Figure 1. Prof. Eric Cheng

Prof. Eric Cheng is a Professor in the Department of Electrical Engineering of The Hong Kong Polytechnic University (PolyU). He is also the Research Group Leader of Utilization of the Department and the Director of the Power Electronics Research Centre of the University. His research areas cover all aspects of power electronics, machines, EMI and drives. With extensive industrial experience, he has published over 200 papers and received numerous awards in this area. Professor Cheng and his team members have successfully made use of the state-of-the-art polymer-boned magnet....click here for more details. 24 January 2008 - Seminar on the Invention of Polymer-Bonded Magnetic Materials for Power Conversion and EMI Screening. Click here for enrollment details. Awards: Gold Medal with Mention in the Brussels Eureka 2007- the 56th World Exhibition of Innovation, Research and New Technology, November 2007 Silver Medal Award in The 16th China Invention Exhibition,  2006

Figure 3a Polymer magnetic materials Figure 3b Polymer bonded material devices

Prof. Cheng and his team members have successfully made use of the state-of-the-art polymer-bonded magnetic device to produce magnetic materials. In using this new method, non-brittle magnetic cores of flexible shapes and different sizes can be made. This breakthrough has versatile applications in a wide range of products, including transformers and inductor components, direct-current-to-direct-current power converters, high frequency power supplies, and screening of electromagnetic wave.

Our research team has successfully made use of state-of-the-art polymer-bonded magnetic materials to produce magnetic devices. In using this new method, light weight, low lost and non-brittle magnetic cores of flexible shapes and different sizes can be made. The Polymer-bonded magnetic material is composed of polymer matrices and magnetic powders which can be produced by traditional polymer processing methods. Hence, it offers significant advantages over the conventional counterparts. One of the important advantages is the ease of molding such as injection molding which can save on manufacturing costs and quality control. Also, recycling and reuse of the polymer-bonded magnetic waste materials are very easy which is very suitable for environmental protection.

Figure 2a The Silver Medal Figure 2b Prof.K.W.E.Cheng in the 16th China Invention Awards Press Briefing Figure 2c The 16th China Invention Awards Press Briefing Figure 2d Students asking questions in the 16th China Invention Awards Press Briefing Figure 2a Dean of Engineering and his visitors (09-Feb-07) Figure 2b Visit of industrial visitors (09-May-07) Figure 2c Head of Department of EE, University of Western Australia (09-May-07) Figure 2d Visit of industrial visitors and the Departmental Advisor Mr. Simon Ho (14-May-07)

 

1)Polymer-bonded magnetic cores for power transformer and inductor

Magnetic Measurement Results (Tested on Ring core (Ø30×Ø15×H12mm), Loop Number N = 45

Process 5 kHz 50 kHz 100 kHz 500 kHz 1 MHz 2 MHz 10 MHz 1100 °C, 20 hours L (μH) 43.98 42.55 42.40 42.03 41.98 42.29 55.54 μr 33.12 32.04 31.93 31.65 31.61 31.84 41.82 1300 °C, 950 minutes L (μH) 56.00 55.45 55.26 54.86 55.01 55.59 80.58 μr 42.17 41.75 41.61 41.31 41.42 41.86 60.68

Typical photos and Magnetic measurement results for a polymer –bonded magnetic ring cores

2) Polymer-bonded magnetic material based charger (Flyback Converter)

Specification for the Polymer-bonded magnetic material based charger (Flyback Converter)

变换器规格(Specification)： 输出功率(Output Power)： 36W 输入电压(Input): 110/220VAC  50/60HZ 工作频率(Operating Frequency)： 500 kHz 输出电压/电流(Output Voltage/Current) : 12V/3A 输出纹波和噪声(Output Ripple & Noise)： 1％最大值(MAX)

3) Polymer-bonded magnetic material based charger (Resonant Load Converter)

Electrical Specification for Polymer-bonded magnetic material based charger (Resonant Load Converter)

Description Symbol Min Typ Max Units Comment Input Voltage VIN 110 VAC Frequency FLINE 50 Hz Output Output Voltage Vout 54 V Output Current Iout 0.93 2.75 A Total Output Power Continuous Output Power Pout 150 W Peak Output Power 200 W Efficiency n   84   % Full Load

Sintering methods are commonly used in conventional magnetic transformers and inductors. The sintering method is much more complicated when compared with polymer injection molding method which is used in our polymer-bonded magnetic cores making. The research team has developed the preparation methods for new materials and cores, the procedures are common in materials science research, and may be easily transferred to standard industrial polymer manufacturing method, such as using injection molding machine. The raw material for the EI40 cores made up the new material is about HKD 0.8 (manufacturing costs are not included); the traditional ferrite core purchase from the market is near HKD1.1 (3C9 approximate). Conventional ferrite has higher permeability and is easily saturated and brittle, and has limited size and shape; commonly an air gap is needed in power applications. The new material have relative low permeability (about 40-70), Non-brittle, flexible shape and size, light in weight, better used in high frequency application, and can handle more power than traditional ferrite. Figure 5a Visited by Dr. Tony Lee and his friends of the Automotive Parts and Accessory system APAS （10-Oct-06） Figure 5b Visit of industrial visitors (8-Nov-06） Figure 5c Visit of Prof. Cheng Yonghong from the Xian Jiaotong University（22-Nov-06） Figure 5d “Hong Kong Electronics Fair 2006” on 13-16 October 2006 at the Hong Kong Convention & Exhibition Centre Figure 6a Figure 6b Figure 6c Figure 6d Figure 6a-6d: Participation in Exhibition: “Hong Kong International Auto Parts Fair 2007” on 28 April- 1 May 2007 at AsiaWorld-Expo International Airport, Hong Kong

 

Prof. K.W.E.Cheng (EE.),
Dr. Y.W.Wong (Applied Physics),
Dr. Kai Ding (EE.),
Mr.Bonus Ho (EE),
Mr.Wei-Tai Wu (EE),
Mr. C.K.Cheong (EE),
Mr. T.K.Cheung (EE)

Prof. Eric Cheng is a professor in the Dept. of Electrical Engineering of the Hong Kong Polytechnic University.  He is the group leader of Utilization of the Department and Director of Power Electronics Research Center of the University. His research interests cover all aspects of power electronics, magnetics, machines, EMI and drives.  He has published over 200 papers and 7 books.  Since he joined the Department in 1997, he has been working on 31 research and development projects as a Principal Investigator with total funding of more than $35 Million. He also has been Principal Investigator for 4 CERG projects. He has also obtained a number of awards from the institution and university.  This includes the best journal paper award, outstanding consultancy award, best teaching, successful patent and valuable consultancy, Hong Kong Eco-Products Award and 16th National Exhibition of Inventions and also the Brussel Eureka Technological Innovation Gold Medal with Mention. He is very active in conducting professional industrial consultancy work for the industries.  The company involved includes both overseas and local company.  So far, he has been the project manager of more than 80 consultancy projects with total project amount over $35 Million.

Dr. Y.W.Wong has been working on polymeric materials and their composites with functional ceramics for a number of years. His recent interest is developing composite material using thermoplastic elastomer and ferroelectric ceramics to make flexible pyroelectric sensor for infrared radiation detection. In addition, he has developed a novel technique using pulsed laser deposition method to prepare magnetic nanocomposite films which exhibit giant magnetoresistant effect. Dr. Wong has also served the industry as consultancy for many years, in particular for electronic packaging industries which require advices on the characteristics of epoxy. In a recent consultancy work joined with the Electrical Engineering Department of PolyU, he served to make advice on the insulating failure of the enamel coated copper wires of the cast resin high voltage transformers. Dr. Wong is the principle investigator of three RGC funded projects of which one has been successfully completed. He is also the co-investigator of two ITF projects which work on the textile applications of the shape memory polymers. He has published over sixty scientific research papers and holds one patent with a collaborator on using X-ray absorption to measure the yarn density distribution.

Dr. Kai Ding  was born in Kaifeng, Henan Province , China, on 1976. He received the B.E., M.E., and Ph.D. degrees from Huazhong University of Science and Technology, Wuhan, China, in 1998, 2001, and 2004, respectively. In 2005, he joined the HongKong Polytechnic University as a Research Fellow. His research interests include Polymer-bonded magnetic devices, multilevel converters, piezoelectric transformer, fuel cell technique, power electronics applications in electric power system and computer simulation.

Mr. Bonus Ho Y. L. Ho obtained his BSc(Eng) from the Hong Kong Polytechnic University and MScMIT from City University. He was employed by the Hong Kong Polytechnic University as research assistant in 2000 and joined Digipower Technology Limited as engineer in 2004. His research interests include resonant converters, topologies of switching circuits, AC drives and high frequency magnetic.

Dr. Wei-Tai Wu, born in Fujian Province of China, majored in Applied Chemical and minored in Computer Science and Technology at University of Science and Technology of China (USTC), and received his B.S. degree in 2003. After that, he began his graduate program as PhD candidate of Condensed Matter Physics in the Structure Research Laboratory of Chinese Academy of Sciences, and Hefei National Laboratory for Physical Sciences at the Microscale (at USTC. Expected graduate date: Summer 2008). He also began work as a research assistant at USTC in September 2003. He joined Prof. KW Eric Cheng’s group at the Hong Kong Polytechnic University in Kowloon as a research assistant in October 2006. He currently studies in field of macromolecular condensed matter physics. His PhD thesis is centered in stimuli-responsive polymers and their derivative functional materials with special emphasis in nanoscience/nanotechnology area. And his research interests also include carbon fiber (cooperated with Beijing University of Chemical Technology etc.), polyolefin (cooperated with Sinopec Yangzi Petrochemical Co Ltd. etc.), and strongly correlated electrons systems. He has authored around 25 scientific papers.

Mr.C.K.Cheong, born in Hongkong, Graduate from The Hong Kong Polytechnic University, majored in Power electronics. He is now a project Assistant in Honkong Polytechnic University.His research interests include resonant load power converter, Polymer-bonded magnetic materials for power converters,etc. Mr. T.K. Cheung T. K. Cheung started his research in power electronics in 2000 at the Hong Kong Polytechnic University. He received the B.Eng and MScMIT from the Hong Kong Polytechnic University and City University of Hong Kong, in 2004 and 2006, respectively. He is currently the Design Engineer at Digipower Technology Ltd., Hong Kong. His current research interests include Polymer-bonded magnetic devices, computer programming, power converter and controller design.

 

Conventional types of magnetic materials suffer from a number of disadvantages including limited size, its brittleness, high loss and high cost. One of the typical problems is that for application in high power conversion such as more than 20kW system, the transformer or inductor required is gets extremely difficult to obtain because of the mechanism of formation of Ferrites or powder iron and is exceptionally expensive. High frequency use of the conventional component is difficult as the permutation is too high. The Polymer-bonded magnetic material is composed of polymer matrices and magnetic powders which can be produced by traditional polymer processing methods. Hence, it offers significant advantages over the conventional counterparts. One of the important advantages is the ease of molding such as injection molding which can save on manufacturing costs and quality control.
The loss can be divided into conductor loss and core loss. The conductor (winding) loss is the resistive loss due to the current passed through the winding and they will also increase dramatically as the frequency increases because of the current distribution in the conductor at high frequency.

Figure 8 Organizing the 2007 internal seminar on Power Electronics 1 June 2007 at The Hong Kong Polytechnic University

Research on Polymer-Bonded Magnetic Materials for a buck converter

Abstract--There is currently a need of magneto-electrical apparatus such as inductor and transformer that operate at hundreds of kHz up to several MHz, or even higher frequency. The material based on polymer-bonded magnetic powder has a feature of the magnetic property and can be used as the power transformer, electromagnetic interference shielding, and power inductor, etc. It is so promising that this material has been found good results for power conversion and screening. In this work, a magnetic ring core was fabricated using the composite of the magnetic powder and polymer, nickel, rare earth materials and poly(methyl methacrylate) (PMMA). This ring core was used as the output filter in a buck converter, and the electrical properties were studied.

Index Terms-- buck converter, magnetic material, polymer, transformer

IN recent years, the advanced properties of polymer bonded magnetic materials continue to be attractive to researchers. These magnetic materials, which are composed of magnetic powder and polymer matrix, combine attractive magnetic properties with superior shaping capabilities. Other advantages of the polymer-bonded magnets are lighter weight, unlimited magnet lengths, and, in many cases, improved cost effectiveness due to net-shape manufacturing[1]-[4].
Soft ferromagnetic materials are those magnetic materials with high permeability, low coercivity and low hysteresis loss, which can be used to amplify the flux density generated by a magnetic field. There are many types of soft ferromagnetic materials; usually, for the power converter application, they could be ferrites (like Ni-Zn ferrites, and Mn-Zn ferrites, etc.), the alloy containing Fe, Co, Ni and other elements (like FeNi alloy, and silicon steel sheet, etc.), and polymer-bonded magnetic materials which are composed of polymer matrix and magnetic powder (like polymer -bonded MSS magnetic powder, polymer -bonded MPP magnetic powder, polymer bonded Fe-Ni powder, etc.), etc. The available range of magnetic properties of soft magnetic materials is continually being expanded. This amounts to reduction in coercivity, increase in permeability and consequently a decrease in hysteresis loss. These advantages make them very popular for the application in power conversion. One of the applications is to produce inductor cores or transformer cores for the power converter. And the loss for the magnetic device usually accounts for 30-40% of the total losses of the converter.[5]-[10]
Conventional magnetic material suffers from a number of disadvantages including limited size, brittle, high loss, and high cost, etc. Particularly, for the application in high power conversion (> 20 kw system), it becomes very difficult and much expensive to fabricate the transformer or inductor cores required. However, polymer-bonded magnets can be produced by traditional polymer processing methods, which offer significant advantages in respect to shaping and cost.[11]
In power conversion, the loss mainly derives from the loss in the conductor and that in the core. The loss in the conductor (winding) is the resistive loss because of the passing current through the winding, and it will increase dramatically with increasing frequency due to the high-frequency current distributed in the conductor. The losses in the magnetic core usually include the hysteresis loss, eddy loss, and residue loss.  The hysteresis loss is the dominant loss when working at the lower frequency (below 50 kHz, hysteresis/eddy loss < 20%), and the eddy loss becomes more important at the higher frequency (above 500 kHz, hysteresis/eddy loss > 60%). The introduction of polymer bonded soft magnetic materials in the core could lower the eddy loss to some extent, thus could extend their applications to a broader range at high frequency area.
This paper is to investigate the electrical properties of a polymer bonded magnetic material applied in power conversion. The polymer bonded magnetic material was prepared by a common mixing method namely “melt mixing method”, and then was molded into ring shape. This ring core was used as the inductor, which is used as the output filter in a buck converter. It is envisioned that the present studies may bring some insight into the application of the polymer bonded magnetic material.

Fabrication  of Magnetic Composite

The filler of the composite materials were cobalt powders (spherical, diameter < 18 mm) or nickel powders (spherical, diameter < 4 mm).  Their surfaces were treated by titanic coupling agent (1.5% weight of filler) to enhance dispersing the magnetic powders and improving the consistent between magnetic powders and the polymer. Poly(methyl methacrylate) (PMMA) was used as the polymer matrix.

The Polymer Bonded Magnetic Ring Core

A Polymer bonded ring core which consist of Nickel micro –powder(18%), Rare Earth Materials(76%) and PMMA is used for analysis. The outer and inner diameters of the core are 3cm and 1.2cim respectively. The height is 1.1cm. Fig.1 shows the core wounded by 150 turns of winding. It can be seen that the core is very similar to other ferrite or powder iron. The strength of the core is also very high but not brittle as the ferrite and powder iron. The inductance of the core wounded by 150turns was measured at 100 kHz, the wounded core has a low relative permeability of 3, and the measured inductance is 87uH.

Application of a buck converter

A. Buck Converter

The buck or step-down converter regulates the average DC output voltage at a level lower than the input or source voltage. This is accomplished through controlled switching where the DC input voltage is turned on and off periodically, resulting in a lower average output voltage. The buck converter is commonly used in regulated DC power supplies like those in computers and instrumentation. The buck converter is also used to provide a variable DC voltage to the armature of a DC motor for variable speed drive applications[12]. As shown in Fig.2, the characteristic of buck converters, in general, is that the DC output waveform is less than the DC input waveform. A buck converter that does not employ a transformer as an isolation stage is referred to as a non-isolated buck converter. The non-isolated buck converter typically includes switching circuitry coupled to an input source of electrical power. The switching circuitry includes at least one active switch. The switching circuitry is coupled to an output inductor and output capacitor which provides the DC output waveform (i.e., an output voltage) at an output of the non-isolated buck converter[13].

B. Experiment
The buck converter shown in Fig.2 was constructed; the inductor based on the polymer-bonded magnetic material was used in the experiment. The electrical specification of the converter is shown in table I. The polymer-bonded magnetic ring core inductor was used as the output filter in the experiment shown in Fig.1. The switching frequency is 100 kHz; the switch duty ratio of the buck converter was set to 50% and 75% respectively. A variable load resistor was used as the load. The load resistor was adjusted to 10ohm. The experiment waveforms are shown in Fig.3 and Fig.4. Fig.3(a) and Fig.4(a) show the PMW signal. For the inductor current measurement, calibration of the current sensors is such that for 1A flowing through the sensor, the output is 0.5V. The signal is amplified by a factor of 2 as shown in Fig.3(b) and Fig.4(b). The capacitor current is measured across a series resistor of 0.1ohm. Hence the actually value of capacitor current is 10 times the value as shown in Fig.3(c) and Fig.4(c).

TABLE I  
The Electrical Specification of the buck converter

(a) PWM signal (b) Inductor current(500mV/1A)  (c) Output capacitor current(100mV/1A)

Fig.3 Experiment results (Duty ratio=50%)

(a) PWM signal (b)Inductor current(200mV/400mA) (c) Output capacitor current(100mV/1A)

Fig.4.Experiment results (Duty ratio=75%)

ACKNOWLEDGMENT

This work is supported by Guangdong-Hong Kong Technology Cooperation Funding Scheme, Innovation and Technology Fund, under project GHP/066/05.

CONCLUSION

Polymer bonded magnetic materials offers many advantages compared to the conventional type of magnetic materials in respect to shape, and cost, etc. A polymer bonded magnetic material was developed.  The fillers of the composite were nickel powders (18% in weight) and rare earth compounds (76% in weight), and PMMA was used as the polymer matrix. A buck converter using an inductor core based on the materials had been prototyped.  The experimental results indicate that the material may be suitable for making inductor core.

Reference

Research on Polymer-Bonded Magnetic Materials for a buck converter

Abstract--Polymer bonded magnetic materials have recently attracted increasing attention from fundamental research to industrial applications in many electromagnetic devices. It is necessary to investigate the properties of these magnetic materials operated under different environmental conditions. In particular, in this work, we investigated the relationship between the temperature and the magnetic properties of a polymer bonded magnetic material, epoxy (EP) resin bonded Co-Ni magnetic material. Experimental results indicate that the coercivity, the remanence magnetic flux density and the saturation magnetic flux density decrease at elevated temperature. It is envisioned that the present studies may offer guidance of the polymer bonded magnetic materials in the power conversion and EMI applications.

Index Terms—magnetic material; magnetic property; polymer bonded; temperature.

Introduction

THE dynamic development in the technology and engineering domains always requests demanding requirements posed to various desired materials. Recently, the polymer bonded magnetic materials has attracted a great deal of attention for the fundamental and applied research in the fields of magneto-electrics, and magneto-optics, etc.[1]-[3] These magnetic materials have also emerged as a potential class of materials for transformer cores, in which high saturation and low losses are desired, and electromagnetic interference (EMI) shielding—an application which becomes more and more important due to electromagnetic smog.[3]-[7]
Generally, the polymer bonded magnetic materials are mainly composed of polymer matrix and magnetic powder, and they can be prepared by two mixing methods namely “melt mixed method” and “cement mixed method”.[3]-[16] There are many types of magnetic powder, including ferritic magnets (such as BaO • 6Fe2O3, SrO • 6Fe2O3, Mn-Zn ferrite, and Ni-Zn ferrite), iron alloys containing Co, Ni and other elements (such as FeNi alloy, and silicon steel sheet), and rare-earth magnets (such as SmCo3, and Sm2M17 (M = Co, Fe, Cu, Ni, Mn, etc.)), etc. The polymer matrix can be thermoplastics, such as polyethylene (PE), polypropylene (PP) and polyamide (PA), or thermosetting resin like epoxy resin (EP) and phenolic resin (PF), etc. It is obvious that the polymer bonded magnetic materials have the advantages in respect to light weight, low cost, design flexibility, versatile electrical and microwave properties, compared with those magnets without polymer.[12]
The devices in which the polymer bonded magnetic materials are used work in various environmental conditions. Under these conditions, the factors, like temperature, change in a broad range and in short time, even stepwise often. Besides, a big problem in using the magnets in transformer cores is their significant calefaction because of the losses.[8]-[16] It is well known that carbon-based polymers typically suffer from a lack of stability when exposed to heat or other external factors, thus may causing more or less changes in the internal structure of the magnetic materials. Technological progress in electrical engineering (production, transport and use of electrical energy) is highly linked to studies made in material science. Therefore, in this work, the goal is to investigate the sensitivity of the magnetic properties of a polymer bonded magnetic material (EP resin bonded Co-Ni) to temperature. It is envisioned that the present studies may make some insight into the effect of environmental factors on the material, which may be helpful for its applications.

Experimental

The magnetic material used in this investigation was prepared by a common mixing method namely “cement mixed method”.[8][17] To obtain the composite materials the Co powder and Ni powder, the mass ratio of Co:Ni = 1:1, were mixed with the thermosetting EP resin powder. The mass ratio of EP resin was ~10% of the total mass of the mixture.
FTIR measurements were performed using a Nicolet Instrument Co. MAGNA-IR 750 FTIR spectrometer with KBr as background. SEM images were obtained on a JEOL JSM-6700F field emission scanning electron microscope using conventional sample preparation and imaging techniques. Differential scanning calorimetry (DSC) measurements of the sample were carried out with a Perkin-Elmer DSC 7 at a heating rate of 10oC/min over a temperature range of 30-300oC.
The magnetic properties of the sample were measured by a microprocessor-controlled vibrating sample magnetometer (VSM). The specimen used in the measurements is about 1×2×3 mm of 0.041 g. In order to protect the sample against oxidation, the sample chamber was evacuated for 15 min and then filled with helium gas to ambient pressure before measurements. Temperature dependence of the magnetic properties of the sample was measured at temperatures ranged from 300 K to 325 K. The saturation magnetic flux density (Bs), remanence magnetic flux density (Br), and coercivity (Hc) are derived from the hysteresis loop obtained.

Results and Discussion

The sample was identified by examining FTIR spectrum (Fig. 1) recorded at room temperature. The evidence of characteristic absorptions located at ~2964.5 cm−1, ~1250.4 cm−1 and ~910.2 cm−1 corresponding to the vibrancy models of the epoxy ring, and those located at ~1606.1 cm−1, ~1508.7 cm−1 and ~828.6 cm−1 corresponding to the vibrancy models of the benzene ring observed in the FTIR spectrum confirm the successful attachment of EP resin to the surface of the magnetic powders.
SEM images in Fig. 2 show the microstructure of the as-prepared sample, revealing the well bonding of the magnetic powders by EP resin. A few crannies were also observed possibly due to the poor compatibility between EP resin and the magnetic powders, resulting in phase separation to some extent. The DSC curve was also recorded as shown in Fig. 3, indicating the glass transition temperature (Tg) of the sample is about 104oC.

The magnetic hysteresis loops of the sample are shown in Fig. 4. All curves exhibit the typical magnetic hysteresis, which indicates the ferromagnetic nature of the Co-Ni particles. The magnetic properties of the sample are measured from certain defined points and derivatives obtained from these hysteresis loops, delivering the saturation magnetic flux density Bs, the remanence Br, and the coercivity Hc, etc.

Fig. 2. Typical SEM images of the sample.

Fig. 5 shows the graphs of magnetic properties versus temperature. The magnetic properties of materials can be divided into two general categories: those that are structure sensitive and those that are structure insensitive. Structure-sensitive properties are those that are drastically affected by changes in materials processing (heat treatment or mechanical deformation) or by small changes in composition. Permeability, coercivity, hysteresis losses, remanence, and magnetic stability are all considered to be structure sensitive. The structure sensitive properties are controlled through processing of the material including mechanical and thermal treatments. It can be seen that the coercivity (Fig. 5a) and the remanence magnetic flux density (Fig. 5b) indeed decreases at elevated temperature.
The hysteresis loss is the area enclosed by the hysteresis loop. It represents the energy consumed per unit volume during one cycle of the hysteresis loop. The hysteresis loss increases as the maximum magnetic field reached during the cycle increases. This loss is closely related to the coercivity so that processing of materials to reduce coercivity also reduces the hysteresis loss. Reduce the remanence magnetic flux density could also reduce the hysteresis loss. Thus, from these points of view, it is envisioned that the available range of magnetic properties of the present material would not be restricted by the calefaction factor; on the contrary, it seems that it favors the application at the higher temperature.[3]

(a) (b) (c) (d) (e) (f)

Fig. 4. Magnetic hysteresis loops of the sample measured at various temperatures: (a) 300 K, (b) 305 K, (c) 310 K, (d) 315 K, (e) 320 K, and (f) 325 K.

Moreover, the coercivity is the parameter which is used to distinguish hard and soft magnetic materials. Traditionally, a material with a coercivity of less than 1000 A/m (equals to 4π Oe) is considered magnetically “soft”, and a material with a coercivity of greater than 10,000 A/m (equals to 40π Oe) is considered magnetically “hard”.[3][18] According to the tendency shown in Fig. 5a, the coercivity (79.28 Oe) obtained at 325 K would further decrease to a much lower value, and the transition to magnetically “soft” may happen; however, we could not give further evidence due to the restriction of our experimental condition, and further studies are still needed to clarify this hypothesis.

(a) (b) (c)

Fig. 5. Temperature dependent magnetic properties of the sample: (a) the coercivity, (b) the remanence magnetic flux density, and (c) the saturation magnetic flux density versus temperature.

Structure insensitive refers to properties not markedly affected by changes in materials processing or composition. Structure-insensitive properties include the saturation magnetization and resistivity. These properties are largely dependent on the composition of the material and are not changed substantially in the processing. On the other hand, in the low temperature range (up to around 0.4-0.5 TC) the temperature dependence of the magnetization M can be described by Bloch’s law [19]:

And

Herein, μ0 is the permeability of free space; H is the strength of the external magnetic field. Fig. 5c shows the saturation magnetic flux density against the temperature, demonstrating that Bloch’s law is well fulfilled in the present experimental temperature range that the saturation magnetic flux density decreases at the elevated temperature.
However, for transformer applications, not only low losses but also high saturation magnetic flux density is desired, since the higher saturation could improve the DC superposition property of permeability and favors the larger transmited power in a strong magnetic field. Under this consideration, the relationship between the saturation magnetic flux density and the temperature may present as a shortcoming in the application of the present material as transformer cores.

Conclusions

Polymer bonded magnetic materials have attracted increasing attention from fundamental research to industrial applications in many electromagnetic devices. The previous discussion underlines the need to investigate these magnetic materials under various environmental conditions. In particular, the aim of this paper was to investigate the relationship between the temperature and the magnetic properties of a polymer bonded magnetic material, EP resin bonded Co-Ni composite material. Experimental results indicate that the coercivity, the remanence magnetic flux density and the saturation magnetic flux density decrease at the elevated temperature. It is envisioned that the present studies may offer guidance for the applications of the polymer bonded magnetic material.

Acknowledgement

This work is supported by Guangdong-Hong Kong Technology Cooperation Funding Scheme, Innovation and Technology Fund, under project GHP/066/05.

References