Electrochemical sElectrochemical synthesisynthesis and and properties of Fe-W powderproperties of Fe-W powder

Professor Professor Dragica M. MinićDragica M. Minić

Faculty of Physical Chemistry, University Faculty of Physical Chemistry, University BelgradeBelgrade

E-mail: E-mail: drminic@gmail.com or or dminic2003@yahoo.comTelephon: 1-512-250-2088Telephon: 1-512-250-2088 or or 1-512-502-2822 1-512-502-2822

• IntroductionIntroductionThe recent intense development of modern powder metallurgy has provoked a sudden interest in amorphous powders, particularly metallic ones. These materials represent a relatively new state of matter with an interesting combination of physical and physical-chemical properties that make them very attractive from the technical point of view. It is due to their great possibilities to be applied in the manufacturing of precise components for various types of equipment by hot and cold sintering.

The amorphous state of matter is, however, structurally and thermodynamically unstable and very susceptible to partial or complete crystallization during thermal treatment or nonisothermal compacting. The latter imposes the need to know its stability in a broad region of temperature.

• The electroplating as a method for producing amorphous metals The electroplating as a method for producing amorphous metals has been suggested in some papers since 1830, but these papers has been suggested in some papers since 1830, but these papers only reported that some plated alloy films had amorphous only reported that some plated alloy films had amorphous structure. structure.

• Our results obtained on Fe or Ni based amorphous alloys prepared Our results obtained on Fe or Ni based amorphous alloys prepared using co-deposition by electroplating showed that chemically using co-deposition by electroplating showed that chemically synthesized Ni82P18 and electrochemically synthesized Fe89P11 synthesized Ni82P18 and electrochemically synthesized Fe89P11 amorphous powder alloys are active hydrogen absorbers in the amorphous powder alloys are active hydrogen absorbers in the temperature range from 100 °C to 300 °C and that they are temperature range from 100 °C to 300 °C and that they are transformed into crystalline state above this temperature range.transformed into crystalline state above this temperature range.

• In order to synthesize an amorphous alloy of increased structural In order to synthesize an amorphous alloy of increased structural stability, with no intention to stabilize additionally the alloy by stability, with no intention to stabilize additionally the alloy by crystallization over a wide temperature range, tungsten was used crystallization over a wide temperature range, tungsten was used as amorphizer instead of phosphorous in the present work.as amorphizer instead of phosphorous in the present work.

• The aim of this work is synthesis and characterization of The aim of this work is synthesis and characterization of amorphous Fe-W powder as well as investigation thermal stability amorphous Fe-W powder as well as investigation thermal stability and structural transformations obtained alloy in broad and structural transformations obtained alloy in broad temperature interval (20-1300temperature interval (20-1300C) as well as in hydrogen C) as well as in hydrogen atmosphere.atmosphere.

• ExperimentalExperimentalThe Fe-W powders of different compositions were obtained by electrolyzing aqueous solutions containing Na2WO4, C2H2O4, glycine and FeSO4 at a current density of 8 A/dm2 by changing the ratio of iron to tungsten but maintaining their total molar concentration of 0.26 M in the solution.

The electrolysis was performed using a Cu cathode and a Pt anode in a stream of purified nitrogen, whose continuous flow was used for stirring the electrolytes in the electrolyzer at 80 °C.

Microscopic analysis showed that 97% of the particles have dimensions 0.5/4.5m

The compositions of the electrolytes and obtained alloys AlloyAlloy Fe/WFe/W

mol.mol. ratio in ratio in electrolyteelectrolyte

Fe/WFe/W

mass ratio inmass ratio in

alloyalloy

Fe/WFe/W

atomic ratio atomic ratio inin

AlloyAlloy

11 1:91:9 76:2476:24 91.2:8.891.2:8.8

22 2:82:8 80:2080:20 92.9:7.192.9:7.1

33 3:73:7 84:1684:16 94.5:5.594.5:5.5

• The thermal stability, the process of crystallization and the The thermal stability, the process of crystallization and the process of hydrogen absorption were investigated by non-process of hydrogen absorption were investigated by non-isothermal thermal analysis (DSC, DTA) using a Du Pont isothermal thermal analysis (DSC, DTA) using a Du Pont Thermal Analyzer (model 1090). Thermal Analyzer (model 1090).

• In this case, samples weighting several milligrams were In this case, samples weighting several milligrams were heated in the DSC cell from room temperature to 500 °C in a heated in the DSC cell from room temperature to 500 °C in a stream of hydrogen at normal pressure and in the DTA cell stream of hydrogen at normal pressure and in the DTA cell from room temperature to 1300 °C in a stream of nitrogen at from room temperature to 1300 °C in a stream of nitrogen at normal pressure. normal pressure.

• The thermomagnetic (TM) curve was measured on weakly The thermomagnetic (TM) curve was measured on weakly compacted material of cylindrical shape with a diameter of 2 compacted material of cylindrical shape with a diameter of 2 mm and thickness of about 1.5 mm placed in a special mm and thickness of about 1.5 mm placed in a special vacuum furnace. The TM measurement was done in a field of vacuum furnace. The TM measurement was done in a field of 3.98 kA/m (50 Oe) with a heating and cooling rate of 4 3.98 kA/m (50 Oe) with a heating and cooling rate of 4 K/min. using an EG&G vibrating sample magnetometer.K/min. using an EG&G vibrating sample magnetometer.

• The X-ray powder diffractogram (XRPD) patterns were obtained The X-ray powder diffractogram (XRPD) patterns were obtained by a Philips PW-1710 automated diffractometer using Cu-tube by a Philips PW-1710 automated diffractometer using Cu-tube operated at 40 kV and 30 mA. operated at 40 kV and 30 mA.

• The instrument was equipped with the diffraction beam curved The instrument was equipped with the diffraction beam curved graphite monochromator and a Xe-filled proportionalgraphite monochromator and a Xe-filled proportional counter. counter.

• The XRPD were collected in the 2The XRPD were collected in the 2 angle range 4-900, counting angle range 4-900, counting for 0.25 and 2.5 seconds, respectively in 0.020 steps. A fixed for 0.25 and 2.5 seconds, respectively in 0.020 steps. A fixed 10 divergence and 0.1 mm receiving slits were used. 10 divergence and 0.1 mm receiving slits were used.

• The XRPD pattern data were proceThe XRPD pattern data were processsed by Philips APD software sed by Philips APD software PW-1844. The unit cell dimensions of FePW-1844. The unit cell dimensions of Fe--W alloys were W alloys were calculated in the Im3m space group from three the most calculated in the Im3m space group from three the most intensive peaks: (110), (200) and (211) as averaged values. intensive peaks: (110), (200) and (211) as averaged values. Obtained values were compared with the corresponding values Obtained values were compared with the corresponding values deposited in the JCPDS-data base (card file 6-0696 for deposited in the JCPDS-data base (card file 6-0696 for -Fe and -Fe and 4-0806 for W). The parameters of crystallite size, i.e. the length 4-0806 for W). The parameters of crystallite size, i.e. the length of coherent order structure (LHKL), were calculated from the of coherent order structure (LHKL), were calculated from the Scherre’s method. The crystallite size dimensions were Scherre’s method. The crystallite size dimensions were measured on the most intensive reflexion with the Miler indices measured on the most intensive reflexion with the Miler indices (110).(110).

• Mössbauer spectra of the powder material were taken in the Mössbauer spectra of the powder material were taken in the standard transmission geometry using a Co57(Rh) source at room standard transmission geometry using a Co57(Rh) source at room temperature and at 20 K. temperature and at 20 K.

• The calibration was done against The calibration was done against -iron foil data. For the spectra -iron foil data. For the spectra fitting and decomposition, the “CONFIT” program package was fitting and decomposition, the “CONFIT” program package was usedused. .

• The computer processing yielded intensities I of components, their The computer processing yielded intensities I of components, their hyperfine inductions Bhf, isomer shifts hyperfine inductions Bhf, isomer shifts and quadrupole splitting and quadrupole splitting . .

• The contents of the iron containing phases are given as intensities The contents of the iron containing phases are given as intensities of the corresponding spectral components (phases with negligible of the corresponding spectral components (phases with negligible iron content are not detectable by Mössbauer spectroscopy). iron content are not detectable by Mössbauer spectroscopy).

• The exact quantification of the phase contents could be done only The exact quantification of the phase contents could be done only when possible differences in values of Lamb-Mössbauer factors when possible differences in values of Lamb-Mössbauer factors were considered. were considered.

• ResultsResults

X-ray diffractograms onX-ray diffractograms on as-prepared as-prepared

samples of:samples of: a) Fea) Fe91.291.2WW8.88.8

b) Feb) Fe92.992.9WW7.17.1

c) Fec) Fe94.594.5WW5.55.5

• ResultsResults

X-ray diffractograms on as prepared samples of:

a) Fe76W24; b) Fe80W20; c) Fe84W16.

• The inspection of the structure and micro structural The inspection of the structure and micro structural parameters of the electrochemically obtained powders of parameters of the electrochemically obtained powders of the Fe-W alloys were done by comparing their XRPD the Fe-W alloys were done by comparing their XRPD patterns with the same parameters given for the pure -Fe patterns with the same parameters given for the pure -Fe and W deposited in the JCPDS data base.and W deposited in the JCPDS data base.

The crystallinity and the enthalpy of the absorption of The crystallinity and the enthalpy of the absorption of hydrogenhydrogenAlloyAlloy

22(())

d-d-valuevalue

CrystalCrystallinitylinity

aa

(nm)(nm)LL(110(110

))

(nm)(nm)

HH

(J/g)(J/g)TmTm

((C)C)

11 43.8743.8755

2.0612.06199

2.662.66 0.2911(10.2911(1))

11.711.7 -24.1-24.1 226.3226.3

22 43.7943.7900

2.0652.06577

4.934.93 0.2920(10.2920(1))

23.823.8 -27.2-27.2 239.2239.2

33 43.7343.7300

2.0682.06844

6.666.66 0.2929(10.2929(1))

35.735.7 -28.2-28.2 251.4251.4

• XRPD patterns of the alloys indicate some amorphization of XRPD patterns of the alloys indicate some amorphization of the iron phase in the presence of tungsten. the iron phase in the presence of tungsten.

• In the alloys, the α-Fe (110) peaks (2θ = 43.8°) have lower In the alloys, the α-Fe (110) peaks (2θ = 43.8°) have lower intensity, they are broadened and shifted towards lower 2θ intensity, they are broadened and shifted towards lower 2θ values due to incorporation of W atoms in Fe lattice. values due to incorporation of W atoms in Fe lattice.

• This can be explained by interfacial regions with partial This can be explained by interfacial regions with partial incorporation of tungsten atoms into the iron crystal lattice incorporation of tungsten atoms into the iron crystal lattice according to Vegrad rule, which causes its deformation, according to Vegrad rule, which causes its deformation, owing to the somewhat larger atomic radius of tungsten. owing to the somewhat larger atomic radius of tungsten.

• It is clear from the obtained grain size values (Ll00) that It is clear from the obtained grain size values (Ll00) that investigated alloys are nanostructured compounds having investigated alloys are nanostructured compounds having different dimensions dependent on synthesis conditions. different dimensions dependent on synthesis conditions.

• Exposing the obtained alloys to annealing at the Exposing the obtained alloys to annealing at the temperature up to 1200 °C during DTA measurement some temperature up to 1200 °C during DTA measurement some structural changes above 400 °C can be seen.structural changes above 400 °C can be seen.

DTA thermograms DTA thermograms

FeFe91.291.2WW8.88.8 for for heating heating

and cooling cycles in and cooling cycles in

argon flow, heating argon flow, heating

rate of 20K/minrate of 20K/min

The DSC thermograms of the alloys The DSC thermograms of the alloys

in the temperature range from 20 in the temperature range from 20 °C to 500 °C show complex °C to 500 °C show complex exotherms. They can be ascribed exotherms. They can be ascribed

to to the reduction of the oxide filmthe reduction of the oxide film formed on the surface of the alloyformed on the surface of the alloy particles during drying after the particles during drying after the synthesis and partially to a process synthesis and partially to a process of poor absorption of hydrogen of poor absorption of hydrogen between 120 °C and 300 °Cbetween 120 °C and 300 °C

DSC thermograms in hydrogen flow DSC thermograms in hydrogen flow of:of:

a) Fea) Fe91.291.2WW8.88.8

b) Feb) Fe92.992.9WW7.17.1

c) Fec) Fe94.594.5WW5.55.5

The MThe Mössbauer spectra of the as prepared Feössbauer spectra of the as prepared Fe91.291.2WW8.8 8.8 at room at room temperature and at 20 Ktemperature and at 20 K

• ParamParameeterterss derived fromderived from M Möössbauerssbauer spespecctratra of the as of the as prepared Feprepared Fe91.291.2WW8.88.8

CComp. omp. sspepecctrtr

aa

II BhfBhf II PhasePhase

SA1SA1

SA2SA2

SA3SA3

SA4SA4

0,040,04

0,010,01

0,050,05

0,060,06

0,070,07

0,090,09

0,010,01

0,210,21

0,010,01

0,180,18

0,050,05

0,010,01

0,130,13

0,210,21

0,070,07

32,9432,94

0,080,08

30,0130,01

26,6726,67

23,8423,84

0,220,22

-Fe(W)+ -Fe(W)+ amoramorphous phasephous phase

SA5SA5

SA6SA6

DA1DA1

DA2DA2

0,050,05

0,070,07

0,240,24

0,100,10

-0,05-0,05

0,290,29

0,130,13

0,490,49

0,070,07

0,430,43

0,510,51

0,420,42

18,4718,47

6,326,320,450,45

Amorphous phaseAmorphous phase++

interfacial regionsinterfacial regions

LA1LA1

LA2LA20.200.20

0.120.12-0,09-0,09

0,210,210,320,32 -Fe-Fe

• The prevailing paramagnetic part is formed by singlets LA1 and LA2 The prevailing paramagnetic part is formed by singlets LA1 and LA2 and doublets DA1 and DA2. The singlets were ascribed to the and doublets DA1 and DA2. The singlets were ascribed to the -Fe -Fe particles. particles.

• The intensity and the components of the paramagnetic part remain The intensity and the components of the paramagnetic part remain stable up to 20K, except temperature shift and slight change in the stable up to 20K, except temperature shift and slight change in the quadrupole splitting. It indicates that the paramagnetic part does quadrupole splitting. It indicates that the paramagnetic part does not represent small superparamagnetic particles.not represent small superparamagnetic particles.

• The The -Fe phase did not transit from paramagnetic to -Fe phase did not transit from paramagnetic to

antiferromagnetic state by cooling down to 20 K.antiferromagnetic state by cooling down to 20 K.

• The doublets together with sextets SA5 and SA6 were identified as The doublets together with sextets SA5 and SA6 were identified as the amorphous phase indicated in the the amorphous phase indicated in the X-ray diffractogram. X-ray diffractogram.

• The magnetic part represented by the sextets SA1–SA4 cannot be The magnetic part represented by the sextets SA1–SA4 cannot be simply ascribed to a crystalline simply ascribed to a crystalline -Fe(W) solid solution identified in -Fe(W) solid solution identified in the X-ray diffraction. The distribution of their partial intensities does the X-ray diffraction. The distribution of their partial intensities does not fit to the values expected for a homogeneous solid solution of not fit to the values expected for a homogeneous solid solution of 8.8 at.%W in the bcc Fe. This can be caused by overlapping of the 8.8 at.%W in the bcc Fe. This can be caused by overlapping of the --Fe(W) components with other components of the magnetic ordered Fe(W) components with other components of the magnetic ordered amorphous phase in the Mössbauer spectrum. The composition of amorphous phase in the Mössbauer spectrum. The composition of the ferromagnetic phase also remains stable after cooling down to the ferromagnetic phase also remains stable after cooling down to 20 K.20 K.

The MThe Mössbauer spectra after thermomagnetic curve össbauer spectra after thermomagnetic curve measurement of Femeasurement of Fe91.291.2WW8.88.8

The MThe Mössbauer spectra of the Feössbauer spectra of the Fe91.291.2WW8.88.8 after heating at 1073 after heating at 1073 KK

CComp. omp. sspepecctrtr

aa

II BhfBhf II PhasePhase

SB1SB1

SB2SB2

SB3SB3

0,510,51

0,040,04

0,040,04

0,000,00

0,020,02

-0,01-0,01

0,000,00

0,010,01

0,010,01

33,1633,16

30,3730,37

28,5828,58

0,590,59 -Fe-W-Fe-W

DB1DB1

DB2DB2

0,200,20

0,110,11

0,020,02

0,900,90

0,350,35

0,980,98

0,200,20

0,110,11

W(Fe(II)W(Fe(II)

Fe(II)Fe(II)

LB1LB1 0.100.10 0,240,24 0,100,10 -Fe2W-Fe2W

• After the heat treatment at 1073 K, the increase in the intensity of After the heat treatment at 1073 K, the increase in the intensity of the magnetic part at the expense of the paramagnetic one was the magnetic part at the expense of the paramagnetic one was observed.observed.

• The distribution of its components SB1-SB3 is close to solid The distribution of its components SB1-SB3 is close to solid solution of W in solution of W in -Fe. The content of the W can be estimated by -Fe. The content of the W can be estimated by comparison with a model of solid solution in bcc comparison with a model of solid solution in bcc -Fe to approx. 3 -Fe to approx. 3 at.%. at.%.

• The components of the paramagnetic part DB1, LB1, and DB2 were The components of the paramagnetic part DB1, LB1, and DB2 were ascribed to the W(Fe), ascribed to the W(Fe), -Fe2W, and Fe-Fe2W, and Fe22+ phases, respective. + phases, respective.

• The result of the Mössbauer phase analysis shows that during the The result of the Mössbauer phase analysis shows that during the annealing decomposition takes place and the detected phases annealing decomposition takes place and the detected phases agree with those in the equilibrium Fe-W phase diagram. agree with those in the equilibrium Fe-W phase diagram.

• The The -Fe2W found in our crystallized sample is paramagnetic down -Fe2W found in our crystallized sample is paramagnetic down to 20 K.to 20 K.

Thermomagnetic curve of Fe91.2W8.8 measured at 3.98 kA/m (50 Oe) with the heating and cooling rate of 4 K/min.

• The TM curve reflects some structural changes during the The TM curve reflects some structural changes during the heating of the sample, especially above 500 °C. heating of the sample, especially above 500 °C.

• The sharp increase in magnetic moment can be ascribed to The sharp increase in magnetic moment can be ascribed to crystallization of the amorphous phase and decomposition into crystallization of the amorphous phase and decomposition into iron-rich α-phase and W rich phases that enlarges the total iron-rich α-phase and W rich phases that enlarges the total magnetic moment of the sample. magnetic moment of the sample.

• The small bulge above the temperature of 200 °C corresponds The small bulge above the temperature of 200 °C corresponds with the shape of the DTA curve and can be explained by with the shape of the DTA curve and can be explained by relaxation of the amorphous structure and/or an annihilation of relaxation of the amorphous structure and/or an annihilation of defects. defects.

• The Curie temperature derived from the curves by increasing The Curie temperature derived from the curves by increasing and decreasing temperatures is approximately 755 °C which and decreasing temperatures is approximately 755 °C which indicates some low amount of W in the solid solution indicates some low amount of W in the solid solution -Fe(W).-Fe(W).

• The transmission picture and diffractogram of complex FeOThe transmission picture and diffractogram of complex FeOWOWO33 particle of the powderparticle of the powder

• ConclusionConclusion

• The investigation of the thermal stability of all three amorphous Fe-W The investigation of the thermal stability of all three amorphous Fe-W alloys prepared by electrolysis of aqueous solutions of corresponding alloys prepared by electrolysis of aqueous solutions of corresponding electrolytes by thermal analysis has shown that poor hydrogen electrolytes by thermal analysis has shown that poor hydrogen absorption takes place, as an exothermal process, in the temperature absorption takes place, as an exothermal process, in the temperature range 100 °C - 300 °C. Obviously, the reducing reaction with oxide range 100 °C - 300 °C. Obviously, the reducing reaction with oxide films takes place as well. films takes place as well.

• According to the X-ray diffractograms, a certain extent of According to the X-ray diffractograms, a certain extent of amorphization can be expected to be present. The hysteresis of the amorphization can be expected to be present. The hysteresis of the DTA curve measured by heating up to 1200 °C in an argon DTA curve measured by heating up to 1200 °C in an argon atmosphere indicated some structural changes above 400 °C. It was atmosphere indicated some structural changes above 400 °C. It was certified by the thermomagnetic curve where crystallization of certified by the thermomagnetic curve where crystallization of amorphous phase and formation α phase can be observed above 500 amorphous phase and formation α phase can be observed above 500 °C.°C.

• In Mössbauer spectra of the as-prepared powder the In Mössbauer spectra of the as-prepared powder the -Fe(W) phase -Fe(W) phase was found. However, the prevailing amount of iron atoms is situated was found. However, the prevailing amount of iron atoms is situated in an amorphous phase and in interfacial regions with distorted in an amorphous phase and in interfacial regions with distorted crystal lattice. After the heat treatment by measurement of the TM crystal lattice. After the heat treatment by measurement of the TM curve, the most pronounced curve, the most pronounced is the is the -Fe with approx. 3 at. % W -Fe with approx. 3 at. % W accompanied by the W(Fe), accompanied by the W(Fe), -Fe2W, and Fe2+ in the FeO-Fe2W, and Fe2+ in the FeOWO3 WO3 phases. The estimated content of the W in the phases. The estimated content of the W in the -Fe is in good -Fe is in good agreement with the Curie temperature determined from the TM agreement with the Curie temperature determined from the TM curve.curve.