                 7472/845° C. 
                 8.4 × 1.26 
                 6.75 × 1.01 
                 310 
                 24 
                 6 
                 243 
                 1.86 
               
               
                   
                 10% G1-00 
               
               
                 58 
                 Zn—1.0% Pr + 
                 1065 
                 7472/845° C. 
                 8.4 × 1.20 
                 6.91 × 0.95 
                 356 
                 25 
                 6.8 
                 249 
                 1.81 
               
               
                   
                 10% G1-00 
               
               
                 59 
                 Zn—1.5% Pr + 
                 1065 
                 7472/845° C. 
                 8.4 × 1.21 
                 6.84 × 0.98 
                 337 
                 25 
                 6.8 
                 233 
                 1.80 
               
               
                   
                 10% G1-00 
               
               
                 60 
                 Zn—0.5% Ag + 
                 1065 
                 7472/845° C. 
                 8.4 × 1.14 
                 7.01 × 0.96 
                 275 
                 24 
                 6.9 
                 259 
                 1.83 
               
               
                   
                 10% G1-00 
               
               
                 61 
                 Zn—1.0% Ag + 
                 1065 
                 7472/845° C. 
                 8.4 × 1.19 
                 6.97 × 0.98 
                 265 
                 25 
                 8.9 
                 258 
                 1.77 
               
               
                   
                 10% G1-00 
               
               
                 62 
                 Zn—1.5% Ag + 
                 1065 
                 7472/845° C. 
                 8.4 × 1.18 
                 6.99 × 0.97 
                 239 
                 24 
                 9.1 
                 305 
                 1.76 
               
               
                   
                 10% G1-00 
               
               
                 63 
                 Zn—0.5% Si + 
                 1065 
                 7472/845° C. 
                 8.4 × 1.16 
                 7.02 × 0.93 
                 277 
                 24 
                 10 
                 305 
                 1.78 
               
               
                   
                 10% G1-00 
               
               
                 64 
                 Zn—1.0% Si + 
                 1065 
                 7472/845° C. 
                 8.4 × 1.14 
                 7.13 × 0.92 
                 312 
                 24 
                 13 
                 277 
                 1.73 
               
               
                   
                 10% G1-00 
               
               
                 65 
                 Zn—1.5% Si + 
                 1065 
                 7472/845° C. 
                 8.4 × 1.19 
                 6.92 × 0.94 
                 238 
                 24 
                 11 
                 358 
                 1.86 
               
               
                   
                 10% G1-00 
               
               
                 66 
                 Zn—0.5% V + 
                 1065 
                 7472/845° C. 
                 8.4 × 1.12 
                 7.09 × 0.92 
                 266 
                 26 
                 10 
                 290 
                 1.63 
               
               
                   
                 10% G1-00 
               
               
                 67 
                 Zn—1.0% V + 
                 1065 
                 7472/845° C. 
                 8.4 × 1.04 
                 7.41 × 0.90 
                 247 
                 24 
                 10 
                 286 
                 1.90 
               
               
                   
                 10% G1-00 
               
               
                 68 
                 Zn—1.5% V + 
                 1065 
                 7472/845° C. 
                 8.4 × 1.06 
                 7.40 × 0.91 
                 270 
                 23 
                 10 
                 263 
                 1.86 
               
               
                   
                 10% G1-00 
               
               
                   
               
            
           
         
       
     
     EXAMPLE 3 
     The chemical coprecipitation method was used to prepare sample ZnO grains doped with at least two species of doping ions as shown in Table 3. The sintering material G1-00 of Example 1 was also used. 
     The sample ZnO grains and the sintering material G1-00 were well mixed in a weight ratio of 100:10 and then the mixture was used to make ZnO varistors under the same conditions as provided in Example 1. The varistors were tested on their varistor properties and the results are listed in Table 3. 
     From Table 3, it is learned that when the sample ZnO grains doped with at least two species of doping ions and mixed with the same sintering material, the varistors have their varistor properties varying with the species of the doping ions doped in the ZnO grains. Meantime, the varistors also have their varistor properties varying with variation of the sintering temperature. 
     Thus, the varistor properties of the ZnO varistor can be adjusted in an enlarged range by changing the species of the doping ions doped in the ZnO grains or by controlling the sintering temperature. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Varistor Properties of ZnO Varistors Made of ZnO Grains Doped with at least 
               
               
                 Two Species of Single Doping Ions and the Same Sintering Material 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Sinter 
                 Silver/ 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                   
                 Temp. 
                 Reduction 
                 Green Size 
                 Grog Size 
                 BDV 
                   
                   
                 Cp 
                   
                 Surge 
                 ESD 
               
               
                 No. 
                 Composition 
                 (° C.) 
                 (° C.) 
                 (mm) 
                 (mm) 
                 (V/mm) 
                 α 
                 I L  (μA) 
                 (pF) 
                 Clamp 
                 (A) 
                 (KV) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 69 
                 Zn—1% Si—0.5% Pr + 
                 1065 
                 7472/845 
                 8.4 × 1.20 
                 6.80 × 0.93 
                 261 
                 26 
                 3.2 
                 348 
                 1.69 
                 164 
                 30 
               
               
                   
                 10% G1-00 
               
               
                 70 
                 Zn—1% Si—0.5% Pr + 
                 1107 
                 7472/845 
                 8.4 × 1.20 
                 6.78 × 0.96 
                 204 
                 22 
                 2.3 
                 426 
                 1.88 
                 160 
                 20 
               
               
                   
                 10% G1-00 
               
               
                 71 
                 Zn—1% Si—0.5% Sn—0.5% Sb + 
                 1065 
                 7472/845 
                 8.4 × 1.23 
                 6.70 × 0.97 
                 691 
                 29 
                 1.9 
                 99 
                 1.33 
                 150 
                 30 
               
               
                   
                 10% G1-00 
               
               
                 72 
                 Zn—1% Si—0.5% Sn—0.5% Sb + 
                 1107 
                 7472/845 
                 8.4 × 1.23 
                 6.69 × 0.96 
                 580 
                 35 
                 2.7 
                 150 
                 1.49 
                 200 
                 20 
               
               
                   
                 10% G1-00 
               
               
                 72a 
                 Zn—1% Si—13.5% Sn—1.5% Sb + 
                 1065 
                 7472/845 
                 8.4 × 1.23 
                 6.82 × 1.03 
                 1354 
                 39 
                 23 
                 78 
                 1.43 
                 180 
                 30 
               
               
                   
                 10% G1-00 
               
               
                 72b 
                 Zn—1% Si—13.5% Sn—1.5% Sb + 
                 1107 
                 7472/845 
                 8.4 × 1.23 
                 6.75 × 1.00 
                 1138 
                 37 
                 207 
                 132 
                 1.52 
                 220 
                 30 
               
               
                   
                 10% G1-00 
               
               
                 73 
                 Zn—1% Si—0.5% Pr-0.5% Li + 
                 1065 
                 7472/845 
                 8.4 × 1.23 
                  6.8 × 0.98 
                 234 
                 25 
                 8.7 
                 382 
                 1.75 
                 150 
                 30 
               
               
                   
                 10% G1-00 
               
               
                 74 
                 Zn—1% Si—0.5% Pr-0.5% Li + 
                 1107 
                 7472/845 
                 8.4 × 1.20 
                 6.76 × 0.97 
                 206 
                 25 
                 3.4 
                 441 
                 1.80 
                 100 
                 30 
               
               
                   
                 10% G1-00 
               
               
                 75 
                 Zn—1% Si—0.5% Pr + 
                 1065 
                 7472/845 
                 8.4 × 1.23 
                 6.80 × 0.98 
                 242 
                 26 
                 4.6 
                 374 
                 1.80 
                 164 
                 30 
               
               
                   
                 10% G1-00 
               
               
                 76 
                 Zn—1% Si—0.5% Pr + 
                 1107 
                 7472/845 
                 8.4 × 1.23 
                 6.77 × 1.03 
                 218 
                 24 
                 10 
                 400 
                 1.75 
                 160 
                 20 
               
               
                   
                 10% G1-00 
               
               
                 77 
                 Zn—1% Si—0.5% Sn—0.5% Sb + 
                 1065 
                 7472/845 
                 8.4 × 1.31 
                 6.72 × 0.98 
                 583 
                 34 
                 8.1 
                 135 
                 1.48 
                 150 
                 30 
               
               
                   
                 10% G1-00 
               
               
                 78 
                 Zn—1% Si—0.5% Sn—0.5% Sb + 
                 1107 
                 7472/845 
                 8.4 × 1.31 
                 6.70 × 0.92 
                 602 
                 32 
                 14 
                 122 
                 1.53 
                 200 
                 20 
               
               
                   
                 10% G1-00 
               
               
                   
               
            
           
         
       
     
     EXAMPLE 4 
     The chemical coprecipitation method was used to prepare sample ZnO grains coded Zn-X29 and Zn-X36, as shown in Table 4. The compositions of Zn-X29 and Zn-X36 are given below: 
     
       
         
           
               
               
            
               
                   
                   
               
               
                   
                 Composition 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 ZnO 
                 V 
                 Mn 
                 Cr 
                 Co 
                 Si 
                 B 
                 Pr 
                 Ag 
               
               
                   
                   
               
            
           
           
               
            
               
                 Zn-X29 ZnO Grain 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 mol % 
                 93 
                 2 
                 0.5 
                 1 
                 1 
                 1.5 
                 0.4 
                 0.3 
                 0.5 
               
            
           
           
               
            
               
                 Zn-X36 ZnO Grain 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 mol % 
                 100 
                 2 
                 0.5 
                 0.5 
                 0.5 
                 — 
                 — 
                 — 
                 — 
               
               
                   
               
            
           
         
       
     
     The chemical coprecipitation method was used to prepare sintering materials numbered G0-00, G1-01, and G1-02, as shown in Table 4. Compositions of the sintering materials G0-00, G0-01, and G1-02 are given below: 
     
       
         
           
               
               
            
               
                   
               
               
                 Sintering 
                 Composition (wt %) 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Material 
                 ZnO 
                 SiO 2   
                 B 2 O 3   
                 Bi 2 O 3   
                 Co 2 O 3   
                 MnO 2   
                 Cr 2 O 3   
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 G1-00 
                 8 
                 23 
                 19 
                 27 
                 8 
                 8 
                 7 
               
               
                 G1-01 
                 10 
                 22 
                 19 
                 26 
                 8 
                 8 
                 7 
               
               
                 G1-02 
                 12 
                 21 
                 19 
                 25 
                 8 
                 8 
                 7 
               
               
                   
               
            
           
         
       
     
     The sample ZnO grains and sintering materials were well mixed in a weight ratio of 100:10 and then the mixture were used to make ZnO varistors under the same conditions as provided in Example 1. The varistors were tested on their varistor properties and the results are listed in Table 4. 
     From Table 4, it is learned that sintering materials significantly affect the varistor properties of the ZnO varistors. For example, different sintering materials lead to very different levels of surge-absorbing ability of the ZnO varistors. 
     Thus, the varistor properties of the ZnO varistor can be adjusted in an enlarged range by changing the sintering material mixed with the ZnO grains. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Varistor Properties of ZnO Varistors Made of ZnO Grains Doped with the Same 
               
               
                 Species of Doping Ions and Different Sintering Materials 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Sinter 
                 Silver/ 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                   
                 Temp. 
                 Reduction 
                 Green Size 
                 Grog Size 
                 BDV 
                   
                 I L   
                 Cp 
                   
                 Surge 
                 ESD 
               
               
                 No. 
                 Composition 
                 (° C.) 
                 (° C.) 
                 (mm) 
                 (mm) 
                 (V/mm) 
                 α 
                 (μA) 
                 (pF) 
                 Clamp 
                 (A) 
                 (KV) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 79 
                 Zn-X29 + 
                 1065 
                 7472/845 
                 8.4 × 1.47 
                 6.55 × 1.03 
                 390 
                 21 
                 9 
                 124 
                 1.77 
                 80 
                 30 
               
               
                   
                 10% G1-00 
               
               
                 80 
                 Zn-X29 + 
                 1065 
                 7472/845 
                 8.4 × 1.24 
                 6.48 × 0.94 
                 414 
                 27 
                 4.6 
                 185 
                 1.77 
                 220 
                 30 
               
               
                   
                 10% G1-01 
               
               
                 81 
                 Zn-X29 + 
                 1065 
                 7472/845 
                 8.4 × 1.22 
                 6.58 × 0.91 
                 357 
                 26 
                 7 
                 220 
                 1.70 
                 300 
                 30 
               
               
                   
                 10% G1-02 
               
               
                 82 
                 Zn-X36 + 
                 1065 
                 7472/845 
                 8.4 × 1.37 
                 6.76 × 1.01 
                 311 
                 17 
                 42 
                 263 
                 1.60 
                 350 
                 30 
               
               
                   
                 10% G1-00 
               
               
                 83 
                 Zn-X36 + 
                 1065 
                 7472/845 
                 8.4 × 1.20 
                 6.73 × 0.93 
                 331 
                 22 
                 15 
                 297 
                 1.81 
                 120 
                 30 
               
               
                   
                 10% G1-01 
               
               
                 84 
                 Zn-X36 + 
                 1065 
                 7472/845 
                 8.4 × 1.18 
                 6.82 × 0.89 
                 348 
                 20 
                 27 
                 297 
                 1.82 
                 300 
                 30 
               
               
                   
                 10% G1-02 
               
               
                   
               
            
           
         
       
     
     EXAMPLE 5 
     The chemical coprecipitation method was used to prepare sample ZnO grains coded Zn-X41, Zn-X72, and Zn-X73, as shown in Table 5. Compositions of Zn-X41, Zn-X72, and Zn-X73 are given below: 
     
       
         
           
               
               
               
            
               
                   
                   
               
               
                   
                 Composition 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 ZnO 
                 Mn 
                 Cr 
                 Co 
                 Si 
                 Sb 
                 Bi 
                 Ag 
               
               
                   
                   
               
            
           
           
               
            
               
                 Zn-X41 ZnO Grain 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 mol % 
                 92.3 
                 1.5 
                 0.5 
                 1.0 
                 1.0 
                 2.0 
                 0.2 
                 1.5 
               
            
           
           
               
            
               
                 Zn-X72 ZnO Grain 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 mol % 
                 93.0 
                 1.0 
                 1.0 
                 2.0 
                 — 
                 2.0 
                 1.0 
                 — 
               
            
           
           
               
            
               
                 Zn-X73 ZnO Grain 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 mol % 
                 92.3 
                 0.5 
                 1.0 
                 1.0 
                 1.5 
                 2.0 
                 1.5 
                 — 
               
               
                   
               
            
           
         
       
     
     The chemical coprecipitation method was used to prepare sintering materials numbered G1-08 and G1-11, as shown in Table 5. The compositions of sintering materials G1-08 and G1-11 are given below: 
     
       
         
           
               
               
            
               
                   
                   
               
               
                   
                 Composition (wt %) 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Sintering Material 
                 ZnO 
                 SiO 2   
                 B 2 O 3   
                 Bi 2 O 3   
                 Co 2 O 3   
                 MnO 2   
                 Cr 2 O 3   
                 V 2 O 5   
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 G1-08 
                 8 
                 23 
                 19 
                 27 
                 4 
                 8 
                 4 
                 7 
               
               
                 G1-11 
                 16 
                 21 
                 17 
                 25 
                 4 
                 7 
                 4 
                 6 
               
               
                   
               
            
           
         
       
     
     The sample ZnO grains and the sintering materials were well mixed in a weight ratio of 100:10 and then the mixtures were used to make ZnO varistors under the same conditions as provided in Example 1, except that the sintering temperature is changed to 950° C. The varistors were tested on their varistor properties and the results are listed in Table 5. 
     From Table 5, it is learned that the ZnO varistors can be made with excellent varistor properties under low sintering temperature by using ZnO grains doped with proper species of doping ions and modifying the compositions of the sintering material. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Varistor Properties of ZnO Varistors Made of ZnO Grains Doped with Doping 
               
               
                 Ions and Sintering Materials 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Sinter 
                 Silver/ 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                   
                 Temp. 
                 Reduction 
                 Green Size 
                 Grog Size 
                 BDV 
                   
                 I L   
                 Cp 
                   
                 Surge 
                 ESD 
               
               
                 No. 
                 Composition 
                 (° C.) 
                 (° C.) 
                 (mm) 
                 (mm) 
                 (V/mm) 
                 α 
                 (μA) 
                 (pF) 
                 Clamp 
                 (A) 
                 (KV) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 85 
                 Zn-X41 + 
                 950 
                 7472/845 
                 8.4 × 1.20 
                 6.50 × 0.89 
                 1317 
                 48 
                 1.1 
                 29 
                 1.40 
                 206 
                 30 
               
               
                   
                 10% G1-08 
               
               
                 86 
                 Zn-X41 + 
                 950 
                 7472/845 
                 8.4 × 1.38 
                 6.07 × 0.94 
                 1079 
                 40 
                 1.1 
                 39 
                 1.59 
                 160 
                 30 
               
               
                   
                 10% G1-11 
               
               
                 87 
                 Zn-X72 + 
                 950 
                 7472/845 
                 8.4 × 1.12 
                 6.93 × 0.92 
                 937 
                 47 
                 1.5 
                 54 
                 1.44 
                 280 
                 30 
               
               
                   
                 10% G1-08 
               
               
                 88 
                 Zn-X73 + 
                 950 
                 7472/845 
                 8.4 × 1.10 
                 7.00 × 0.87 
                 1063 
                 42 
                 0.7 
                 42 
                 1.58 
                 400 
                 30 
               
               
                   
                 10% G1-08 
               
               
                   
               
            
           
         
       
     
     EXAMPLE 6 
     The chemical coprecipitation method was used to prepare sample ZnO grains coded Zn-X144, doped with 2 mol % of Si. The sintering material G1-08 as described in Example 5 was also prepared by means of the chemical coprecipitation method. 
     The sample ZnO grains and the sintering material G1-08 were well mixed in a weight ratio of 100:5 and then the mixture was used to make ZnO varistors under the same conditions as provided in Example 1, except that the sintering temperature is changed to 1,000° C. The varistors were tested on their varistor properties and the results are listed in Table 6. 
     The varistors were also tested on their thermistor properties and the results are listed in Tables 7 and  FIG. 10 . 
     From Tables 6 and 7, it is learned that the ZnO varistors can be made with varistor properties and thermistor properties by using ZnO grains doped with proper species of doping ions and by modifying composition of the sintering material. In addition, from the statistics of  FIG. 10 , the resultant ZnO varistors have NTC (Negative Temperature Coefficient) thermistor properties. 
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                 Varistor Properties of ZnO Varistors Made of ZnO Grains Doped with Si and 
               
               
                 G1-08 Sintering Material 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Sinter 
                 Silver/ 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                   
                 Temp. 
                 Reduction 
                 Green Size 
                 Grog Size 
                 BDV 
                   
                 I L   
                 Cp 
                 Surge 
                 ESD 
               
               
                 No. 
                 Composition 
                 (° C.) 
                 (° C.) 
                 (mm) 
                 (mm) 
                 (V/mm) 
                 α 
                 (μA) 
                 (pF) 
                 (A) 
                 (KV) 
               
               
                   
               
               
                 89 
                 Zn-X144 + 
                 1000 
                 7472/845 
                 8.41 × 1.11 
                 6.88 × 0.87 
                 736 
                 23 
                 7.4 
                 144 
                 100 
                 30 
               
               
                   
                 5% G1-08 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 7 
               
             
            
               
                   
               
               
                 NTC Properties of ZnO Varistors Made of ZnO Grains Doped with Si and 
               
               
                 G1-08 Sintering Material 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 25° C. 
                 35° C. 
                 45° C. 
                 55° C. 
                 65° C. 
                 75° C. 
                 85° C. 
                 B Value 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Resistance 
                 4000 
                 3800 
                 3500 
                 3000 
                 2800 
                 2100 
                 1400 
                 1867 
               
               
                 (M ohm) 
               
               
                   
               
            
           
         
       
     
     EXAMPLE 7 
     The chemical coprecipitation method was used to prepare sample ZnO grains coded Zn-X141, doped with 2 mol % of Ag. A sintering material coded G1-38 whose composition is given below was also prepared by means of the chemical coprecipitation method. 
     
       
         
           
               
               
            
               
                   
               
               
                 Sintering 
                 Composition (wt %) 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Material 
                 Bi 2 O 3   
                 B 2 O 3   
                 Sb 2 O 3   
                 Co 2 O 3   
                 MnO 2   
                 Cr 2 O 3   
                 V 2 O 5   
               
               
                   
               
               
                 G1-38 
                 32 
                 4 
                 15 
                 15 
                 15 
                 15 
                 4 
               
               
                   
               
            
           
         
       
     
     The sample ZnO grains and the sintering material G1-38 were well mixed in a weight ratio of 100:10 and then the mixture was used to make ZnO varistors under the same conditions as provided in Example 1. The varistor was tested on its varistor properties and the results are listed in Table 8. 
     The varistors were also tested on its thermistor properties and the results are listed in Table 9 and  FIG. 11 . 
     From Tables 8 and 9, it is learned that the ZnO varistors can be made with varistor properties and thermistor properties by using ZnO grains doped with proper species of doping ions and modifying composition of the sintering material. In addition, from the statistics of  FIG. 11 , the resultant ZnO varistor possesses PTC (Positive Temperature Coefficient) thermistor properties. 
     
       
         
           
               
             
               
                 TABLE 8 
               
             
            
               
                   
               
               
                 Varistor Properties of ZnO Varistors Made of ZnO Grains Doped with Ag and 
               
               
                 G1-38 Sintering Material 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Sinter 
                 Silver/ 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                   
                 Temp. 
                 Reduction 
                 Green Size 
                 Grog Size 
                 BDV 
                   
                 I L   
                 Cp 
                 Surge 
                 ESD 
               
               
                 No. 
                 Composition 
                 (° C.) 
                 (° C.) 
                 (mm) 
                 (mm) 
                 (V/mm) 
                 α 
                 (μA) 
                 (pF) 
                 (A) 
                 (KV) 
               
               
                   
               
               
                 90 
                 Zn-X141 + 
                 1060 
                 7472/845 
                 8.41 × 1.0 
                 7.55 × 0.83 
                 846 
                 9 
                 48 
                 156 
                 630 
                 20 
               
               
                   
                 5% G1-38 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 9 
               
             
            
               
                   
               
               
                 PTC Properties of ZnO Varistors made of ZnO Grains Doped with Ag and 
               
               
                 G1-38 Sintering Material 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                   
                   
                 B 
               
               
                   
                 25° C. 
                 35° C. 
                 45° C. 
                 55° C. 
                 65° C. 
                 75° C. 
                 85° C. 
                 Value 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Resistance 
                 1700 
                 2100 
                 2600 
                 3050 
                 4100 
                 5000 
                 5000 
                 −1918 
               
               
                 (M ohm) 
               
               
                   
               
            
           
         
       
     
     EXAMPLE 8 
     ZnO grains of two formulas, A and B, which were doped with different doping ions and mixed with different sintering materials were used. Therein, Formula A contains Zn-X144 ZnO grains of Example 6 mixed with 5% of G1-08 sintering material. After sintering, Formula A gave strong varistor properties and considerable NTC properties (yet has high resistance at 25° C.). 
     Formula B contains Zn-X144 ZnO grains of Example 6 mixed with 30% of N-08 sintering material by weight. After sintering, Formula B gave meaningful NTC properties (yet has high resistance at 25° C.) but had inferior varistor properties. Therein, N-08 has the below composition. 
     
       
         
           
               
               
            
               
                   
                   
               
               
                   
                 Composition (wt %) 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Sintering Material 
                 Co 2 O 3   
                 MnO 2   
                 Cr 2 O 3   
                 NiO 
                 SiO 2   
                 V 2 O 5   
               
               
                   
               
               
                 N-08 
                 23 
                 37 
                 10 
                 23 
                 5 
                 2 
               
               
                   
               
            
           
         
       
     
     Formula A and Formula B were respectively added with a binder and a solvent, and then were ball ground and pulped so as to be made into green tapes having a thickness of 20-60 μm through a tape casting process. 
     According to the know approach to making multi-layer varistors, the green tapes of Formula A and Formula B were piled up and printed with inner electrode, to form green tape  10  for the dual-function chip as shown in  FIG. 12 . After binder removal, the green tape  10  was placed into a sintering furnace to be heated at 900-1050° C. for 2 hours. 
     Then two ends of the green tape  10  were coated with silver electrode and sintered at 700-800° C. for 10 minutes to form the dual-function chip element. Measurement of electricity of the dual-function chip element indicates that the chip element possesses varistor properties and excellent NTC thermistor properties (with low resistance at room temperature). 
     Then electrical properties of the chip element, including ESD tolerance and thermistor properties, were also tested and are provided in Tables 10 and 11. 
     From Tables 10 and 11, it is learned that the chip element is capable of enduring 20 times of ESD 8KV applied thereto and has 10.2K ohm of NTC thermistor properties while presenting low resistance at room temperature. Thus, the chip element is a dual-function element possessing both varistor properties and thermistor properties. 
     
       
         
           
               
             
               
                 TABLE 10 
               
             
            
               
                   
               
               
                 Varistor Properties of Dual-Function Element Made of Two 
               
               
                 Formulas containing ZnO grains doped with different Species 
               
               
                 of Doping Ions and Different Sintering Materials 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Sinter 
                   
                   
                   
                   
                   
                   
               
               
                   
                   
                 Temp. 
                 Reduction 
                 Green Size 
                 Grog Size 
                 BDV 
                 Cp 
                 ESD 
               
               
                 No. 
                 Composition 
                 (° C.) 
                 (° C.) 
                 (mm) 
                 (mm) 
                 (V/mm) 
                 (pF) 
                 (KV) 
               
               
                   
               
               
                 91 
                 Zn-X141 + 5% G1-38 
                 1000 
                 845 
                 1.95 × 0.97 
                 1.6 × 0.795 
                 14 
                 376 
                 pass 
               
               
                   
                 Zn-X144 + 30% N-08 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 11 
               
             
            
               
                   
               
               
                 NTC Properties of Dual-Function Element Made of Two Formulas 
               
               
                 containing ZnO Grains Doped with Different Species of Doping Ions and 
               
               
                 Different Sintering Materials 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 25° C. 
                 35° C. 
                 45° C. 
                 55° C. 
                 65° C. 
                 75° C. 
                 85° C. 
                 B Value 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Resistance 
                 10.2 
                 8.6 
                 7.5 
                 5.4 
                 4.2 
                 3.3 
                 2.7 
                 2367 
               
               
                 (K ohm) 
               
               
                   
               
            
           
         
       
     
     EXAMPLE 9 
     Zn-X300 ZnO grains of Table 12 were made by immersing ZnO powder of 0.6 micron into a solution containing doping ions, and drying and sintering the doped ZnO powder at 1050° C. for 5 hours, and grinding the sintered product into fine grains. Zn-X300 ZnO grains have the composition shown below: 
     
       
         
           
               
            
               
                   
               
               
                 Zn-X300 ZnO Grain 
               
            
           
           
               
               
               
            
               
                   
                 Composition 
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Zn 
                 Sn 
                 Si 
                 Al 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 mol % 
                 0.97 
                 0.01 
                 0.02 
                 0.000075 
               
               
                   
                   
               
            
           
         
       
     
     The chemical coprecipitation method was used to prepare a sintering material numbered G-200, as shown in Table 12. The composition of the sintering material G-200 is given below: 
     
       
         
           
               
               
            
               
                   
               
               
                 Sin- 
                   
               
               
                 tering 
                 Composition (wt %) 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Material 
                 Bi 2 O 3   
                 Sb 2 O 3   
                 MnO 2   
                 Co 2 O 3   
                 Cr 2 O 3   
                 Ce 2 O 3   
                 Y 2 O 3   
               
               
                   
               
               
                 G-200 
                 20 
                 20 
                 20 
                 20 
                 10 
                 6 
                 4 
               
               
                   
               
            
           
         
       
     
     The sample ZnO grains and the sintering material were well mixed in a weight ratio of 100:17.6 and then ground. The ground product was used to make ZnO varistors under the same conditions as provided in Example 1, except that the sintering temperature was changed to 980° C. and 1020° C. The resultant ZnO varistors were tested on their varistor properties and the results are listed in Table 12. 
     
       
         
           
               
             
               
                 TABLE 12 
               
             
            
               
                   
               
               
                 Varistor Properties of Multi-Layer Varistor Made of Zn-X300 Grains 
               
               
                 and G-200 Sintering Material 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Sinter 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                   
                 Temp. 
                 Green Size 
                 Grog Size 
                 BDV 
                   
                 I L   
                 Cp 
                   
                 Surge 
                 ESD 
               
               
                 No. 
                 Composition 
                 (° C.) 
                 (mm) 
                 (mm) 
                 (V/mm) 
                 α 
                 (μA) 
                 (pF) 
                 Clamp 
                 (A) 
                 (KV) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 92 
                 Zn-X300 + 
                 1020 
                 8.4 × 1.20 
                 6.78 × 0.94 
                 530 
                 29 
                 15 
                 261 
                 1.42 
                 264 
                 30 
               
               
                   
                 17.6% G-200 
               
               
                 93 
                 Zn-X300 + 
                 980 
                 8.4 × 1.20 
                 6.79 × 0.96 
                 660 
                 28 
                 16 
                 193 
                 1.38 
                 398 
                 30 
               
               
                   
                 17.6% G-200 
               
               
                   
               
            
           
         
       
     
     EXAMPLE 10 
     Zn-X301 ZnO grains of Table 13 was made by immersing ZnO powder of 0.6 micron into a solution containing doping ions, and drying and calcining the doped ZnO powder at the sintering temperature of 850° C. for 30 minutes in air or in argon gas, and grinding the sintered product into fine grains. Zn-X301 ZnO grains have the composition as below: 
     
       
         
           
               
            
               
                   
               
               
                 Zn-X301 ZnO Grain 
               
            
           
           
               
               
               
            
               
                   
                 Composition 
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Zn 
                 Sn 
                 Si 
                 Al 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 mol % 
                 0.983 
                 0.006 
                 0.001 
                 0.0003 
               
               
                   
                   
               
            
           
         
       
     
     The chemical coprecipitation method was used to prepare a sintering material numbered G-201, as shown in Table 13. 
     The composition of G-201 sintering material is given below: 
     
       
         
           
               
               
            
               
                   
               
               
                 Sin- 
                   
               
               
                 tering 
                 Composition (wt %) 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Material 
                 Bi 2 O 3   
                 Sb 2 O 3   
                 MnO 2   
                 Co 2 O 3   
                 Cr 2 O 3   
                 Ce 2 O 3   
                 Y 2 O 3   
               
               
                   
               
               
                 G-201 
                 32 
                 16 
                 16 
                 16 
                 10 
                 6 
                 4 
               
               
                   
               
            
           
         
       
     
     The sample ZnO grains and the sintering material were well mixed in a weight ratio of 100:15 and then ground. Then, the conventional technology for making multi-layer varistors was implemented while pure silver was taken as the material for inner electrode and inner electrode printing was conducted for two or four times. The product was sintered at low temperature (sintering temperature of 850° C.) to form multi-layer varistors having 0603 specifications. Varistor properties of the multi-layer varistors made by two and four times of inner electrode printing were both measured and the results are given in Table 13. 
     From Table 13, it is learned that the varistor made by two times of inner electrode printing has a 30 A tolerance to surge of 8/20 μs, while the varistor made by four times of inner electrode printing has a tolerance up to 40 A against the same surge. Thus, the ZnO varistors can be made with excellent varistor properties under low sintering temperature by controlling the number of times where inner electrode printing is conducted. 
     
       
         
           
               
             
               
                 TABLE 13 
               
             
            
               
                   
               
               
                 Properties of Multi-Layer Varistor Made by Sintering Zn-X301 + 15% G-201 at 
               
               
                 Low Temperature (Sintering Temperature at 850° C.) 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Ag 
                 Sinter 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                   
                 Coating 
                 Temp. 
                 Green Size 
                 Grog Size 
                 BDV 
                   
                 I L   
                 Cp 
                   
                 Surge 
                 ESD 
               
               
                 No. 
                 Composition 
                 Times 
                 (° C.) 
                 (mm) 
                 (mm) 
                 (V/mm) 
                 α 
                 (μA) 
                 (pF) 
                 Clamp 
                 (A) 
                 (KV) 
               
               
                   
               
               
                 94 
                 Zn-X301 + 
                 2 
                 850 
                 1.95 × 0.97 
                 1.6 × 0.8 
                 35.5 
                 33 
                 1.1 
                 34 
                 1.38 
                 30 
                 8 
               
               
                   
                 15% G-201 
               
               
                 95 
                 Zn-X301 + 
                 4 
                 850 
                 1.95 × 0.97 
                 1.6 × 0.8 
                 32.3 
                 35 
                 0.5 
                 98 
                 1.33 
                 40 
                 8 
               
               
                   
                 15% G-201