characterisation of the sol?gel process in the superconducting ndba2cu3o7?y system

Characterisation of the sol–gel process in the superconductingNdBa2Cu3O72y system{

B. Schoofs,*a D. Van de Vyver,a P. Vermeir,ab J. Schaubroeck,b S. Hoste,a G. Hermana and I. Van Driesschea

Received 28th September 2006, Accepted 20th December 2006

First published as an Advance Article on the web 6th February 2007

DOI: 10.1039/b614149h

The objective of this paper is to obtain a better insight into the sol–gel mechanism of water-based

precursors for the development of thin NdBa2Cu3O72y (NBCO) superconducting films. The

influence of metal complexation behaviour on the formation of transparent and homogenous gels

after the combination of different metal salts and ligands has been studied for several metal salts

(Cu2+, Nd3+ and Ba2+). Two inorganic aqueous sol–gel precursors have been studied: a metal

nitrate–citric acid-based and a metal acetate–triethanolamine-based solution. The characteristics

of the precursor solution are based on the determination of the stability constants by the

computer program Superquad. The prediction of the complex stability in this solution was related

to the complexation of the three metal ions (Cu2+, Nd3+ and Ba2+) with a certain ligand.

IR-spectroscopy was used for the determination of the gel. This resulted in a better understanding

of the composition of the solution and could be used for preparation of more stable sol–gel

precursors for the synthesis of homogeneous end products. These sol–gel systems were used for

the deposition of highly textured superconducting thin films on SrTiO3 substrates by dip-coating.

Using detailed thermal analysis, it is shown that the morphology of the films can be optimised by

adjusting the parameters during thermal treatment, resulting in dense and highly textured thin

films. Special attention is given to the microstructure of the thin film because of its relevance to

the superconducting transport properties of the coated conductor system.

1 Introduction

A large number of synthetic routes for the preparation of

(RE)1Ba2Cu3O72y (123) superconducting oxides are reported

in the literature (RE = rare earth).1 The normal state and

superconducting properties of polycrystalline (123) materials

and textured (123) thin films depend on the morphology,

which is in turn determined by the preparative methods.

The use of sol–gel techniques to prepare glasses and electro-

ceramics has received increasing interest during the last

decade. The sol–gel process has become an attractive technique

for the preparation of oxide ceramics with a great variety of

chemical compositions, because of the possibility of producing

very homogeneous multi-component materials at temperatures

much lower than used in melt-based procedures.2 Most

industrial processes are based on inorganic precursors, such

as metal salts in aqueous solutions. Such precursors are much

cheaper and easier to handle than metal alkoxides in organic

solvents. The sol–gel process involves the evolution of

inorganic networks through the formation of a colloidal

suspension (sol) and gelation of the sol to form a three-

dimensional network in a continuous liquid phase (gel). From

a chemical point of view, sol–gel chemistry is based on the

hydrolysis and condensation of metal complexes (Fig. 1).3 In

the aqueous method described in this work, the metal salts are

dissolved in water with formation of unstable metal aqua

complexes. Fast hydrolysis of these species will then lead to

precipitation of metal hydroxides. To prevent this, the rate of

hydrolysis needs to be controlled by adding complexing agents,

leading to the formation of metal chelates and thus, stabilisa-

tion of the precursor solution.

By slow evaporation of the solvent (water), condensation of

these complexes will take place, leading to the formation of a

homogenous gel network. It can be seen that this condensation

mechanism requires stable M–OH species and therefore it will

only occur with a judicious selection of several parameters: i)

the value of pH, depending on the metal source, ii) the choice

of complexing agent and iii) the metal to complexing agent

aDepartment of Inorganic and Physical Chemistry, Ghent University,Krijgslaan 281 (S3), 9000 Ghent, BelgiumbDepartment of Industrial Sciences, Hogeschool Ghent, AssociationGhent University, Jozef Kluyskensstraat 2, 9000 Ghent, Belgium.E-mail: [email protected]{ Electronic supplementary information (ESI) available: Additionaltables and figures. See DOI: 10.1039/b614149h.

Fig. 1 Sol–gel reactions (with M = Metal, z = positive charge, m =

negative charge, A = complexing agent)

PAPER www.rsc.org/materials | Journal of Materials Chemistry

1714 | J. Mater. Chem., 2007, 17, 1714–1724 This journal is � The Royal Society of Chemistry 2007

ratio. For our sol–gel precursors, it should also be mentioned

that the function of a complexing agent is twofold: i)

stabilising the precursor solution through formation of

metal hydroxo complexes and ii) acting as a driving force for

the gelation because these hydroxo complexes benefit the

condensation process. Since the quality assurance of the final

product is dependent on the homogeneity and the bonding

behaviour of the metal complexes within the sol and the gel,

a systematic study of the properties of the gels derived

from varying operational parameters has been undertaken in

this paper.4

The paper describes the collection of these data using

potentiometric titration curves and SUPERQUAD software

instead of estimating them from experimental trial-and-error

synthesis. These data lead to a better understanding of the

composition for each solution and can be used for the prepara-

tion of more stable sol–gel precursors for the synthesis of

homogenous end products. The conversion into a gel network

was studied using IR spectroscopy.

In this way, the chemistry of two water-based sol–gel routes

is described in detail, both starting from different metal salts

(acetates and nitrates) and different complexing agents

(triethanolamine and citric acid). From these solutions, both

bulk NBCO material and thin NBCO films on SrTiO3 were

prepared and analysed using TGA–DTA–MS measurements

to study the thermal decomposition behaviour of the gel

network. Texture analysis of the NBCO layers was performed

using XRD scanning and SEM imaging. Electrical resistivity

measurements were applied to determine the critical transition

temperature of the superconductor.

2 Experimental

2.1 Reagents

All reagents were of analytical grade and used as received.

Distilled and deionised water (Milli-Q quality, specific

conductance , 0.05 mS cm21) was used throughout for all

solutions.

Titrisol ampoules were used to obtain carbonate-free

potassium hydroxide solutions (# 0.2 M). The metal-ion

stock-solution was prepared from metal nitrate and was

standardised by titration with the disodium salt of ethylene-

diaminetetraacetic acid (EDTA) in the presence of a small

amount of the Hg(EDTA) complex using appropriate condi-

tions and electrodes (mercury and calomel).5 All final solutions

for the potentiometric and spectrophotometric titrations were

made up to an ionic strength of 0.1 M with KNO3.

2.2 Synthesis

Based on experimental results, two water-based inorganic

NBCO sol–gel precursors were prepared. The stability and

thus homogeneity of these solutions was studied using

potentiometric measurements.

2.2.1 Metal acetate–triethanolamine (TEA) route. Nd , Ba

and Cu acetates (Nd(CH3COO)3?4H2O, Ba(CH3COO)2 and

Cu(CH3COO)2?1H2O) are dissolved in a 4 : 1 water–acetic

acid mixture with a total metal concentration of 0.6 M. After

refluxing at 90 uC for 6 h, a clear blue solution is obtained. To

25 ml of this solution, 3 ml of triethanolamine (98%) as a

complexing agent is added while stirring. The solution is

allowed to cool, then the pH of the solution is adjusted to 5

using ammonia. After water is evaporated slowly at 60 uC, a

clear blue, homogenous, viscous gel is formed. This sol–gel

precursor solution can be stored for 2 weeks. After this period,

precipitation of large blue crystals appears. As these pre-

cipitates can lead to off-stoichiometric phase formation

and impurities in the final oxide film, the stability of this

solution was analysed using potentiometric titration curves, as

described in Section 3.

2.2.2 Metal nitrate–citrate route. Nd , Ba and Cu nitrates

(Nd(NO3)3?6H2O, Ba(NO3)2 and Cu(NO3)2?3H2O) were dis-

solved in water with a total metal concentration of 0.6 M.

Then citric acid is added as a complexing agent in a ratio of

metal ion to citric acid of 1 : 3. The pH is adjusted to 7 using

ammonia. After water is slowly evaporated at 60 uC, a glassy

state, homogenous gel is formed. This NBCO precursor

solution can be stored for several months but using the results

of potentiometric titration curves, a further optimisation of

this precursor will be described in Section 3.

To prepare superconducting bulk material via powder

synthesis, the precursor gels were decomposed at 880 uC and

pressed into pellets before being sintered at 990 uC for 30 h in a

1% O2/Ar reduced oxygen atmosphere. The final step consists

of annealing in pure oxygen at 400 uC for 20 h.

To produce thin films on polished and cleaned SrTiO3 (001)

single crystals, the precursor gels were dip-coated at room

temperature at a speed of 170 mm min21 in a class 10 000

clean room. After gelling at 60 uC for 3 h, these amorphous

layers were converted to dense, crystalline NBCO films at

990 uC for 6 h in a 1% O2/Ar atmosphere. The final step

consists of annealing at 400 uC in pure oxygen for 5 h.

2.3 Characterisation techniques

2.3.1 Potentiometric measurements. The potentiometric

measurements were performed with standard dilute KOH

using a Schott pH meter and a 5 ml Schott T-burette (total

volume). The pH meter was connected with a Schott H2680

glass electrode and a Schott B3410 calomel electrode with a

second salt bridge filled with 0.1 M KNO3. Each aqueous

system under consideration was measured in a 100 ml

jacketed cell thermostatted at 25 uC ¡ 0.1 uC by a refrigerated,

circulated water bath. All systems were studied under

anaerobic conditions, established by a stream of pre-saturated

nitrogen, obtained by bubbling the inert gas through a 0.1 M

KNO3 solution. The ionic strength was adjusted to 0.10 M

by the addition of KNO3 as supporting electrolyte. The

concentrations of all the experimental ligand solutions were in

the range of 1.0 6 1023 M. The program Titrate, slightly

modified, was used to monitor the titration.6

The titration data were processed using Gran’s method7

in order to calculate the standard cell potential (Eu), the

dissociation constant of water (Kw), together with the

correction terms for changes in the liquid junction potential

in strong acid medium, aj (2log[H+] , 2.5), and for the

This journal is � The Royal Society of Chemistry 2007 J. Mater. Chem., 2007, 17, 1714–1724 | 1715

non-linear electrode response in a strong alkaline medium,

bj (2log[H+] . 11.5), as described previously.

Experimental runs were performed by adding increments

of standard base to a ligand solution containing an excess of

HNO3.

For each chemical species MpLqHr in the solution equilibria

there is its formation constant, which is expressed (charges

were omitted for simplicity) as follows:

bpgr~MqLpHr

� �

M½ �p L½ �q H½ �r

Because the thermodynamic definition of a formation

constant utilises activities (and not concentrations, as given

above), the quotient of the activity coefficients was kept

constant by performing the experiments in a medium of

constant ionic strength by adding potassium nitrate.

The stability constants of several aqueous species were

calculated by Superquad using a numerical analysis of all

experimental titration data.8,9 These data were evaluated by

testing a number of chemically acceptable complexation

models. These models not only consist of metal ion–ligand

complexes (MpLqHr) with r = 0), also some protonated

complexes (r . 0) and hydroxo complexes (r , 0) can appear.

After successive attempts, the best models were selected

according to the best agreement between observed and

calculated data by means of an accurate statistic analysis of

the global s-value for the refinement, by goodness of fit (x2)

and by the standard deviation of each formation constant as

calculated by Superquad. The simulated titration curves and

species distribution diagrams for each given set of aqueous

species were computed from the equilibrium constants with the

program Equil.10

2.3.2 IR spectra. IR spectra (4000–400 cm21) of the gel

samples were taken in KBr pellets at room temperature using a

Mattson Unicam FTIR spectrometer. Each measurement ran

over 64 scans, at a resolution of 4 cm21.

2.3.3 Other techniques. The thermal decomposition beha-

viour of the gel network was investigated separately by TGA–

DTA measurements (Stanton–Redcroft STA 1500) on bulk

samples with the same composition as the dip-coated layers.

The microstructure of the deposited layers was characterised

by X-ray diffraction (XRD; Siemens D5000, CuKa) using h–2h

geometry in combination with pole figures for determination

of the degree of biaxial texturation of the layer. The overall

morphology of the thin films was characterised by optical

microscopy (Leitz) and SEM (Philips 501). Local defects in the

layer and orientation of the material at the substrate–layer

interface were identified using high resolution TEM measure-

ments (Jeol JEM 3010). The critical temperature of the

superconductive layers was determined by resistivity measure-

ments using a custom-made four-point test device (Keithley).

3 Results and discussion

3.1 Potentiometric results

3.1.1 Acetate and triethanolamine system. TEA is a hydro-

philic ligand with a nitrogen donor atom substituted with three

side arms bearing OH-groups. Because of its chelating ability,

this ligand generally forms monomeric tricyclic structures that

are water-soluble (Fig. S1, ESI{).11 The TEA complexes were

formed by the reaction of TEA and the metal salt in water.

i. Protonation constants of triethanolamine (TEA). The

potentiometric equilibrium curve of TEA?HNO3 was found

to possess an inflection at a = 1 (where a = mol of base added/

mol of ligand), which corresponds to the completion of the

neutralisation of the ammonium ion. The inflection at a = 0

matches the end of the neutralisation of the excess HNO3

added to the ligand solution. The protonation constant

obtained for TEA is listed in Table S1, ESI{.

ii. Cu(II) TEA complexes. The complexometric titrations with

Cu(II) were performed in four different molar ratios (1 : 1, 1 : 2,

1 : 3 and 1 : 4). Due to the coordinative properties of the metal

ion and the structure of the ligand, the metal ion being

surrounded by a maximum of two ligands was suggested. The

potentiometric equilibrium curve for the 1 : 1 Cu2+/TEA

titration shows an inflection point at a = 3, corresponding to

the formation of a mononuclear dihydroxo Cu(II) complex.

The Cu(II) complexation curve indicates a good complexation

strength because the curve profile is under the ligand

deprotonation curve after titration of the strong acid HNO3

(a . 0). The chemical model was further refined with three

other mononuclear species. The equilibrium curves for the

other molar ratios show inflection points at a = 2, 1.68 and

1.50. All of these inflections refer to the formation of the same

mononuclear dihydroxo Cu(II) complex, followed by a proton

neutralization step, corresponding to deprotonation of the

excess ligand in the solution. Dinuclear Cu(II) complexes

were all rejected by Superquad. These four different titrations

result in a chemical model of four mononuclear Cu(II) species

(Table S2, ESI{). This chemical model differs from the model

characterised by Bjerrum et al.12 and Hancock et al.,13 which

also includes ML2 hydroxo and M2L2 hydroxo species.

The corresponding species distribution diagrams only differ

in intensity of the species. The distribution diagram of the 1 : 3

system (Fig. 2) shows that the mononuclear Cu(II) species

begin to form at about pH 4 and reach a maximum percentage

Fig. 2 Species distribution diagram showing the species formed as a

function of pH when 4.06 6 1023 M TEA and 1.35 6 1023 M Cu(II)

(metal/ligand mol ratio 1 : 3) are equilibrated at 25 uC and I = 0.10 M

(KNO3).


of 22% at a pH of 6. Other complexes that form as the pH is

further increased are CuLOH+, CuL(OH)2 and CuL(OH)32.

The hydroxo Cu(II) complexes form at and above pH 5.0,

and are significant species at higher pH. The monohydroxo

complex was formed in the region between pH 5–10 and lead

to a maximum of 76%. A further increase in pH leads to the

replacement of another water molecule to form the dihydroxy

species with a peak maximum of 95% at a pH of 9.

A possible Cu(II) complex configuration uses the nitrogen

and the three oxygen donor atoms of TEA. The preference of

the Cu(II) ion for fivefold instead of sixfold coordination,

which occurs for other cations of comparable charge and size,

results from a Jahn–Teller destabilisation of the octahedral

complex.14 Three donor atoms must be located in the

equatorial plane, but one has to be located axial. Elongation

through the z-axis is therefore difficult and this leads to a

decrease in complexation strength. The other coordination site

is occupied by a water molecule, which will be replaced with

increasing pH by a hydroxide ion. Another possibility is the

deprotonation of the alcohol groups with increasing basicity,

but a potentiometric measurement alone cannot give any

confirmation for this chemical concept.

iii. Ba(II) TEA complexes. The thermodynamic stability of

complexes of Ba(II) with TEA are too weak to determine

stability constant values (logb = 0.36).

iv. Nd(III) TEA complexes. The potentiometric equilibrium

curve for the 1 : 1 Nd3+/TEA titration indicates a interruption

at a pH above 7. This disturbance in the titration curve implies

the precipitation of metal hydroxides, which suggests a weak

interaction between Nd(III) and TEA. This assumption is

strengthened by the profile of the complexation curve, which is

characterised by a series of pH values only slightly different

from those of the ligand deprotonation curve at a . 0 (Fig. 3).

Therefore, several metal/ligand titrations were performed, but

in all of these experiments precipitation occurred during the

titration. Even with a metal/ligand ratio of 1 : 6, a slight

disturbance in the curve was found above pH = 8, referring to

the formation of precipitated material. Therefore, it was only

possible to collect suitable stability data in the pH region

below 8 (for the overall formation constants (b) (Nd(III)–TEA)

for the interaction of Nd(III) with TEA, see Table S3, ESI{)

v. Characteristics of the precursor solution. Based on previous

potentiometric measurements, the pH of the precursor solu-

tion used for gel formation was chosen to be 6.75. Assuming

that the results obtained in the previous sections can be used to

discuss the behaviour of mixtures, at a pH of 6.75 only the

Cu2+ has been complexed by TEA. Only less then 10% free

Cu2+ is still present in the solution. The species distribution

diagram of Nd(III)–TEA shows that almost no Nd(III) has

been coordinated. Ba(II) will not form complexes with TEA.

This means that the addition of triethanolamine only affects

the complexation of Cu(II) and prevents it from precipitation

as its acetate salt. This means that, under the experimental

conditions used (pH = 6.75) the monohydroxy species is the

main complex formed in the solution before the gelation takes

place and can act as a precursor for the gelation mechanism.

Different studies of sol–gels indicate that a polymerisation

network is built during the drying step (Fig. 1). This three-

dimensional network consists of a –O–M–O–M–O– network,

and the gel formation starts with a hydrolysis reaction. The

species distribution diagram (Fig. S2, ESI{) now shows that

the monohydroxy species Cu(TEA)(OH)+ is the main species

formed in the pH range 6–8 with a maximum at pH = 6.75.

It also appears that a stable gel is formed in the same region.

Therefore this monohydroxy Cu(II) complex can be considered

as the precursor which controls the start of the polymerisation

reactions. The pH adjustment of the precursor solution

will allow us to manage the percentage hydrolysis of the

mononuclear species Cu(TEA)2+ and to influence the forma-

tion of the three-dimensional network in an efficient way.

Furthermore Fig. S2 (ESI{) shows the results of the influence

of increasing metal/ligand ratio to the percentage of the

monohydroxy Cu(II) species; it shows that the monohydroxo

complex reaches a maximum point at 1 : 3 and then starts

to decrease.

3.1.2. Nitrate citrate system.

i.Protonation constants of citric acid. In preparation for the

quantitative determination of the stability constants of the

metal ions with citric acid (CA), very precise protonation

constants were obtained. The titration curve of citric acid

has an inflection point at a = 3, which corresponds to the

completion of the neutralisation of the carboxylic acid groups.

The protonation constants for citric acid and the literature

values are listed in Table S4 (ESI{). Comparison of the two

sets of results for citric acid indicates good agreement for the

mono- through tri-protonated stepwise equilibrium constants.

ii. Cu(II) citrate complexes. The potentiometric equilibrium

curve for the 1 : 1 Cu2+/citric acid titration possesses a sharp

inflection at a = 4, matching the formation of the mononuclear

hydroxo species CuCA(OH). Two other titrations in a 1 : 2 and

1 : 4 molar ratio confirm the results of the first titration. This

leads to a chemical model based on a series mononuclear

species, with formation constants given in Table S5 (ESI{).

These results are in good agreement with earlier titrations,15

although dinuclear species are not present in the final chemical


function of pH when 4.07 6 1023 M TEA and 0.68 6 1023 M Nd(III)

(metal/ligand mol ratio 1 : 6) are equilibrated at 25 uC and I = 0.10 M

(KNO3).


model. It is apparent from the species distribution diagram

that the mononuclear Cu(II) species begin to form at a pH , 3.

It is interesting to note that a protonated metal complex was

found in the theoretically accepted model. As the mononuclear

complex is formed the three acidic protons of the carboxylic

groups are all deprotonated. With further increasing pH, the

hydroxo Cu(II) complex is being formed and is the dominating

species at higher pH (pH . 5). This hydrolysis mechanism

is described using the distribution curves of Cu(II) with citric

acid as the ligand. The importance of this hydroxo complex is

clear, and has the same precursor function as described in the

TEA system.

iii. Nd(III) citrate complexes. The complexation of Nd(III) with

citric acid was studied in a 1 : 1, 1 : 2 and 1 : 4 metal/ligand mol

ratio. The overall results are published in Table S6 (ESI{). The

metal ion Nd(III) forms mononuclear and polynuclear com-

plexes. The simulated distribution diagram shows a dominant

ML2 complex species in the pH region 3–8. With increasing

pH, polynuclear hydroxo complexes become more important.

iv. Ba(II) citrate complexes. Ba(II) gives rise to a weaker

complexation in comparison with Cu(II) and Nd(III). The

complexation of Ba(II) with citric acid will lead to mono-

nuclear complexes, with a stability constant of 2.55,15 resulting

in a decrease of the free Ba2+ concentration of 40%. Because of

the lower stability constant of the Ba(II) citric acid complexes,

related to Cu(II) and Nd(III) complexes, these phenomena will

be of less importance in the optimisation of stable precursor

solutions.

v. Characteristics of the precursor solution. Preliminary

experiments showed that with a pH of 6.75 a stable precursor

solution and a clear gel were formed. The species distribution

diagram simulated from the stability constant data shows that

two metal ions, Cu(II) and Nd(III), are completely coordinated,

while Ba(II), because of its low stability constant with the

ligand, is only partially coordinated. Due to the low solubility

of the Ba(II)-hydroxide, the pH of the precursor solution

cannot be raised too high. The species distribution diagram of

Cu2+ and Nd3+ (Fig. 4 and Fig. 5) demonstrate that the mono

hydroxo species CuCA(OH)22 and the mononuclear species

NdCA232 are the main species formed in the pH range 6–8. It

can be presumed that the role of the mono hydroxo Cu(II)

species is to manipulate the polymerisation reactions. The pH

fine-tuning of the precursor solution will then permit us to

handle the percentage hydrolysis of the mononuclear species

CuCA2 and to influence the formation of the three-dimen-

sional network in an efficient way, with respect to the

solubility of the different species in the precursor solution.

3.2 IR spectroscopy

The main requirement for the sol–gel approach is to achieve

a very high level of precursor homogeneity. The acetate–

triethanolamine and nitrate–citrate routes give rise to the

formation of certain metal complexes and ensure the homo-

geneous distribution of metal species in the sol–gel precursor.

The potentiometric data allow us to identify complex species

which can have a large influence on the gel mechanism. The

presence of hydroxo complex species facilitates the olation

mechanism during the heating of the precursor solution. The

slow evaporation of the solution initiates condensation

reactions finally resulting in a three-dimensional gelified

network. IR analysis of the solution during gelation can be

important to obtain further insight into the properties of the

materials obtained and the reactions that take place. The IR

spectrum of the precursor solution and gel (obtained by drying

the solution for 24 h at 60 uC), based on the acetate–TEA

route, is presented in Fig. S3 (ESI{). A few characteristic

regions can be discussed: 3750–2700, 1850–1200, 1150–825 and

800–515 cm21. All of these regions represent one or more

characteristic bands, referring to the different functional

groups that are present in the gel. The stretch vibrations of

–CH3, –CH2, –CH2OH, –COOH and –N give rise to some very

typical bands, which are indicated in Table S7 (ESI{).

A motivating correlation was observed when the gels were

allowed to form using different time stages. The positions of all

peaks in these different IR spectra of NBCO gels occur in the

same region. However, one major change between these IR

spectra was observed. If the gelation mechanism uses olation

to build up a three-dimensional network, then the formation of

the hydroxo bridge M–OH–M (Fig. 1) must lead to a change in


function of pH when 3.47 6 1023 M citric acid and 1.14 6 1023 M

Cu(II) are equilibrated at 25 uC and I = 0.10 M (KNO3).


function of pH when 3.42 6 1023 M citric acid and 1.15 6 1023 M

Nd(III) are equilibrated at 25 uC and I = 0.10 M (KNO3).


the polymeric OH band observed in the spectra between 3200

and 3400 cm21.16 Therefore, Fig. 6 compares fragments of IR

spectra of synthesised gels for the region of 4000–2400 cm21.

As seen, the intensity of the broad band attributable to the

O–H stretching is dependent on the synthesis time. During the

drying time of the solutions, the polymeric OH band becomes

more intense up to 5 h. Further increasing drying time does not

increase the peak intensity. This observed saturation effect is

presented in Fig. S4 (ESI{). The slight increase in the

intensities of the O–H absorption peaks in the IR spectra,

which also includes a concentration effect, suggests that

condensation in NBCO aqueous acetate–TEA solution occurs

through the olation mechanism in good agreement with our

stoichiometric information obtained by potentiometry.

3.3 Bulk and thin film synthesis

Using the results obtained from potentiometric analysis, 2

stable aqueous NBCO sol–gel precursor solutions were

synthesised. For the metal nitrate–citric acid-based precursor,

a 1 : 3 ratio was used and the pH was adjusted to 7. For the

metal acetate–TEA-based sol, a 1 : 3 ratio and pH 6.75

was used. When heated at 60 uC, the clear blue solution

will transform into a glass state gel with the metal ions

homogenously distributed in the three-dimensional gel net-

work which consists of metal complexes linked by weak ionic

bonding. The importance of having all of these metals

homogeneously distributed inside this network will be shown

in the following analysis part.

3.3.1 Thermal decomposition precursor.

i.Thermal decomposition of the nitrate–citrate gel network. The

TGA–DTA measurement of the metal nitrate–citrate-based

sol–gel route is shown in Fig. 7. The atmosphere used is

identical to the one used in the sintering procedure of the

amorphous gel coatings, containing a reduced partial oxygen

pressure of 1% O2/Ar. The heating rate was 5 uC min21.

Knowledge about the thermal decomposition behaviour of the

gel network allowed us to determine the appropriate thermal

procedure to obtain a smooth, crack-free thin film, avoiding

particularly violent exothermic reactions coupled with a large

weight loss.

In order to obtain an acceptable morphology of the thin

films prepared by this nitrate–citrate sol–gel procedure, it is

thus important to suppress these strong exothermic reactions

by applying a slow heating rate of 1 uC min21.

Three main areas can be distinguished in the thermogram

shown in Fig. 7. The first broad endotherm can be correlated

to the evaporation and the release of gel network water just

below 200 uC. The exotherm at 220 uC coupled to a large loss

in mass can be ascribed to an auto combustion reaction due to

the presence of nitrate and citrate groups. This redox reaction

is an auto-catalytic propagation reaction, which is accom-

panied by the release of several gasses, mainly NOx, H2O and

COx. These factors may cause heavy foaming of the gel during

the combustion phase and will obviously have detrimental

effects on the morphology of the final layer.17 The second

exotherm at 450 uC corresponds to the decomposition of the

intermediary products, itaconic acid and metal citrate, formed

during the decomposition of the metal citrate complex. This

final combustion of the network mainly involves the release of

a relatively large quantity of CO and CO2 gasses. After this

reaction, the remaining species consist of oxides of the Nd, Cu

Fig. 6 IR fragments of the NBCO–TEA–acetate gels.

Fig. 7 TGA–DTA measurement of the metal nitrate–citrate-based

sol–gel route


and Ba metal ions. These metal oxides then convert to the

desired NBCO phase at the sintering temperature of 940 uC.

ii. Thermal decomposition of the metal acetate–TEA gel

network. The TGA–DTA spectrum of the metal acetate–

TEA sol–gel system is shown in Fig. 8 (heating rate 5 uC min21,

1% O2/Ar atmosphere). Again, the broad endotherm beneath

200 uC corresponds to the evaporation and release of gel

network water. Above 200 uC, three exothermic peaks are

observed, corresponding to the stepwise decomposition of the

gel network at 220, 350 and 400 uC. After the last decom-

position step at 400 uC, the remaining species are formally

metal oxides of Nd, Cu and Ba, and no further weight loss

is noticed.

This decomposition behaviour of the gel network is

completely different to the thermal decomposition behaviour

of the separate metal acetates given in Fig. 9. Most remarkable

is the decomposition from the metal complexes to the

respective oxides, which now takes place at much lower

temperatures in comparison to the separate metal acetates

where the formation of the oxides takes place at 300 uC for Cu

acetate, 500 uC for Ba acetate and 650 uC for Nd acetate.

Similar behaviour was found for the separate metal nitrates in

comparison to the metal nitrates–citric acid gel network. These

results are in correlation with the IR spectra, where no peaks

related to free metal salts inside the gel network were visible.

This high reactivity of the amorphous precursor is probably

caused by the very homogenous distribution of the metal ions

inside the gel network and can be a real advantage for sol–gel

systems because phase formation can now take place at much

lower temperatures.

In comparison with the decomposition behaviour of the

nitrate–citrate gel, the main difference for this route is the

absence of an uncontrollable auto combustion reaction. Also,

the loss in weight is spread over the whole range of the three

exothermic reactions, thus smoothing out the evolution of

gaseous compounds during calcination. Therefore, using this

sol–gel procedure, one can apply a higher heating rate of

10 uC min21 during thermal treatment of the thin film.

3.3.2 Morphology. Fig. 10 shows the SEM images of the

NBCO layer deposited from the nitrate–citrate precursor

solution. It can be clearly seen that the layer obtained has a

discontinuous and very porous surface morphology. This is

undoubtedly related to the vigorous auto combustion reaction

as described Section 3.3.1. The autocatalytic nature of this

reaction renders it uncontrollable, and coupled with the release

of high quantities of gasses this will result in the creation of

pores during calcination.

These observations lead us to conclude that the nitrate–

citrate sol–gel method is not the appropriate sol–gel route to

deposit thick (.500 nm) superconductive layers with high Jc

performance. Nevertheless, earlier investigations proved that

this nitrate–citrate method could be used successfully for the

deposition of thin CeO2 buffer layers as long as the deposited

layer does not exceed the critical thickness of approximately

50 nm.17 Also, the preparation of very fine powders exhibiting

nanoscale grain size and good superconducting properties was

demonstrated using this sol–gel precursor system.

Fig. 11 shows the SEM micrographs of the NBCO layer

deposited from the acetate–TEA precursor. In stark contrast

to the nitrate–citrate method, thick homogeneous and crack-

free layers are obtained here. This is to be correlated with the

absence of an aggressive auto combustion reaction during the

thermal decomposition of the gel network. As mentioned

earlier, we also suppressed decomposition reactions with exten-

sive mass loss by applying a slow heating rate of 1 uC min21

up to 500 uC during thermal treatment of the amorphous gel

and by careful selection of ligand molecules.

3.3.3 Microstructure. Fig. 12 shows the h–2h XRD spectrum

of the sintered and annealed NBCO layer dip-coated on a (l00)

polished STO substrate. From the intensity of the different

(00l) peak reflections, a strong c-axis orientation of the layer

can be observed. No peaks that could be attributed to other

orientations of the NBCO phase or remaining BaCuO2 phase

were observed.

However, when a NBCO thin film is synthesised using a

precursor solution without applying the optimum parameters

derived from potentiometric analysis (in this case a pH value

of 5 was used instead of 6.5), impurity phases are formed

during phase conversion to NBCO, as can be seen in Fig. 13.

This phenomenon emphasises the importance of good metal

complex formation inside the precursor solution and thus the

necessity for stipulating the optimum complexation para-

meters using potentiometric titration curves. If the precursor

solution contains free metal ions, precipitation and impurity

phase formation is encouraged during synthesis.

Fig. 14 shows the (103) pole figure for this layer and

exemplifies a highly in-plane textured NBCO phase. This is

Fig. 8 TGA–DTA spectrum of the metal acetate–TEA sol–gel system

Fig. 9 Thermal decomposition behaviour of the separate metal

acetates


Fig. 10 (a), (b) and (c): SEM images of the NBCO layer deposited from the nitrate–citrate precursor solution at different magnifications

Fig. 11 (a), (b) and (c): SEM micrographs of the NBCO layer deposited from the acetate–TEA precursor at different magnifications


confirmed by the very low degree of mis-orientation angles

(2.67u) calculated from the FWHM, given in the accompanied

phi-scan in Fig. S5 (ESI{).

It is well known that the good lattice match with the STO

single crystal substrate induces the textured growth of the

NBCO phase. In vacuum techniques this is thought to occur in

an atomic layer-by-layer growth. However, using a sol–gel

system, one starts from a bulky amorphous layer which is

already present on the substrate and which has to be converted

to the final crystalline phase afterwards. To induce texture in

this layer, this means that the initial nucleation of the NBCO

phase has to take place at the interface between the substrate

and the amorphous layer.

This assumption was corroborated by HR-TEM measure-

ments at the interface (see Fig. 15a) where the orientation of

the HTSC layer starts immediately at the interface. In fact, this

study demonstrates that the quality of the biaxial textures

obtained in films deposited by an aqueous sol–gel method are

at least comparable to those obtained in vacuum-based

deposition technologies.

The diffraction pattern given in Fig. 15b corresponding to

this interface proves that the correct growth of both the cubic

single crystal substrate and the orthorhombic NBCO layer on

top occurs.

This nucleation mechanism at the interface is also confirmed

by the TEM picture given in Fig. 16, proving that the growth

of the NBCO layer follows the defects present at the substrate

surface for thousands of packed unit cell layers.

It can therefore be safely concluded that the textured

growth of the superconductor coating probably starts with

nucleation of the material at the interface followed by the

development of a crystallization front proceeding through the

amorphous gel.

3.3.4 Superconductivity. First, NBCO bulk material was

synthesised for Tc analysis using both sol–gel routes

described in Section 2.2. The resistivity measurements in

Fig. 12 XRD of NBCO bulk material with random orientation

(full line) and of a NBCO layer dip-coated on a (l00) polished STO

substrate with c-axis orientation (dotted line)

Fig. 13 XRD of NBCO thin film derived from a non-optimised (by

potentiometric titration) precursor solution.

Fig. 15 (a): HR-TEM picture of NBCO–STO interface, (b): diffraction pattern of interface.

Fig. 14 (103) Pole figure of NBCO thin film


liquid nitrogen given in Fig. 17 clearly show for both

samples a sharp superconducting transition of the NBCO

material at 94 K.

For the thin film deposition, we applied the same synthesis

conditions as used for the bulk material. The film synthesised

from the metal nitrate–citric acid precursor, however, could

not be measured because of the porous, discontinuous

morphology as described in Section 3.3.2. Fig. 18 shows the

resistivity curve of the acetate–TEA-based layer.

A broad transition at 89 K is visible and needs to be looked

into before technological optimisation is possible. The further

study and optimisation of superconducting properties of the

thin films will be the subject of a future paper.

Conclusions

We prepared NBCO thin films of 1 micron thickness by dip-

coating aqueous sol–gel solutions on polished (100) STO single

crystal substrates. Two aqueous inorganic sol–gel routes were

explored: a metal nitrate–citric acid-based solution and a metal

acetate–triethanolamine acid-based solution.

Potentiometric data were employed to calculate formation

constants of complexes with the metal ions Ba2+, Cu2+

and Nd3+. This stability data and the corresponding distribu-

tion curves gave a good indication of the different species

present in the precursor solutions. It also facilitates the

preparations of the solutions. The distribution curves also

showed the pH region where monohydroxy species were

formed. These species can be of great importance during the

gelation because they facilitate the start of the polymerisation

reactions.

IR spectra were used to prove the olation mechanism. The

variation in peak intensities of the polymeric OH bands gave

a good indication of the existence of these polymerisation

reactions.

The thermal decomposition behaviour of both solutions

was investigated and correlated to the final morphology of

the deposited layers. By adjusting the different parameters

during thermal treatment, we were able to gain control over

the layer morphology. Starting from aqueous-based solutions

using metal acetates and triethanolamine (as a complexing

agent), 1000 nm thick highly textured NBCO films were

deposited with an in-plane mis-orientation angle of less than

4u. The synthesised NBCO material showed a superconducting

transition temperature of 94 K for bulk and 89 K for thin

films. The importance of a good metal complex formation

inside the precursor solution and thus the necessity for

stipulating the optimum complexation parameters using

potentiometric titration curves was shown by XRD phase

analysis.

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Fig. 18 Resistivity measurement in liquid nitrogen of NBCO thin

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characterisation of the sol?gel process in the superconducting ndba2cu3o7?y system

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