Properties of Ceramic Layer Formed by Centrifugal Thermit Reaction with Silicon Sludge Replacement

Transcription

1 Materials Transactins, Vl. 49, N. 8 (2008) pp t 1873 #2008 The Japan Institute f Metals Prperties f Ceramic Layer Frmed by Centrifugal Thermit Reactin with Silicn Sludge Replacement M. T. Le 1;2, D. J. Kim 1; *, J. R. Lee 3, C. G. Kim 2 and H. S. Chung 1 1 Divisin Minerals & Materials Prcessing, Krea Institute f Gescience and Mineral Resurces (KIGAM), Daejen , Krea 2 Department Materials Science and Engineering, Chungnam Natinal University, Daejen , Krea 3 Department Gesystem Engineering, Kangwn Natinal University, Chunchen, Kangwn-d , Krea A ceramic-lined steel pipe has been prepared by the thermit reactin f aluminum-ferric ide under centrifugal frce with the partial replacement f aluminum by the silicn sludge btained frm a semicnductr wafer cutting prcessing. The ceramic layer n the pipe inner surface cnsists f the crystalline structures f mainly crundum (-Al 2 O 3 ) and hercynite (FeAl 2 O 4 ). The sludge replacement increases the layer density frm 2.9 g/cm 3 t 3.5 g/cm 3 and the hardness frm 1450 Hv t 1780 Hv. The amrphus phases f mullite (Al 6 Si 2 O 13 ) frmed between the crystalline structures are fund t be respnsible fr a significant imprvement f the ceramic layer density. [di: /matertrans.mra ] (Received April 9, 2008; Accepted May 30, 2008; Published July 9, 2008) Keywrds: ceramic-lined pipes, cmpsites, centrifugal thermit reactin, densificatin 1. Intrductin The imprtant advantage f the thermit prcess is t use ethermic reactins fr material synthesis. Thermit reactin typically generates mlten prducts due t its high ethermic nature. The mlten ceramic and metal in a steel pipe prduced by thermit reactin can be separated in layers by virtue f their specific gravity difference under centrifugal frce. After cling, the ceramic lining cnsists f an inner ceramic layer and an intermediate irn ne n the steel pipe surface. This technique knwn as centrifugal-thermit prcess was used t prduce the ceramic-metal cmpsite pipe. 1 3) The cmpsite pipe is gd against crrsin, and thermal and mechanical shck, 4) and has been recmmended fr the industrial applicatins t the transprt f abrasive and crrsive materials such as cal, cke, cal cinder, mineral pwder, cement mrtar, mlten aluminum, and il water miture. The service life f the cmpsite pipe is abut ten times lnger than that f steel pipe used befre. 5,6) There were many researches using a thermit miture f Fe 2 O 3 and Al system. Aluminum ide and irn are prduced by the reactin f: Fe 2 O 3 þ 2Al ¼ Al 2 O 3 þ 2Fe ð1þ where the enthalpy f frmatin is 836 kj/ml. The thermit technique that has been applied fr the ceramic lining f pipes withut the use f additives hld sme demerits, mstly due t the frmatin f large pres within the ceramic layer causing the lw dense structure. Varius additives such as MgO, SiO 2,Na 2 B 4 O 7, etc. have been used t slve this prblem by reducing the melting temperature. 1,5 8) Silicn is knwn as a strng reducing element, leading t the thermit reactin f: 2Fe 2 O 3 þ 3Si ¼ 3SiO 2 þ 4Fe: ð2þ *Crrespnding authrs, djkim@kigam.re.kr where the enthalpy f frmatin is 311 kj/ml. If silicn is used as a partial replacement f Al in the thermit system f Fe 2 O 3 -Al, it wuld wrk as a reductant fr Fe 2 O 3 as well as a surce f SiO 2. In ther wrds, even thugh it cntributes t the heat generatin needed fr melting the mitures less than Al, it prvides the cmpsitin f SiO 2 lwering the melting temperature, which is believed t make the ceramic lining structures denser. Ceramic lining n the inner surface f a steel pipe was carried ut by the centrifugal thermit technique. Silicn sludge prduced during the cutting and plishing prcess f silicn wafers practiced in a lcal cmpany was selected as a surce f Si. In the present investigatin, a miture f Fe 2 O 3 and Al was used as the base and the silicn sludge was added in place f aluminum. The micrstructure f the ceramic layers and their densificatin mechanism were studied. 2. Eperimental Prcedure A miture f thermit material was prepared by miing stichimetrically aluminum metal (<100 mm with 39 mm average size, 99.7% purity, Alca Brazil) and ferric ide (<5 mm with 1.6 mm average size, 99:0% purity, EWIC Krea). In subsequent tests, aluminum was replaced by silicn sludge in the range 5 50 mass%. The sludge cnsisted f 70 mass% SiC and 30 mass% Si accrding t the chemical analysis and its average particle size was abut 8 mm. The raw materials were mied fr 1 hur in a barrier mier. A carbn steel pipe 300 mm lng with an inner diameter f 54 mm and a wall thickness f 3 mm was munted in a hrizntal centrifuge apparatus. The steel pipe was filled with the miture at different filling ratis. A ciled tungsten wire in 0.5 mm diameter was electrically heated at ne pint f the miture t start the thermit reactin. Upn the cmpletin f the reactin, the centrifuge machine was allwed t rtate fr 10 mre minutes until the reactr bdy cled dwn. A piece f the ceramic layer was taken ut frm the pipe and munted n a thermsetting plastics, then grund and

2 Prperties f Ceramic Layer Frmed by Centrifugal Thermit Reactin with Silicn Sludge Replacement 1869 Fig. 1 XRD pattern f ceramic layer prepared at a filling rati f 40% and a centrifugal frce f 120 G, with (a) 0%, (b) 5%, (c) 10%, (d) 20%, (e) 30%, (f) 50% silicn sludge. plished with different particle sizes f diamnd pwder fr the purpses f SEM and EDX (JEOL JSM-6380LA) analysis. The ceramic layer was crushed int pwder and its phase cmpsitin was identified by X-ray diffractin (XRD, Rtafle Ru-200B CuK, Rigaku). Its hardness was measured by the Vickers methd (Hardness tester, MVK-E, Akashi). 3. Results and Discussin 3.1 Crystalline structures Figure 1 represents the X-ray diffractin pattern f the ceramic layers prepared with the replacement f silicn sludge in the range 5 50%. The majr intensity peaks f crundum (-Al 2 O 3 ) and hercynite (FeAl 2 O 4 ) are clearly fund in the layer synthesized withut the replacement f the sludge (Fig. 1(a)). The same crystalline structure peaks appear with the sludge replacement up t 10% f the ttal miture as shwn in Figs. 1(b) and 1(c). Hwever, the crystalline peaks crrespnding t the sludge frm 20% t 50% d nt clearly appear as indicated in Figs. 1(d), 1(e), and 1(f) as fr the nes at 10% and less. The higher the silicn sludge replacement int the thermit miture is, the lwer is the reactin temperature. The silicn sludge increase may nt prvide an enugh time t maintain high temperature sufficient t cnvert all cmpnents in the ceramic layer int crystalline structures during the cling. In ther wrds, the X-ray peaks f crundum and hercynite are nt strng r disappeared at mre than 10% silicn sludge replacement. Interestingly, the intensity peaks f silicn and carbn cmpunds derived frm Si and SiC in the sludge are nt fund at all as given in Fig. 1. The melt frmed at the high cmbustin temperature frm the thermit reactin f the Fe 2 O 3 -Al-Si system slidifies as cling prceeds. Hwever, Si r C cmpunds besides crundum and hercynite structures seem t remain in the amrphus state. 3.2 Apparent density and hardness The thermit pwder miture filled in a steel pipe was pressed against the inner wall by centrifugal frce. After the ignitin, the cmbustin prpagated thrughut the thermit tunnel within a few secnds. The mlten material prduced at ver 3,000 C mved twards the pipe surface. Sme evlved gases during the reactin remained in the melt, which ultimately lwered the density f the ceramic layer frmed by a quick slidificatin activity within a few minutes. Figure 2 depicts the density variatin in the ceramic layer with different silicn sludge replacement. The density f the ceramic layer is 2.9 g/cm 3 withut silicn sludge and increases t 3.5 g/cm 3 with the 5% and 10% replacement. The mlten phase frmed with the sludge replacement is fund t be maintained fr a relatively lng perid f time in the curse f cling under a centrifugal frce and it helps prmte a dense ceramic layer. Hwever, its density decreases dwn t 2.8 g/cm 3 with further replacement f the silicn sludge. This wuld be due t the inclusins f pres, which will be discussed in the Sectin 3.3 under micrstructure.

3 1870 M. T. Le, D. J. Kim, J. R. Lee, C. G. Kim and H. S. Chung (a) (b) Apparent density (g/cm 3 ) Hardness (Hv) Silicn sludge (%) Centrifugal frce (G) 200 Fig. 2 Effect f silicn sludge additin n the apparent density f ceramic layer with 40% f filling rati, centrifugal frce f 120 G. Fig. 4 Effect f centrifugal frce n hardness f the ceramic layer with 40% filling rati. (a) withut silicn sludge, (b) 10% silicn sludge. Apparent density (g/cm 3 ) Figure 3 shws the apparent density f the ceramic prduct varied at different filling ratis. The filling rati herein refers t the rati f the weight f thermit filled in a steel pipe t the weight f the steel pipe. The density f the prepared layer at the filling rati f 30 50% withut silicn sludge is in g/cm 3. On the ther hand, it is shwn that the 10% silicn sludge replacement significantly raises the density up t the maimum value 3.5 g/cm 3. The Vickers hardness f a specimen f the ceramic layer prepared was measured with a 50-Kg lad and the results are shwn in Fig. 4. The hardness fr the ceramic layer prepared withut sludge replacement is abut 1,450 Hv. Hwever, in the presence f the 10% replacement at a 40% filling rati, the hardness increases t 1,780 Hv by 23%. It is nted that the hardness value remains cnstant at centrifugal frce ver 120 G (G: the acceleratin due t gravity). As epected, the hardness f the ceramic layer has a similar trend t its density. 3.3 Micrstructure The micrstructure f the ceramic layer was bserved by SEM is shwn in Fig. 5. The micrgraphs taken in the mde f back scattered electrn diffractin reveal gray clred 40 Filling rati (%) Fig. 3 Effect f filling ratis n the apparent density f ceramic layer with a centrifugal frce f 120 G. (a) withut silicn sludge, (b) 10% silicn sludge. 50 (a) (b) phases f -Al 2 O 3 and lighter-clred phases f FeAl 2 O 4. The gray grain-shaped -Al 2 O 3 are surrunded by spinel-like structured FeAl 2 O 4. In the Fig. 5(a), withut silicn sludge, the alumina grains are distributed in the ceramic layer with large dark pres. Because the melting temperature f -Al 2 O 3 (2054 C) is higher than FeAl 2 O 4 (1780 C), alumina seems t nucleate and grw first. Then, FeAl 2 O 4 get slidified and crystallized n the already frmed -Al 2 O 3 grains. Accrding t the SEM images btained at different lcatins in the crss-sectins f the ceramic layers using, it was generally fund that the innermst part epsed t air has carse grains f -Al 2 O 3, while the utermst part cntacted t the pipe surface has fine nes (Figs. 5(b), 5(d)). The micrstructure f the middle ceramic layer parts in the directin f thickness was bserved by SEM as shwn in Fig. 5(c). A dense micrstructure f the ceramic layer with dendritic structure f -Al 2 O 3 grains surrunded by FeAl 2 O 4 is frmed using 10% silicn sludge and the significant decrement f the pre is nticed in Fig. 5(c), cmpared t the case f n sludge. The sludge includes tw majr cmpsitins f Si and SiC; therefre, the SiO 2 cmpsitin is prduced frm the thermit reactin f Fe 2 O 3 and Si accrding t eq. (2). In ther wrds, the SiO 2 -Al 2 O 3 system btained frm the verall thermit reactin f Al-Si-Fe 2 O 3 likely frms the amrphus silicate materials which remain as a liquid phase f high fluidity fr an etended time perid belw the slidifying temperature f FeAl 2 O 4 prviding a great effect n the ceramic layer structure. The structure rearrangement f the ceramic layer as well as the evlved gas remval becmes easier and results in its micrstructure denser after all. During the thermit reactin between Al and Fe 2 O 3 by eq. (2), it als ccurs between Si present in the silicn sludge and Fe 2 O 3. The silica material frmed by eq. (2), subsequently, will react with SiC present in the sludge at high temperatures as fllwings: 9) SiC þ 2 SiO 2! 3 SiO (gas) þ CO (gas) ð3þ 2 SiC þ SiO 2! 3Siþ2CO (gas) ð4þ

4 Prperties f Ceramic Layer Frmed by Centrifugal Thermit Reactin with Silicn Sludge Replacement KV µ m KIGAM (a) 20 KV µ m KIGAM (b) 20 KV µ m KIGAM 20 KV µ m KIGAM (c) (d) Fig. 5 Micrgraphs f ceramic layer at 40% filling rati and centrifugal frce f 120 G. (a) withut silicn sludge; (b) innermst part, (c) middle, and (d) utermst part f ceramic layer with 10% silicn sludge. The idatin f SiC may als take place at high temperatures by: SiC þ 3 2 O 2! SiO 2 þ CO (gas) It is epected that the silica material in eq. 5 can be cnsumed t react with SiC accrding t eq. 3 and eq. 4. This is the reasn why any X-ray intensity peaks f SiC are nt bserved in Fig. 1. The SiO 2 cmpnents are ging t certainly prmte the frmatin f liquid phase, resulting in the dense ceramic layer structure. Hwever, it is thught that the generated gases likely create mst f the pres within the ceramic layer as shwn in Fig. 5(c), greatly affecting its density and hardness. Especially, the develpment f CO at the high cncentratin f the sludge ver 20% wuld seriusly decrease the density f the ceramic layer. 3.4 Amrphus phases Figure 6 shws the EDX mappings f the verall ceramic layer. Highly cncentrated aluminum clearly appears n alumina grains and less cncentrated ne is scattered between the grains as seen in Fig. 6(b). Irn element heavily lies between the alumina grains in Fig. 6(d), and the irn regin cntains cncentrated silicn, t, as shwn in Fig. 6(e). Very rarefied carbn in Fig. 6(f) signifies a cmplete reactin f SiC. Oygen is evenly distributed all ver the surface f the ceramic layer (Fig. 6(c)). Based n the abve bservatin, there shuld be the FeO-Al 2 O 3 -SiO 2 system n the bundary f alumina grains. ð5þ Accrding t the XRD data, it can be assumed that, as cling prceeds, the FeO-Al 2 O 3 preferentially precipitates nt alumina grains frm the thermit melt and then frms hercynite (FeAl 2 O 4 ) which is a spinel-like crystalline structure. Other unknwn cmpsitins which culd be silicates, hwever, are nt fund in the XRD patterns. They hardly get any time t be crystallized by the rapid temperature drp. In ther wrds, it can be deduced that the amrphus silicate materials mst likely eist between the hercynite layers surrunding alumina grains in accrdance with the silicn distributin map in Fig. 6(e). T identify the cmpsitins f amrphus silicn cmpunds in the ceramic layer, heat treatment is carried ut fr the ceramic specimens. Specimens in 1:5 cm 2 cm taken frm the ceramic layer were calcined in an electric furnace at 1400 C fr 4 hr. The X-ray analyses fr the heat treated specimens are shwn in Fig. 7. Besides Al 2 O 3 and FeAl 2 O 4 in the ceramic layer, a new phase f mullite (Al 6 Si 2 O 13 )is fund. The slid phase having a mullite cmpsitin is assciated with SiO 2 at 1600 C at which the mlar rati f Al 2 O 3 t SiO 2 is apprimately 2.5 t ) In ther wrds, the mlten material f a mullite cmpsitin remains fluid dwn t 1600 C, which helps the ceramic layer dense. 4. Cnclusins The prperties and structure f ceramic layer frmed by the centrifugal thermit prcess f aluminum and irn ide were investigated by the use f single crystal silicn wafer

5 1872 M. T. Le, D. J. Kim, J. R. Lee, C. G. Kim and H. S. Chung (a) (b) (c) (d) (e) (f) Fig. 6 EDX mapping fr the ceramic layer prepared with 10% silicn sludge at 40% filling rati f and 120 G centrifugal frce. Al 2 O 3 FeAl 2 O 4 Al 6 Si 2 O 13 Intensity (Arbitrary unit) Theta Fig. 7 XRD patterns f ceramic layers prepared with 10% Silicn sludge at 40% filling rati and 120 G centrifugal frce after heat treatment at 1400 C. sludge cnsisting f 30 mass% Si and 70 mass% SiC prduced frm a slicing and plishing prcess. Al 2 O 3 alng with FeAl 2 O 4 crystalline structure was bserved in the ceramic layer. The silica cnstituents prduced by the reactin in the Si-Fe 2 O 3 thermit system as well as the idatin f SiC in the sludge are likely cnsumed t frm an amrphus cmpnent having a lw melting temperature. The amrphus phase in the mullite cmpsitin (Al 6 Si 2 O 13 ) is fund t prmte the rearrangement f the layer structure during the curse f cling, resulting in the imprvement f density and hardness f the ceramic layer. SiC in the sludge wrks as an imprtant surce f silicn.

6 Prperties f Ceramic Layer Frmed by Centrifugal Thermit Reactin with Silicn Sludge Replacement 1873 The replacement f the silicn sludge up t 10% f aluminum imprves the density f the ceramic layer frm 2.9 t 3.5 g/cm 3 and the hardness frm 1,450 t 1,780 Hv, but ver 10% replacement significantly deterirates the layer structure. The sludge, a waste material f semicnductr industry, culd be cnsidered as a suitable replacement f aluminum t enhance the layer s mechanical prperties. Acknwledgements This wrk was supprted by the Krea Fundatin fr Internatinal Cperatin f Science & Technlgy (KICOS) thrugh a grant prvided by the Krean Ministry f Science & Technlgy (MOST) in 2007 (N. K E ). REFERENCES 1) O. Odawara: Key Eng. Mater (1996) ) A. G. Merzhanv: Inter. J. SHS. 10 (2001) ) L. L. Wang, Z. A. Munir and Y. M. Maimv: J. Mater. Sci. 28 (1993) ) O. Odawara: Trans. Jpn. Inst. Met. 26 (1985) ) S. G. Zhang, X. X. Zhu and D. H. Quian: Inter. J. SHS. 11 (2002) ) Z. Z. Du, H. G. Fu, H. F. Fu and Q. Xia: Mater. Lett. 59 (2005) ) Q. S. Meng, S. P. Chen, J. F. Zha, H. Zhang, H. X. Zhang and Z. A. Munir: Mater. Sci. Eng. A 456 (2007) ) J. R. Lee, M. T. Le and H. S. Chung: Mater. Trans. 48 (2007) ) V. G. Brisv and B. F. Yudin: Ogneupry 3 (1968) ) S. Aramaki and R. Ry: J. Am. Ceram. Sc. 42 (1959) 644.