Electroplating: An Alternative for Producing Low Magnetic Loss Amorphous Alloys

Author: from Hydro-Québec IREQ, Varennes, Québec, Canada from Hydro-Québec LTE, Shawinigan, Québec, Canada from Hydro-Québec IREQ, Varennes, Québec, Canada from Hydro-Québec LTE, Shawinigan, Québec, Canada from Hydro-Québec IREQ, Varennes, Québec, Canada from Hydro-Québec IREQ, Varennes, Québec, Canada

Posted on: 12/3/2010

In recent years, Hydro-Québec’s Research Institute has developed, through technological innovation, a program to improve energy efficiency in the generation, transmission and distribution of energy. Among the various projects, one was focused on the production of soft magnetic materials to improve the energy efficiency of distribution transformers. The aim of the project was to produce, by electrodeposition, free-standing foils or plates of amorphous Fe-P material having similar, or better, mechanical and low loss magnetic properties compared to iron-based amorphous alloys produced by rapid solidification. To meet this goal, an electroplating process was developed to produce a well-defined binary Fe-P amorphous alloy. The key electrochemical parameters, ancillary needs, critical monitoring and process controls were investigated. We succeeded in producing a fully amorphous soft magnetic material with very low power frequency losses and other advantageous properties such as a thickness range from 20 to 75 µm with a high production rate. Using thicker material, the production of stacked cores could be possible. In addition, electroplating offers the possibility of forming complex 2D shapes for rotating machines.

Keywords: Fe-P alloys, amorphous, soft magnetic properties, low magnetic losses, power distribution transformers

 

Introduction and background

 

Hydro-Québec as an electrical utility is continuously developing new methods and new technologies for improving the generation, transmission and distribution of electrical power. In December 2001, Hydro-Québec submitted its first Energy Efficiency Plan (EEP) to reduce consumption growth and carbon emissions. Hydro-Québec’s Research Institute has developed a Technological Innovation Program to contribute to the development of more energy-efficient processes and products. Among the various projects, one was focused on the production of soft magnetic materials to improve the energy efficiency of distribution transformers. 

 
Amorphous or nanocrystalline soft magnetic materials that exhibit high saturation induction, high permeability, low coercivity and low core loss are desired for manyalternating current (AC) applications. Rapid solidification technology development has facilitated the usage of metallic glasses. These alloys are usually made of about 20% metalloids such as silicon, phosphorus, boron or carbon and about 80% iron. Typical commercial thin ribbons have a thickness of 25 µm and a width of 210 mm.1,2 The transformer core loss of these alloys at an induction level of 1.4 tesla (T) are about 0.3 W/kg compared to 0.7 W/kg for best grade grain-oriented electrical steel, the commonly used material for core transformers.2 Amorphous alloys can also be prepared by electrodeposition, vacuum deposition, sputtering and plasma spraying.
 

Electrodeposition of amorphous or nanocrystalline alloys based on iron group metals is one of the most important developments over recent decades in the field of soft magnetic thin films3 and nanotechnology applications.4 Iron-phosphorus alloys deserve special attention as cost effective soft magnetic materials for many applications. Electrodeposition has been extensively used to produce Fe-P alloy thin films, permitting control of the coating composition, microstructure, internal stress and magnetic properties, by using suitable plating conditions.5-11 Electroplating has also been used to produce amorphous Fe-P coatings and electroforms for inductive devices such as amplifiers.12

 
The aim of the present work relates to the electrodeposition of thick free-standing foils or plates (20-75 µm) made of an amorphous Fe-P soft magnetic alloy, destined for large scale applications in electrical power equipment (transformers, rotating machines, etc.), that show high saturation induction, low coercive field and low power frequency losses.
 
Experimental
 
Electroplating conditions
 
Thick samples of Fe-P alloys were produced in a chloride bath. Table 1 shows the electroplating bath composition and the operating conditions used for three plating systems. The bath was an aqueous solution of FeCl2•4H2O and NaH2PO2•H2O at different concentration ranges depending on the plating system. Calcium chloride was added as a supporting electrolyte salt to improve solution conductivity for experiments conducted at 40-60°C. Some experiments were done by adding CuCl2•2H2O in the bath to electroplate a ternary Fe-P-Cu alloy. The pH of the solution ranged from to 0.9 to 1.4 and was adjusted by the addition of hydrochloric acid. In all the experiments, a preliminary bubbling with nitrogen was performed in the aqueous plating solution and a small argon gas flow was maintained over the aqueous plating solution to prevent entry of oxygen.
 
At low temperatures (40-60°C), we used an electroplating system equipped with a rotating disk electrode (RDE). The cathode was a titanium disk 1.3 cm2 in area and the anode was a dimensionally stable anode (DSA) 4 cm2 in area. The rotation rate of the working electrode was mainly 900 rpm. The electroplating was performed using a DC current of 3-10 A/dm2.
 
 
 
Table 1 - Composition of the bath and operating conditions.
 
Chemicals and operating conditions
RDE cell
Parallel plate cell
Low current density,
high solution velocity
LCD-HSV
Parallel plate cell
High current density,
high solution velocity
HCD-HSV
FeCl2•4H2O (M)
1.0
1.0 - 1.5
1.0 - 1.5
NaH2PO2•H2O (M)
0.035 - 0.5
0.05 - 0.35
0.5 - 0.75
CaCl22H2O (M)
0.5
0.5
---
CuCl22H2O (mM)
0.0 - 0.3
0.0 - 0.02
---
Current density (A/dm2)
1 - 5
4 - 30
30 - 110
Temperature (°C)
40 - 60
40 - 70
70 - 95
pH
1.1 - 1.4
0.9 - 1.2
0.9 - 1.2
Rotation rate (rpm)
500 - 3000
---
---
Electrolyte velocity (cm/sec)
---
100 - 350
100 - 350
Cell or bath volume (L)
0.5
400
40
Cathode: material, dimension (cm2)
Ti disk, 1.3
Ti plates, 150
Ti plates, 10
Anode
DSA
DSA or graphite
DSA or graphite
Reference electrode
SCE
---
---
 
For the LCD-HSV (Table 1) system at temperatures ranging from 40 to 70°C, a flow-through rectangular polypropylene cell with a large volume plating bath and equipped with parallel plates of 10 × 15 cm was used. For the HCD-HSV (Table 1) system at temperatures ranging from 70 to 95°C, a flow-through rectangular Kynar (PVDF) cell system with a 40-liter volume plating bath and equipped with parallel plates of 2 × 5 cm was used. The anode, made of DSA or graphite, had a surface area equal to that of the titanium cathode or was adjusted to a value allowing for control of any edge effect on the Fe-P deposit. The interelectrode gap between the working electrode and the anode was 5 mm. Electroplating was performed using DC current ranging from 4 to 110 A/dm2. The systems operated at a high concentration of the active species and at high plating solution velocities ranging from 100 to 350 cm/sec with a large ratio of bath to cell volume. The concentration of the hypophosphite and the pH at the cathode were closely controlled.
 
The titanium cathode was mechanically polished before use. The surface roughness was adjusted to be able to peel off the Fe-P foil, while keeping adhesion sufficiently high to avoid the detachment of the deposit during the process.
 
Hydrogen removal, annealing and Fe-P characterization
 
In the present work, production of Fe-P plates comprises one or more additional steps in order to improve their magnetic properties. First, a thermal treatment was performed at 120°C for 10 min for hydrogen removal after the amorphous foil was peeled from the working electrode. Second, a further annealing treatment from 200 to 325°C of the amorphous Fe-P foil was performed for stress relief.
 
The XRD characterization was made by using a Bruker D8 Advance x-ray diffractometer with CuKα radiation. Scattering angles (2θ) from 30° to 60° were measured and the amorphousness analysis was based on the presence or absence of α-Fe diffraction peaks attributed to large crystals. The composition of the alloys was determined by the inductively-coupled plasma emission spectral analysis (Perkin-Elmer® Optima 4300 DV), using appropriate standards and after dissolution of the sample in 5% nitric acid.
 
The thermal stability of the alloys as a function of temperature was determined by differential scanning calorimetry (DSC) using a Perkin-Elmer DSC-7 with a temperature scanning rate ranging from 5 to 20 K/min.
 
The magnetic measurements fall into three categories. First, using an ADE EV7 vibrating sample magnetometer (VSM), the measurements of the basic physical materials properties such as the saturation magnetization and the corresponding coercive field Hc in quasi-static conditions, were performed. Second, using an in-house integrating magnetometer, the performances of many similar samples (disk or 1 × 4 cm samples) were compared, at power frequencies (around 60-64 Hz) for a sine wave-applied magnetic field (up to around 8000 A/m), and by obtaining the losses and corresponding induction and an estimate for Hc. Third, an in-house integrator for a no-load transformer configuration similar to a four-leg Epstein frame, with the primary and secondary windings wound tightly onto each leg, was used. The measurements were carried out by integrating the pick-up voltage of the secondary of the sample and of a calibrated air core transformer in series with the sample in order to obtain waveforms for the magnetic induction and applied field strength respectively. A feedback system ensured as near as possible a sine wave induction in the sample. The hysteresis (B-H) loops were then integrated to obtain the losses. To allow for a small overlap of each leg at the corners of the sample, the weight used to obtain the losses was reduced to that calculated using the path length multiplied by the cross section (which was previously calculated from the total weight divided by the density and by the total length). The power frequency losses and the corresponding value of Hc from the analysis of individual hysteresis (B-H) loops were then obtained. Measurements were confirmed for consistency using a commercial hysteresis measurement apparatus (Walker AMH20).
 
Saturation induction (Bs). This magnetic parameter was measured using a commercial VSM or from the transformer measurement (in-house integrator and Walker AMH20).
 
Low coercive field (Hc). This parameter was quantified using a VSM (physical measurement) and an in-house integrating magnetometer (comparative measurement) and a transformer configuration (to obtain Hc as a function of peak induction).
 
Power frequency losses (W60, hysteresis, eddy current and anomalous losses). This parameter was quantified as a function of peak induction using the in-house transformer configuration and compared between samples using the in-house magnetometer measurement for inductions near to saturation.
 
Electrical resistivity (ρdc). This physical parameter was measured with a four-contact direct current method on short samples, with gauge length of about 1 cm (HP current supply, Keithly® nanovoltmeter).
 
 
Results and discussion
 
Fe-P alloy composition and structure
 
A number of foils were prepared with a rotating disk electrode cell at temperatures ranging from 40 to 60°C to determine the influence of the phosphorus content on the microstructure and on the magnetic properties of the Fe-P foil. Figure 1 shows the relation between the at% P in foils of 50 µm thickness versus the concentration of the phosphorus precursor in the plating bath and the cathodic coulombic efficiency (CE). The phosphorus content, ranging from 12-16 at% in the foil, increased linearly with the phosphorus content in solution and was inversely related to the cathodic CE. In this range, a preferential deposition of phosphorus was observed. At phosphorus contents higher than 16 at%, the CE drops rapidly and is associated with a greater difficulty in adding phosphorus in the alloy.
 

 
Figure 1 -    Relation between the phosphorus content in Fe-P foils of 50 µm thickness with the concentration of hypophosphite in the aqueous plating bath and the coulombic efficiency: 1.0M FeCl2, 0.025-0.5M NaH2PO2, 0.5M CaCl2, 40°C, 4.0 A/dm2.
 
 
It is well known that Fe-P produced by electrodeposition gradually changes from a microcrystalline to an amorphous structure by increasing the phosphorus content in the alloys.13-14 For RDE-electrodeposited samples in the present work, the amorphous structure as shown by x-ray diffraction was obtained with an at% P higher than 15% (Fig. 2). The x-ray diffraction pattern at 17 at% P revealed no crystalline peaks except for the small region surrounding the foil (due to an edge effect).
 
From the analysis of Fig. 1 and the XRD characterization, the transition between the two electro-deposition regimes seems to correspond to the change from microcrystalline to amorphous structure.
 
 
Figure 2 -    X-ray diffraction patterns of Fe-P foils of 50 µm thickness as-deposited with different phosphorus contents: (a) 12 at% P, (b) 13.5 at% P, (c) 15 at% P, (d1) 17 at% P with the edge effect and (d2) 17 at% P without the edge effect.
 
 
To determine the range of annealing temperatures required to avoid the crystallization of the amorphous matrix, the thermal stability of the present amorphous Fe-P samples was studied by differential scanning calorimetry (DSC). Figure 3 shows that the amorphous Fe-P foil exhibits only one strong exothermic peak at around 410°C. A short low-temperature thermal treatment (from 200 to 325°C) of the amorphous Fe-P foils was performed with and without the presence of an applied magnetic field (Fig. 3 insert). The insert shows a decrease of the coercive field Hc by 64% as the temperature is increased from 25°C to around 300°C. This drastic change in Hc occurs at a temperature below the crystallization temperature. After crystallization, the material loses its soft magnetic behavior. The treatment time and temperature were optimized to obtain the lowest coercive field values. The treatment time was around 10 sec at 300°C, to around one hour at 200°C. For instance, it would be about 15 min around 265°C. The mechanism responsible for the decrease of Hc was probably a stress relief of the structure.11
 
Before focusing on the electroplating of a binary Fe-P alloy, some attempts were made at electroplating a ternary Fe-P-Cu alloy. The DSC trace of the amorphous Fe85-P14-Cu1 foil shows the presence of two exothermic peaks at around 366 and 383°C. The electroplating of the ternary alloy was postponed because of the difficulty of controlling the copper content, since its deposition is diffusion-controlled.
 
 
Figure 3 -    Differential scanning calorimetry traces obtained with an amorphous Fe-P foil compared with an amorphous Fe-P-Cu foil (scanning rate 20 K/min.); (Insert: Hc vs. temperature for Fe-P alloy; VSM measurements ±10%).
 
 
The samples considered in Figs. 1 and 2, annealed for 30 min at 250°C under a magnetic field, were used to determine the phosphorus content range in the foils necessary to obtain the lowest coercive field (Hc magnetometer measurement) while maintaining a high saturation induction. Figure 4 shows a minimum Hc value near 17 at% P, corresponding to the amorphous structure of the alloy (as seen in Fig. 2). These results reveal that a good compromise for the phosphorus content can be found that ensures a low Hc value with a good coulombic efficiency while maintaining a high saturation induction (which depends on keeping the iron content as high as possible). At different bath temperatures, this optimum value will change in accordance with variation in the Hc and the coulombic efficiency family of curves.
 
 
Figure 4 -    Relationship between the phosphorus content in Fe-P foils of 50 µm thickness with the coulombic efficiency and the relative coercive field Hc (magnetometer measurement) after annealing for 30 min at 250°C (electroplating conditions as in Fig. 1).
 
 
High solution velocity and high deposition rate
 
The performance of electrochemical processes depends on several factors, such as charge transfer and electrode kinetics, and fluid dynamic conditions, which are the most critical as they control the mass and heat transport. The productivity of the RDE cell is poor due to operation at low current densities and with a cell design resulting in a low space-time yield. Thus, the technical challenge facing our work was to design an electrochemical cell that allowed for producing a cathode deposit of homogeneous phosphorus content with high coulombic efficiency while maintaining a low coercive field. The cell design in our work consisted of a flow-through rectangular cell containing one cathode and one anode both 10 × 15 cm (40-70°C) or 2 × 5 cm (70-95°C). The entry and exit flow channel lengths of the cell were adjusted to attain fully developed turbulent electrolyte flow. This cell design, operating at a high solution velocity of up to 350 cm/sec and high current densities, allowed good uniformity of electroactive species concentration at the electrode surface while minimizing local pH variation and other concentration changes due to electrode reactions.
 
Fe-P electrodeposition: Low current density and high solution velocity (LCD-HSV)
 
The development work with the flow-through rectangular cell started with the same bath composition and operating conditions except for the fluid dynamic conditions. For instance, sample foils of 10 × 15 cm were produced at a low current density of 4.0 A/dm2 at 60°C with a solution velocity of 300 cm/sec with an inter-electrode gap of 5 mm. The plating solution had the following composition: 1.0M FeCl2•4H2O, 0.1M NaH2PO2•H2O, 0.02mM CuCl2•2H2O and 0.5M CaCl2•2H2O. The pH was 1.15. An analysis of one foil gave a composition Fe81.8-P17.8-Cu0.4. The coulombic efficiency was 53% for a thickness of 70 µm. The homogeneity of the phosphorus content across the thickness was 18.4 ± 0.24 at% P and the SEM micrographs show low porosity. With these conditions, easy peel off and manipulable 30-70 µm thick samples were obtained.
 
Although the high solution velocity Fe-P deposits containing 17-20 at% P had acceptable characteristics for temperatures ranging from 40 to 70°C, increasing the current density for these bath conditions led to highly stressed deposits. This was explained by the formation of amorpho-crystalline deposits because of the difficulty of further increasing the phosphorus content in this set of conditions.
 
Fe-P electrodeposition: High current density and high solution velocity (HCD-HSV)
 
For high space-time yield production of Fe-P deposits containing 17-20 at% P, the concentration ratio P/Fe in solution was increased, and the phosphorus content in the foils was investigated as a function of bath temperature, solution velocity and current density. Sample foils of 2 × 5 cm were produced at high current densities as described in Table 1. Figure 5 shows the variation of the phosphorus content in the foil for different solution velocities at 70°C and at a current density of 20 A/dm2. For a velocity increase from 110 cm/sec to 160 cm/sec, the phosphorus content increased by approximately 1.0 at% in the foil.
 
 
Figure 5 -    Effect of solution velocity on the phosphorus content of the Fe-P foils; 1.0M FeCl2, 0.5M NaH2PO2, 70°C, 20 A/dm2 with the HCD-HSV system.
 
 
Figure 6 shows the variation of phosphorus content in the foils at different current densities for three different temperatures. For the same electrolyte conditions and solution velocity, increasing the temperature from 70°C to 95°C permitted a three-fold increase in the current density while keeping the alloy composition in the range of 17-20 at% P. At a temperature of 95°C, easy peel off, easy handling, low porosity, low stress foils of 50-75 µm in thickness were obtained at current densities as high as 100 A/dm2 with coulombic efficiencies of 64 ± 4%.
 
 
 
Figure 6 -    Effect of temperature and current density on the phosphorus content of the Fe-P foils; 1.0M FeCl2, 0.5M NaH2PO2, solution velocity=180 cm/sec with the HCD-HSV system)
 
 
Amorphous characterization properties of Fe-P foils
 
Specimen foils were produced under both LCD-HSV and HCD-HSV conditions and annealed to minimize Hc. Figure 7 shows the x-ray diffraction patterns obtained as-deposited and at different temperatures: 275, 288 and 425°C. The x-ray diffraction patterns reveal an amorphous structure for the as-deposited and annealed samples at 288°C. Annealing a foil at temperatures higher than the exothermic peak around 400°C induced a transformation from the amorphous phase to a crystalline BCC Fe and Fe3P structure. Table 2 shows the optimization of the annealing temperature for minimizing the coercive field.
 
 
Figure 7 -    X-ray diffraction patterns of 50 µm thick Fe-P foils. An as-deposited sample is compared with annealed ones below (288°C) and above (425°C) the DSC transformation peak (Conditions: HCD-HSV deposition).
 
 
 
Table 2 -     Effect of annealing temperature on the variation of the coercive field (VSM measurements ± 10%).
 
Annealing
Temperature (°C)
LCD-HSV
Fe82-P17.5-Cu0.5
Hc (A/m)
HCD- HSV
Fe82.5-P17.5
Hc (A/m)
As-deposited
26
51
275
20
---
288
15
13
425
3700
> 3000
 
 
Fe-P power frequency losses
 
LCD-HSV specimens 1 × 4 cm were cut from a 10 × 15 cm foil and were annealed for 15 min at 265°C under an argon atmosphere and in a magnetic field produced by permanent magnets that completed a magnetic circuit with the samples.
 
Figure 8 shows the power frequency losses (W60), in a reduced size Epstein frame configuration, with corresponding values of coercive field (Hc) as a function of the peak induction Bmax. The actual losses presented in the figure are estimated to be about 5% higher due to the overlap section of the sample segments. The power frequency loss (W60) at peak induction of 1.35 T was 0.39-0.41 W/kg. The coercive force (Hc) after an induction of 1.35 T was 13 A/m ± 5%. The saturation induction was 1.5 T ± 5%. The electrical resistivity (ρdc) of the foil was 165 ± 15% μΩ•cm.
 
 
Figure 8 -    Power frequency losses (W60) and corresponding values of coercive field (Hc) as a function of the peak induction Bmax (Conditions: LCD-HSV deposition).
 
 
HCD-HSV specimens 1 × 4 cm were cut from several different 2 × 5 cm sample foils and annealed as previously described to construct another Epstein transformer configuration sample.
 
Figure 9 shows the power frequency losses (W60) and corresponding values of the coercive field (Hc) as a function of the peak induction Bmax. The actual losses presented in the figure are estimated to be about 10% higher due to the overlap section of the sample segments. The power frequency loss (W60) at peak induction of 1.35 T was 0.395-0.434 W/kg. The coercive force (Hc) after an induction of 1.35 T was 9.9 A/m ± 5%. The saturation induction was 1.4 T ± 5%. The average electrical resistivity (ρdc) of these foils was 142 ± 15% μΩ•cm.
 
 
Figure 9 -    Power frequency losses (W60) and corresponding values of coercive field (Hc) as a function of the peak induction Bmax (Conditions: HCD-HSV deposition).
 
 
These power frequency results shows that we have succeeded in producing low loss thick amorphous Fe-P foils by electrodeposition at low and at high deposition rates.
 
Conclusions
 
The aim of the present work relates to the electrodeposition of thick foils or plates (20-75 µm) of an amorphous Fe-P soft magnetic alloy, destined for large scale applications in electrical power equipment (transformers, rotating machines, etc.), that show high saturation induction, low coercive field and low power frequency losses.
 
The conditions for producing good amorphous Fe-P material have been systematically studied. In particular, a high current density, high solution velocity (HCD-HSV) process has been identified which could lead to a competitive and economical industrial process based on electrodeposition.
 
The 60 Hz AC loss properties are very encouraging at around 0.4 W/kg at 1.35 T maximum induction. This places the losses between those of metallic glasses and grain-oriented electrical steels.
 
Further advantages of these electrodeposited materials, for developing power applications, are that much greater thicknesses than metallic glasses can be achieved and that complex 2D shapes for rotating machines could be envisioned. Further evaluation of technical as well as economic aspects and a pilot production demonstration will follow.
 
Acknowledgements
 
This work was sponsored by Hydro-Québec Technological Innovation Program. The authors gratefully acknowledge helpful discussion with Dr. Ashok Vijh. We also thank F. Brochu, M. Provencher, R. Rioux, R. Veillette, J. Beaudry, I. Montplaisir and P. Noël for their technical assistance.
 
 
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*Corresponding author:

Dr. Robert Lacasse
Hydro-Québec
IREQ Research Institute
1800 boul. Lionel-Boulet
Varennes, Québec, Canada, J3X 1S1
Phone: (450) 652-8383
E-mail: lacasse.robert@ireq.ca

 

 

 

 

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