Iec tr 62518 2009

15 Figure 10 – Time dependence of irreversible flux loss for SmCo5 magnet exposed at different temperatures [22] .... 16 Figure 11 – Time dependence of irreversible flux loss for SmCo5 m

Reversible flux loss

The magnetic properties of rare earth magnets undergo reversible changes with temperature, primarily due to variations in saturation magnetization This reversible flux loss represents a change in magnetization that can be restored once the disturbing factor, such as temperature, is eliminated.

Irreversible flux loss

Irreversible flux loss occurs when the magnetization fails to revert to its original value after the disturbing influence is removed Factors such as temperature changes, local temperature fluctuations, and external magnetic fields can cause this disturbance However, this irreversible flux loss can be completely recovered through the process of remagnetization.

Permanent flux loss

Permanent flux loss occurs due to irreversible changes in the metallurgical state, which are typically influenced by time and temperature Factors contributing to this loss include precipitation and growth, oxidation, annealing effects, and radiation damage Unlike temporary flux loss, permanent flux loss cannot be restored to its original magnetization value through remagnetization.

Figure 1 illustrates the different types of flux losses, as schematically presented by R Tenzer [7, 8] It depicts the relationship between magnetic flux density B and temperature, along with demagnetization curves at various temperatures, accompanied by a specific load line to elucidate the three categories of losses.

The curves depicted in Figure 1(a) are relevant for brief temperature changes, such as from 25°C to θ °C The magnetic flux density B varies reversibly along a demagnetization curve at different temperatures The difference B d (25) - B d (θ) is referred to as the "reversible flux loss," which can be completely recovered by returning the magnet to room temperature.

In larger temperature excursions, as shown in Figure 1(b), a portion of the flux change can be recovered during cooling, represented by B d ’(25) - B d ’(θ) The remaining part, B d (25) - B d ’(25), is recoverable through remagnetization and is referred to as the “irreversible flux loss.”

When temperatures exceed several hundred degrees Celsius, the magnet's microstructure undergoes changes, and surface oxidation occurs, leading to additional flux loss that cannot be recovered through remagnetization This phenomenon is illustrated by the difference between B d (25) and B d ”(25) in Figure 1(c).

The concept of "permanent flux loss" is closely associated with the reversible relationship between magnetic field strength (B) and temperature (θ), which mirrors the changes in saturation magnetization and remanence This relationship is typically represented as a straight line, known as the "reversible temperature coefficient."

LICENSED TO MECON Limited - RANCHI/BANGALORE FOR INTERNAL USE AT THIS LOCATION ONLY, SUPPLIED BY BOOK SUPPLY BUREAU.

Figure 1 – Change of magnetic flux density operating on a load line during elevated temperature ageing after R Tenzer (schematic) [7, 8]

5 Long term ageing of rare earth magnets

Newly magnetized magnets exhibit a gradual decay in flux, known as open circuit flux or operating-point induction, when observed over an extended period This decay typically follows a time function, as illustrated schematically in Figure 2 after K J Strnat [9].

The process of change occurs in three distinct stages Initially, there is a rapid loss of flux, denoted as ab This is succeeded by an extended phase of stability, referred to as the "plateau," represented by bc, during which a consistent irreversible flux loss is typically observed per logarithmic time cycle.

The time dependency of B on the plateau is proportional to log t, as demonstrated by the magnetic after-effect results from Street et al [10] To illustrate this constant flux change, the concept of "irreversible flux loss per decade" is utilized, which measures the flux change per decade in percentage terms This irreversible flux loss per decade represents the flux loss over a specified time period.

1 h to 10 h, from 10 h to 100 h or from 100 h to 1 000 h

At elevated temperatures, certain magnets experience a significant and sometimes catastrophic decline in flux, particularly rare earth bonded magnets with inadequate surface treatment in harsh environments In contrast, rare earth sintered magnets show only minor flux variations This report does not address the temperature stability related to the complex corrosion behavior of these magnets.

Initial flux loss: mainly magnetic change, recoverable by remagnetizing

Plateau: constant flux loss per logarithmic time cycle

Initial flux loss: mainly magnetic change, recoverable by remagnetizing

Plateau: constant flux loss per logarithmic time cycle

Figure 2 – Long term ageing of rare earth magnets (schematic) [9]

The dimensions of the specimens used are 10 mm in diameter and 12 mm in length (diameter:

D, L: length), D = 10 mm and L = 7 mm, D = 10 mm and L = 3 mm and D = 10 mm and L = 2 mm The relationship between length to diameter ratio (L/D) and the permeance coefficient

(P c ) for cylinders was obtained by Du-Xing Chen et al [11] The permeance coefficient is a ratio of the magnetic flux density, B d , to its self-demagnetisation field, H d , P c = B d /μ 0 H d

The initial magnetization was performed with a pulsed magnetic field of 4,1 MA/m to 4,5 MA/m

The peak field ranged from 5.1 T to 5.6 T, with a rise time exceeding 1 ms Open circuit flux was measured using a closely fitting pickup coil and a digital integrating fluxmeter In this experiment, unlike typical procedures where specimens are removed from the coil, the coil remained fixed while the specimens passed through it.

After measuring the initial flux, specimens were placed in a controlled oven environment at ±1 °C, ensuring they were spaced 150 mm apart to prevent magnetic interaction Following long-term stability tests, the specimens were allowed to cool to room temperature for 1 hour before flux measurements were taken Any temperature variations from 23 °C during these measurements were corrected using the reversible temperature coefficient.

Irreversible flux losses were assessed using the equation: \[\Delta \phi_{\text{irr}} = \left( \frac{\phi - \phi_0}{\phi_0} \right) \times 100 \, (\%)\]In this equation, $\Delta \phi_{\text{irr}}$ represents the irreversible flux loss, $\phi_0$ denotes the initial flux, and $\phi$ indicates the flux after being subjected to elevated temperatures.

Permanent flux losses were determined with the following equation: Δφper= {(φremag – φ0)/φ0} × 100 (%)

The document IEC 383/09, intended for internal use by MECON Limited in Ranchi/Bangalore, discusses the permanent flux loss, denoted as Δφper, which occurs due to structural changes It also highlights the remagnetization process, where the flux, represented as φremag, is achieved using a pulsed field of 4.1 MA/m (5.1 T).

The temperature coefficient of flux was assessed by measuring the temperature dependence of the flux in magnets using a fluxgate type digital integrating fluxmeter, as illustrated in Figure 3 [13] To ensure accuracy, the ambient noise level in the laboratory was evaluated beforehand, revealing a noise level of less than ±80 mA/m, while the signal exceeded 100 A/m The flux measurements were conducted on specimens with a diameter of 10 mm.

7 mm in length were employed The specimen was set in the furnace at room temperature

(23 °C) and heated up to 30 °C The temperature cycling procedure was heating up to 50 °C from 30 °C at the heating rate of 3 °C/min followed by cooling to 30 °C at the cooling rate of

3 °C/min and heating up to 100 °C followed by cooling to 30 °C and so on (23 °C

Flux change due to temperature

The changes of flux during temperature cycling for SmCo 5 [16], Sm 2 Co 17 [16] and Nd-Fe-B

The magnetic properties of the magnets used in this experiment, including SmCo 5, Sm 2 Co 17, and Nd-Fe-B, are detailed in Table 1 These magnets can withstand maximum exposure temperatures of 200 °C for SmCo 5 and Sm 2 Co 17, and 150 °C for Nd-Fe-B The changes in magnetic flux after exposure to elevated temperatures are influenced by factors such as temperature, coercivity, and the specimen's shape, specifically the length-to-diameter ratio for cylindrical samples After being exposed to 150 °C, the flux changes observed were -1.5% for SmCo 5, -1.5% for Sm 2 Co 17, and a decrease for Nd-Fe-B.

Reversible temperature coefficients of magnetic flux can be determined from this data

Reversible temperature coefficients, as shown in Table 2, highlight two selected temperature ranges for comparison These coefficients tend to align with the temperature dependence of the saturation magnetization of various magnets Specifically, the reversible temperature coefficients of magnetic flux for SmCo 5, Sm 2 Co 17, and Nd-Fe-B magnets are discussed.

–0,05 %/°C, –0,04 %/°C and –0,13 %/°C, respectively in the temperature range from 30 °C to

Table 1 – Magnetic properties of the rare earth magnets employed for the open circuit flux measurements to determine the reversible temperature coefficient of the magnetic flux

Table 2 – Reversible temperature coefficient of the magnetic flux determined by temperature cycling

Materials Reversible temperature coefficient of magnetic flux

Figure 4 – Temperature dependence of flux for SmCo 5 magnet ( L/D = 0,7) [16]

Figure 5 – Temperature dependence of flux for Sm 2 Co 17 magnet ( L/D = 0,7) [16]

Figure 6 – Temperature dependence of flux for Nd-Fe-B magnet ( L/D = 0,7) [17]

Figure 7 – J-H demagnetization curves of Nd-Fe-B magnet measured at different temperatures [18]

Effect of temperature on B r and H cJ (demagnetization curves at different temperatures)

(demagnetization curves at different temperatures)

The J-H demagnetization curves for a Nd-Fe-B magnet, measured at various temperatures, are illustrated in Figure 7 Additionally, Figure 6 presents the temperature dependence of flux for this magnet The temperature coefficient of B r (α(B r)) for the Nd-Fe-B magnet, calculated from the B r data at 20 °C and 150 °C, is –0.122 %/°C, which closely aligns with the reversible temperature coefficient value of –0.142 %/°C listed in Table 2.

The J-H demagnetization curves at different temperatures of a Nd-Fe-B magnet [18] are shown in Figure 8 The magnetic properties at 25 °C are shown in Table 3 With the data in

Figure 8 the temperature dependence of normalized B r and H cJ for the Nd-Fe-B magnet [19] can be obtained The same procedure for SmCo 5 and Sm 2 Co 17 magnets was followed

Nd-Fe-B magnet measured at different temperatures [19]

Figure 9 – Temperature dependence of normalized B r and H cJ for SmCo 5 , Sm 2 Co 17 and Nd-Fe-B magnets [19]

Figure 9 shows the temperature dependence of normalized B r and H cJ for SmCo 5 , Sm 2 Co 17 and Nd-Fe-B magnets for which the magnetic properties at 25 °C are shown in Table 3

The values of \$B_r\$ and \$H_{cJ}\$ were normalized at 25 °C, and their temperature coefficients, \$\alpha(B_r)\$ and \$\alpha(H_{cJ})\$, are presented in Table 3 Notably, the \$\alpha(B_r)\$ value for the Sm$_2$Co$_{17}$ magnet is the lowest among the three materials, which can be attributed to its higher Curie temperature compared to the others.

The non-linearity of the temperature dependence of B r and H cJ is given in Annex B

The α(H cJ ) of the three materials range from –0,22 %/°C to –0,50 %/°C, and –0,22 %/°C for

Sm 2 Co 17 magnets exhibit the lowest coercivity, which is regulated through "pinning by precipitates." The α(H cJ) value of these magnets can be adjusted by modifying their composition and heat treatment In contrast, the α(H cJ) of SmCo 5 and Nd-Fe-B magnets remains largely unchanged due to their coercivity mechanism based on "nucleation."

“nucleation” type magnets is determined by the nucleation of a reverse domain after removal of the domain walls, while the movement of existing domain walls within a grain is relatively easy [20, 21]

Table 3 – Temperature coefficients of B r and H cJ for SmCo 5 , Sm 2 Co 17 and Nd-Fe-B magnets (temperature range for the coefficient: 25 °C to 150 °C)

The time effects at constant temperature (influence of temperature exposure

The time dependence of the irreversible flux loss for SmCo 5 [22], Sm 2 Co 17 [22] and Nd-Fe-B

The magnetic properties of various specimens at different temperatures are illustrated in Figures 10, 12, and 14, with detailed data provided in Table 4 Initial flux loss is primarily influenced by factors such as coercivity, the length to diameter (L/D) ratio of the magnets, and the exposure temperature For SmCo 5 magnets, the initial flux losses are observed to range from -0.46% to a specified value.

2,83 % after exposure from 50 °C to 150 °C The initial flux losses for Sm 2 Co 17 magnets range from –0,16 % to –1,86 % after exposure from 50 °C to 150 °C The initial flux losses for

Nd-Fe-B magnets experience irreversible flux loss ranging from –0.16% to –7.32% when exposed to temperatures between 50 °C and 150 °C Additionally, specimens with a higher initial flux loss tend to show greater irreversible flux loss per decade, irrespective of the material used.

Figure 10 – Time dependence of irreversible flux loss for SmCo 5 magnet exposed at different temperatures [22]

Figure 11 – Time dependence of irreversible flux loss for SmCo 5 magnets with various L/D s [24]

Figure 12 – Time dependence of irreversible flux loss for Sm 2 Co 17 magnet exposed at different temperatures

Figure 13 – Time dependence of irreversible flux loss for Sm 2 Co 17 magnets with various L/D s (Material 2) [24]

Figure 14 – Time dependence of irreversible flux loss for Nd-Fe-B magnet exposed at different temperatures [23]

Figure 15 – Temperature dependence of irreversible flux loss after exposure for

100 h for Nd-Fe-B magnets with various

The time dependence of irreversible flux loss for SmCo 5 and Sm 2 Co 17 magnets with varying L/D values is illustrated in Figures 11 and 13 Additionally, Figure 15 presents the temperature dependence of irreversible flux loss after 100 hours of exposure for Nd-Fe-B magnets with different L/D values The data indicates that specimens with the lowest L/D values exhibit distinct characteristics in flux loss behavior.

L/D exhibit the highest irreversible flux loss The lower L/D means that a higher demagnetisation field is applied to magnets by their own magnetization and a higher irreversible flux loss results

The irreversible flux loss of rare earth magnets is influenced by factors such as coercivity (H cJ), the length-to-diameter ratio (L/D), exposure temperature, and the uniformity of the field strength (H k), which indicates the squareness of the demagnetization curve However, this report does not consider the squareness of the demagnetization curve as a parameter for controlling irreversible flux loss.

Table 4 – Magnetic properties of the specimens for the experiments to evaluate the effects of temperature and L/D on irreversible flux loss

The influence of H cJ on the irreversible flux loss for Sm2Co17 magnets

The time dependence of irreversible flux loss for Sm 2 Co 17 magnets with H cJ = 0,48 MA/m [26],

Figures 16, 17, and 18 illustrate the irreversible flux loss, with values of 1.19 MA/m and 1.97 MA/m, representing the average of two samples By applying the methods outlined in references [2], [3], and [4], the critical magnetic field strength (H cJ) can be enhanced through adjustments in composition and heat treatment The magnetic properties of the Sm 2 Co 17 magnets used in this study, which assess the impact of H cJ on irreversible flux loss, are detailed in Table 5 Notably, high coercivity Sm 2 Co 17 magnets present challenges in magnetization due to the "pinning" coercivity mechanism, where domain wall movement is significantly hindered by pinning at the phase boundary of the precipitate, while small reverse domains persist continuously.

[20, 29] From the data in Figures 16, 17 and 18, it is concluded that irreversible flux loss can be reduced by an increase in coercivity under conditions which retain the squareness level

Comparison of the irreversible flux losses for Sm 2 Co 17 magnets with different H cJ is shown in

Figure 23 The irreversible flux losses shown in Figure 23 are after an exposure for 1 000 h

Increasing the coercivity (H cJ) of Sm 2 Co 17 magnets from 0.48 MA/m to 1.97 MA/m significantly reduces irreversible flux loss from -13% to -1% when exposed to 200 °C for 1,000 hours This enhancement in coercivity greatly improves the temperature stability of Sm 2 Co 17 magnets, making them the most stable option among the three materials evaluated.

Figure 16 – Time dependence of irreversible flux loss for a Sm 2 Co 17 magnet with H cJ = 0,48 MA/m and L/D = 0,7

Figure 17 – Time dependence of irreversible flux loss for a Sm 2 Co 17 magnet with H cJ = 1,19 MA/m and L/D = 0,7

Figure 18 – Time dependence of irreversible flux loss for a Sm 2 Co 17 magnet with H cJ = 1,97 MA/m and L/D = 0,7 [28]

Table 5 – Magnetic properties of the Sm 2 Co 17 magnets for the experiment to evaluate the influence of H cJ on the irreversible flux loss

The influence of H cJ on the irreversible flux loss for Nd-Fe-B magnets

The time dependence of irreversible flux loss for Nd-Fe-B magnets with H cJ = 1,16 MA/m [30],

Figures 19, 20, 21, and 22 illustrate that magnets with higher coercivity, such as those measuring 1.66 MA/m, 2.17 MA/m, and 2.45 MA/m, demonstrate reduced irreversible flux loss While the coercivity of Nd-Fe-B magnets can be enhanced by substituting Dy or Tb for Nd, this comes at the cost of a decrease in Br Table 6 presents the magnetic properties of Nd-Fe-B magnets, highlighting that significant substitutions of other rare earth elements can improve HcJ, albeit with a trade-off in Br.

Figure 19 – Time dependence of irreversible flux loss for a Nd-Fe-B magnet with H cJ = 1,16 MA/m and L/D = 0,7 [30]

The irreversible flux losses were measured after holding the sample at a certain temperature for 1 000 h The irreversible flux losses were measured after holding the sample at a certain temperature for 1 000 h

Figure 23 – Comparison of irreversible flux loss for Sm 2 Co 17 magnets with different

Figure 24 – Comparison of irreversible flux loss for Nd-Fe-B magnets with different

A comparison of irreversible flux losses in Nd-Fe-B magnets with varying H cJ values is illustrated in Figure 24 Increasing the H cJ from 1.16 MA/m to 2.45 MA/m significantly reduces irreversible flux loss from -43% to -2% after 1,000 hours of exposure at 200 °C.

Currently, the α(H cJ) of Nd-Fe-B magnets remains unchanged despite ongoing research However, magnets with enhanced coercivity of up to 2.8 MA/m have been successfully developed for high-temperature applications.

A summary of the temperature stability graphs is given in Table A.1

Table 6 – The magnetic properties of Nd-Fe-B magnets for the evaluation of the influence of H cJ on irreversible flux loss measured by a pulse recording fluxmeter

Irreversible flux loss per decade

Figure 25 illustrates the correlation between irreversible flux loss per decade and initial flux loss, based on data from this technical report The values for irreversible flux loss per decade were derived through a least squares fit, utilizing data collected over periods ranging from 1 hour to 1000 hours Notably, the analysis reveals that magnets with higher initial flux loss tend to exhibit greater irreversible flux loss per decade.

The irreversible flux loss per decade is similar across the three materials, and the reasons behind this trend in sintered magnets remain unclear.

Permanent flux loss

The permanent flux loss of Sm 2 Co 17 and Nd-Fe-B magnets after exposure at 100 °C to

200 °C for 1000 h is tabulated in Tables 7 and 8 The permanent flux loss for Sm 2 Co 17 magnet ranges from –1,15 % to +0,377 % and sometimes an increase in flux was observed

Permanent flux loss in Nd-Fe-B magnets appears to result from changes in the morphology of the precipitate and/or oxidation after exposure to temperatures of 100 °C.

At 200 °C for 1000 hours, the permanent flux loss of magnets ranges from –1.1% to +0.14%, primarily due to the oxidation of the R2Fe14B main phase and the R-rich phase (R: rare earth elements) Although the results are not entirely systematic, the absolute values of permanent flux loss cluster around 1% It is concluded that magnets subjected to higher temperatures experience greater permanent loss.

The thickness of the surface oxidized layer in (Nd, Dy)-Fe-B sintered magnets increases proportionally with the square root of exposure time, following a t^{1/2} relationship This behavior aligns with the observed permanent flux loss, confirming the t^{1/2} law in the oxidation process of these magnets After 1,000 hours of exposure at 200 °C, the calculated thickness of the oxidized layer remains below 1 μm.

NOTE All data in this figure are collected from the figures on time dependence of irreversible flux loss in this technical report

Figure 25 – Relationship between irreversible flux loss per decade and initial flux loss

Table 7 – The permanent flux loss of Sm 2 Co 17 magnets after exposure for 1 000 h at different temperatures

Exposed at 100 °C Exposed at 150 °C Exposed at 200 °C

NOTE The specimens were remagnetized after the experiments shown in Figures 16 to 18

Table 8 – The permanent flux loss of Nd-Fe-B magnets after exposure for 1 000 h at different temperatures

Exposed at 100 °C Exposed at 150 °C Exposed at 200 °C

++：Permanent loss increased but exact values were not obtained

NOTE The specimens were remagnetized after the experiments shown in Figures 19 to 22

Irr f lu x lo ss/d ecad e( % /d ecad e)

The temperature characteristics of SmCo 5, Sm 2 Co 17, and Nd-Fe-B sintered magnets are significantly influenced by their magnetic properties and the temperature dependence of their base intermetallic compounds Among these, Sm 2 Co 17 sintered magnets exhibit the highest temperature stability, evaluated through parameters such as T c, α(B r), and α(H cJ) However, sintered Nd-Fe-B material continues to be the most favored choice for permanent magnet applications.

Co substitution can enhance the value of α(B r) in Nd-Fe-B sintered magnets; however, excessive substitution may lead to a reduction in crystalline anisotropy and H cJ To improve the thermal stability of Nd-Fe-B sintered magnets, increasing H cJ is essential, as there are no viable methods to lower α(H cJ).

For the best temperature stability of rare earth sintered magnets, a high coercivity and good squareness of the demagnetization curve are required

This technical report presents findings on rare earth sintered magnets subjected to dry air environments It is important to note that the behavior of these magnets in humid conditions will differ from the results outlined in this report.

Table 9 – Basic magnetic properties of the three intermetallic compounds

Basic magnetic properties of intermetallic compound Materials Crystal structure

Summary of temperature stability graphs

The graphs to elucidate the temperature stability for three kinds of rare earth sintered magnets are summarised in Table A.1

Table A.1 – Summary of temperature stability graphs

Figure Magnet materials Magnet L/D Time h

Irreversible flux loss vs temperature

23 Sm 2 Co 17 Irreversible flux loss vs temperature (parameter: H cJ )

24 Nd-Fe-B Irreversible flux loss vs temperature (parameter: H cJ )

Non-linearity of temperature dependence of B r and H cJ

Figure B.1 illustrates the non-linear relationship between temperature and the parameters B_r and H_cJ This temperature dependence is represented by a quadratic function, highlighting the complexity of the relationship.

H cJ (θ)/H cJ(25 °C) = 1,33 × 10 –5 θ 2 + 7,45 × 10 –3 θ + 1,17 for a temperature dependence of B r and H cJ , respectively A straight line drawn between

25 °C and 120 °C shows clearly that the temperature dependence of B r and H cJ is not linear

In this case, θ and θref are 120 °C and 25 °C, respectively

H cJ (θ)/H cJ (25 °C) = – 5,52 × 10 –3 θ + 1,14 for a temperature dependence of B r and H cJ , respectively Calculated temperature coefficients α(B r) and α(H cJ) in the temperature range from 25 °C to 120 °C using the linear functions are

Figure B.1 – Temperature dependence of normalized B r and H cJ to show the non- linearity (see data for Nd-Fe-B magnets in Figure 9)

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Tiêu đề	Rare earth sintered magnets – Stability of the magnetic properties at elevated temperatures
Chuyên ngành	Electrical Engineering
Thể loại	Technical report
Năm xuất bản	2009
Thành phố	Geneva

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Số trang	32
Dung lượng	1,24 MB