Sol-gel derived Si/C/O/N-materials: Molecular model compounds, xerogels and porous ceramics

Article abstract of DOI:10.1002/aoc.1825

Polymeric Si/C/O/N xerogels, with the idealized polymer network structure comprising [Si-O-Si(Ni-Ci-N)₃]_n moieties, were prepared by reactions of hexachlorodisiloxane (Cl₃Si-O-SiCl₃) with bis(trimethylsilyl)carb

Full text of DOI:10.1002/aoc.1825

Full Paper
Received: 2 July 2011
Revised: 6 July 2011
Accepted: 6 November 2011
Published online in Wiley Online Library: 9 September 2011
(wileyonlinelibrary.com) DOI 10.1002/aoc.1825
Sol–gel derived Si/C/O/N-materials: molecular
model compounds, xerogels and porous
ceramics†
a,b
b
b∗
b
¨
Hui-Jie Cheng , Katrin Lippe , Edwin Kroke , Jorg Wagler ,
Gerrit W. Festerb, Ya-Li Lia, Marcus R. Schwarzb, Tatyana Saplinovab,
Stefanie Herkenhoff , Vladislav Ischenko and Jorg Woltersdorf
c
c
c
¨
Polymeric Si/C/O/N xerogels, with the idealized polymer network structure comprising [Si–O–Si(N
C
N)3]n moieties, were
prepared by reactions of hexachlorodisiloxane (Cl3Si–O–SiCl3) with bis(trimethylsilyl)carbodiimide (Me3Si–N
BTSC). NMR and FTIR spectra indicate the existence of -N N- and–Si–O–Si- units in the xerogels and also in the ceramic
materials obtained upon pyrolysis. The feasibility of this reaction protocol was confirmed on the molecular level by the
deliberate synthesis of the macrocyclic compound [SiPh2 –O–SiPh2(N N)]2, the crystal structure and spectroscopic data of
C N–SiMe3,
C
C
which are reported. The influence of pyridine as a catalyst for the cross-linking reaction was studied. The degree of cross-linking
increased within the polymers with the addition of pyridine. It was shown by the reaction of hexachlorodisiloxane with excess
pyridine that the latter appears to activate only one out of the two -SiCl3 moieties under formation of hexacoordinated silicon
compounds. The crystal structure of Cl3Si–O–SiCl3(pyridine)2 is presented. Quantum chemical calculations are in support of
this adduct being a potent◦ial intermediate in the pyridine catalyzed sol–gel process. The ceramic yield after pyrolysis of the
Si/C/O/N-xerogels at 1000 C, which reaches values up to 50%, was found to depend on the aging protocol (time, temperature),
whereas no correlation◦was found with the amount of pyridine added for xerogel synthesis. The Si/C/N/O-ceramics obtained
after pyrolysis at 1000 C under NH3 are completely amorphous. Chemically they have to be considered as hybrids between
an ideal [SiOSi(NCN)3]n network and glass-like Si2N2O. The products are mesoporous with closed pores and a broad pore size
c
distribution. Copyright ꢀ 2011 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: Si/C/O/N; sol–gel process; silicon carbodiimide; hexachlorodisiloxane; nonhydrolytic; quantum mechanical calculations
Introduction
Although such sol–gel processes are nonoxidic and nonhy-
drolytic, there is a characteristic analogy to the oxide systems.
Owing to the pseudo-chalcogenic character of the carbodiimide
group, it adopts the role of the oxygen atom, whereas the
trimethylsilyl group can be considered as an analog of the protons
in a hydrolytic process.[13,16,17] Several materials in the systems
Si/C,[18] B/C/N,[19,20] Si/C/N,[21,22] Si/B/C/N,[23–25] and Si/Al/C/N[26]
possessing high thermal resistance and chemical stability, have
been synthesized using this particular kind of sol–gel route. Sev-
eral further reports on related work on BTSC- derived element
carbodimides and their use as precursors for hybrid and ceramic
The sol–gel route to inorganic materials and ceramics has
the advantages of flexible processing, product homogeneity,
diversity of accessible elemental compositions, as well as much
lower processing temperatures compared with conventional
techniques.[1–3] In the past decades, this synthesis method has
been investigated in detail and developed to prepare various
porous as well as nonporous materials. Most of the research in this
field has dealt with hydrolytic sol–gel routes to oxides. Numerous
reports have been published on the incorporation of hetero-
elements E such as boron[4] or aluminum[5] into the gels in oder to
modify and improve the properties of the final Si/E/C/O-ceramics.
Since the discovery of gel-forming reactions of tetrachlorosilane
∗
Correspondence to: Edwin Kroke, Institut fu¨r Anorganische Chemie, TU
Bergakademie Freiberg, Freiberg 09596, Germany.
E-mail: Edwin.Kroke@chemie.tu-freiberg.de
[8,9]
SiCl4,[6,7] methyltrichlorosilane MeSiCl3
and other trichlorosi-
lanes RSiCl3 (with R = H, alkyl, aryl)[10,11] as well as selected
dichlorosilanes[12–14] with bis(trimethylsilyl)carbodiimide (BTSC),
this novel anhydrous technique has been further investigated and
applied to produce organic–inorganic hybrid materials[15] and
various nonoxide ceramics [eqn (1)].
a
b
c
Key laboratory of Advanced Ceramics and Machining Technology, Tianjin
University, Ministry of Education, Tianjin 300072, China
Institutfu¨rAnorganischeChemie,TUBergakademieFreiberg,FreibergD-09596,
Germany
pyridine
Max-Planck-Institut fu¨r Mikrostrukturphysik, Weinberg 2, D-06120 Halle,
Germany
n
1.5n
+ 3n
gel
+
RSiCl3
(CH3)3Si NCN Si(CH3)3
RSi(NCN)1.5
(CH3)3SiCl
n
R = H, Cl, CH3, C6H5, SiCl3, etc.
(1)
¨
In memoriam Professor Jorg Woltersdorf.
†
c
Appl. Organometal. Chem. 2011, 25, 735–747
Copyright ꢀ 2011 John Wiley & Sons, Ltd.

H.-J. Cheng et al.
materials have been published. Among these studies are gallium
carbodiimides[27] and several transition metal compounds.[28] Re-
cent studies on polysilylcarbodiimides were devoted to their
enthalpy of formation[29] and use as stabilizers.[30]
This solid sticks to the walls of the Schlenk tubes, and can there-
fore easily be removed before gelation by transferring the reaction
solution to another flask. In the samples with larger fractions of
pyridine added, the formation of notable amounts of solid pre-
cipitates was observed, resulting from the formation of adducts
of hexachlorodisiloxane with pyridine (see below). The reaction
mixtures were then incubated at 45, 75 or 90 ◦C (without stirring)
until gelation occurred. Gelation points were visually determined
simply by turning the flask over. The gelation time depends on
the temperature applied and the amount of the catalyst used. For
further cross-linking, different aging times (ranging between 7 and
30 days) were applied. The liquid components of the gels (i.e. the
by-product chlorotrimethylsilane, the solvent cyclohexane, as well
as residual BTSC ◦and pyridine) were removed in vacuum followed
by drying at 160 C/10−2 mbar/4 h to obtain the xerogels.
Neither the controlled incorporation of oxygen into silyl-
carbodiimide gels, nor the controlled incorporation of car-
bodiimide groups into conventionally-produced polysiloxanes,
leading to the hybrid polysiloxane/polysilylcarbodiimide gels
and related Si/C/O/N ceramic materials, has been reported
so far. In particular, Si/C/O/N ceramics derived from the
above-mentioned hybrid gels might be technologically in-
teresting, combining enhanced oxidation resistance com-
pared with Si/(C)/N materials, and improving thermo me-
chanical properties compared with Si/(C)/O glasses and
ceramics.
In the present work, the hybrid polysiloxane/polysilyl-
carbodiimide gels were synthesized by the reaction between
BTSC and hexachlorodisiloxane, with the aim to produce suitable
precursors for the preparation of Si/C/O/N ceramics upon their
subsequent pyrolysis. The effect of pyridine as a catalyst on the
cross-linking degree of the Si/C/O/N-polymers as well as on the
composition, structure and morphology of the derived ceramics
has been studied along with the aging temperature and the aging
time.
Synthesis of Si/C/O/N Ceramics
The xerogels were transferred to quartz boats and pyrolyzed in a
stream of dry ammonia in a quartz Schlenk tube. Applied heating
rates were 5 ◦C min−1. The time of exposure at 1000 ◦C was 2 h in
all cases.
Synthesis and Characterization of cyclo-[Ph2Si–O–SiPh2
(NCN)]2
A solution of 1,3-dichloro-1,1,3,3-tetraphenyldisiloxane (5.37 g,
11.9 mmol) and triethylamine (2.41 g, 23.8 mmol) in THF (70 ml)
was stirred at 0 ◦C and a solution of cyanamide (0.50 g, 11.9 mmol)
in THF (8 ml) was added dropwise (causing the immediate
precipitation of triethylamine hydrochloride). Upon complete
addition of the cyanamide solution the mixture was heated
and kept under reflux conditions for 1.5 h and then stored at
room temperature overnight. The triethylamine hydrochloride
was then filtered off and washed with THF (20 ml). From the
combined filtrate and washings the solvent was removed under
reduced pressure, and the solid (but somewhat oily) residue was
recrystallized from a mixture of hexane (5 ml) and THF (10 ml).
After some crystalline product had formed additional hexane
(5 ml) was layered over the product solution before it was then
stored at 6 ◦C for one week. The crystalline colorless product was
filtered off, washed with hexane (3 ml) and dried in a vacuum
(2.34 g). From the combined filtrate and washings some solvent
(ca. 5 ml) was removed under reduced pressure and the solution
was again stored at 6 ◦C to give a second fraction of the product
(0.72 g). A third fraction of the product was isolated after cooling
the solution to −25 ◦C (0.30 g). Yield: 3.36 g (3.99 mmol, 67%).
M.p. 197–199 ◦C (in sealed capillaries). 1H NMR (400.1 MHz, CDCl3)
δ = 7.27 (m, 16 H, meta), 7.39 (t, 8 H, 7.4 Hz, para), 7.57 (d,
16 H, 7.4 Hz, ortho). 13C NMR (100.6 MHz, CDCl3) δ = 122.0
Experimental
General Considerations
Owing to the air and moisture sensitivity of silylcarbodiimides,
all reactions were performed in heat-gun dried glass vessels in
a glovebox (N2 atmosphere, O2 < 0.1 ppm, H2O < 0.1 ppm) or
in an argon flow, using the Schlenk technique.[31] Hexachloro-
disiloxane (Sigma-Aldrich Chemie GmbH, 96%) and 1,3-dichloro-
1,1,3,3-tetraphenyldisiloxane (ABCR) were used without further
purification. Bis(trimethylsilyl)carbodiimide was prepared from
hexamethyldisilazane (Me3Si–NH–SiMe3, Sigma-Aldrich Chemie
GmbH, 99%) and cyanoguanidine [HN C(NH2)NHCN, Sigma-
Aldrich Chemie GmbH, 99%] according to the literature.[32] Dried
cyclohexane (Sigma-Aldrich Chemie GmbH, 99.5%) was stored in
a glovebox and used directly as a solvent without further pu-
rification. Tetrahydrofuran (THF), toluene and triethylamine were
distilledfromsodium/benzophenoneketylandstoredunderargon
atmosphere.
Preparation of Si/C/O/N Gels
Synthesis of the gels proceeds according to the following idealized
reaction [eqn (2)]:
(N
C N), 127.9 (ortho), 130.4 (ipso), 133.7 (para), 134.2 (meta).
pyridine
29SiNMR(79.5 MHz, CDCl3)δ = −42.9. Elementalanalysiscalcdfor
C50H40N4O2Si4 (841.2 g mol−1): C, 71.4; H, 4.79; N, 6.66. Found: C,
70.9;H,4.91;N,6.77.X-raystructureanalysis(CCDC781 896):crystal
dimensions 0.50 × 0.38 × 0.18 mm, C50H40N4O2Si4, Mr = 841.22,
T = 180(2) K, triclinic, space group P <− 1, a = 9.2415(7),
b = 10.6542(7), c = 12.4571(9) Å, α = 75.691(2), β = 73.965(2),
γ = 78.193(2)◦, V = 1129.94(14) Å3, Z = 1, ρcalcd = 1.236
Mgm−3, µ(Mo–Kα) = 0.176 mm−1, F(000) = 440, 2θmax = 54.0◦,
n
3n
+ 6n
gel
Cl3Si
O
SiCl3
+
(CH3)3Si NCN Si(CH3)3
liquid
SiOSi(NCN)3
(CH3)3SiCl
n
liquid
(2)
A homogeneous mixture of 1.20 ml (4.46 mmol) of hexachloro-
disiloxane and 4.10 ml (18.06 mmol) of BTSC was prepared in
a Schlenk tube prior to addition of pyridine (Py). The different
amounts of Py [molar ratios of Py/BTSC = 0, 0.05, 0.10, 0.15, 0.20;
with cyclohexane (1.2 ml) as solvent, or without any solvent] were
dropped into the mixture of starting materials while shaking the
Schlenk tube at the same time. Upon addition of small amounts
of pyridine traces of solid pyridine hydrochloride precipitated,
owing to very small amounts of HCl present in the chlorosiloxane.
20 014 collected reflections, 4803 unique reflections (Rint
=
0.0271), 307 parameters, S = 1.076, R1 = 0.0372 [I > 2σ(I)],
wR2 (all data) = 0.1076, maximum/minimum residual electron
density +0.360/−0.239 e Å−3
.
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Sol–gel derived Si/C/O/N-materials
Synthesis and Characterization of Cl3Si–O–SiCl3(Pyridine)2
N2, at 30–300 ◦C), and analyzed using the BET (Brunauer-Emmett-
Teller) model. The pyrolysis of the xerogels was studied by thermal
analys◦isunderanargonflow(TG/DTA22,Seiko◦Instrument,heating
rate 5 C min−1, maximum temperature 1000 C).
Themorphologyofthemesoporesinthepyrolyzedsampleswas
investigated by high resolution transmission electron microscopy
(HR-TEM) using a CM 20 FEG electron microscope (Philips,
Eindhoven, Netherlands; operating voltage of 200 kV), equipped
with a post-column electron energy filter (Gatan Imaging Filter GIF
200, model 667, Pleasanton, CA).
To a cooled solution (dry ice/iso-propanol, −78 ◦C) of hexachloro-
disiloxane (1.43 g, 5.00 mmol) in toluene (20 ml) was added pyri-
dine (1.58 g, 20.0 mmol), whereupon a white precipitate formed.
The mixture was stirred at −78 ◦C for 1 h and then allowed to
adapt to room temperature. The white solid product was then
filtered off, washed with toluene and dried in a vacuum. Yield:
1.55 g (3.50 mmol, 70%). This compound exhibits poor solubility
in nonpolar solvents such as toluene or hexane, whereas it dissoci-
ates into hexachlorodisiloxane and pyridine in chloroform. Hence,
characterization was limited to the solid state. 13C CP/MAS NMR:
δ = 124.7, 130.1, 145.1; 29Si CP/MAS NMR: δ = −53.5 (OSiCl3),
−180.5 (OSiCl3Py2). Elemental analysis calcd for C10H10Cl6N2OSi2
(443.1 g mol−1): C, 27.8; H, 2.31; N, 6.52. Found: C, 27.8; H, 2.33;
N, 6.48. X-ray structure analysis (CCDC 781 895): crystal dimen-
sions 0.38 × 0.20 × 0.02 mm, C10H10Cl6N2OSi2, Mr = 443.08,
T = 100(2) K, monoclinic, space group P21/c, a = 13.0395(14),
b = 6.9499(8), c = 19.675(2) Å, β = 95.897(3)◦, V = 1773.6(3) Å3,
Z = 4, ρcalcd = 1.659 Mg m−3, µ(Mo–Kα) = 1.101 mm−1, F(000)
= 888, 2θmax = 50.0◦, 9094 collected reflections, 3128 unique re-
flections (Rint = 0.0563), 204 parameters, S = 1.034, R1 = 0.0496
[I > 2σ(I)], wR2(all data) = 0.1087, maximum/minimum residual
electron density +0.523/−0.379 e Å−3. The data set was collected
from a slightly twinned crystal. De-twinning was performed after
initial structure solution and refinement using TwinRotMat as im-
plemented in WinGX Platon and a HKLF5-file was written for the
final stages of the refinement, which revealed twin populations of
96 and 4%. The twin law found, ca. (1 0 0)(0 −1 0)(−0.333 0–1),
corresponds to a 180◦ rotation about axis [1 0 0].
Theoretical Methods
Geometry optimization and single point energy calculation
have been performed with GAMESS-US[33] program in the
framework of the density functional theory (DFT). In case of the
isodesmic reaction, geometry optimization and zero-point energy
calculations were carried out with a 6-31G∗ basis set along with
the B3LYP hybrid exchange correlation functional. For single point
energy calculations the B3LYP functional was used with a TZV +
(2df,1p) basis set.
The B3LYP hybrid exchange-correlation functional was used
along with a 6-31G∗ Gaussian-type basis set for the geometry
optimization of the pyridine–adduct complexes. Thereby, in a first
run the Si–N bond length was fixed at 1.8 Å, whereas the other
parts of the molecule were fully optimized. In successive cycles
the restraint on the Si–N bond was lifted as well. prior to full
optimization of all parameters including the Si–N bond. Single-
point energy calculations have been performed with a 6-311G +
(2d,p) basis set along with the B3LYP hybrid exchange correlation
functional. The natural charges were obtained from calculations
with NBO 5.0.[34]
Si/C/O/N Xerogel and Ceramics Characterization
The carbon content of the products was determined by combus-
tion analysis (LECO RC-412 carbon analyzer). Oxygen and nitrogen
contents were measured by hot gas extraction at 2500 ◦C in he-
lium atmosphere (LECOTC-436). Additionally, C/H/N-combustion
analyses were performed using a CHN analyzer (Micro Cube, Ele-
mentar, Hanau). For the determination of the chlorine content the
xerogels were dissolved in aqueous NaOH solution at pH 11, then
analyzed by ion chromatography, using a standard NaCl solution
as reference.
The chemical structures of the products were investigated by
FT-IR spectroscopy (Varian Excalibur 3100 with 4 cm−1 resolution)
using the KBr method, Raman spectroscopy (Bruker RFS 100/s
with capillary glass, laser wavelength 1064 nm) and cross-
polarization/magic-angle-spinning nuclear magnetic resonance
spectroscopy (CP/MAS NMR Bruker Avance TM WB 400 MHz
spectrometer with a 29Si resonance frequency of 79.51 MHz,
spinning rate 5 kHz). Inside a glove box the xerogels were
compressed in small KelF inserts prior to placing them in 7 mm
zirconia spinners for 29Si and 13C CP/MAS NMR measurements.
Scanning electron microscopy (SEM, LEO 1530 FEG, Carl Zeiss
NTS GmbH, Oberkochen, Germany, with acceleration voltage of
12 kV, and Tescan VEGA II SBH, 20 kV) was used to characterize
the microstructure of the xerogels. The samples were handled
under an inert atmosphere to avoid hydrolysis. For EDX analysis
during the SEM studies a Bruker AXS X-Flash4010 with silicon drift
detector and a 8 µm ultrathin polymer-coated Be window (for
light elements) was used together with Quantax 400 software for
spectrum recording and data processing.
Results and Discussion
Synthesis of a Molecular Model Compound for Siloxane/
silylcarbodiimide Containing Oligomers and Polymers
In a previous report we have shown that from diphenyldichlorosi-
lane oligomeric (cyclic) silylcarbodiimides can be prepared
via different routes, i.e. via condensation with BTSC and via
base-supported HCl-elimination with cyanamide.[13] (Scheme 1,
top). The latter route proved also feasible to create mixed
siloxane/silylcarbodiimide-functionalized heterocycles with
alternating Si–O–Si and Si–N
C N–Si sequences, the use
of chlorosiloxanes being a prerequisite (Scheme 1, bottom).
This synthesis route delivered the macrocyclic compound
cyclo-[Ph2Si–O–SiPh2(NCN)]2 in good yield (67% isolated). A 12-
memberedcycleconsistingofa[Si–O–Si–(NCN)]2 sequencecould
thus be also a favorable structural motive in the formation of car-
bodiimide gels from 1,3-(oligo)chloro-functionalized disiloxanes.
The 29Si NMR spectrum of cyclo-[Ph2Si–O–SiPh2(NCN)]2 (δ =
−42.9 ppm in CDCl3) is very similar to that of octaphenyl-
cyclotetrasiloxane (δ = −43.0 ppm in dioxane) and the fully
carbodiimide bridged system [Ph2Si(NCN)]4 (δ = −44.4 ppm in
dioxane)[13]. Considering a [Si–O–Si(NCN)]2 12-membered cyclus
in an isodesmic reaction;[35] this reaction can be written like
in Scheme 2, where the tetramer carbodiimide and the cyclote-
trasiloxane act as counterparts. Such an isodesmic reaction can be
used to quantify the stability of mixed siloxane/silylcarbodiimides
by calculating the reaction enthalpy of this hypothetic reaction
according to eqn (3). The reaction enthalpy for the reaction in
Nitrogen adsorption isotherms of xerogels were determined by
an AREA meter (Stro¨hlein GmbH und Co. Pressure 0.5–1.0 mbar,
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H.-J. Cheng et al.
Scheme 1. Synthesis of phenyl-substituted cyclo-tetrasilylcarbodiimide using bis(trimethylsilyl)carbodiimide (BTSC) or cyanamide/triethylamine as
starting materials (top) and the first ‘mixed’ siloxane/silylcarbodiimide using the educt 1,3-dichloro-1,1,3,3-tetraphenyldisiloxane (bottom).
H2
Si
H2
Si O
H2
Si
SiH2
O
SiH2
O
Si
H2
O
SiH2
O
= -N=C=N-
H2Si
H2Si
H2Si
Si
O
Si
H2
H2
Scheme 2. Hypothetic isodesmic reaction to form cyclo-[H2Si–O–SiH2(NCN)]2 starting from cyclo-tetrasilylcarbodiimide and cyclo-tetrasiloxane.
Scheme 2 has been calculated as 0.91 kcal mol−1, and therefore
the reaction should be endothermic in the gas phase. This en-
thalpy is quite small and may be reverted to exothermic value
by solvent effects, thus supporting a certain relevance of mixed
siloxane/silylcarbodiimide macrocyclic systems also in the case of
partially hydrolyzed silylcarbodiimide gels.
N2
Si2*
C1
Si1
O1
Si2
N1
Hrct. = 2 ꢀH◦ [(H2Si)4O2(NCN)2]
f
− {ꢀH◦ [(H2Si)4(NCN)4] + ꢀH◦ [(H2Si)4O4]}
(3)
f
f
N2*
The molecular structure of cyclo-[Ph2Si–O–SiPh2(NCN)]2 was
determined by single crystal X-ray diffraction analysis (Fig. 1). The
discussion of the bonding properties of the carbodiimide moiety
is limited by a twofold disorder (cross-wise about C1). Despite
this effect, the structure reveals an almost planar arrangement
of this 12-membered heterocycle. The bonding parameters (Si–O
and Si–N bond lengths) are in the expected range for siloxanes
and silylcarbodiimides, respectively, and the wide Si–O–Si and
Si–N–Canglesreflectsimilarbehaviourasinoctaphenyldisiloxane
and [Ph2Si(NCN)]4.
N2
C1
N1
Si1
O1
Si2
Formation of Siloxane/silylcarbodiimide Xerogels – Cl3Si–O–
SiCl3(py)2 as an Intermediate
The hybrid polysiloxane/polysilylcarbodiimide gels were synthe-
sized by reactions between BTSC and hexachlorodisiloxane. BTSC
was used in excess, since it was observed in former studies that
this leads to lower chlorine contents of the gels and xerogels
in many cases. Table 1 summarizes the experimental conditions.
The time to the gelation point depends strongly on the amount
of the catalyst (pyridine) used and on the reaction temperature
(see below). Nevertheless, given sufficient reaction time, gels were
obtained for all reaction batches without exception.
Figure 1. ORTEP diagram (thermal ellipsoids with 30% probability) of
cyclo-[Ph2Si–O–SiPh2 –(NCN)]2 inthecrystal. Theasymmetricunitconsists
of a half-molecule, which is located around a center of inversion. Selected
atoms are labeled; asterisked labels represent symmetry equivalents.
The carbodiimide group is two-fold disordered (site occupancy 0.5 each
position); only one orientation is depicted. Selected bond lengths (pm)
and angles (deg): Si1-O1 162.3(1), Si2-O1 162.7(1), Si1-N1 171.7(2), Si2-N2∗
171.6(2), N1-C1 120.5(2), N2-C1 120.5(2), Si1-O1-Si2 151.1(1), Si1-N1-C1
157.9(5), Si2∗-N2-C1 145.4(5), N1-C1-N2 176.5(5).
Mixtures without pyridine remain liquid for several hours and
then turn into homogeneous gels. However, from the first drop of
pyridine added at room temperature, the mixtures react violently,
especially if no solvent is present. This exothermic reaction can
be attributed to the formation of chlorosilane-pyridine adducts of
the type [RSiCl3· 2Py] (R = H, alkyl, aryl, Cl, etc.). These adducts
containing hexa-coordinated silicon atoms tend to precipitate
owing to their low solubility in many organic solvents.[36]
Formation of related adducts of chorosilanes has already been
studied.[37–40] Incaseofhexachlorodisiloxane, however, questions
arise concerning the composition of the adduct (pyridine attached
to one or two of the silicon atoms) as well as its relevance
in a system containing an excess hexachlorodisiloxane but
under-stoichiometric amounts of pyridine. The first question
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Sol–gel derived Si/C/O/N-materials
phase), which was determined to be 146 4◦. This Si–O–Si
bond angle in Cl3Si–O–SiCl3(Py)2 allows for Cl-capping of one of
the tetrahedral faces of Si2 by Cl3 from a distance of 343.6(2)
pm, which might also contribute to the upfield shift of the
29Si NMR signal. With respect to Si–Cl bond activation by
pyridine coordination, one can discern a notable Si–Cl bond
lengthening upon hexacoordination (from ca. 202 to ca. 220 pm).
The differences between the longer bonds Si1–Cl1, Si1–Cl2 (223
and222pm, respectively)andthesomewhatshorterbondSi1–Cl3
(219 pm) is mainly based on packing effects. Whereas the former
establish 5 Cl... H contacts each within a distance of ca. 320 pm,
the environment of the latter exhibits only four of them.
The stability of pyridine adducts and in turn their relevance as
intermediates in the pyridine catalyzed sol–gel-process was also
investigated by computational methods. For the monodentate
ligand pyridine there are four different types of 1 : 1 complexes
(Ia–d, Fig. 3) theoretically possible. In the case of the 1 : 2
complexes there are three isomers with only one silicon atom
coordinated by two pyridine molecules (IIa–c, Fig. 3) as well as
10 additional different complexes if the pyridine molecules are
coordinated at both silicon atoms. Out of these complexes only
the one shown in Fig. 3 (IId) has been calculated. For complexes
with a 1 : 4 silane : pyridine ratio complex IV in Fig. 3 out of all six
theoretically possible complexes was considered.
Optimization converged to the geomtries depicted in Fig. 4 and
selected parameters thereof are presented in Table 2. Whereas the
pentacoordinate silicon complex with pyridine and the–OSiCl3
moiety in axial positions (Ia) proved less stable and converged
to molecular structures with a long (>3 Å) Si–N bond, those
with an angle smaller than 180◦ between pyridine and–OSiCl3
turned out to have positive relative energies with respect to
hexachlorodisiloxane and pyridine. At the hexacoordinated silicon
atoms (IIa–c) and the pentacoordinated Si atoms in IId the
respective configuration was retained during optimization, and
the structure IIa with trans disposed N atoms proved to be
the most stable. According to these data in a reaction from
hexachlorodisiloxane and pyridine the complex IIa should be
formed, which is confirmed by the above results of the X-ray
diffraction analysis of Cl3Si–O–SiCl3(pyridine)2.
Table 1. Experimental conditions of the formation of Si/C/O/N
gels from hexachlorodisiloxane (O) and bis(trimethylsilyl)carbodiimide
(BTSC) with or without cyclohexane as solvent and pyridine (Py) as
catalyst. The syntheses were conducted at different temperatures (45,
75 and 90 ◦C) in nitrogen atmosphere. As an example, the table shows
the data for 75 ◦C
Sample
no.
O/BTSC/Pya
(molar ratio)
The time to
gelation point (h)
Aging time
(days)
1
2
3
4
5
1 : 3 : 0
1 : 3 : 0.15
1 : 3 : 0.3
1 : 3 : 0.45
1 : 3 : 0.6
18
9
7 and 30
7 and 30
7 and 30
7 and 30
7 and 30
5
4
2
a Since pyridine reacts with chlorosilane to form adducts which
precipitate, the concentration of pyridine in solution is lower than
the provided values.
Cl2
C7
C6
C8
C9
Cl1
N2
Si1
Cl3
C1
N1
C5
C2
C10
C3
O1
C4
Cl4
Si2
Cl6
Cl5
Figure 2. ORTEP diagram (thermal ellipsoids with 50% probability) of
Cl3Si–O–SiCl3(Py)2 in the crystal. The trichlorosilyl group is two-fold
disordered (site occupancies 0.86 and 0.14); only the predominant part is
depicted. Selected bond lengths (pm) and angles (deg): Si1-Cl1 222.8(2),
Si1-Cl2 221.6(2), Si1-Cl3 218.9(2), Si1-N1 195.4(4), Si1-N2 197.0(4), Si1-O1
170.0(3), Si2-O1 158.0(3), Si2-Cl4 202.8(2), Si2-Cl5 202.3(2), Si2-Cl6 202.0(2),
Si1-O1-Si2 149.0(2), O1-Si1-Cl2 177.6(2), Cl1-Si1-Cl3 178.1(1), N1-Si1-N2
179.0(2).
Finally, optimization of IV confirmed that the addition of four
pyridine molecules under formation of a disiloxane with two hexa-
coordinate Si atoms should be less favorable, thus being in accor-
dance with the isolation of the adduct Cl3Si–O–SiCl3(pyridine)2.
could be answered by deliberate synthesis of the complex
Cl3Si–O–SiCl3(pyridine)2. Although excess pyridine was utilized in
this synthesis, only one of the two silicon atoms is hexacoordinate,
whereas tetracoordination of the other O–SiCl3-group is retained.
ThiswasconfirmedbybothX-ray diffractionanalysis(Fig. 2)andby
29Si CP/MAS NMR analysis for the gross product. The latter clearly
shows a peak characteristic of tetracoordinate OSiCl3 groups (at
−53.5 ppm, slightly upfield with respect to hexachlorodisiloxane,
which has a signal at −45.45 ppm[41]) and a significantly upfield-
shifted resonance at −180.5 ppm for the hexacoordinate silicon
atoms. The upfield shift of the tetracoordinate Si atom’s NMR
signal may be interpreted as a result of Si–O bond strengthening.
Whereas the Si–O bond to the hexacoordinate Si atom is
rather long [170.0(3) pm], this bond lengthening upon pyridine
coordinationgivesrisetoashorteningofthesecondSi–Obond[to
158.0(3) pm], whereas for hexachlorodisiloxane a slightly longer
Si–O bond length of 159.2 1.0 pm was reported (results of
gas phase electron diffraction analysis).[42] The Si–O–Si angle
[149.0(2)◦] is similar to that of hexachlorodisiloxane (in the gas
Characterization of Siloxane/Silylcarbodiimide Xerogels
Obtained from Hexachlorodisiloxane and BTSC
Si/C/O/N gels and xerogels were synthesized by a standard
procedure. Details concerning synthesis parameters including
temperature, aging time, drying conditions are summarized in
the Experimental section. The elemental analysis data of a typical
xerogelobtainedfromhexachlorodisiloxaneisasfollows:C,15–20;
N, 25–30; O, 8.0–8.5; H, 1.0–1.8; Cl, 1.0–3.0 (weight-%). The
ideal composition of the silyloxycarbodiimide [SiOSi(NCN)3]n is:
C, 18.8; N, 43.8; O, 8.3. The deviation from the ideal formula can
(at least in part) be attributed to remaining -N
C N–SiMe3
substituents.[43,44] Incomplete substitution and condensation
reactions cause both the residual chlorine content and hydrogen
content.
29Si NMR spectra for samples without pyridine as a catalyst
(Fig. 5a) revealed several distinct signals: The broad resonance
with high intensity at ca. −103 ppm is assigned to silicon atoms
c
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H.-J. Cheng et al.
Pyr
Si
Cl
Si
Cl
Id
Cl
Si
Pyr
Si
Cl
Cl
Pyr
Cl
Cl
Cl
Cl
Cl3SiO
Cl
Pyr
Cl
OSiCl3
Cl
OSiCl3
Ic
OSiCl3
Ia
Ib
Pyr
Si
Pyr
Si
Pyr
Si
Cl
Cl
Cl
Pyr
Cl
Cl
Pyr
Cl
Cl
Cl
Cl3SiHO
Cl3SiHO
Pyr
IIa
Cl
OSiCl3
IIb
IIc
Pyr
Si
Cl
Si
Pyr
Pyr
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Si
Si
O
O
Cl
Pyr
Pyr
Pyr
IId
IV
Figure 3. Possible coordination modes of the monodentate ligand pyridine to interact with the two silicon atoms in hexachlorodisiloxane.
Ia
Ib
Ic
Id
IIa
IIb
IIc
IId
IV
Figure 4. Calculated structures of pyridine–hexachlorodisiloxane adducts shown in Fig. 3.
c
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Sol–gel derived Si/C/O/N-materials
The 13C NMR peak at around 0.6 ppm results from the residual -
Si(CH3)3 endgroups. Additionalsignalsattributedtopyridinewere
observed in the samples with catalyst at about 145 ppm.[13,15]
The FT-IR and Raman spectra of the as-synthesized Si/C/O/N
xerogels shown in Fig. 6a and 6b clearly indicate the presence
Table 2. Selected geometrical parameters of complexes shown in
Fig. 3
ꢀE
Complex (kcal mol−1 a
d(Si–N) d(Si–O) d(Si–O)b
(N–Si–N)
<
)
(Å)
(Å)
(Å)
(deg)
of N
C N units with the characteristic valence vibrations at
Si2OCl6
Ia
–
–
–
1.633
1.626
1.638
1.608
1.617
1.596
1.607
1.609
–
–
–
2168 and 1533 cm−1, as well as the corresponding deformation
vibration at 800 cm−1. An Si–O–Si absorption band appears at
−0.10
+4.23
+12.33
+12.52
−0.56
+4.68
+4.89
+11.29
+14.20
3.661
2.248
1.970
1.960
2.038
2.134
2.126
2.242
2.049
1.641
1.673
1.739
1.664
1.745
1.746
1.718
1.659
1.695
Ib
–
1100 cm−1 [49,50]
The characteristic valence signal for cyanamide
.
Ic
–
N–C N groups at 2264 cm−1. The signal at 570 cm−1 is caused
by deformation vibrations of Si–NCN moieties and/or valence
vibrations of the Si–Cl groups. A weak signal at 1270 cm−1 is
attributed to Si–CH3 units.[51]
Id
–
IIa
179.4
83.9
86.6
–
IIb
IIc
Like other silylcarbodiimides,[9,13] Si/C/O/N xerogels are also
sensitive to moisture. After 2 h in air, the xerogel hydrolyzes
to form Si–O–Si/Si–OH groups and cyanamide (H2N-CN) or
its oligomers, e.g. dicyanamide or melamine as depicted in
eqn (4) and Fig. 6(c).[15,52] The IR signals at 3250 cm−1 appearing in
the spectrum in Fig. 6(c) indicate absorptions owing to Si–OH
and N-H groups. The expected absorption owing to valence
vibration for the cyano moieties at 2264 cm−1 is also present.
The additional signals around 1700 cm−1 are caused by N–H
deformation vibrations and carbonyl valence vibrations of further
hydrolysis products of the cyanamide such as urea.
IId
IV
–
172.5
a Relative energies (with respect to reactants).
b Noncoordinated silicon atom.
bonded to three carbodiimide groups and one oxygen atom.
This is deduced from the chemical shifts of Si(NCN)4 (−104 ppm)
and SiO4 units (−110 ppm).[43,45,46] The signals centered at ca.
−85, −68 and −45 ppm indicate the presence of silicon atoms
in ClSiO(NCN)2, Cl2SiO(NCN) and Cl3SiO moieties, respectively, in
the gel structure.[45] An additional signal at 3.0 ppm results from
trimethylsilyl substituents, i.e. NCN-Si(CH3)3 end groups.[47]
In contrast, for the xerogels obtained with pyridine as a
catalyst,thesignalsrelatedtoClSiO(NCN)2,Cl2SiO(NCN)andCl3SiO
groups are much less pronounced, implying an obviously higher
crosslinking degree. The strongest signal at ca. −103 ppm is
O
NCN
Si NCN
NCN
+ H2O
+ H2O
O
Si NCN
NCN
SiO2
Si
O
Si
....
- H2N-CN
- H2N-CN
n
n
(4)
However, it is at present not clear, how the hydrolytic
sensitivity of Si–NCN units is influenced by the number of oxygen
atoms bonded to the same silicon atom. This is a matter for
further investigation. After thermal treatment at 1000 ◦C Si/C/O/N
ceramics are obtained, which are stable against oxidation and
hydrolysis in air (see below).
assigned to OSi(N
C N)3 units and the peaks around 18 ppm to
NCN–Si(CH3)3 units.[48] The latter assignment appears a little
inconsistent with the above-mentioned signal at 3 ppm (for
specimensobtainedwithoutpyridine). However, thesaidchemical
shift range is characteristic for trimethylsilyl groups as reported in
literature.[47,48]
Morphology and Thermal Behavior of Si/C/O/N xerogels
The 13C NMR chemical shifts for the NCN groups were detected
between 120 and 125 ppm (Fig. 5b).[8,9,13,15] Without pyridine, a
more complex structure is formed, as can be seen from the 29Si
CP/MASNMR, withdifferentlybondedNCNunits. Thisismostlikely
the reason that the generally weak 13C signal for the carbodiimide
group is not visible in Fig. 5b (top).
After different aging times, the gels were dried at 160 ◦C in
vacuum. Analysis of nitrogen adsorption data according to the
Brunauer–Emmett–Teller model, shows that the xerogels aged
at 75 ◦C for 30 days with pyridine as catalyst have a surface
area of 46 m2 g−1. This is a rather low value compared with
(a)
(b)
Cl2SiO(NCN)
Cl3SiO
(CH3)3SiNCN
Figure 5. (a) 29Si CP/MAS NMR spectrum of Si/C/O/N xerogels at 75 ◦C aging for 30 days; (b) 13C CP/MAS NMR spectrum of Si/C/O/N xerogels at 75 ◦C
aging for 30 days.
c
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H.-J. Cheng et al.
Figure 6. Spectra of a Si/C/O/N xerogel synthesized at 75 ◦C, aging for 30 days: (a) Raman; (b) FTIR of a xerogel as synthesized; and (c) FT-IR for the same
xerogel hydrolyzed in air for 2 h.
Figure 7. SEM micrographs of Si/C/O/N xerogels showing microstructure typically observed for samples prepared with and without pyridine as a catalyst.
typical data for oxide xerogels, which frequently show specific
surface areas in the range of up to 1000 m2 g−1 and above.[1–3]
However, carbodiimide-based xerogels have been reported to
showcomparativelylowsurfaceareavalues.[11] Thismayberelated
to the hydrolytic sensitivity of these materials, which might lead
to pore closure resulting from surface reactions (partial hydrolysis
and condensation at the surface).
Morphologies of the obtained xerogels were investigated by
means of SEM and HR-TEM. SEM micrographs (Fig. 7) indicate the
formation of xerogels without any macro-porosity in both samples
with pyridine and without pyridine.
Figure 7(a) shows a smooth surface of the xerogel, indicating
a homogenous gel process without pyridine. In contrast, with
pyridine as catalyst (Fig. 7b), the gel appears to be more
heterogeneous. The above-discussed formation of adducts of the
type [RSiCl3· 2Py] may be responsible for this observation. Nev-
ertheless, the reaction with pyridine proceeds more completely
than without it in the same period of time, which agrees with the
solid-state NMR results (see above).
(Fig. 8). It should be pointed out that these studies are not
directly comparable with the bulk ceramisation performed under
ammonia. It is known that pyrolysis of Si/C/N and related
Si/E/C/N precursor polymers under NH3 induces substitution and
transamination reactions as well as the formation of volatile CxHy
species, resulting in a decreased carbon and an increased nitrogen
contentoftheceramics.[53,54] TheresultsoftheTGAmeasurements
show that the ceramic yield depends on the synthesis temperature
and aging time of the gels but is not influenced by the content of
catalyst.
Figure 8(a) shows that the ceramic yields of the xerogels with
or without pyridine as catalyst are the same, even though the
decomposition paths are different. With pyridine, the degree of
cross-linking should be higher. However, the removal of residual
pyridine in the xerogel can lead to an increased weight loss. Under
higher aging temperature and after longer aging time, the ceramic
yield increases from about 43 to 51%, as can be seen in Fig. 8(b
and c), which indicates that the cross-linking reactions to form
gels are more complete in these xerogels. It should be pointed out
that the TGA investigations performed under argon atmosphere
are not directly comparable with the bulk pyrolyses discussed in
The thermal behavior of the obtained Si/C/O/N xerogels
was investigated by thermal gravimetry (TGA) under argon
c
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Sol–gel derived Si/C/O/N-materials
Figure 8. Thermogravimetry curves of Si/C/O/N xerogels: (a) aging for 7 days at 75 ◦C with or without pyridine; (b) with pyridine aging for 7 days at 75
and 90 ◦C; and (c) with pyridine at 75 ◦C aging for 7 and 30 days.
the following chapter. Argon acts as an inert gas which does not
undergo any chemical reactions with the xerogels, ceramics or
gaseous side products,
The 29Si NMR resonance centered at −50 ppm for the same
sample (Fig. 9c) is in the chemical shift range reported for
amorphous silicon nitride, Si2N2O as well as Si2N2(NCN).[11] No
further details can be concluded owing to the broadness and
featureless nature of the resonance (see below). This indicates
that rather random Si/C/O/N networks with a low degree of order
and crystallinity are characteristic for the samples synthesized
at 1000 ◦C, as expected. The predominantly amorphous nature
with only weak indications for partial nanocrystallinity of these
products was confirmed by powder XRD measurements (Fig. 10)
as well as several HR-TEM studies (see below).
The position of the 29Si NMR signal together with the strong
absorption band in the FTIR spectrum (Fig. 9a) around 950 cm−1
indicate that the network is primarily composed of Si–N and Si–O
bonds. Si–C units are not present after pyrolysis at 1000 ◦C (an FT-
IR absorption at 700–800 cm−1 would be expected). The carbon
is obviously primarily bonded to nitrogen in the form of NCN units
(in the carbodiimide and/or cyanamide form). No free carbon was
detectable by the applied methods (no G or D band in the FT-IR
spectrum; no graphitic structures in HR-TEM, see below).
Chemical Structure of Silylcarbodiimide-derived Si/C/O/N
ceramics
Bulk pyrolyses of the xerogels were performed under ammonia at-
mosphere. This induces transamination and substitution reactions
reducing the number of -NCNSiMe3 end groups and Si–Cl untis.
At higher temperatures the carbon content is decreased while
nitrogen is incorporated.[53] FT-IR spectra of products obtained
from gels synthesized with and without pyridine and pyrolyzed
at 1000 ◦C show absorption bands ν(NCN) at ∼2200 cm−1, which
indicates that carbodiimide units are still present (Fig. 9a). This
is supported by solid-state 13C NMR spectra showing a broad
resonance signal centered at ∼100 ppm, which is characteristic
for N
C
N units (Fig. 9b). However, the broadness of this 13C
NMR signal and the weakness of the FT-IR signal seem to indicate
that a major part of the carbodiimide units is decomposed. A
rearrangement to the isomeric cyanamide group N–CN is also
possible. Such cyanamide carbons typically appear at 115 ppm in
the 13C NMR spectra.[13] Nevertheless, a surprisingly high thermal
stability of the NCN moiety up to 1000 ◦C was also found for other
silylcarbodiimides.[11] However, it should be pointed out that
most authors examining the pyrolysis of polysilylcarbodiimides
used argon and nitrogen atmospheres.
Furtherinformationonthechemicalstructureoftheamorphous
ceramics obtained upon pyrolysis at 1000 ◦C may be gained
from detailed analysis of the elemental composition and element
distribution. Several samples where therefore investigated by
combustion analyses, EDX and EELS. Chlorine was not detected in
the samples. Depending on the various processing parameters
which include synthesis of the xerogel precursors (catalyst,
c
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H.-J. Cheng et al.
(a)
N=C=N
Si-N / Si-O
(b)
250
200
150
100
50
0
ppm
(c)
10
0
-10 -20 -30 -40 -50 -60 -70 -80 -90 -100 -110 -120 -130ppm
Figure 9. FTIR (a), 13C (b) and 29Si (c) solid state NMR spectra of the Si/C/O/N xerogel pyrolyzed under NH3 at 1000 ◦C.
temperature, aging time, solvent) and pyrolysis conditions (NH3
atmospheres, powder or bulk sample, heating rate, annealing
temperature and holding time) the overall composition varied
significantly in the range of Si, 36–49; C, 2–24; N, 20–28; O, 10–19
(weight-%). This has to be compared with the ideal composition
of a [SiOSi(NCN)3]n with completely intact carbodiimide groups
and Si2N2O which may be obtained upon full decomposition of
the carbodiimide units and complete removal of the carbon,
i.e. Si, 29.1; C, 18.8; N, 43.8; O, 8.3 and Si, 56.0; C, 0.0; N,
28.0; O, 16.0, respectively. In general, we found a decreasing
carbon content with increased pyrolysis temperatures especially
in ammonia atmospheres. This is expected owing to the reactions
of NH3 to form volatile carbon compounds such as cyanamide
H2N–CN and its dimer cyanoguanidine and trimer melamine, HCN
or methane, which all lead to a decreased carbon content of the
solid residue.[53,54] Selected ceramic products were thoroughly
investigated with EELS. The EEL-spectra of samples pyrolyzed
under NH3 at 1000 ◦C show very similar features to Si2N2O, which
contains the analogous OSiN3-units (Fig. 11).
Morphology of Silylcarbodiimide-derived Si/C/O/N ceramics
HR-TEM studies on xerogel samples synthesized at 75 ◦C and
pyrolyzed at 1000 ◦C under ammonia atmosphere clearly show
that mesoporous Si/C/O/N materials are formed. Typical TEM
images are shown in Fig. 12(a, b). An amorphous and very porous
microstructure was observed in powdered as well as bulk samples.
Nitrogen adsorption measurements did not show any significant
porosity, indicating that the pores are closed.
A statistical analysis of the pore sizes (open as well as closed
pores observed by TEM) indicates a rather narrow size distribution
with an average pore diameter of 16.4 0.4 nm (Fig. 13). This
c
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Sol–gel derived Si/C/O/N-materials
composition of the starting polymer is the major point of interest
for our future work. While for Si/C/O materials a purely statistical
distribution among possible SiC4−xOx sites has been reported,[55]
a strong influence of the precursor structure on the ceramic
structure has been found for Si/C/N materials, especially when
silazanes are compared with silylcarbodiimides.[56]
Conclusions
The following conclusions can be drawn from the reported
investigations:
(1) The sol–gel preparation of hybrid polysiloxane/poly-
silylcarbodiimide gels, i.e. the controlled introduction of oxy-
gen into Si/C/N xerogels and ceramics, is reported for the
first time. The pyridine-catalyzed gel formation and cross-
linking between hexachlorodisiloxane (Cl3Si–O–SiCl3) and
bis(trimethylsilyl)carbodiimide can be performed at temper-
atures between 40 and 90 ◦C.
Figure 10. XRD pattern of the sample pyrolyzed in NH3 at 1000 ◦C,
indicating the amorphous nature of the solid.
(2) Amorphous, hydrolytically sensitive xerogels are obtained,
which consist of a random network of Si–O–Si and
Si–NCN–Si units. The same structural feature is present in
the macrocyclic molecule [SiPh2 –O–SiPh2(NCN)]2. NMR data
and a single crystal structure of this compound provide
detailed information as a model for the chemical structure of
the Si/C/O/N gels.
(3) The gelation time is decreased with increasing temperature
and increasing the amount of pyridine. In the latter
case, precipitates of pyridine–chlorosilane adducts were
observed, consisting of the hexacoordinated chlorosilane
corresponds well with investigations on the preparation of
silylcarbodiimide-derived Si/C/N membranes.[22]
Preliminary investigation of the pyrolysis behavior above
1000 ◦C indicates that pore sizes significantly decrease to values
around 10 nm. Graphitic carbon is not detectable, but nano-sized
carbon domains segregate around 1400 ◦C. Silicon nitride starts to
crystallize in the same temperature range. A detailed investigation
ofthecrystallizationbehavioroftheamorphousSi/C/O/Nceramics
and the dependence of the Si/C/O/N network structure after
annealing at various temperatures on the structure and chemical
(a)
(b)
pyrolysed in NH3
1000°C
Si2N2O
pyrolysed in NH3
1000°C
Si2N2O
100
110
120
130
140
150
400
410
420
430
440
450
Energy [eV]
Energy [eV]
(c)
pyrolysed in NH3
1000°C
Si2N2O
530
540
550
560
570
580
Energy [eV]
Figure 11. EEL-spectra of Si/C/O/N ceramics obtained by pyrolysis of the xerogel in NH3 at 1000 ◦C in comparison with the spectra of silicon oxynitride
Si2N2O. Si–L2,3 edge (a); N–K edge (b); and O–K edge (c).
c
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H.-J. Cheng et al.
Figure 13. Pore size distribution derived by measuring the diameters of
200 of the pores shown in the HR-TEM micrographs in Fig. 8. An average
pore diameter of 16.4 0.4 nm was determined.
above 1000 ◦C and in different atmospheres to form (partially)
crystalline ceramics.
Acknowledgment
This work was supported by the Deutsche Forschungsgemein-
schaft, DFG (Project KR 1739/18–1 and WO 482/19-1), the CSC
(China Scholarship Council) project and a cooperation between
TU Bergakademie Freiberg and Tianjin University, China. The au-
thors thank the colleagues of the Departments of Chemistry and
of Material Science at TU Bergakademie Freiberg, Germany, for
the analysis work (NMR, XRD, FTIR, RAMAN, SEM, TEM, elemental
analyses, and nitrogen adsorption method).
Figure 12. TEM (a) and HR-TEM (b) micrographs of a typical Si/C/O/N
xerogel obtained by a pyridine-catalyzed reaction of hexachlorodisiloxane
with BTSC after aging and drying followed by pyrolysis under dry ammonia
at 1000 ◦C. The amorphous material is characterized by homogeneously
distributed, irregular mesopores as marked by the arrow in (a).
Supporting information
adduct Cl3Si–O–SiCl3(Py)2, which was structurally analyzed
by single crystal X-ray diffraction. Interestingly, only one of
the two Si atoms is activated by coordination to pyridine.
The latter observation was also supported by theoretical
investigations.
Supporting information may be found in the online version of this
article.
(4) There is neither a negative effect of the Cl3Si–O–SiCl3(Py)2
precipitates on the gelation process, nor a positive effect of
the pyridine-catalyzed cross-linking on the ceramic yield after
pyrolysis of the xerogels. Instead, the reaction temperature
and aging time are crucial for complete cross-linking.
However, there are significant differences in the structure
and morphology of the xerogels obtained with and without
pyridine.
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