Optimisation of a sol-gel synthesis route for the preparation of MgF2 particles for a large scale coating process

Article abstract of DOI:10.1039/c5dt02196k

A synthesis route for the preparation of optically transparent magnesium fluoride sols using magnesium acetate tetrahydrate as precursor is described. The obtained magnesium fluoride sols are stable for several months and can be applied for antireflective coatings on glass substrates. Reaction parameters in the course of sol synthesis are described in detail. Thus, properties of the precursor materials play a crucial role in the formation of the desired magnesium fluoride nanoparticles, this is drying the precursor has to be performed under defined mild conditions, re-solvation of the dried precursor has to be avoided and addition of water to the final sol-system has to be controlled strictly. Important properties of the magnesium fluoride sols like viscosity, particle size distribution, and structural information are presented as well.

Full text of DOI:10.1039/c5dt02196k

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Optimisation of a sol–gel synthesis route for the
preparation of MgF2 particles for a large scale
coating process
Cite this: DOI: 10.1039/c5dt02196k
K. Scheurell,a J. Noack,a R. König,a J. Hegmann,b R. Jahn,b Th. Hofmann,b
P. Löbmann,b B. Lintner,c P. Garcia-Juan,d J. Eicherd and E. Kemnitz*a
A synthesis route for the preparation of optically transparent magnesium fluoride sols using magnesium
acetate tetrahydrate as precursor is described. The obtained magnesium fluoride sols are stable for
several months and can be applied for antireflective coatings on glass substrates. Reaction parameters in
the course of sol synthesis are described in detail. Thus, properties of the precursor materials play a
crucial role in the formation of the desired magnesium fluoride nanoparticles, this is drying the precursor
has to be performed under defined mild conditions, re-solvation of the dried precursor has to be avoided
and addition of water to the final sol-system has to be controlled strictly. Important properties of the
magnesium fluoride sols like viscosity, particle size distribution, and structural information are presented
as well.
Received 9th June 2015,
Accepted 17th September 2015
DOI: 10.1039/c5dt02196k
www.rsc.org/dalton
Hence, a large scale coating process is strongly limited.
1 Introduction
Similar to the trifluoroacetic acid method Bass et al.8 describe
In contrast to the commonly used CVD, PVD and ALD techno- a thermally activated chemical solution deposition as easy
logies, sol–gel processing is a considerably less expensive method to tune the refractive index. The resulting materials
method to produce antireflective coatings on optical sub- demonstrate a range of tuneable compositions with varying
strates.1,2 Using different techniques functional coatings con- refractive indices (1.08–1.2). These data are very surprising
sisting of SiO2 were developed and up-scaled for the because MgF2 and MgOi(OH)j are formed during the synthesis
production of self-cleaning windows, lenses, long-life phos- but MgO is known for his high refractive index (1.73). Further-
phor lamps etc. For an optimum performance of the antireflec- more, the authors don’t give any information about the mecha-
tive λ/4 films the porosity of the coated material is very nical properties of such extreme highly porous materials. A
important.2 Beside SiO2 (refractive index = 1.5) MgF2 is an very simple method to synthesise MgF2 nanoparticles is given
attractive material due to its excellent optical properties (refrac- by Nandiyanto et al.9 They used a liquid-phase method in
tive index = 1.38).15 In order to create a suitably low refractive aqueous solution starting from MgCl2 and NH4F. Control of
index of 1.23 for the entire AR-layer, the porosity can be the particle size was accomplished only by changing the com-
limited and hence, an enhanced thermal stability and better position of the reactants. Nanocrystalline MgF2 particles in
hardness is expected.3 MgF2 can be synthesised by the so suspensions can easily be synthesised by a polyol-mediated
called trifluoroacetic acid method developed by Fujihara et al. approach.10 Unfortunately, such suspensions are not suitable
and Joosten et al.4–7 The disadvantage of this method is that for the coating of optical glasses, because the minimum vis-
even after treatment at 973 K impurities (OH and COO−) were cosity of a 5 wt% MgF2 suspension in water is in a region of
experimentally proven in the MgF2 solids5 and a lot of toxic ∼100 mPa s. Several patents reflect the great interest on anti-
gaseous products (CF3COF, COF2 and HF) were formed. reflective coatings consisting of MgF2.11,12 There are reports on
the synthesis of MgF2 particles with an amorphous silicon
oxide-based binder for the preparation of optical thin films on
aDepartment of Chemistry, Humboldt-Universität zu Berlin, Brook-Taylor-Str. 2,
12489 Berlin, Germany. E-mail: erhard.kemnitz@chemie.hu-berlin.de;
Tel: +49 30 20937555
suitable substrates.11 Recently, a patent was published about
the chemical solution process to deposit porous coatings con-
taining magnesium fluoride and/or oxidefluoride together
with a surfactant porogen.12 A comprehensive discussion of
different strategies to obtain antireflective properties is given
in.2,13 The processing of the formation of the antireflective
bFraunhofer-Institut für Silicatforschung ISC, Neunerplatz 2, 97082 Würzburg,
Germany
cPRINZ OPTICS GmbH, Simmerner Strasse 7, 55442 Stromberg, Germany
dSOLVAY FLUOR, Hans-Böckler-Allee 20, 30173 Hannover, Germany
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films is described such as porous λ/4 layers, multilayer fluorination, 3 to 5 mol% Al(OiPr)3 or TMOS and 1 mol% tri-
interference-type films and index-gradient like materials. fluoroacetic acid were added to the MgF2 sol.
Using trifluoroacetic acid as fluorination agent, MgF2 and
2.2 Material characterisation
quaternary Mg–F–Si–O sol–gel particles were prepared for coatings
on large area glass substrates and solar collectors.14,15 The
optical properties of these coatings are very impressive but
here, as mentioned above, a large scale industrial process is
strongly limited.
The kinematic viscosity of the sols was measured with a capil-
lary viscometer (Schott AVS 400) at 25 °C. The capillary con-
stant was 0.02908.
The X-ray powder diffractograms of all samples were
recorded on a XRD 3003 TT diffractometer (Rich. Seifert & Co.,
Freiberg) using Cu-Kα-radiation (λ = 1.542 Å).
Most of the above discussed MgF2 synthesis methods start
from water containing educts (Mg(OAc)2·4H2O, HF(aqueous) etc.)
or are performed in water containing solutions. It is known
that water promotes the gelation in the sol–gel process and
during the thermal treatment of the MgF2-coatings additional
oxide containing byproducts (MgO or MgOxFy) might be
formed. For that purpose a non-aqueous flurolytic sol–gel
method has been developed by our group for the preparation
of transparent MgF2 sols.16,17 Thereby the crucial point is the
exclusion of water (water-free educts and anhydrous HF). This
synthesis route leads to MgF2 particles with a particle size in
the range of 5–10 nm and the coatings, prepared from these
sols, are very homogeneous and exhibit a low refractive index
∼1.32.18,19 Unfortunately, the synthesis route starting from
Mg(OMe)2 is not recommended for a large scale industrial syn-
thesis and coating process, because in the first step of the reac-
tion between magnesium and methanol a lot of hydrogen is
released and furthermore methanol is a toxic solvent. On the
other hand, all attempts to start from commercially available
magnesium ethoxide failed since this is insoluble in methanol
The dried Mg(OAc)2 was studied using Differential Thermal
Analysis/Thermogravimetry (DTA-TG) measurements in
a
Netzsch STA 409C/CD thermobalance (heating rate 10 K min−1
under argon up to 700 °C).
1
19
The H and
F NMR spectra of the sols were carried out
using a Bruker Avance II 300 spectrometer (Lamor frequencies
300.13 MHz for 1H and 282.4 MHz for 19F). The 1H and 19F iso-
tropic chemical shifts are given with respect to the C6D6 and
CFCl3 standards.
Dynamic light scattering experiments (DLS) were performed
using a Zetasizer Nano ZS (Malvern Instruments, Worcester-
shire, UK) in disposable PMMA cuvettes. The hydrodynamic
diameters were calculated from the correlation functions using
the Malvern Nanosizer Software.
3 Results and discussion
or in ethanol and hence, no transparent sols are available due The classical non-aqueous fluorolytic sol–gel method deve-
to the deposition of a protective MgF2-layer onto the sus- loped in our group for the preparation of transparent MgF2 sols
pended solid Mg(OEt)2 precursor particles. The alternative starts with the dissolution of metallic Mg in dry methanol
preparation method with MgCl2 as educt19 is very easy to under Schlenk conditions.18,20 In a second step the resulting
perform but suffers from the stoichiometric formation of HCl Mg(OMe)2 is reacted with HF dissolved in MeOH to form a
from MgCl2 as result of the reaction with HF. Since HCl is very clear MgF2 sol (conc. 0.25–0.3 mol L−1). For a large scale appli-
corrosive,
recommended.
a
practical application of such sols is not cation this synthesis procedure is inappropriate because of the
formation of large quantities of hydrogen in the first synthesis
In spite of these drawbacks we were interested in exploring step. Furthermore, methanol as solvent is toxic and should be
a non-toxic, non-corrosive and low-cost precursor for a sol–gel avoided in an industrial process. Hence, an alternative prepa-
synthesis of pure MgF2 particles, which can be used for a large ration method based on a non-toxic solvent, such as EtOH, is
scale antireflective coating process.
strongly required. Commercially available alkoxides, such as
Mg(OMe)2 and Mg(OEt)2 are insoluble in ethanol and other
alcohols. It is therefore not possible to prepare transparent
MgF2 sols in higher concentration via the reaction of these
alkoxides with HF. We speculate, if the precursor is not dissolved
the desired product MgF2 is formed preferentially at the
2 Experimental part
2.1 Sample preparation
Route1. For the acetate synthesis route, the precursor surface of the solid alkoxide resulting in the formation of
Mg(OAc)2 was obtained by drying the Mg(OAc)2·4H2O (Aldrich larger aggregates that cannot be broken off afterwards. Even
99.8%) at 210 °C for 6 h in a vented furnace. The MgF2 sols limitation of the reaction due to the formation of a protective
were prepared by a stoichiometric reaction of the dried pre- MgF2-layer on solid Mg(OR)2 particles has to be taken into
cursor Mg(OAc)2 (56 g, 0.4 mol) suspended in 1 L ethanol account. Another source for complication is that the commer-
(Roth 99.8%) with HF (anhydrous, Solvay Fluor GmbH, 15–20 cial alkoxides are partly hydrolysed by reaction with moist air,
molar in ethanol, Roth 99.8%) under vigorous stirring. To the and hence, HF under these reaction conditions (nonprotic
resulting sol 10 mol% trifluoroacetic acid was added in order to organic solvent) is not able to break off Mg–O–Mg moieties in
suppress agglomeration of the reactive MgF2 particles formed.
the resulting Mg(OR)x(OH)y phases.
Route 2. In an alternative synthesis route, Mg(OAc)2·4H2O
However, Mg(OAc)2·4H2O is a commercially available, less
was dried under mild conditions (100 °C, vacuum). After the expensive solid and should be suitable as starting material for
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the sol–gel synthesis. The solubility of Mg(OAc)2·4H2O in EtOH
is excellent and the reaction with HF to MgF2 is a very fast
process. In order to obtain clear and transparent MgF2 sols
starting from Mg(OAc)2·4H2O as educt, some important syn-
thesis parameters have to be respected and abided. In general
these synthesis parameters are:
- Precise dehydration of the Mg(OAc)2·4H2O precursor
- Resolvation of Mg(OAc)2·4H2O precursor in ethanol before
fluorination
- Suppress water contamination of the sols
3.1 Dehydration of the Mg(OAc)2·4H2O precursor
The water content in the magnesium acetate tetrahydrate pre-
cursor causes a relatively rapid gelation of the sols meaning
long time stability of such MgF2-sols is inacceptable. In Fig. 1
(left) a clear and transparent MgF2 sol is shown made from a
pre-dried, nearly water free Mg(OAc)2. After addition of water
(ca. 7 vol%) a rapid gelation within a few seconds was observed
(Fig. 1 right). This is also known from the classical sol–gel
reactions in oxidic systems such as SiO2. Milea et al. observed
an increasing gelation with raising water contents.21 An inter-
action of the initially formed silica particles with water leads
to the formation of a gel-network into which the solvent
(alcohol) is embedded. In the course of the nonaqueous fluori-
nation process used here and starting from dried Mg(OAc)2
and anhydrous HF the formation of the desired product MgF2
is very fast and no hydrolysis process occurs during the syn-
thesis. Nevertheless, by addition of water an increase of the vis-
cosity resulting finally into gelation can be observed.
Obviously, water (e.g. from the Mg(OAc)2·4H2O) binds to the
MgF2 particle surface by adsorption resulting in a very limited
surface hydroxylation. These surface OH-groups might cause
condensation reactions, and hence, an interaction (agglomera-
tion) between the particles becomes possible finally leading to
the formation of a particle network and gelation starts.
Fig. 2 Viscosities of the MgF2 sols depending on the solution age.
substrates over a longer period. Consequently, in order to sup-
press gelation tendency and to improve the long term stability
of the resulting MgF2 sols water has to kept out of the sol-
system as completely as possible. Hence, the precursor
Mg(OAc)2·4H2O was dehydrated before the reaction with HF.
However, dehydration at 210 °C in air generates a product
which is only partially soluble in EtOH and other alcohols. The
XRD patterns of the dried Mg(OAc)2 show reflections of a
phase mixture consisting of α-Mg(OAc)2 (PDF 14-802) and a
minor “unknown phase” (Fig. 3).
Unfortunately, it was not at all possible to process clear
transparent MgF2 sols from these pre-dried acetate precursors.
The same effect was observed when using commercially avail-
able Mg(OAc)2 as starting material; under no circumstances
clear MgF2 sols were obtained. We speculated that the
unknown phase detected in the powder XRD might be the one
that does not dissolve, and hence, remains unconverted thus
being the origin for non-transparent, opaque sols. Therefore,
much effort had been spent to synthesise and characterise this
unknown phase. Finally, this unknown phase was be identified
as a basic magnesium acetate, Mg5(µ3-OH)2(OAc)8·1.19H2O.22
This compound (insoluble in water and alcohols) exhibits an
The viscosities of different MgF2 sols depending on their
solution age and water content are given in Fig. 2. It clearly
demonstrates a more pronounced viscosity increase in case of
the precursor Mg(OAc)2·4H2O. Because of the high viscosity,
this sol is inappropriate for a dip-coating procedure on glass
Fig. 1 Photographs of a MgF2 sol (left) and MgF2 gel with air bubbles (right).
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tures and so the reaction with HF during the sol–gel synthesis
process is limited. Therefore, for a successful synthesis of
transparent MgF2 sols it is very important to reduce the time
between dissolution of the precursor material, Mg(OAc)2, and
the following reaction with HF according the fluorolytic sol–gel
route in order to suppress undesired re-solvation processes.
3.3 Suppress water contamination of the sols
The reaction of pre-dried Mg(OAc)2 with HF in ethanol leads to
the formation of acetic acid which can consecutively react with
ethanol forming the corresponding ethyl acetate and equi-
molar amounts of water (eqn (1) and (2)).
Fig. 3 X-ray diffraction patterns of dried Mg(OAc)2 (a) and the basic
magnesium acetate Mg5(µ3-OH)2(OAc)8·1.19H2O (b).
MgðOOCCH3Þ2 þ 2HF ! MgF2 þ 2CH3COOH
CH3COOH þ C2H5OH ! CH3COOC2H5 þ H2O
ð1Þ
ð2Þ
As this reaction progresses the water content in the system
increases continuously and, as described above (point 3.1),
causes gelation and/or agglomeration within a few days.
1H-NMR investigations demonstrate the time dependence of
this undesired consecutive reaction (Fig. 5).
After one week the ratio of acetic acid to ethyl acetate is
97 : 3 but after four weeks it is already 84 : 16. The question
arises, how much water can be tolerated before gelation
occurs. Our investigations on sols obtained from water-free
magnesium acetate as precursor showed that a water content
above ca. 7 vol% leads to an immediate gelation (see chapter
3.1), above 5 vol% gelation starts after 2 days, above 2.5 vol%
gelation occurs after 2 weeks and at ca. 0.5 vol% gelation
needs about 3 months. Any water content below 0.1 vol% can
be tolerated, meaning the sols are stable against gelation over
open cage structure with channels along the a, b, and c axes
which probably is the reason for its insolubility in water and
alcohols. Since this basic acetate phase is formed in the
course of drying magnesium acetate tetrahydrate, a classical
thermal dehydration process is not recommended for the
fluorolytic sol–gel process when clear transparent MgF2 sols
are desired. However, soluble Mg(OAc)2 can be obtained by
extracting the water under mild conditions (100 °C, vacuum).
As can be seen from Fig. 4 the mass loss up to 300 °C is only
0.16%, that is the magnesium acetate is nearly water free. The
formed product is X-ray amorphous, soluble in alcohols, and
consequently, yields clear MgF2 sols after fluorination.
3.2 Resolvation of Mg(OAc)2·4H2O precursor in the ethanol
When dissolving the precursors Mg(OAc)2·4H2O or Mg(OAc)2 several months (>3). In spite of these facts it is very amazing
in EtOH, already after 2 hours solvated magnesium acetates that sol–gel syntheses for the preparation of clear and low
Mg(OAc)2(H2O)3(EtOH) or Mg3(OAc)6(EtOH)2·2EtOH are viscous MgF2 sols starting from the magnesium acetate tetra-
formed.23 Unfortunately, these solvates are hardly soluble hydrate or syntheses in water containing solutions with
because of their oligomeric, one- or three dimensional struc- aqueous HF are described in the literature.11,14,15,24 Of course,
Fig. 4 Thermogravimetric analysis (TG) and differential thermal analysis (DTA) of dried Mg(OAc)2 (100 °C, vacuum).
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grow crystals from these unknown species for the identifi-
cation by single crystal X-ray analysis. The formation of such
partially fluorinated intermediates is supported by the classi-
cal fluorolytic sol–gel reaction which starts with magnesium
methoxide, Mg(OMe)2, as precursor. There, the intermediate
formation of dicubane-like intermediates was evidenced by
19F-NMR spectroscopy of the sols and by powder XRD investi-
gations of the dried xerogels as well as single crystal determi-
nation.20 When the HF to Mg(OAc)2 ratio is further increased
(1.6 and 1.7) the 19F-NMR signal of MgF2 in rutile structure
(−198 ppm) becomes visible. The relatively high signal width
(∼1–2 kHz) indicates the formation of MgF2 nanoparticles.
During agglomeration and/or gelation of the MgF2 particles
with HF/Mg ratios of 1.8 and higher the mobility of the fluor-
ine species is suppressed (obviously strongly adsorbed at the
NPs) and the signals in the 19F-NMR spectra become very
broad and are almost invisible.
Fig. 5 1H-NMR spectra of the CH3-groups of the resulting ethyl acetate
in comparison to the acetic acid depending on the aging time of the
MgF2 sols.
Directly after the fluorination of the Mg(OAc)2 the formed
MgF2 sols are turbid because of the interaction and agglomera-
tion of the very active MgF2 particles. After an aging time of
several hours, sometimes days or even weeks (this depends on
the concentration and the quantity of the sol) the sols clear
up, thus getting totally transparent. Fig. 7 demonstrates the
change in particle size distribution depending on the sol age
determined by dynamic light scattering (DLS).
In some cases in the liquid NMR spectra of freshly syn-
thesised MgF2 sols an additional small signal at −180 ppm
can be detected. There are several strong hints that this signal
originates from immobilised HF that is adsorbed on the par-
ticles surface. Although just traces of this unconverted HF are
bound via hydrogen bridges, probably this HF and/or the
corresponding unconverted M-OAc-sites are the origin for
agglomeration by bridging surfaces of the particles. If, during
the aging process, the unconverted and bridging HF reacts
with the remaining acetate sites, these bridges break off, and
hence, the sol clears up.
all these publications do not reflect details about viscosities of
sols after a prolonged life time. However, our results prove that
the MgF2 sols in alcoholic solvents tolerate only small contents
of water ∼0.1–0.2 vol%. Thus, MgF2 sols described in the
above mentioned publications shouldn’t be stable for a long
time. At water concentrations above ca. 0.1 vol%, gelation is a
matter of fact and it is impossible to coat any substrates with
MgF2. Consequently, any carboxylate based magnesium pre-
cursor will form the respective carboxylic acid in the reaction
with HF, and thus, potentially creates the basis for water for-
mation as a result of esterification.
To better understand the reaction progress, NMR-spectra of
reaction mixtures with increasing HF to Mg(OAc)2 rations were
taken (see Fig. 6). At very low fluorine to magnesium ratios
(<0.5 eq.) Mg(OAc)2 reacts initially to partially fluorinated
intermediates, MgFx(OAc)2−x. These acetate fluorides are
soluble in EtOH to be seen from several narrow signals
between −170 and −190 ppm (cf. Fig. 6). At this stage no MgF2
can be detected. Unfortunately, so far it was not possible to
Consequently the clearing up process of freshly prepared
sols can be significantly accelerated just by addition of suitable
dissolved alkoxides such as TEOS/TMOS (a) or Al(OiPr)3 (b).
In this way the formerly adsorbed HF can react, and thus,
weak agglomerates break down. The same effect can be
achieved by addition of an understoichiometric amount of HF
to the Mg(OAc)2 (c).
(a) In case of TMOS as reactant, in addition to the broad
signal of the MgF2 nanoparticles at −198 ppm several signals
in the region between −140 to −160 ppm appear (Fig. 8).
These very small signals originate from dissolved SiFx(OR)y
(R = OMe) species formed by the reaction of TMOS with
adsorbed/unreacted HF. The special impact of the alkoxides
(e.g. TMOS) is still not fully understood, but evidently, the
interaction between TMOS and the fluoride on the MgF2 par-
ticle surface leads to a deagglomeration of aggregates formerly
formed. Within a few hours such sols clear up, being transpar-
ent, low viscous and stable over several months.
Fig. 6 19F-NMR spectra of MgF2 sols with HF to Mg(OAc)2 ratios 0.5,
1.6, 1.7, 1.8, and 1.9.
(b) Fig. 9 represents the size distribution of the hydro-
dynamic diameters of MgF2 nanoparticles after addition of
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Fig. 7 Particle size distribution (DLS) depending on the sol age.
of particles with 1000 nm. This effect is represented in Fig. 9
(right), size distribution depending on the volume of the par-
ticles. Here the majority of the particles with 10 nm is clearly
demonstrated. Such
a particle size distribution can be
obtained after addition of the alkoxides Al(OiPr)3 or TEOS/
TMOS shortly after the fluorination.
(c) In the classical oxidic sol–gel chemistry, usually immedi-
ately after the hydrolysis and condensation process clear sols
are formed under appropriate conditions. With time and as a
result of Ostwald ripening the particles can grow to larger
domains and the sols become turbid or gelation occurs. In
contrast to that, in the course of the fluorolytic sol–gel syn-
thesis the fresh sols usually are not clear but clear up after
several hours or even several days and parallel the viscosity
decreases. Finally, optically transparent, clear MgF2 sols are
formed. Fig. 10 displays the 19F-NMR spectra of a fresh and a
3 days aged understoichiometric (MgF1.8(OAc)0.2) sol (HF/Mg-
ratio 1.8). The very broad signal of the fresh sol indicates the
presence of larger agglomerates which disaggregate after
around 3 days (see the smaller 19F-NMR signal in Fig. 10).
Expectedly, sols formed from understochiometric HF/
Mg(OAc)2 rations clear up without addition of any additional
alkoxide because no excessive unreacted HF is present at the
MgF2 particle surface. Alternatively, the time of this clearing
process can be reduced by a treatment of freshly prepared sols
in an autoclave as discussed by Murata et al.24 However, for a
Fig. 8 19F-NMR spectra of a MgF2 sol stabilised with TMOS.
Al(OiPr)3 measured by dynamic light scattering (DLS). A distri-
bution of two size classes with hydrodynamic diameters
around 10 and 1000 nm was obtained (Fig. 9, left). It is known
from Rayleigh’s approximation that the intensity of the scatter-
ing of particles is proportional to their diameter with a coeffi-
cient of I = d6. So, the intensity of larger particles seems to be
higher than those of smaller particles. In reality it can be
ascertained that the number of smaller particles with a hydro-
dynamic diameter of 10 nm is much larger than the number
Fig. 9 Hydrodynamic diameters of particles in MgF2 sols; size distribution by intensity (left) and size distribution by volume (right).
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Fig. 11 MgF2 sol stabilised by Al(OiPr)3 after 2 days of fluorination pre-
pared in a 10 L reactor.
Fig. 10 19F-NMR spectra of a MgF1.8(OAc)0.2 sol fresh prepared and
after an aging time of 3 days.
Acknowledgements
This project was funded by the German Federal Ministry of
Economics and Technology (grant 0329800, synonym: TRex).
The authors would like to thank Dr Thoralf Krahl for the large
scale preparation of the MgF2 sols (10 L). S. Bässler is kindly
acknowledged for the DTA-TG measurement.
large scale preparation of MgF2 sols for antireflective coatings
this procedure is inappropriate.
4 Conclusions
These comprehensive investigations of the fluorolytic sol gel-
synthesis conditions for the preparation of MgF2 sols as anti-
reflective coating material starting from magnesium acetate as
precursor demonstrate that commercial magnesium acetate
tetrahydrate is an appropriate starting material. However,
several important pre-requisites with significant impact on the
performance of the final sol should be addressed:
Notes and references
1 M. A. Aegerter, R. Almeida, A. Soutar, K. Tadanaga,
H. Yang and T. Watanabe, J. Sol-Gel Sci. Technol., 2008,
47, 203.
(i) The precursor must be dried under well controlled mild
conditions (100 °C vacuum) in order to prevent the formation
of a basic magnesium acetate that was just recently identified.
To our experience, the industrial drying process does not
prevent the formation of this unwanted phase, hence, com-
mercially available dried magnesium acetate can not be used
for the preparation of clear MgF2 sols. However, optimisation
of the industrial drying process should overcome this present
restriction.
(ii) The time between dissolving the precursor and reaction
with HF should be as short as possible in order to prevent re-
solvation, because solvated magnesium acetates are heavily
soluble, and hence, do not completely react with HF.
2 P. Löbmann, Antireflective Coatings and Optical Filters in
Chemical Solution Deposition of Functional Oxide Thin Films,
ed. T. Schneller, R. Waser, M. Kosec and D. Payne,
Springer, Wien, Heidelberg, New York, 2013, 707.
3 M. Pietrowski and M. Wojciechowska, J. Fluorine Chem.,
2007, 128(3), 219.
4 S. Fujihara, M. Tada and T. Kimura, Thin Solid Films, 1997,
304, 252.
5 S. Fujihara, S. Ono, Y. Kishiki, M. Tada and T. Kimura,
J. Fluorine Chem., 2000, 105, 65.
6 S. Fujihara, H. Naito and T. Kimura, Thin Solid Films, 2001,
389, 227.
7 P. H. Joosten, T. J. A. Pompa, H. J. P. Nabben, H. A. M. Van
Hal and J. Haisma, US 4492721, 1983.
8 J. D. Bass, C. Boissere, L. Nicole, D. Grosso and C. Sanchez,
Chem. Mater., 2008, 20, 5550.
9 A. B. D. Nandiyanto, F. Iskandar, T. Ogi and K. Okuyama,
Langmuir, 2010, 26(14), 12260.
(iii) The MgF2 sol system tolerates only a very small content
of water (0.1–0.2 vol%) otherwise gelation occurs after a short
time, resulting in sols that cannot stored for a longer period,
and hence, will not be suitable for practical applications.
Finally, strict consideration of these reaction parameters
enables the upscaling of the synthesis conditions for 10 F. Waltz, M. A. Swider, P. Hoyer, T. Hassel, M. Erne,
optimised MgF2 sols (cf. Fig. 11) which can be stored and
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K. Möhwald, M. Adlung, A. Feldhoff, C. Wickleder,
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coating of glass substrates under industrial conditions. The 11 I. Hitoshi and N. Shunsuke, US 20110122497A1, 2011;
optical performance is better and the mechanical properties M. Bockmeyer and I. Henze, WO 2011116980A1, 2011.
are comparable to the classical SiO2 coatings as described in.25 12 S. Jewhurst and N. Kalyankar, US 20140147594A1, 2014.
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