DOCTORAL THESIS ABSTRACT STUDY OF PHASE EQUILIBRIA IN ... Ion Alexandru_EN.pdf · DOCTORAL THESIS ABSTRACT STUDY OF PHASE EQUILIBRIA IN NANOALLOY SYSTEMS Scientific advisor, CS I

ROMANIAN ACADEMY

„Ilie Murgulescu” Institute of Physical Chemistry

DOCTORAL THESIS

ABSTRACT

STUDY OF PHASE EQUILIBRIA IN NANOALLOY

SYSTEMS

Scientific advisor,

CS I Dr. Tănăsescu Speranţa Valeria

PhD Student,

Milea Ion Alexandru

BUCHAREST

2015

TABLE OF CONTENTS

Part I ACTUAL STAGE OF RESEARCH............................................................................................................................ 1 Chapter 1 INTRODUCTION……………….……………........................................................................................................... 2 Chapter 2 ACTUAL STAGE OF RESEARCH REGARDING THE NANOALLOYS USING NON-EQUILIBRIUM METHODS....................................................................................................................................................

7

2.1 Nanoalloys: general characteristics, synthesis methods.................................................................... 7 2.1.1 Types and structures of nanoalloys............................................................................................ 8 2.1.2 Synthesis methods in non-equilibrium conditions...................................................................... 10

2.1.2.1 Rapid quenching.....................................................……........................................................ 11 2.1.2.2 Laser processing................................................................................................................... 11 2.1.2.3 Vapor deposition................................................................................................................. 12 2.1.2.4 Radiolysis............................................................................................................................. 12 2.1.2.5 Thermal plasma processing................................................................................................. 13 2.1.2.6 Mechanical alloying............................................................................................................. 13

2.2 Phase equilibria. Phase diagrams....................................................................................................... 17

2.2.1 Thermodynamics of metastable phase’s formation................................................................... 18

2.2.1.1 The free energy of the phases in the case of one element.................................................. 19

2.2.1.2 The free energy of alloy phases........................................................................................... 20

2.2.1.3 The mechanism of formation of metastable phases in nanocrystalline state..................... 22

2.2.1.4 Free energy determination of metastable phases............................................................... 24

2.2.2 Thermal stability of nanocrystalline materials. Particularities of grain growth process............. 27

2.2.2.1 Comments on the stability of grain size............................................................................... 28 2.2.2.2 Grain growth process........................................................................................................... 28

2.3 Effect of grain size and the presence of the interfaces on thermodynamic properties, thermal expansion and electrical properties...................................................................................................

34

2.3.1 Heat capacity............................................................................................................................... 34 2.3.2 Thermal expansion...................................................................................................................... 37 2.3.3 Electrical properties.................................................................................................................... 38

2.4 Alloy thermodynamics in Ag-Cu system............................................................................................. 39 2.5 The need for additional contributions in the research field............................................................... 45

Part II EXPERIMENTAL.....................….…...................................................................................................................... 46

Chapter 3 MATERIALS AND EXPERIMENTAL CHARACTERISATION TECHNIQUES...................................................... 47 3.1 Materials............................................................................................................................................ 47

3.2 Synthesis............................................................................................................................................. 48

3.3 Experimental characterization techniques. ……………….…………....……….............................................. 49 3.3.1 Experimental techniques for structural and morphological analysis.......................................... 49

3.3.1.1 X-ray diffraction (DRX)......................................................................................................... 49 3.3.1.2 X-ray photoelectron spectroscopy (XPS)............................................................................. 49

3.3.1.3 Electron microscopy (SEM).................................................................................................. 50

3.3.2 Thermal analysis techniques....................................................................................................... 51

3.3.2.1 Drop calorimetry.................................................................................................................. 51

3.3.2.2 Differential scanning calorimetry........................................................................................ 55

3.3.2.3 Thermo-mechanical analysis (TMA)..................................................................................... 56

3.3.3 Experimental techniques for measuring the electrical properties. Impedance spectroscopy...............................................................................................................................

57

Part III CONTRIBUTIONS.............................................................................................................................................. 59

Chapter 4 STUDY REGARDING THE MICRO AND NANOSTRUCTURED PHASES OBTAINED BY MECHANICAL ALLOYING IN AG-28%CU SYSTEM..............................................................................................................

60

4.1 Structural, morphological and surface characterization of micro and nanocrystalline powders.......................................................……………………………..................….….……..........................

60

4.1.1 Particle size distribution obtained after different times of milling......................................…….. 60

4.1.2 XPS spectroscopy measurements.........………………………..…..................……................................. 62

4.1.3 X-ray diffraction measurements...........................…………........…....….......................................... 64

4.1.4 Electron microscopy measurements (SEM+EDX)........................................................................ 68

4.2 The study of phase equilibria in microstructured and nanostructured systems of Ag-28%Cu.......... 68

4.2.1 Experimental results obtained using differential scanning calorimetry (DSC)............................ 69

4.2.2 Measurements using drop calorimetry technique...................................................................... 72

4.2.2.1 Relative enthalpy variation HT-H298 with temperature for samples with euthectic composition obtained by mechanical alloying with processing times of 20 and 80 hrs.....

72

4.2.2.2 Determination of thermodynamic functions: specific heat, relative entropy, Gibbs free energy function....................................................................................................................

76

4.2.4.3 Determination of the enthalpy of formation using direct synthesis................................... 79 4.2.2.4 Crystallite size influence on thermodynamic properties..................................................... 79 4.2.2.5 The variation of thermodynamic properties of micro and nanostructured phases during grain growth process...........................................................................................................

81

4.3 Thermal expansion evolution for Ag-28%Cu...................................................................................... 83 4.4 Determination of electrical properties using impedance spectroscopy measurements.................... 85 4.5 Correlations between thermodynamic properties, thermal expansion and electrical properties..... 87 4.6 Partial conclusions. Original contributions......................................................................................... 89

Chapter 5 STUDY OF THE MICRO AND NANOSTRUCTURED PHASES OBTAINED BY MECHANICAL ALLOYING IN AG-50%CU SYSTEM....................................................................................................................................

91

5.1 X-ray diffraction measurements..………....................................……….…................……......................... 91 5.2 The study of phase equilibria in microstructured and nanostructured systems of Ag-50%Cu.......... 93

5.2.1 Experimental results obtained using differential scanning calorimetry (DSC)............................ 93 5.2.2 Measurements using drop calorimetry technique...................................................................... 95

5.3 Thermal expansion evolution for Ag-50%Cu...................................................................................... 99 5.4 Determination of electrical properties using impedance spectroscopy measurements.................... 101 5.5 Partial conclusions. Original contributions......................................................................................... 102

Chapter 6 GENERAL CONCLUSIONS. ORIGINAL CONTRIBUTIONS.............................................................................. 104 Prospects for further research.................................................................................................................................... 107 REFERENCES................................................................................................................................................................. 108 List of published works in thesis research field............................................................................................................ 118 List of oral/poster communications presented at national/international scientific events......................................... 118

Keywords: Nanoalloys, Drop calorimetry, Phase equilibria

1

INTRODUCTION

The reduction of particle size and the possibility to obtain alloys at nanometric scale has lead

to a remarcable enhancement of alloy properties, new opportunities arising for technological

applications in various domains: electronics, catalysis, magnetic devices, optoelectronics etc. These

new properties due to the unique effect of the nanostructure raise new questions important for

both, basic research and applications – regarding the relation between composition, structure and

properties.

One of the key factors responsible for understanding the physical and chemical modified

properties associated with the rise of the surface/volume ratio at a nanometer scale is the change of

the energetic parameters. However, the literature is rather scarce as concerns the thermodynamic

properties of these alloy systems, the majority of information being based largely on theoretical

calculations.

Ag-Cu is one of the reference systems for classical alloy systems. It is well known that Ag and

Cu are mutually immiscible in solid state and the heat of mixing is positive. But using non-

equillibrium techniques such as rapid quenching, vapor deposition, plasma processing or mechanical

alloying, one can obtain metastable phases of this nanoalloy, yet each of these methods will leave

the own mark on the properties of the resulting nanosystem. For the Ag-Cu system there is no

experimental data regarding thermodynamic functions relevant for the thermodynamic evaluation of

the processes accompanying the formation of nanoalloy phases associated with a specific synthesis

method. Current studies regarding thermal stability refer to limited domains of compositions and

temperature.

In order to bring original contributions to the proposed subject, within the present thesis was

realized a systematic thermodynamic study of micro and nanostructured phase equillibria in Ag-Cu

systems synthesized by mechanical alloying in different conditions of processing (different milling

times). For this study, two compositions situated in distinct areas of the phase diagram of Ag-Cu

system have been selected: 72% Ag - 28% Cu corresponding to eutectic composition, and 50% Ag –

50% Cu. This study has been carried out on the basis of a compex thermodynamic approach, taking

into consideration the following aspects: the influence of the synthesis parameters on the

thermodynamic properties; the correlation of thermodynamic parameters with structure,

composition (Ag/Cu ratio), thermal expansion and electrical properties in large domains of

temperature; the identification of energetic parameters which favour the stability of nanostructure

phases.

The objectives of the current study are:

1. Structural and morphological characterization of powders obtained by mechanical alloying. In

order to determine the alloying degree of Ag and Cu, as well as particle and crystallite size, a

couple of measurements have been performed: X-ray diffraction measurements (XRD), electronic

microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). XRD analysis was carried out at

room temperature, as well as by in situ heating of sample in order to determine the constituent

phases and the phase transformations during heating.

2. Thermodynamic and thermochemical study of phase equillibrium in 72% Ag – 28% Cu and 50%

Ag – 50% Cu systems obtained by mechanical alloying. Thermodynamic properties (relative

2

enthalpy, specific heat, relative entropy and Gibbs free energy) in isothermal regime have been

obtained by drop calorimetry using a multi-detector high temperature calorimeter SETARAM

MHTC-96 operating in the drop mode. The thermochemical behaviour to heating was analyzed

using the DSC technique. These measurements allowed to evidence the influence of particle size

and synthesis parameters on the thermodynamic and thermochemical behaviour of the studied

nanomaterials.

3. Thermal expansion and electrical properties study of 72% Ag - 28% Cu and 50% Ag – 50% Cu

obtained by mechanical alloying. Depending on the temperature, the following characteristics of

the materials have been determined: thermal expansion coefficient, conductivity and electrical

resistivity.

4. Study of the corelations between composition-structure-properties and the highlighting of the

role of energetic parameters in the control of the material stability on a nanometric scale

Thesis structure:

The PhD thesis is structured in 6 chapters included in 3 distinct parts:

Part I: Current level of research (chapters 1 and 2)

Part II: Experimental part (chapter 3)

Part III: Original contributions (chapters 4, 5 and 6)

The thesis concludes with bibliographic references, followed by the list of ISI published papers, as

well as communications at national/international scientific events.

In Chapter 1, Introduction, after a short presentation of the importance of the approached

domain, the scope, specific objectives and thesis structure are presented.

Chapter 2 contains a description of the current level of research in the nanoalloy field,

insisting on the possibility of obtaining metastable phases in non-equillibrium conditions. Based on

bibliographic research, the main theoretical and experimental aspects regarding the role of the

energetic parameters in the understanding and the control of the nanostructured material’s stability,

especially in the case of nanoalloys are critically discussed. At the same time the need for

contributions in this domain is identified.

Chapter 3 contains the presentation of the studied materials together with the description of

the synthesis method and the methodology and experimental techniques used for of physico-

chemical characterization(XRD, XPS, SEM, DSC, TMA, drop calorimetry), insisting on the

thermodynamic and thermochemical methods.

Chapter 4 contains the original contributions regarding the experimental results obtained

from the study carried out on the system with the eutectic composition 72% Ag – 28% Cu. The

powders were structurally and morphologically characterized, the thermodynamic stability of micro

and nanostructured phases was studied, thermal expansion and electrical properties.

Chapter 5 contains original contributions regarding experimental results obtained from the

study that was carried out on the system with the composition of 50% Ag – 50% Cu. The structure,

thermodynamic stability, thermal expansion and electrical properties of micro and nanostructurate

phases were analyzed.

3

Chapter 6 contains the final conclusions resulting from the present study.

STRUCTURAL CHARACTERIZATION OF NANOMETRIC POWDER SYSTEMS Ag - 28%Cu AND Ag -

50%Cu

Using Brookhaven 9Plus/BI-MAS technique it was determined that only samples milled for 80

hrs presented nanometric particle size distribution.

XPS spectroscopy measurements registered the photoelectrons high frequency spectra of the

most prominent transitions for C1s, O1s, Cu2p and CuLMM. The binding energies in case of Ag3d and

Cu2p are assigned to the formation of the Ag-Cu alloy.

Using X-ray diffraction (XRD) measurements, the structures of the Ag-Cu alloys milled for 80

hrs having atomic ratio Ag/Cu = 1 and eutectic composition were analyzed. In both cases, the most

intense diffraction line associated with Ag-Cu alloys is shifted compared to the position of the pure

Ag line, showing the formation of a solid solution AgCuss. The medium crystallite size at room

temperature and during heating until 973 K was calculated. The crystallite size variation with

temperature is not monotonous. Until 503 K the crystallite size is stable. After this temperature,

crystallite size rises with the temperature. Also using XRD analysis, it was observed experimentally

that the lattice parameter tend to higher values for compositions with a higher concentration in Cu.

THERMODYNAMIC STUDY OF MICRO AND NANOSTRUCTURED PHASE EQULIBRIA IN METASTABLE

POWDER SYSTEMS OF Ag-Cu OBTAINED BY MECHANICAL ALLOYING

Ag - 28%Cu system

For the thermodynamic study were used differential scanning calorimetry in dynamic regime

of temperature (DSC) and drop calorimetry in isothermal conditions.

1. DSC diagrams of powders processed at different times of milling revealed the following:

- The effect associated with the relaxation of microstresses observed at low temperatures, around

450-480 K, process specific only to samples obtained through mechanical alloying. This effect is

more obvious for the sample milled for 80 hrs.

- Large exothermic signals around 480-973 K specific to the phase separation and grain growth

processes. For the first time in this thesis we made a distinction between the domains of

temperature in which these processes evolve, using DSC and XRD results.

- For the first time it was observed the shifting of melting temperature point to lower values with

the reduction in grain size.

2. Thermal effects from DSC diagrams were evaluated using drop calorimetry and the following

aspects were observed:

- The relative enthalpy variation with temperature is not monotonous, thermal effects revealed by

DSC curves were also obtained using drop calorimetry. For the first time in the literature we

reported the Ag-28%Cu enthalpy increments for samples obtained in different processing

conditions (different milling times) (Fig. 1).

4

400 500 600 700 800 900 1000 1100

50

100

150

200

250

300

525 550 575 600 625 650 675 700

60

70

80

90

100

110

120

MA80

MA20

HT

-H298

(J g

-1)

T (K)

Ag-28% Cu(MA80)

Ag-28% Cu(MA20)

HT

-H298 (

J g

-1)

T (K)

Fig.1 Relative enthalpy variation with temperature for a microstructured sample Ag-28%Cu(MA20)

and for a nanostructured sample Ag-28%Cu(MA80)

- We determined thermodynamic functions: specific heat, relative entropy, Gibbs free energy

function. The enthalpy of formation of an alloy Ag-28%Cu by using direct synthesis method (in

calorimeter) has been also determined. The obtained value of -48.14 J/g shows that the process

of formation of an nanoalloy in mechanical alloying conditions is possible.

- The higher values for enthalpy and specific heat at low temperatures, around 400-450 K, are

explained by the nanocrystalline structure generated by processing using ball milling technique.

High values of entropy around 473-548 K are characteristic for the high degree of disorder

present at the grain boundaries (Fig.2).

Fig.2 Temperature dependence of enthalpy increments, specific heat and relative entropy for Ag-

28%Cu(MA80) sample

- In the temperature domain related to the grain growth process (573-1048 K), specific heat of

nanometric powders, milled for 80 hrs, has a different behavior compared to micrometer

samples (Fig.3). A first observation is that until 673 K, specific heat of nanostructured sample has

a higher value compared to the microstructured samples. A second observation is that the

specific heat of nanostructured sample has a weak dependence on the temperature compared to

micrometer sized samples. This behavior can be explained by the particularities of grain growth

process in nanocrystalline samples; an important role in the stabilization of the nanostructure is

5

played by the decrease in interfacial energy and the presence of stress in the crystalline

structure.

600 700 800 900 1000

0.15

0.20

0.25

0.30

0.35

0.40

0.45

Cp

(J g

-1 K

-1)

T (K)

Ag-28% Cu(MA20)

Ag-28% Cu(MA80)

bulk

Fig.3 The temperature dependence of specific heat in the temperature domain of the grain growth

process. Comparison between nanometer and micrometer sized powders.

- The smaller values of free energy over 548 K shows the tendency of the structure to become

thermodynamically stable after the elimination of structure microstress and the phase

separation processes.

CRYSTALLITE SIZE INFLUENCE ON THE EVOLUTION OF THE THERMODYNAMIC PROPERTIES

Results obtained using drop calorimetry were correlated with the results obtained from XRD.

It was observed an unusual variation of crystallite size during heating. Until temperatures around

500-550 K, crystallite size has a weak dependence of temperature. Meanwhile, relative enthalpy and

Gibbs function have minimum values at these temperatures, values correlated with the medium

crystallite size values (Fig 4a and 4b). This study shows that nanocrystalline materials can exhibit the

stabilization of crystallite size phenomenon until higher temperatures. The onset temperature for

grain growth process and the energy of activation for this process have higher values compared to

the conventional materials.

400 500 600 700 800 900 1000

20

40

60

80

100

Dimensiunea medie a cristalitelor

Incrementii de entalpie Ag-28% Cu(MA80)

T (K)Dim

en

siu

ne

a m

ed

ie a

cri

sta

lite

lor

(nm

)

20

40

60

80

100

120

140

160

180

200

220

HT

-H2

98 (J

g-1

)

(a)

6

400 500 600 700 800 900 1000 1100

-0.63

-0.60

-0.57

-0.54

-0.51

-0.48

-0.45

-0.42

Functia Gibbs

Dimensiunea medie a cristalitelor

T (K)

(GT-H

298)/T

(Jg-1

K-1

)

30

40

50

60

70

80

90

100

Dim

ensiunea medie a cristalitelor (nm

)

(b)

Fig.4 Crystallite size and relative enthalpy variation with temperature (a), and free energy variation

with temperature for a nanostructured sample Ag-28%Cu(MA80) (b)

Ag-50%Cu system

DSC measurements showed also that in the case of a Ag-Cu system with a higher

concentration of Cu the thermal effects associated with the relaxation of microstress, phase

separation and grain growth are also present with the specification that phase separation process

occurs at lower temperatures in case of a system richer in Cu.

As in the case of the eutectic composition sample, thermodynamic parameters represented

by relative enthalpy, specific heat, relative entropy and Gibbs free energy function were also

determined.

During a comparison with the eutectic system the following aspects were observed:

- The sample with a higher concentration of Cu has a higher enthalpy on the whole temperature

domain (Fig. 5).

- The relative enthalpy and specific heat also present higher values at low temperatures, result

explained by the nanostructure characteristic disorder (Fig. 5 and Fig. 6).

- Higher values of entropy in the case of both concentrations are seen for nanostructured samples

compared to microstructured samples on the whole temperature domain.

- The free energy minimum is shifted to lower temperatures for the samples with a high Cu

content. In the temperature domain associated with the grain growth process the free energy

values are approximately the same (Fig.7).

7

300 400 500 600 700 800 900 1000 1100

-2.0

-1.8

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

Ag-50% Cu(MA80)

Ag-28% Cu(MA80)

(GT-H

29

8)/

T (

Jg

-1K

-1)

T (K)

Fig. 7 Gibbs free energy evolution of Ag-50%Cu(MA80) and Ag-28%Cu(MA80) samples

The results obtained in the comparative study of thermodynamic properties of

nanostructured samples of different compositions show that the effect of composition on

thermodynamic parameters is more important at low temperatures, before the beginning of grain

growth process. The conclusion is important for the selection and optimization of materials in the

view of applications.

CORRELATION BETWEEN THERMODYNAMIC PROPERTIES, THERMAL EXPANSION AND ELECTRICAL

PROPERTIES

- For the first time there were made thermal expansion measurements for Ag-Cu alloys and have

been determined the coefficients of thermal expansion on the temperature range associated

with the thermodynamic stability and the crystallite size stabilization domains. The coefficient of

thermal expansion of nanometer samples Ag-28%Cu(MA80) and Ag-50%Cu(MA80) are 44x10-6 K-1

and 39x10-6 K-1, respectively. These values are approximately 2.5 times higher than those of

micrometric samples (18x10-6 K-1), confirming theoretical data, as well as some literature results

Fig. 5 The relative enthalpy of two samples milled for 80 hrs, but having different

composition: Ag-50%Cu(MA80) and Ag-28%Cu(MA80)

300 400 500 600 700 800 900 1000 1100

50

100

150

200

250

300

Ag-50% Cu(MA80)

Ag-28% Cu(MA80)

HT

- H

29

8 (

Jg

-1)

T (K)

300400

500600

700800

9001000

1100

0.15

0.20

0.25

0.30

0.35

0.40

0.45

20

30

40

50

60

Ag-50% Cu(MA80)

Ag-28% Cu(MA80)

Cp

(Jg

-1K

-1)

Concentratie

Cu (%

)

T (K)

Fig. 6 Specific heat variation with temperature for samples with different compositions: Ag-

50%Cu(MA80) and Ag-28%Cu(MA80)

8

regarding the thermal expansion coefficient of some metallic materials and alloys with

nanometer structures.

- Electrical resistivity measurements revealed that the variation of relative resistivity with the

temperature is dependent of crystallite size and Cu concentration. Until temperatures close to

600 K, the relative resistivity of the two samples decrease. At 560 K, nanometric sample Ag-

50%Cu(MA80) shows a relative resistivity minimum, after that it begins to increase, having a

similar behavior with that of thin films Cu(Ag) copper rich and of micrometer Cu. Relative

resistivity of nanometric sample Ag-28%Cu continues to decrease until around 800 K, after which

it remains at a constant value until 1000 K.

- In Fig. 8 and Fig. 9 the correlation between the average crystallite size, thermal expansion,

electrical properties and the free energy has been presented. The temperature range where the

minimum energy values were observed is indicative for the temperature domain of the crystallite

size stabilization and where the grain growth process is inhibited.

300 400 500 600 700 800 900 1000

-50

0

50

100

150

200

250

300

L - L0

Dimensiunea medie a cristalitelor

Conductivitate

Rezistivitate relativa

T (K)

0

L -

L0 (

m)

Dim

en

siu

nea

med

ie a

cri

sta

lite

lor

(nm

)

30

40

50

60

70

80

90

100

(S

m-1

)

0

1000

2000

3000

4000

5000

6000

7000

8000

0.0

0.2

0.4

0.6

0.8

1.0

Fig. 8 Correlation between average crystallite size, thermal expansion, conductivity and relative

resistivity for Ag-28%Cu(MA80) sample

400 500 600 700 800 900 1000 1100

0.0

0.2

0.4

0.6

0.8

Rezistivitatea relativa

Functia Gibbs

Dimensiunea medie a cristalitelor

T (K)

-0.64

-0.62

-0.60

-0.58

-0.56

-0.54

-0.52

-0.50

-0.48

-0.46

-0.44

-0.42

(GT

-

H2

98

)/T

(J

g-1

K-1

)

0

20

40

60

80

100

Dim

en

siu

ne

a m

ed

ie a

cri

sta

lite

lor

(nm

)

Fig. 9 Correlation between free energy, relative resistivity and average crystallite size in Ag-

28%Cu(MA80) sample

9

It was shown that in mechanically alloyed nanosized samples, the thermodynamic properties

there are not only the driving force for grain growth process, but also may be used to explain the

experimentally observed stability and the change of properties in a large domain of temperature.

CONCLUSIONS

The study has lead to the following conclusions and original contributions:

For the first time the thermodynamic behavior of the micro and nanostructured phases for the

Ag-28%Cu(MA80) corresponding to the eutectic composition has been studied in a large domain

of temperature by coupling calorimetric measurements in both dynamic (DSC) and isothermal

regimes(drop calorimetry).

For the first time have been determined the thermodynamic functions (relative enthalpy, specific

heat, relative entropy and Gibbs energy function) relevant for the thermodynamic evaluations of

processes accompanying the formation and transformation of phases in nanoalloys.

Correlations existent between composition, structure and the thermodynamic behavior have

been determined. Transformations related to microrelaxation, phase separation and grain

growth processes have been determined.

For the first time measurements of thermal expansion for Ag-Cu nanoalloys have been

performed and the coefficients of thermal expansion on the temperature domain corresponding

to the stabilization of crystallite size have been obtained.

For the first time has been studied the correlation between energetic parameters that controls

the grain growth process in nanoalloys samples, the electrical properties and the thermal

expansion.

The results obtained in the present study give for the first time evidence that, the concept of

thermodynamic nanostructure stabilization is a real phenomenon for the mechanically alloyed

Ag-Cu nanopowders.

The results are important for both fundamental and practical reasons:

The study highlighted new aspects related to thermodynamic stability of investigated materials,

thermodynamic behavior being explained not only by structural changes due to the synthesis

method, but also by the fact that energetic parameters are extremely sensible to the variation of

particle size.

It’s essential to understand the role of energetic parameters in the control of nanostructured

materials stability from a fundamental standpoint, but also from the perspective of finding new

ways to control the enhanced properties.

A major importance is the identification of thermodynamic stability conditions with

consequences over the grain growth process for applications in which the maintenance of

nanocrystalline structure until higher temperatures is essential.

The understanding of the structure-properties correlations allows the exploration of new

opportunities to obtain nanocrystalline materials using non-equlibrium processes – such as

mechanical alloying.

10

List of ISI published works in the thesis research field

1. A. Milea, O. Gîngu, S. Preda, G. Sima, C. Nicolicescu, S. Tănăsescu, Thermodynamic

measurements on Ag-28% Cu nanopowders processed by mechanical alloying route, J. Alloy.

Compd., 629, 2015, 214-220; I.F. 2.726

2. O. Gîngu, P. Rotaru, A. Milea, A. Marin, C. Nicolicescu, G. Sima, S. Tănăsescu, In-situ synthesis

of AgCu/Cu2O nanocomposite by mechanical alloying: The effect of the processing on the thermal

behavior, Thermochim. Acta, 606, 2015, 1-11; I.F. 2.105

3. S. Tănăsescu, A. Milea, O. Gîngu, F. Maxim, C. Hornoiu, S. Preda, G. Sima, Correlation

between thermodynamic properties, thermal expansion and electrical resistivity of Ag-28% Cu

nanopowders processed by the mechanical alloying route, Phys. Chem. Chem. Phys., 2015, DOI:

10.1039/C5CP01390A; I.F. 4.198

List of oral/poster communications presented at national/international scientific events

Oral communications:

Alexandru Milea, Oana Gîngu, Petre Rotaru, Speranța Tănăsescu, Thermodynamic data of

the Ag-Cu nanoalloys processed by mechanical alloying route, 22th Symposium of Thermal

Analysis and Calorimetry, 12 February 2013, Bucharest.

Alexandru Milea, Oana Gingu, Anca Sofronia, Florentina Maxim, Speranta Tanasescu,

Thermodynamics of nanoalloys synthesized by the mechanical alloying method, 12th Edition of

National Seminar of Nanoscience and Nanotechnology, Romanian Academy, 16 May 2013,

Bucharest.

Poster communications:

Speranța Tănăsescu, Oana Gîngu, Petre Rotaru, Alexandru Milea, Andreea Neacșu,

Thermodynamic data of the Ag-Cu nanoalloys processed by mechanical alloying route, COST

Action MP0903 NANOALLOY, 19-21 November, 2012, Antalya, Turkey.

Speranța Tănăsescu, Oana Gîngu, Petre Rotaru, Alexandru Milea, Thermodynamic and

thermomechanical measurements on Ag-Cu nanopowders processed by mechanical alloying

route, COST Action MP0903 NANOALLOY, Final Conference, 5-9 April, 2014, Santa Marguerita

Ligure, Spain.

Alexandru Milea, Oana Gîngu, Petre Rotaru, Speranța Tănăsescu, Thermodynamic data of

the Ag-Cu nanoalloys processed by mechanical alloying route, International Conference of Physical

Chemistry ROMPHYSCHEM15, 11-13 September, 2013, Bucharest, Romania.