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MINISTRY OF EDUCATION AND TRAINING VIETNAM ACADEMY OF SCIENCE AND TECHNOLOGY GRADUATE UNIVERSITY OF SCIENCE AND TECHNOLOGY  NGUYEN HOANG HA INVESTIGATION OF FABRICATION, MAGNETIC

MINISTRY OF EDUCATION AND TRAINING

VIETNAM ACADEMY OF SCIENCE AND TECHNOLOGY

GRADUATE UNIVERSITY OF SCIENCE AND TECHNOLOGY



NGUYEN HOANG HA

INVESTIGATION OF FABRICATION, MAGNETIC PROPERTIES

AND MAGNETOCALORIC EFFECT ON Fe-Zr BASE

AMORPHOUS ALLOYS

Major: Electronic materials Code: 9.44.01.23 SUMMARY OF DOCTORAL THESIS IN MATERIALS SCIENCE

HA NOI - 2022

The thesis was completed at the Key Laboratory for Electronic Materials and Devices, and Laboratory of Magnetism and Superconductivity, Institute of Materials Science, Vietnam Academy of Science and Technology

Supervisor: 1 Assoc.Prof.PhD Nguyen Huy Dan

2 Assoc.Prof.PhD Nguyen Manh An

Time: , , 2022

Thesis can be further referred at:

- National Library of Vietnam

- Library of Graduate University of Science and Technology

- Library of Institute of Materials Science

MỞ ĐẦU

Today, global warming and rising costs of energy require the development

of new cooling technologies to replace the conventional compression/expansion refrigeration In response to this demand, the magnetic cooling technology based on the magnetocaloric effect of the material is a good candidate This technology can be used to obtain extremely low temperatures, as well as application in refrigerators in the room temperature region The magnetic refrigerator is more efficient than the conventional vapour-cycle one The cooling efficiency of magnetic refrigerators can achieve up 70% of the ideal Carnot cycle The gas-compression refrigerators, meanwhile, just only obtain an efficiency of about 40% Moreover, magnetic refrigeration is an environmentally friendly cooling technology It does not use ozone-depleting chemicals or greenhouse gases

gas-Magnetocaloric effect (MCE) is the heating or cooling of a magnetic material under variation of a magnetic field This effect holds a potential for magnetic refrigeration applications As compared to conventional gas compression refrigeration techniques, the magnetic refrigeration has advantages of avoiding environmental pollution and saving energy Many scientists have focused on finding magnetocaloric materials for magnetic refrigeration at ambient temperature To explore magnetic refrigeration in the room temperature region, magetocaloric materials must have a magnetic phase transition at room temperature, because the MCE (the isothermal magnetic entropy change or the adiabatic temperature change) is often maximal at the magnetic phase transition temperature (TC) In most cases, the MCE of a material can be assessed through its magnetic entropy change (ΔSm) and the refrigeration capacity (RC), which is the product of the magnetic entropy change and the working temperature range In addition, some other properties such as the low specific heat capacity (the large adiabatic temperature change), high thermal conductivity (for fast heat-exchange), low electrical conductivity (reducing power losses due to Foucault currents), high durability and low magnetic hysteresis are also needed for application of the magnetocaloric materials A large number of magnetocaloric materials for room-temperature magnetic refrigeration have been developed, including Gd-containing alloys (GdSiGe, GdCo), As-containing alloys (MnAsSb, MnFePAs), and La-containing alloys (LaFeSi)

Many researchers have focused on the exploitation of magnetocaloric

materials with an amorphous or a nanocrystalline structure The main advantages of amorphous materials are low coercivity, high resistivity, room temperature magnetic phase transition, and low cost, which are necessary for practical applications Among these materials, Fe-Zr based alloys have attracted growing attention from scientists By adding other elements to the alloys, their ferromagnetic-paramagnetic (FM-PM) transition temperature

(Curie temperature, TC) can be easily tuned to a desired temperature range while retaining the good magnetocaloric properties.Although the Fe-Zr based amorphous alloys have the maximum magnetic entropy smaller than that of other GMCE materials (such as Gd-containing alloys, La-Fe-Si alloys, Heusler alloys…) but they own a wide working temperature range leading to large RC, which are necessary for practical application To change the Curie temperature and improve glass forming ability (GFA) of this material, other elements such as Co, Ni, B, Y, Cr, Mn have been added However, the effect of the additional elements on the GFA and TC of the alloy is widely various For example, the Curie temperature of the Fe90-

xMnxZr10 system is decreased from 210 K (for x = 8) to 185 K (for x = 10) with increasing Mn concentration While that of the Fe89-xBxZr11 is increased from  310 K (for x = 2.5) to 370 K (for x = 10) with increasing B concentration Therefore, with the goal of adjusting the working temperature

of the alloy to room temperature, the study of the influence of additional elements is very necessary for this alloy system

In Vietnam, there have been some research groups interested in magnetocaloric materials such as University of Natural Sciences, University of Engineering and Technology, Hanoi University of Science and Technology, Institute of Materials Science These research groups have had a number of scientific publications in the domestic and international journals The research

on the magnetocaloric materials in Vietnam is relatively close to the progress in the world However, due to insufficience of equipment, funding and human resources to the research, the results including both basic and applied researches are limited Therefore, structure, magnetic properties and MCE of the magnetocaloric materials are still necessary to study

From the above reasons, we have chosen the research topic of the thesis as

following: “Investigation of fabrication, magnetic properties and magnetocaloric effect of Fe-Zr amorphous alloys”

The research objectives of the thesis:

The Fe-Zr amorphous alloy systems: Fe-(Pr,La,Nd)-Zr, (Nd,Gd,Co)-Zr

Fe-(Cr,Cu)-The goal of the thesis:

Fabrication, investigation of structure, magnetic properties and magnetocaloric effect of Fe-Zr amorphous alloys, in order to find the magnetocaloric materials that have applicability for magnetic refrigeration in room temperature region

The research contents of the thesis:

- Investigation of fabrication of Fe-(Pr,La,Nd)-Zr, (Nd,Gd,Co)-Zr possessing large magnetocaloric effect

Fe-(Cr,Cu) Studying the relationship between structure, magnetocaloric properties and magnetocaloric effect of alloys

- Taking working temperature of the alloys to room temperature range

Research Methods:

The thesis was carried out by experimental methods The samples were prepared by melt-spinning method Some ribbon samples were annealed to stabilize or create desired structure phases The structure of the ribbons was analyzed by X-ray diffraction (XRD) Magnetic properties of the alloys were investigated by magnetic hysteresis and thermomagnetization measurement Magnetocaloric effect is assessed indirectly through determination of the magnetization versus magnetic field, M(H), at various temperatures

Scientific meanings of the thesis:

The results of the thesis contribute to the search for magnetocaloric materials that used in magnetic refrigeration technology at room temperature This is an advanced technology that can be applied in practice and has been attracting a lot of scientists The clarification of the relationship between GMCE and structural and magnetic phase transitions in the magnetocaloric materials is also an interesting object for fundamental research

The layout of the thesis:

The main content of the thesis is presented in four chapters The first chapter is an overview of magnetocaloric effect on rapidly quenched alloys The second chapter displays the experimental techniques for fabrication methods, structural characteristics and magnetic properties of the materials The remaining chapters present the obtained research results of Fe-(Pr,La,Nd)-Zr, Fe-(Cr,Cu)-(Nd,Gd,Co)-Zr alloys

Main results of the thesis:

Successfully synthesized the sample systems: Fe90-xNdxZr10 (x = 1 - 5),

Fe90-xPrxZr10 (x = 1 - 3), Fe90-xLaxZr10 (x = 1 - 3), Fe84-xCr2+xB2Co2Zr10 (x = 1

- 5), Fe90-xCoxZr7Cu1B2 (x = 0 - 4), Fe81-xCr4+xB2Nd3Zr10 (x = 1 - 5) và Fe

82-xCr4+xB2Gd2Zr10 (x = 1 - 5) The structure, magnetic properties and magnetocaloric effects of Fe-Zr-based amorphous alloys were systematically investigated Almost the samples have a nearly amorphous structure All the ribbons reveal soft magnetic behavior with low coercive force.The Curie temperature of the Fe-Zr base alloys can be controlled to be near room temperature by changing concentration of Pr, La, Nd, Gd, Co, Cu, Cr and B The quite high maximum magnetic entropy change (Smmax > 1,5 J.kg-1.K-1with H = 12 kOe), large refrigerant capacity (RC > 130 J.kg-1) and wide working range around room temperature (δTFWHM > 100 K) reveal application

potential in magnetic refrigeration technology of this alloy

The thesis was carried out at the Key Laboratory for Electronic Materials and Devices, and Laboratory of Magnetism and Superconductivity, Institute of Materials Science, Vietnam Academy of Science and Technology

CHAPTER 1 OVERVIEW OF MAGNETOCALORIC EFFECT OF

RAPIDLY QUENCHED ALLOYS 1.1 Overview of magnetocaloric effect

The magnetocaloric effect is defined as the change in the adiabatic temperature of magnetic materials as the external magnetic field apply on them changes

In the case of ferromagnetic materials, they heat-up as magnetized and cool-down as demagnetized Basically, the MCE is directly related to the

magnetic entropy change, ∆S m (T, H), and the adiabatic temperature change,

ΔTad(T, H) They are determined by the following equations:

H

M T H T

RC = |ΔSM|×δTFWHM, (1.22)

where δTFWHM = T2 – T1 is the full-width-at-half maximum of the ΔSm(T)

curve and it corresponds to the amount of heat that can be transferred between the cold and hot parts of the refrigerator in an ideal thermodynamic cycle

For second-order phase transition ferromagnetic materials, during the

transition phase FM-PM, at vicinity of the critical temperature TC, the

variations of M S (T) and 0 -1 (T) versus temperature thus obey the asymptotic

1 2 Overview of magnetocaloric material

Magnetocaloric materials have been used and developed since the early 20th century Since then, the study of this material has concentrated on two areas of application The first area studies materials that have high MCE at low temperatures to use in very low temperature techniques The second area studies materials that high MCE around room temperatures to use in chillers Nowadays, a number of magnetic materials having large MCE have been discovered, such as Gd-containing alloys, As-containing alloys, La-Fe-Si alloys, Heusler alloys, Fe and

Mn based rapidly quenched alloys, the ferromagnetic perovskite maganites…

Although the Fe-Zr based amorphous alloys have the maximum magnetic entropy smaller than that of other GMCE materials, they own a wide working temperature range leading to large RC, which are necessary for practical application

CHAPTER 2 EXPERIMENTAL TECHNIQUES 2.1 Fabrication of the samples

Alloy ingots with nominal compositions of Fe90-xNdxZr10 (x = 1 - 5),

Fe90-xPrxZr10 (x = 1 - 3), Fe90-xLaxZr10 (x = 1 - 3), Fe84-xCr2+xB2Co2Zr10 (x = 1

- 5), Fe90-xCoxZr7Cu1B2 (x = 0 - 4), Fe81-xCr4+xB2Nd3Zr10 (x = 1 - 5) và Fe

82-xCr4+xB2Gd2Zr10 (x = 1 - 5) were prepared from pure (> 3 N) components of

Fe, Pr, La, Nd, Gd, Co, Cr, Cu and Zr on an arc-melting furnace A spinning method was then used to fabricate the ribbon samples

melt-2.2 Methods of struture analysis, magnetic properties and magnetocaloric effect

2.2.1 Structure analysis by X-ray diffraction

Powder X-ray diffraction (XRD) was used to study the structure of samples Through XRD patterns, we cuold determine the structural characteristics of the lattice such as: lattice type, crystalline phase and lattice parameters From the XRD schema, we could also evaluate the amorphous and crystalline crystal fraction of samples

2.2.2 Study of magnetic properties and magnetocaloric effect by hysteresis and thermomagnetic measures

The dependences of magnetization on the temperature and magnetic field were investigated by a vibrating sample magnetometer (VSM) and a superconducting quantum interference device (SQUID) The values of magnetic entropy change (Sm) caused by a variation of applied magnetic field was calculated by using the formula:

M S

In order to assess the applicability of the magnetocaloric material, the refrigerant capacity (RC) of the material usually is usually used:

RC = |Sm|max  TFWHMwhere TFWHM is full width at half maximum of entropy change peak

CHAPTER 3 STRUCTURE, MAGNETIC PROPERTIES AND

MAGNETOCALORIC EFFECT IN Fe-(Pr,Nd,La)-Zr THREE-

COMPONENTS ALLOYS 3.1 The Fe 90-x Pr x Zr 10 alloy system

Figure 3.1 shows XRD patterns of Fe90-xPrxZr10 (x = 1, 2 and 3) ribbons We can see that, all the patterns appear an XRD peak corresponding

to FeZr2 phase at 2 of 43.2o However, intensity of this XRD peak is low That means volume fraction of the crystalline phase in the ribbons is small Except the XRD peak of the sample with x = 3, which is a litle bit sharp, the other ones are broad, characterizing for nearly-full amouphous structure in

the alloy ribbons

Figure 3.1 XRD patterns of Fe 90-x Pr x Zr 10 (x = 1, 2 and 3) rapidly quenched

alloy ribbons

The measurements of magnetization versus temperature are carried out and illustrated in Figure 3.2 As seen from the graph, ferromagnetic-paramagnetic transition (FM-PM) temperature of the alloy ribbons is depended on Pr concentration With x = 3, no magnetic phase transition is observed in the thermomagnetization curves M(T) While, the M(T) curves

of samples with x = 1 and 2 demonstrate a quite sharp FM-PM phase transition at 282 K and 302 K, respectively Thus, for the x = 2 sample, the phase transition temperature is in room temperature region

Figure 3.2 Thermomagnetization curves of Fe 90-x Pr x Zr 10 (x = 1, 2 and 3)

alloy ribbons in an applied magnetic field of 100 Oe

Temperature dependence of the magnetic entropy change ΔSm(T) in magnetic change ∆H = 4, 6, 8, 10 and 12 kOe are depicted in Figure 3.6 The

|ΔSm|max determined for the sample with x = 1 is 0.92 J.kg-1.K-1 at 282 K (with ΔH = 12 kOe) The working temperature range (FWHM), which is defined by full width at half maximum (FWHM) of magnetic entropy

change peak, of this ribbon is 69 K As for the sample with x = 2, |ΔSm|max is 0.99 J.kg-1.K-1 at 302 K (with ΔH = 12 kOe), and the working temperature range is 70 K Refrigerant capacity (RC) of the samples, which is defined as product of maximum magnetic entropy change and working temperature range (FWHM), is determined (Table 1) We can realize that, the working temperature of the these alloy ribbons is about 70 K and their refrigerant capacity RC is larger than 64 J/kg at near room temperature with Pr concentration of 1 - 2%

Figure 3.6 Temperature dependence of magnetic entropy change of Fe

90-x Pr x Zr 10 alloy ribbons with x = 1 (a) and 2 (b) in various magnetic field change

3.2 The Fe 90-x La x Zr 10 alloy system

Figure 3.2 shows the XRD diffraction pattern of Fe90-xLaxZr10 alloy ribbons at room temperature The results

reveal that characteristic of the XRD

patterns of the samples is quite similar

All the ribbons have a coexistence

amorphous and crystalline phases

Diffraction peaks corresponding to the

crystalline phase of -Fe and Fe2Zr are

observed in these patterns However,

these diffraction peaks are very weak

That means the alloy ribbons are almost

amorphous

Figure 3.9 shows the reduced thermomagnetization curves in magnetic field of 100 Oe of the Fe90-xLaxZr10 (x = 1, 2 and 3) ribbons The results indicate that La-concentration clearly influences the Curie phase transition

temperature of the alloy These

ribbons have a magnetic phase

transition in the temperature range

of 250 - 350 K corresponding to

the amorphous phase As for the

samples with x = 1, 2 and 3, the TC

value is 262, 302 and 305 K,

respectively Thus, the magnetic

transition phase temperature of the

alloy ribbons was increased to

room temperature with the

Figure.3.12 -ΔS m (T) curves for magnetic field changes of 4 - 12 kOe of

Fe 90-x La x Zr 10 with x = 1 (a) and x = 2 (b)

The values of Ms(T) and -1(T) as functions of temperature T are plotted for the Fe90-xLaxZr10 ribbons (Fig 3.13) In accordance with equations (2) and (3) for Ms(T) and -10(T), the power law fittings are used

Figure 3.9 Thermomagnetization curves

in an applied magnetic field of 100 Oe

of Fe 90-x La x Zr 10 (x = 1, 2 and 3)

ribbons

0 0.5 1 1.5

to extract ,  and TC (Fig 3.13) The resulted values of  and  were then used to calculate the  parameter based on equation (1.24) As resulted, the sample with x = 1 has the critical parameters of   0.438, γ ≈ 0.835, δ ≈ 3.37 and TC ≈ 262 K Similarly, for the sample with x = 2, those values are   0.492,

γ ≈ 1.1786, δ ≈ 3.23 and TC ≈ 301 K Thus, the values of TC of the alloy obtained from the fittings are mostly equal to that directly determined from the thermomagnetization measurements That means the procedures of deducing and fitting data are corrected

In comparison with some standard models such as mean field theory (

= 0.5,  = 1 and  3D-Heisenberg model ( = 0.365,  = 1.336 and

 and 3D-Ising model ( = 0.325,  = 1.241 and our critical parameters attained in this method for the Fe90-xLaxZr10 ribbons are close to those of the mean field theory of long-range ferromagnetic orders This means the sample is mainly of long-range ferromagnetic orders However, all the critical parameters of the sample fall between those of mean-field model and 3D Heisenberg model revealing a part of short-range orders coexists with long-range orders of ferromagnetic interactions

in the material The coexistence of the short- and long-range orders is in good agreement with the above-mentioned multi-magnetic phase behavior

Figure 3.13 Temperature dependence of spontaneous magnetization

M s (T) and inverse initial susceptibility χ 0 -1 (T) of the Fe 90-x La x Zr 10 with x =

1 (a) and x = 2 (b)

3.3 The Fe 90-x Nd x Zr 10 alloy system

Figure 3.14 shows XRD patterns of the Fe90-xNdxZr10 (x = 1, 2, 3, 4 and 5) ribbons We can see that, all the patterns have diffraction peaks corresponding to Fe2Zr and α-Fe phases However, these peaks are quite

0 5 10 15 20 25 30 35

0 30 60 90 120

0 20 40 60

broad and low in intensity This

indicates the amorphous structure

of the ribbons prepared

In order to study the effect of

Nd concentration on the Curie

temperature of the Fe90-xNdxZr10

(x = 1, 2, 3, 4 and 5) ribbon alloys,

magnetization versus temperature

(M-T) measurements were carried

out and illustrated in Figure 3.15

As shown in the figure, the

FM-PM transition temperature of the

alloy is dependent on Nd concentration The samples with x = 1, 2, 3 and 4 exhibit a quite sharp FM-PM

phase transition at 262 K, 280 K,

302 K, and 310 K, respectively

This indicates that the Curie

temperature TC of the Fe-Zr alloy

can be tuned to room temperature

concentration

Figure 3.19 shows the

temperature dependence of ΔSm(T)

of Fe90-xNdxZr10 alloy ribbons for

various magnetic field changes As

expected, the maximum magnetic

entropy change |ΔSm|max is

achieved near TC for the samples

For comparison, Figure 3.19f shows the temperature dependence of ΔSm(T) for all the samples for ΔH = 12 kOe |ΔSm|max of the alloy is found to decrease with increasing the Nd concentration The |ΔSm|max values of the samples with x = 1 and 5 are about 1.56 and 1.02 J/kg.K, respectively (Table 1) We have found that the working temperature range (the full width at maximum of the ΔSm(T) curve, FWHM) of these alloy ribbons is around 70 K and their refrigerant capacity RC exceeds 60 J/kg in the room temperature

region as Nd concentration was varied in the range of 1 - 5%

Hình 3.14 Giản đồ XRD của hệ hợp kim Fe 90-x Nd x Zr 10 (x = 1, 2, 3, 4 và 5)

Figure 3.15 (a) M-T curves taken in

an applied magnetic field 100 Oe of

Fe 90-x Nd x Zr 10 (x = 1, 2, 3, 4 and 5) alloy ribbons, respectively

0 0.2 0.4 0.6 0.8 1 1.2

x=1

x=2 x=3

Figure 3.19 Temperature dependence of the magnetic entropy change of

Fe 90-x Nd x Zr 10 alloy ribbons with x = 1 (a) and 2 (b), 3 (c), 4 (d) and 5 (e) for

various magnetic field changes

0 100 200 300 400 500 600

260 280 300 320 340 360 10

20 30 40 50 60

10kOe 12kOe

0

1 1.5

0 1.3

0 2

0 50 100 150 200 250 300

0 100 200 300 400 500 600

0 40 80 120 160 200 240 280