Electron transport and kinetics of transformations in the amorphous alloy Ni66B34

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Electron transport and kinetics of transformations in the amorphous alloy Ni66B34

F. Machizaud, F.-A. Kuhnast, J. Flechon, B. Auguin, A. Defresne

To cite this version:

F. Machizaud, F.-A. Kuhnast, J. Flechon, B. Auguin, A. Defresne. Electron transport and kinetics of transformations in the amorphous alloy Ni66B34. Journal de Physique, 1981, 42 (1), pp.97-106.

�10.1051/jphys:0198100420109700�. �jpa-00208995�

97

Electron transport ^{and kinetics of} transformations in the amorphous alloy Ni66B34

F. Machizaud, ^F.-A. Kuhnast, ^{J. Flechon}

Laboratoire de Physique ^des Dépôts Métalliques, Université de Nancy I, C.O. 140, ⁵⁴⁰³⁷ Nancy Cedex, ^France

B. Auguin ^and A. Defresne

Service de Chimie Physique, ^C.E.A. Saclay, ^France

(Reçu ^{le 23 mai} 1980, révisé le 7 juillet, accepté ^{le 15} septembre 1980)

Résumé.

L’alliage amorphe Ni66B34 préparé ^{par voie} chimique ^{à 20° C} apparait ^{comme constitué d’un} grand

nombre d’agrégats ^{de 8} Å environ, noyés ^{dans une matrice} désordonnée, ^et qui ^{lui confère un} ordre local voisin de celui du borure Ni3B.

La cinétique ^des transformations qui accompagnent ^tout traitement thermique du matériau à une température supérieure ^{à 20 °C est} déduite de l’étude des isothermes de décroissance de la résistivité électrique qui ^{en résulte.}

Deux domaines d’accroissement linéaire de l’énergie d’activation apparente Ea ^{sont définis en fonction de la tem-} pérature ^{de recuit : le} premier ^{dans le domaine de} températures ^où l’alliage ^{conserve son caractère} amorphe,

le deuxième au-delà de 350 °C qui correspond ^{à la} précipitation ^des phases cristallines.

Abstract.

The amorphous alloy Ni66B34 prepared by chemical methods at 20 °C appears ^to be formed of a large number of clusters of about 8 Å, surrounded by ^a disordered matrix, the former giving ^to the latter local order similar to that of the boride Ni3B.

The kinetics of transformations which accompany all thermal treatments of the material at a temperature ^above 20 °C are deduced from study ^{of the} resulting isotherm of decreasing electrical resistivity.

Two regions ^{of linear} increase in apparent activation energy Ea ^{are defined versus} annealing temperature : ^{the first} in the temperature domain where the alloy maintains its amorphous character; the second beyond 350 °C, ^cor- responding ^to precipitation ^{of the} crystallized phases.

J. Physique ⁴² (1981) ^{97-106 JANVIER} 1981,

Classification

Physics ^Abstracts

61.40

1. Introduction.

The Ni-B metallic alloys ^obtain-

ed by oxido-reduction in the liquid phase ^{have been}

of increasing ^{interest over the} past ^several years.

For these materials we have defined the preparation

conditions in thin films or in powders ^and specified

their composition [1-4].

Several methods allow study of the structural modifications and the irreversible phase changes

found in these alloys during ^the appropriate ^thermal

treatments :

- X-ray diffraction [2, 5-9],

-

electron microscopy ^and microdiffraction [4, 10],

-

differential thermal analysis [6, ^{8, 9,} 11],

-

electrical resistivity measurement [2, 4, 5, 7, 10,12].

In the present ^{work we} propose developing ^this

last analytical method, ^to compare the results with those of differential thermal analysis (D.T.A.) [11]

and to interpret ^them by ^a detailed structural study.

2. Expérimental techniques.

^{2.1 SAMPLES.}

The deposited thin films are made at 20 °C in a

thermostated bath on the microscope ^holder foils;

their mean thickness (between ^{500 A and 1 500} A) ^is

calculated by measurement of the mass on a Mettler balance with ± 2 pg accuracy. Our study essentially

concerns films whose thickness is greater than 1 000 A

and which _appear as a continuous monolayer ^{of islands}

on the electron micrographs [4].

The samples containing ³⁴ % ^{boron atoms are cut}

in a rectangle ^{of 40 mm x 10 mm in order to} study

their electrical resistivity.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0198100420109700

2.2 ELECTRICAL RESISTIVITY MEASUREMENT.

All

resistivity measurements were made on a Nanorac Sefram BMP recorder, using ^standard potentiometric

methods with ^a measuring ^{direct current of 1 mA.}

To suppress the parasitic electro-motive forces the connection wires and the evaporated electrical contacts are made of nickel. The current in the sample ^{is stabi-}

lized to 10- 5 A. The voltage variations are recorded

continuously. The electrical resistance of a film is known with a relative uncertainty ^{of 10- 3.}

For the calculation of resistivity, ^{where the} predo-

minant source of error is the determination of layer thickness, ^the precision ^{is then of} the order of 2 %.

Thermal treatment of the samples ^{is done under a}

vacuum of 10-6 torr in a pyrex laboratory ^{tube. The} heating ^{is done} by ^an electrical oven suitably regulated

so that the temperature gradient is less than 1/10 degree

per ^{cm over a} distance of about 7 ^cm. The regulation

system permits temperature stabilization to within 1 degree.

The temperature ^{is measured} by ^a potentiometric

recorder. The thermocouple (Ni-Cr, ^Ni alloy) ^is placed

under the sample ^{in a} groove of the sample holder

prepared ^{for this} purpose. The temperatures ^thus recorded ^are defined to 0.25 %.

The evolution of the electrical resistivity is measured

continuously :

-

either during different linear heatings (the ^rate

of heating ^a

dT/dt ^is constant) : ^this technique, according ^to Damask and Dienes [13], permits display

of the annealed temperatures characterizing ^a given stage ⁱⁿ the evolution of the material and permits

isolation of the different types of transformation

leading ^{to the increase} of electrical conductivity (Fig. 1),

Fig. ^1.

Ni,,B34 ^{film thickness 1200} A : resistivity annealing

curves with différent constant rates of heating ^{(x. D.T.A. curve of} powder N166B34.

-

or during different successive isothermal anneal-

ings of 5 h each (Fig. 2) : ^the slope ratio method [14]

for two successive isotherms then permits calculation of the apparent activation energy Ea.

Fig. ^2.

Ni66B34 ^{film thickness 1300 À :} isothermal curves of

resistivity.

Experimental results have led us to limit the dura- tion of each isothermal annealing ^{to 5} h, ^{for which we}

obtain about 90 % of the total resistivity ^variation

measured on a stabilized sample ^{at the same} tempe-

rature. A longer annealing ^{seems to} congeal ^the

material in its structural state and retards all higher temperature transformations.

3. Method of analysis.

^In solids it is often observ- ed [15] ^{that some} physical properties ^modified by heating of the material obey ^{a rate} equation ^{of thé}

form :

where _p represents the measured property, t ^the time,

T the absolute temperature, kB Boltzmann’s constant Ea ^the apparent activation energy, the q’s being _some

constants or independent ^{variables of t and T} but dependent chiefly ^{on the} material’s history.

Indeed, the Arrhenius equation corresponding ^to homogeneous ^{kinetics can} be extended to certain

heterogeneous ^reactions [16], although ^the validity

cannot be demonstrated. One can write :

where x represents ^the transformation degree, C,

the nucleation frequency, y the apparent transforma- tion order (which ⁱⁿ heterogeneous kinetics loses all objective significance), (1 - x) ^the untransforme4

fraction of the solid body.

99

If equation (2) is satisfied and if the property p is

proportional ^{to the} concentration n of defects :

and if it is written that the concentration n is pro-

portional ^{to the} untransformed fraction (1

x) :

relation (2) ^{becomes :}

and therefore (3) is written :

so :

putting : fl

RD ^v.

For each isotherm at the temperature T, Co ^is supposed ^{constant, but} Co ^can vary from one isotherm to an other following ^the transformation undergone by ^{the material. T} being ^{constant, the sole variable} parameter is the time t ; the integration of (6) gives :

with :

3.1 KINETICS DETERMINATION.

Determination of the transformation kinetics is possible through a

detailed analysis of isothermal measurements of the

electrical resistivity ^{carried out on} thin film alloys.

It for each isotherm :

p being the electrical resistivity ^{at time t,} p; the initial resistivity, pf the extrapolated ^final resistivity, ^then

the isothermal decreasing ^of p satisfies (7) ^{which is}

written :

with :

Relation (8) is verified for each isotherm for which Co

and Ea ^are given characterizing structural transfor- mations.

Indeed it is always possible ^{to choose a} positive

value of M in order that the curve

be a straight line, ^the slope 1/1 - ^y allowing ^{us to}

calculate the apparent order of the transformation.

Fig. ^3.

Ni66B34 film thickness 1 150 A : determination of appa- rent reaction order _y for isothermal curve of resistivity ^{at 200 °C :} a) Log (p - pf)

f(t); b) Log (p - Pc)

f(t ⁺ M), ^M

^100.

The calculation done for each isotherm defmes _an apparent ^order equal ^{to 2} (Fig. 4) ; ^the slight ^fluctuar

tions observed may be due ^to the fact that in the

integration ^of (6) ^we supposed Ea ^constant along ^the

isotherm.

Fig. ^4.

Apparent reaction order _y versus isothermal temperature of annealing.

3. 2 APPARENT ACTIVATION ENERGY DETERMINATION.

- 3.2. 1 Ratio of slopes ^method.

Equations (6)

and (8) having ^been experimentally verified for the electrical resistivity isotherms, the ratio of slopes

method [14] ^{can be used to} define the apparent ^trans- formation activation _energy Ea and its variation with the treatment temperature ^{of the} alloy :

with :

Rl, R2 being ^the slopes ^{at the common} point ^of twg successive isotherms recorded at the temperatures 1¡

and T2 ^and characterizing the variation of the mea-

sured physical property ^p. The value of Ea obtained.

from (9) characterizes the transformations eflecte4

at the temperature Ti, the transformations which should occur during ^{the isotherm} T2 hardly being

started at the common point Ti, T2.

Figure ⁵ reports the results relative to Ea ^{for the} alloy ^{studied : two} regions ^{of linear} variation versus temperature appear with ^a sudden discontinuity ^near

300 °C. The increase of Ea ^{with the} temperature shows that the material being ^{annealed is accom-} panied by ^several transformations associated with

différent activation energies. ^{The value of} Ea, deterr

mined at a given temperature, ^{is an} average value

depending ^{on the} sample history : age of the samples

duration of the thermal treatment, temperature ^diffe,

rence between two isotherms. The results obtained, ^are reproducible ^{for about ten} samples ^{from 1100 A to}

1 200 A of thickness, samples recently prepared, ^the

duration of thermal treatment being ^{fixed at 5 hours,} temperature difference between 2 isotherms being ^a

maximum of 50 °C.

3.2.2 Kissinger’s ^method [17].

^{This allowed}

us [11] ^to determine, by ^{D.T.A. on} powder samples, ^the apparent activation _energy Ea ^and nucleation fre- quency Co upon crystallization.

The theory ^{is based on the} similarity of the D.T.A.

and D.T.G. (differential thermogravimetry) ^curves.

Murray ^{and White} [18], [19] and Sewell [20] admii

that the maximum reaction rate dx/dt given by

relation (2) ^{and indicated on the D.T.G.} curves cor-

responds ^{to the maximum} of the D.T.A. peak.

Knowing ^{that at thç} maximum reaction rate

differentiation of (2) ^{allows us to write :}

(x

dT/dt heating ^rate, T. temperature ^{at the D.T.A.}

peak ^maximum (Fig. 1).

Equation (2) ^{can be} integrated assuming ^{a constant} heating ^{rate in order to} obtain the extent of reaction

as a function of temperature. ^A satisfactory approxià

mation was obtained by Murray ^{and White} [21] by

successive integration by parts : ^a rapidly converging

series results, ^{so that all terms} after the second _may be

dropped. These results combined with equation (10)

give ^an expression independent of the transformation order :

For the crystallization temperatures determineà

from the thermograms ^{carried out at various} heating

rates a, relation (11) is written :

Ea.y,t ^and co , characterizing the observed transfor-

mation, ^{in this case the} crystallization, ^are supposed independent ^{of the} temperature T., provided ^{that the} heating ^rates retained allow crystallization ^{inside a}

limited temperature range (658 ^{K K} T mcryst ^{708 K}

for a rate of 1 oC/min. ^{oc K 20} °C/min.). By ^diffe-

rentiation of (1 1’), ^this permits ^{us to obtain :}

The apparent activation _energy is obtained from the curve of Log (cxIT’.) ^versus 1/Tm (Fig. 6).

Fig. ^5.

Apparent activation energy Ea ^versus isothermal temperature ^of annealing.

101

Fig. ^6.

Kissinger’s method : determination of apparent ^activa- tion _energy Ea.

The figure ^{6 indicates also the} temperature ^variation T. of the D.T.A. peak ^{maximum with the} heating

rate a.

3.2.3 Comparison of the values of Ea given by ^the

two methods.

Kissinger’s ^{method is} applied ^{to the}

exothermic D.T.A. peak, the maximum Tm cryst ^oi

which can vary from 385 °C to 435 °C as a function of

heating ^{rate a} [11].

Thus values of E.acryst ^{can be} compared only ^over

this temperature range; this _range belongs ^{to the}

boride Ni2B crystallization.

The results (Figs. 5-6) reveal remarkable agreement between the apparent activation energies ^deter-

mined on the films by the ratio of slopes ^method (1.90 eV/at. Eacryst ^{2.05 eV/at.) and on the pow-}

ders by Kissinger’s ^method _{(Ea cryst}

^2.00 eV/at.).

3. 3 CALCULATION OF THE NUCLEATION FREQUENCY : ^:

Co.

^The nucleation frequency COcryst ^produced

near 400 °C by the exothermal transformation has been calculated for this transformation, knowing ^the

value of Ea cryst determined by Kissinger’s ^method (c.f. ^relation (11’)) [11] : the calculation gives

Knowing ^{that the} apparent ^order y has been found to equal 2, ^relations (8’) ^and (8") ^{allow us to} determine :

so that :

for each isotherm based on experimental ^data of p;, pf, M and Ea (Fig. 7).

The higher isothermal decrease of electrical resisti-

vity appears ^at 400 °C (Fig. 2), ^{and at the same} tempe-

rature as that of the sudden drop ^of resistivity ^at

Fig. ^7.

Nucleation frequency Co ^and ColP ^{versus isothermal} temperature ^of annealing (film thickness 1 250 À).

constant heating ^rate (Fig. 1). ^{Both are} associated with the exothermic crystallization peak. Thus, ^{the value of}

(Co/P)cryst

^{7.4 x} 1016 S-1 U -1 m-1 can be obtained

by interpolation ^{to 400 °C.} Knowledge ^of

allows calculation of the constant fl and therefore Co

based on (13’) ^{for each isotherm at the} temperature ^T (Fig. 7).

4. Structural study ^{of the} amorphous alloy Ni6sB34.

- A freshly prepared hlm, ^{the mean} thickness of which is 800 A, appears ^{as a} monolayer ^formed by ^some aggregates ^{from 750} A to 1 000 A ⁱⁿ diameter, quite uniformly ^{opaque to} the electrons beam [4]. Only very fine filaments, starting ^{from the centre of each} aggregate and giving ^{them the} aspect ^{of « mimosa} flowers »,

seem to indicate a radial growth; ^{the boundaries}

between aggregates ^{are clear and} rectilinear (Fig. 8a).

Electron microdiffraction photographs (Fig. 8b)

reveal some broad and diffuse rings characterizing

an amorphous ^or imperfectly crystallized ^matrix.

The local order can be defined by Fourier transfor- mation of precise intensity measurements of X-rays

diffused by powders ^{of the same} composition ^{as the}

films.

We used the C.G.R. theta 60 diffractometer equipped

with a step by step device and a print-out. ^{The same} sample is then studied successively using ^{two mono-}

chromatic MQKa ^and CuKa beams. After correction of experimental diffused intensities (fluorescence, pola- rization, Compton diffusion, correlation of the MoKa

and CuKa diagrams, rectification for the absolute scale with normalization after Kaplov et ^al.’s

method [22]), ^{the reduced} repartition ^{function :}

is obtained by Fourier transformation of the reduced

interference function :

Fig. ^8.

Transmission electron micrographs ^and electron diffraction patterns ^of Ni66B34 ^{film thickness 750} Â : a), b) deposited ^at 20°C, c), d) heat treated at 200°C.

where s = 2 sin 0 () ^and 1 s ^{is the} expérimental ^{p inter-}

ference function (Fig. 9a).

The repartition ^function (Fig. 10a) shows well defmed short-range ^order by ^the position ^{of the} first-neighbour peaks ^{and leads us to} consider the presence of clusters in the alloy Ni66B34 ^{as we had}

done for the alloy Ni77P 23 [23].

For reasons of chemical affinity ^{one can assert} that,

in all the nickel borides, the metalloid has exclusively

nickel atoms as immediate first neighbours. ^{Thus if}

the different cluster types ^{used for the} theoretical calculation of I(s) ^{we consider one boron atom}

surrounded by ^{nickel atoms.}

Referring ^{to the} alloy Ni77p23 [24], ^{where we have}

shown the existence of icosaedric clusters, ^a hypothesis

which allowed Briant [25] ^{to describe the structure of} amorphous metals, ^{we have} calculated [2] ^{the inter-}

ference functions of icosaedric and pseudo-icosaedric arrangements containing ^{12 nickel atoms} surrounding

one central boron atom (Fig. 9d, f) :

This relationship involves the number N of metal atoms in the cluster, ^their participation ^{in the diffusion} through ^{the a factor} (calculated by taking ^{into account}

the metalloid percentage ^{of the} material), ^{and the}

interatomic distances _{rNi - Ni} and _{rNi - B} in the unit cell.

The interference function characterizes, ^for large

values of s, ^{the local} order, and thus the clusters. The differences observed between the experimental a) ^and

calculated d ) and f ’) functions, ^{as well as the} positions

and form of the maxima, oblige ^{us to} reject ^{an icosae-}

dral or pseudo-icosaedral ^{structure for} the clusters.

To take local chemical order into account in this kind of alloy, ^{we have also} calculated [2] ^{the inter-}

ference functions for clusters ^« Ni3B », ^« N’7B3 » corresponding ^to 1 B and its surrounding ^{9 Ni as in}

borides Ni3B ^and Ni7B3, ^{and for a cluster «} Ni2B » containing ^{1 B and its} surrounding ^{8 Ni as in boride} Ni2B (Fig. ^{9c, e,} g). Only the interference function c) explains quite well the oscillations of the experimental i(s) ^at large ^s.

However, ^{the «} Ni3B » ^unit brings ^into play only

interatomic distances less than 4.4 A and contains no

pair corresponding ^{to the well} distinguished peak

of the third neighbours ^{noted at 4.90 À on the} repar- tition function (Fig. 10a).

In order to take greater distances into account we

103

Fig. ^9.

Interference functions : a) Ni66B34 powder amorphous, b) calculated for a microcrystal Ni3B (96 ^{Ni and 32} B) with contri- bution of interatomic distances less than 5.5 Á, c) d) e) f ) g) ^cal-

culated for different clusters : « Ni3B », pseudo-icosaedron,

« NiB3 », icosaedron, ^« Ni2B ^».

Fig. ^10.

Repartition functions : a) Ni66B34 powder amorphous, b) calculated for a microcrystal Ni3B of 36 Ni and 12 B.

have calculated an interference function relative to a

microcrystal ^of Ni3B (96 Ni, ³² B) involving only

distances less than a fixed value. The best result is obtained with only ^the contribution of atomic pairs

less than or equal ^{to 5.5} Á, which include those corres-

ponding ^{to the} peak ^{of the 1 st, 2nd, and 3rd} neigh- bours, and which define a mean dimension of the clusters of about 8 Á (5.5 Á ^{+ 2 x 1.24} Á, - ^{1.24 Á} being Goldsmidt’s radius for the nickel atom).

The repartition ^function p(r) (Fig. 10 b) ^deduced by

Fourier transformation of the theoretical interference function for a microcrystal Ni3B of 36 Ni and 12 B

correctly explains the presence of the lst, 2nd and 3rd

neighbours ^{and the} general form of the experimental

function p(r) ^{for r} smaller than 5.5 A.

Thus the amorphous alloy Ni66B34 ^{appears to be}

formed of a great number of clusters immersed in a

disordered matrix and giving ^a short-range ^{order to}

the alloy ^{similar to that} existing ⁱⁿ Ni3B. ^It may be

suggested that there exist in the neighbourhood ^of

the clusters some low density, ^boron rich, ^zones presenting ^a deviation from the local order, ^these

relaxes zones surrounding the clusters and extending throughout the matrix.

5. Structural évolution and discussion.

During ^the

thermal treatments, the evolution of the amorphous phase constituting the material and the amorphous crystal phase transition can be examined :

-

by study ^{of the} morphological variations of the aggregates constituting the metallic films by ^electron micrography,

-

by ^the interpretation of electron microdiffraction and X-ray diffraction carried out at ambient tempe-

rature on the material after treatment at appropriate temperatures.

This evolution should allow us to ’interprete ^the

results of § ^3.

5. 1 FIRST DOMAIN (AMORPHOUS) : AMBIENT-275 °C.

-

The frequency nucleation Co ^{near zero at ambient} temperature reveals the alloy’s stability. The increase

of Ea ^versus temperature ^{is linear} (Fig. 5).

The exothermic phenomenon ^{observed is} slight ^and spread ^{out. The decrease} of resistivity ^is regular (Fig. 1).

The alloy maintains the amorphous character and

no notable change ^{is visible on the X} diffraction

diagrams ^or the microdiffractions. A slight ^diffusion

exists inside the aggregates : ^{a fibrous} grating begins

to form some grain outlines. The ^mean aggregate ^size remains unchanged (Fig. ^8c, d). ^{For this domain it}

seems that the amorphous ^material rearranges itself,

the short-range ^order remaining ^{that of} Ni3B.

5.2 SECOND DOMAIN : : PRECIPITATION : : 275 OC- 3S0 °C.

No slope change ^{is observed} during ^the regular variation of the resistivity. ^{The sudden increase} of Ea and that of the nucleation frequency Co ^coincides

with the formation of the borides Ni3B ^and Ni7B3.

It seems that three phases ^are present : ^one amorphous

phase ^{and two} microcrystallized phases Ni3B ^and Ni7B3, ^{as confirmed} by the diffraction diagrams :

-

the _presence of two shoulders, ^one ahead of the first amorphous ring ^near the intense reflexions (210), (002) ^of Ni7B3, the other ahead of the second ring

and covering the reflexions (312), (232) ^of Ni3B ^and (213), (500) ^of Ni7B3. ^One ray appears well differen- tiated from the continuous background ^{between the}

2nd and 3rd rings. ^It corresponds ^{to the} reflexions (340)

and (422) ^of Ni3B (Fig. 11). ^{Its width at half} height permits estimation of the mean dimension of the

crystallites ^{at about 80 A.}

-

the microdiffractions _appear as the superpo- sition of spot diagrams ^{and the} rings ^{of the} amorphous

material. The aggregates, always ^{of the same size,}

are now clearly constituted of grains of dimension

75Â-100Â (Fig. 12ab).

5.3 THIRD DOMAIN : : CRYSTALLIZATION : : 350 OC- 600 °C.

The refming ^of X-ray diffraction diagrams

and the extension of microdiffraction spots ^{in recir} procal space, confirm the crystalline growth ^{of the} precipitated phases and facilitate their identification.

The links between the aggregates disappear pro-

gressively ; ^at 500 °C the coagulation phenomenon

is total (Fig. 12c, d), (Fig. 13).

Fig. 11. - X-ray diffraction intensities measured with Ni66B34 sample, heat treated at 300 °C. Intensities in arbitrary ^units.

The small second exothermic phenomenon begin- ning ^{at 275 °C} (associated with the formation of the borides Ni3B ^and N’7B3) ^{extends to 400 °C, where}

its growth ^is completed. An intense peak ^{succeeds it,}

Fig. ^12.

Transmission electron micrographs and electron diffraction pattems of Ni66B34 film thickness 750 A : a), b) heat treated at 300 °C,

c), d) heat treated at 500 OC.

105

centred ^{on a} temperature ^{near 400} °C, and the resisti-

vity immediately ^falls rapidly (Fig. 1). This kind of

peak, ^called explosive, perfectly reveals the sudden transition from an amorphous phase ^{to a} crystallized phase [26].

Fig. ^13.

X-ray diffraction intensities measured with N166B34 sample, heat treated at 500 °C. Intensities in arbitrary ^units.

The residual amorphous phase precipitates ^here

under the form of N’2B, the appearance of which coincides with that of the _rays (110), (310), (200)

which characterize it, ^{and the} intensity ^{of these} rays increases to 450 °C (Fig. 14).

This crystallization domain is also characterized by ^{a new} linear increase of Ea (Fig. 5) ^{in the same}

fashion as for the nucleation frequency Co (Fig. 7).

A treatment of the material at a temperature superior

to 500 oC makes the metastable bolide ?783 disap-

pear :

A simultaneous growth ^{of the} crystallized phases Ni3B ^and Ni2B ^results, whereas the reflections (002), (212), (701) of the orthorhombic boride Ni4B3 appear.

Though ^the representative point ^{of the} alloy Ni66B34

in the binary equilibrium diagram Ni, ^B is situated

a little beyond ^the Ni2B composition, the presence of Ni3B ^can only ^be explained by those of borides

Ni7B3 ^{and then} Ni4B3, and indicates that the initial local composition ^is inhomogenous ^{in boron atoms.}

Fig. ^14.

Intensity variations of characteristic X-ray reflections for Ni3B, Ni7B3, Ni2B, ^versus temperature. Intensities in arbitrary

units.

6. Conclusion. - A detailed analysis of isothermal measurements of resistivity ^of amorphous’ alloy Ni66B34 ^films [27] ^{has allowed us to} define the trans..

formation kinetics which accompany the material

annealing from ambient to 500 °C.

The observed variations of apparent ^activation

energy shows that several transformations, ^with

which are associated different activation energies,

are at the origin ^{of the} material’s evolution. The structural study ^{allows us to} provide ^an interpreta-

tion.

Two domains of linear variation of Ea are ^observed.

They ^are separated by ^{a sudden increase of} Ea ^which

exists between 275 OC and 350 OC, simultaneous with the formation and growth of borides Ni3B ^and Ni,B3 :

-

the hrst (ambient - ²⁷⁵ OC) corresponds ^to slight ^atomic diffusions in the amorphous phase, ⁱⁿ

which the local order remains that of Ni3B. ^{It seems}

that the amorphous ^{material is} rearranged,

- the second is associated with the precipitation

of Ni2B, starting from the residual amorphous phase,

whereas at 400 OC the growth of borides Ni3B ^and Ni,B3 ^is hnished, ^{and at} 500 OC the decomposition

of Ni7B3 ^to Ni3B ^and Ni2B begins.

Finally, the values of apparent activation energy

Ea (1.90 eV/at. Ea ^2.05 eV/at.), determined in the precipitation ^{domain of} Ni2B ^{based on the}

electron’s transport properties ^{measured on the} films,

coincide with those (2.00 eV/at.) calculated by D.T.A.,

which is referred to atomic transport ^{in the} pulverized samples for which the composition ^{is the same as}

that of the films.

References

[1] FLECHON, J., VOIRIOT, F., ^{Bull. Soc. Chim. Fr. 93} (1963) ^509.

[2] KUHNAST, F. A., Thesis, Nancy (1979).

[3] FLECHON, J., KUHNAST, ^F. A., Bull. Soc. Chim. Fr. 140 (1976)

739.

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