Effect of cerium addition on microstructure and texture of aluminum foil for electrolytic capaci

JOURNAL OF RARE EARTHS,Vol.28,No.1,Feb.2010,p.134F j y I M I S &T y j ()I M T y R &D j W NG y (f 6@63)DOI 6S ()666Effect of cerium addition on microstr ucture and texture of aluminum foil for electr olytic capacitor sWANG Haiy an (王海燕)1,2,L I Wenx ue (李文学)1,REN Hu iping(任慧平)1,HUANG L iy in g (黄丽颖)1,WAN G Xia ngy ang(王向阳)1(1.School of Material and Metall urgy Engineering,Inner Mongolia University of Science and Technology,Baotou 014010,China;2School of Materials S cience and En-gineering,University of Science and Technology B eijing,Beijing 100083,China)Received 23October 2008;revised 27August 2009Abstract:Anode foil of aluminum electrolytic capacitor,which requires large surface area for high capacitance,were prepared by rolling,an-nealing and electrochemical etching.Effects of cerium addition on the capacitance of aluminum electrolytic capacitors were investigated.Mi-crostructure of the aluminum foil surface was observed by optical microscopy (OM)and scanning electron microscopy (SEM).Electron back scattered diffraction (EBSD)was also employed to reveal texture evolvement of cold-rolled aluminum foil after annealed.The results showed that addition of cerium contributed to higher <100>texture and higher etch pit density as well.Moreover,the distribution of etch tunnels was more uniformcompared with that without cerium addition.Perfect specificsurface area was obtained with 0.0074%addition of cerium.Keywords:electrolytic capacitor;aluminum foil;rare earthsAluminum electrolytic capacitors are widely used in all types of electronic equipments.High voltage electrolytic ca-pacitors consist of an electrolyte put on an anode made from a 40–100μm thin high-purity aluminum foil with an oxide layer serving as dielectric [1].The capacitance of dielectrics depends on the dielectric constant,the thickness,and the surface area.Among those factors,especially for aluminum electrolytic capacitors,the enlargement of the surface area is most effective from the practical viewpoint because of the trend toward capacitor size miniaturization [2,3].The effective surface can be greatly increased through an etching process that forms narrow channels along the crystallographic <001>directions into the foils.Thus,with regard to the cube-tex-ture in the aluminum foils for electrolytic capacitors,produc-tion has to be optimized so as to provide maximum number of cube-nuclei as well as good growth prospect of the cube-orientation [1,4].A strongly textured material consisting of grains with <001>direction parallel to the sheet normal is favorable.In practical application this is achieved by pro-ducing sheets with a strong cube-texture {001}<100>.To increase the surface area,investigation of pit formation seems very important.Many studies have discussed the pits using high-purity aluminum foils during DC etching in HCl solutions,including the shape,morphology of pitting corro-sion,the tunnel pit growth and the effect of impurities on etching behavior [5].In addition,since the thickness of these foils is only 100–110μm,the tunnel length must be con-trolled to less than 50μm,in order to form a non-piercing layer after etching on double surfaces of Al foil [6].Thus,it is necessary to study the principle of tunnel growth to controltunnel length of pits and maintain a non-piercing layer.It has been recognized that the solution composition in-duces certain mechanical,physical and chemical changes,which can lead to activation of chemical and electrochemical reactions,thus result in accelerated material removal and corrosion [7].In general,hydrochloric acid is used for the etching solution,but the size and distribution of etch pits are not uniform with low density as a result of high corrosive-ness.However,effect of solution composition,especially for rare earth element on pitting corrosion,has not been exten-sively or systematically studied,it can be expected that addi-tion of rare earth during electrochemical etching of alumi-num may induce optimum etch structure,and produces high surface of aluminum foil.Therefore,to increase the specific internal area by reducing the rate of dissolution concentrated on local area and producing dense etch pits,studies of rare earth addition was carried out in this work.1ExperimentalHigh-purity (>99.9972%)aluminum foils with different cerium additions of 0.006wt.%,0.0074wt.%,and 0.012wt.%were used.The impurities in the aluminum are as follows (wt.%):Fe 0.00071,Si 0.00070,Cu 0.00125,Zn 0.00014,and Ti 0.00005.The thickness of aluminum ingot was 18mm,after homogenized treatment,reheating,hot rolling and reversibly cold rolling with a thickness reduction of as much as 98%to a final thickness of 0.11mm.Finally,all speci-mens were isothermally annealed for various times in salt bath furnace.For DC electrolytic etching test,specimensound at ion it em:P ro e ct supported b the nne r ongolia mporta nt cie nc e e chnolog Pro ec t 20071911a nd nner ongolia e chnolog ese arch evelopment Pro ec tCorre sponding a uthor :A H ai an E-ma il:w ind :10.101/1002-07210900-1WANG Haiyan et al.,Effect of cerium addition on microstructure and texture of aluminum foil for electrolytic capacitors 135were cut into 10mm ×55mm,and exposed with an area of 1cm 2to the etching solution.The rest of the specimen was masked with polyester tape.All the specimens were pre-treated in HCl+H 2SO 4(3:1)solution and then rinsed with deionized water before electrolytic etching test.Surface morphology of the recrystallized foils and etch tunnels were observed by means of optical microscopy and scanning electron microscopy (SEM).The evolution of cube-texture during isothermal annealing of the cold rolled foils was tracked by crystallographic texture using electron back scattered diffraction (EBSD)micro texture analysis [8].2Results and discussionFig.1(a –d)show the shape and distribution of the etch pit formed in the solutions with cerium additions of 0wt.%,0.006wt.%,0.0074wt.%,and 0.012wt.%respectively.It is found that there are noticeable differences in the morpholo-gies of etch pits,depending on the additive.The intensity of the cube-orientation {001}<100>changed with the increas-ing of cerium content.As shown in Fig.1(a),when cerium is not applied,about 20%of the tunnels observed at 15μm become alive in the process of tunnel growth,while more tunnels are observed with cerium addition and the survival rate increases in Fig.1(b),(c)and (d).Compared with those with additives,the density of etch pits is low in pure alumi-num foil.The length,widths and depths of individual pits weremeasured for each specimen.As shown in Fig.1,under the conditions without additive and with 0.006wt.%Ce,0.0074wt.%Ce,and 0.012wt.%Ce,the density of etch pit is 0.9×105,3.2×105,4.3×105and 6.1×105pits/cm 2,and the av-erage size is 14.0,12.2,9.0and 8.6μm,respectively.The number of etch pits generated in the etching solutions with rare earth additives is about twice as much as that formed without additive as shown in Fig.1(a).This system is typical of the Al-RE (rare earth)series.Ce-rium exists as stable intermetallic compounds in aluminum,and with a negative enthalpy of mixing in the liquid phase [9].There are some kinds of micro impurity,such as Fe,Si,Mn in aluminum,Ce will form compound of CeFe 2Al 10,CeSi 2A13,CeMn 4Al 8,Ce 2Mn 7Al 10,(CeMg)Al 2,etc.[10]These com-pounds can purify the matrix and eliminate the obstruction of impurity,especially the effect of Fe on nucleation and growth of cube orientation grains during annealing process,and thus increase the cube orientation of aluminum foil,which agrees with the etch pits measurement shown in Fig.1.Fig.2shows the texture evolution of samples with differ-ent rare earth contents cold-rolled and annealed at 530°C for 600s.In Fig.2(a),some minor intensities of cube-orienta-tion {001}<100>are observed.By contrast,with cerium addition,the texture has changed in Fig.2(b).The ODF (ΔΦ=5°)is characterized by very sharp cube-orientation dominating over the rolled texture,for the aluminum foil with 0.0074wt.%pare to the Fig.2(b),the 0.019wt.%Ce addition showed weaker cube-orientationtextureFig.1Recrystallized microstructureof aluminumfoil without Ce (a),with 0.006wt.%Ce (b),with 0.0074wt.%Ce(c)and with 0.012wt.%Ce(d)F T x f f 53f 6(),%()%()ig.2e tureevolution o the samples a terannealed at 0o C or 00s without Ce a with 0.0074wt.Ce b and with 0.012wt.Ce c136JOURNAL OF RARE EARTHS,Vol.28,No.1,Feb.2010(Fig.2(c)).To sum up,annealed samples with cerium addi-tion have higher intensity of cube-orientation {001}<100>and the cube texture reaches the maximum with 0.0074wt.%Ce.With further addition of Ce,cube texture density weak-ens.In order to develop greater understanding of texture evolvement,{111}pole figures,determined from EBSP,for the above specimens with different rare earth contents cold-rolled and annealed at 530°C,are shown in Fig.3,where separate measurements were made on small and large grains.The sample of cold rolled and annealed high purity alu-minum had a random texture with few grains exhibiting cube,S and Cu orientations (see Fig.3(a)),while addition of 0.0074wt.%cerium resulted in sharp cube texture,i.e.,more than 90%{100}<001>orientations with little other texture component are observed in Fig.3(b).With more Ce addi-tions,Fig.3(c)indicates that the orientations are similar to those of the grains shown in Fig.3(b),i.e.,cube texture with minor other texture components.However,the cube texture is less sharp,the contour line tend to sparse,with other ori-entations starting to be presented.It is known that addition of rare earth can refine crys-tal [11,12],and induce boundary corrosion,since higher bound-ary density reduces segregation of impurity atoms and dis-tribution inhomogeneity of current density.Therefore,it is possible to increase the specific electric capacity through de-creasing the grain size under special corrosion conditions [4].With increasing rare earth content,higher defect density and corresponding local concentration of corrosion current den-sity around grain boundaries induce preferred corrosion of rge corrosion holes or channels will appear frequently along the grain boundaries,which lead to the in-homogeneity of corroded structure.Taking the microstructure and texture evolution into ac-count,we conclude that control of cerium addition produces different recrystallization textures during annealing of cold-rolled aluminum,from which the favorite corrosion orienta-tion can be found.When aluminum is electrochemically etched,cubic pits are initially formed and turned into tunnels as the pits grow up.The SEM images of the aluminum etched pit shown in Fig.4corroborate the microstructural evolvement.Fig.4(a)shows that the tunnel length is approximately 20μm.Fig.4(b)indicates that many dark and small tunnels distribute along the tunnel depth in the etched aluminum foils with 0.0074wt.%Ce addition.It is easy to verify that cerium additive induces higher density of small etch pits and contributes their further growth into tunnels by enhancing the mass transport inside one-dimensional tunnel structure.Uniform tunnel length distribution increases the surface area which favors capacitance.It should be noted,however,that large etch pit forms in Fig.4(c)which is rooted in the connections of many small pits ，thus the tunnel wall along its depth will pare to the smaller pits,the surface of tunnel will decrease,as a result of blocking of small pits or tunnels by oxides with the increasing of cerium.The specific capaci-tance obtained by using alternating current impedance [13]shows that the capacitance at first increases,and then de-creases with increasing rare earth addition,it means that the content of rare earth should be controlled to certain extent,and the highest specific surface area was obtained from the specimen with 0.0074wt.%Ce.The effect of cerium on the corrosion of aluminum has been investigated by incorporating cerium into aluminum substrates and observing the subsequent anodic oxidation behavior of the alloys [14].The corrosion resistance of alumi-Fig.3{111}pole figures for sample after annealed at 530°C for600s without Ce (a),with 0.0074wt.%Ce (b)and with 0.012wt.%Ce(c)F S M f f (),%()%()ig.4E images o the aluminum oil without Ce a with 0.0074wt.Ce b and with 0.012wt.Ce cWANG Haiyan et al.,Effect of cerium addition on microstructure and texture of aluminum foil for electrolytic capacitors137num in aqueous media can be attributed to rapid formation of surface oxide film[15].Hydrochloric acid is used as etching solution,and Cl–plays an important role in spot corrosion and development,which has great effect on the oxide scale dissolution of the aluminum foil surface.Cl–is adsorbed on the special location of oxide surface,dissolves the oxide scales and formed Al(OH)2Cl,Al(OH)Cl2,AlCl2+,AlCl3, Al(OH)Cl+,finally produces etch pits on the surface of Al. Cerium and aluminium are oxidized at the alloy/film inter-face and incorporated into the anode film.In general,the oxide layer spread over the macroscopic surface,improves active hole forming[16],thus increases the specific internal area.However,when the content of cerium keeps increasing, the distribution inhomogeneity induces the substantial in-crease of chemistry activity,and results in large number of etching pits,which generates film breakdown over alumi-num surface.During the growth of tunnel,metal dissolution occurs from its surface,while the tunnel sidewalls are cov-ered with oxide film which restrain corrosion,which results in inferior capacitance[17].As mentioned above,we can estimate that the introduction of cerium reduces the thickness of oxide film on aluminum metal and induces the nucleation of etch pits.Therefore,ad-dition of rare earth has certain potential to be used for the application of aluminum electrolytic capacitors,and the in-volved researches are still underway to be carried out.3ConclusionsCerium was introduced to control the etch pits and tunnel length distribution on aluminum foil for electrolytic capaci-tors.The dissolution of cerium during electrochemical etch-ing might induce the optimum etch structure,produce high surface area and finally increase the capacitance,since addi-tion of cerium prohibited initial oxide film formation during etching but favors the formation of corrosion pits.In this work,the most perfect surface area was obtained when the addition of Ce reached0.0074wt.%.References:[1]Engler O,Huh M Y.Evolution of the cube texture in high pu-rity aluminum capacitor foils by continuous recrystallization and subsequent grain growth.Mater.Sci.Eng.A,1999,271(1): 371.[2]Scherer J,Magnussen O M,Ebel T,Behm R J.Initial stages ofthe anodic etching of aluminum foils studied by atomic force microscopy.Corros.Sci.,1999,41(1):35.[3]Oh H J,Lee J H,Ahn H J,Jeong Y S,Park N J,Kim S S,ChiC S.Etching characteristics of high-purity aluminum in hy-drochloric acid solutions.Mater.Sci.Eng.A,2007,449-451(25):348.[4]Mao Weimin,Chen Lei,Sa Liman,Yu Yongning,Li Yunfeng.Influence of grain boundaries on corrosion structure of low voltage aluminum foil.T he Chinese Journal of Nonferrous Metals(in Chin.),2004,14(1):1.[5]Osawa N,Fukuoka K.Pit nucleation behavior of aluminiumfoil for electrolytic capacitors during early stage of DC etching.Corros.Sci.,2000,42(3):585.[6]Wang Mei,He Yedong.Growth and passivation of aluminumetch tunnels at on-off controlling direct current in6wt.%HCl solution.Rare Metals,2008,27(2):205.[7]Liu Guili,Zhang Guoying,Li Rongde.Electronic theoreticalstudy of the interaction between rare earth elements and impu-rities at grain boundaries in steel.J.Rare Earths,2003,21(3): 372.[8]Baker I,Li J.An EBSP study of isothermally-annealed cold-rolled nickel.Microscopy Research and T echnique,2004,63: 289.[9]Kang Y B,Peltona A D,Chartranda P,Fuerst C D.Criticalevaluation and thermodynamic optimization of the Al-Ce,Al-Y, Al-Sc and Mg-Sc binary put.Coupling Phase Diagrams Thermochem.,2008,32:413.[10]Deng Yunlai,Zhang Xinmin,Liu Chunming,Chen Zhiyong,Zhou Zhuoping.The effect of trace RE on the evolution of tex-tures in high voltage aluminum capacitor foils.Functional Material,2002,33(1):60.[11]Li Chunlong,Wang Yunsheng,Chen Jianjun,Liu Chengjun,Jiang Maofa.Effects of rare earth on structure and mechanical properties of clean BNbRE steel.J.Rare Earths,2005,23(4): 470.[12]Lin Qin,Guo Feng,Zhu Xingyuan.Behaviors oflanthanum and ce-rium on grain boundaries in carbon manganese clean steel.J.Rare Earths,2007,25(4):485.[13]Wang Xiangyang.Study of the Electrolytic Etching of Ce Alumi-num Foil for Gigh Voltage Anode of Electrolytic Capacitor.Inner mongolia:InnerMongoliaUniv.Sci.&Tech.,2008.5.[14]Tomcsany L,V arga K,Bartik Ietal.Study of the interaction ofCl–with A passive film on Al and initiation of pitting corrosion.Electrochim.A cta,1989,34(5):855.[15]Mishra A K,Balasubramaniam R.Corrosion inhibition of alu-minium by rare earth chlorides.Mater.Chem.Phy s.,2007, 103(2-3):385.[16]Crossland A C,Thompson G E,Skeldon P,Wood G C,SmithC J E,Habazaki H,Shimizu K.Anodic oxidation of Al-Ce Al-loys and inhibitive behavior of Cerium species.Corros.Sci., 1998,40(6):871.[17]Kang J W,Shin Y H,Tak Y S.Growth of etch pits formedduring sonoelectrochemical etching of aluminum.Electro-chimica A cta,2005,51(5):1012.。