a-semana 1-carrazana ferrimagnetic paper in situ method
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IEEE TRANSACTIONS ON MAGNETICS, VOL.
3 1 ,
NO. 6 NOVEMBER 1995
agnetic Paper Obtained By In Situ Synthesis Of Substituted Ferrites
by: Jorge A. Carrazana-Garcia; M. Am uo Lbpez Quntela ; Jose Rivas Rey.
(Physical-Chemistry Dept.,
Univ of
Santiagode Compostela, Spain,
postal
code: 15706, Spain)
Absfruct-
A procedure for prod ucing magnetic paper by in
situ synthesis
of
substituted (CO,
Ni) ferrites is described
Ferrites are formed in the presence of cellulose fibers and then
forced
to
enter into the fiber lumen where they stay protected
from mechanical influences. The remaining particles are
washed out of the fibers leaving its external surface clean,
so
that its papermaking properties are not affected. Multilayer
papers were prepared in which the colored magnetic layer
is
hidden behind white printable surfaces. Finally, the magnetic
properties of the different kinds of obtained papers are
compared with typical com mercial stan dards, like subway
tickets and airport boarding passes.
I. INIXODUCTION
The magnetic properties are an added value to the
traditional characteristics of paper, &ng it an attractive
objective in such diverse fields
as
recording media, novel
handling systems and
security
paper uses.
In
the literature
there are reports about :;uperparamagnetic paper obtained
through in situ synthesis of femtes [1,2,3] and concerning
ferrimagnetic paper obtained by lumen loading its fibers
[2,3.4] with commercial magnetic pigments (magnetite and
maghemite). It has been established that in order to diminish
the loss of mechanical strength and the poor retention of the
inorganic particles, it is necessary to accomplish the
introduction of the powder into the lumen of cellulose fibers
and then effectively wash the outer surface
[5 6 7].
In this
way, the lumen loading c m be considered a well-established
method for introducin g powder ad ditives into cellulose fibers
to give them important characteristics mith mini
damage to their paperrnaking behavior E8f.The in situ
approach allows a better control of the paper magnetic
properties without
a
considerable injury to their mechanical
properties. At the same time, it
permits
a high variety in the
type of charge loaded into the lumen of the fibers. We report
in this work the obtainment of papers
with
sigmficant values
of coercive field combining the two procedures (in situ
synthesis and lumen loa&ng). Substituting a fraction of
ferrous ions by cobalt, nickel and strontium ions in the
reaction of synthesis of the magnetic material with the
presence of cellulose fibers, the paper obtained exhibits
coercive fields, saturation and remanent magnetization
comparable to commercial standards obtained by more
complicated non i n situ procedures, like a magne tic band of a
subway ticket (as shows
fig.l),
or an i rport boarding pass.
In
our
work w e have used pape r pulp obtained from
Manuscript received Fr eb uay 17 ,1995
J.A. Carrazana; e - ma il u s c q f , j c g @c e s g a . e s Th i s w or kw a s suppo r t ed
by the Sunta de Galicia with the project XU GA 206 12B93 and thanks
to
a
doctoral fellowship of the Insti lkto de Coo pe rac ih Iberoamericano.
sugar cane bagasse. This is, to our knowledge , the first work
reported
wth fibers derived
from
plants other than trees.
11.
EXPERIMENTAL
Mdm zk.
Chemicals reagent grade were used: (F eCI 24H 20,
CoC1~6H20,
BaC12.2H20) from Merck and (NaOH.
Ni N03)26H20) from Aldrich. Suspension and solutions
were prepared with bidistilled water. In generai, no special
care was taken to isolate the system from atmospheric
oxygen. In fact, the u lterior oxidation from Fe2+ to Fe3+
(necessary or the femte formation) is achieved by means of
the dissolved 02 mainly during stirring. In some
experiments we use kr ft pulp from
eucalyptus ssp
but the
pM cipal f i k r source was chemical paper pulp o'otain2d from
sugar
cane bagasse both bleached and not bleaclted.
A
diagram of the process for malung the
magnetic paper
ispresented
in
fig.2.
m
M
i
H 1 - 1 0
Fig.1. Hystsrssis
imps-
.M m d g of
f m t e ) Vs.
Oe) at roan
temperature
*.-*- maget ic band of
a
subway ticket,
-
aper
I
see
t a b k
I)
A. Magnetic
Paper Formation
Syn fAeh .A l g 3g sample of pulp is dispersed i water
200
- 5
mL) using a laboratory mechanical stir:cr with
digital speed control. The temperature is fixed a t tke Gesired
value introducing the reactor in a thermostatic tath or using
a jacketed glass. Quantities of inorganic chemicals were
calculated s that the final fem te concentration was'in the
range from 0.04
M
to
0.1
M. Cations were added, generally
as
hydrated chloride salts, after which (about five minutes
later for ion exchange) stoichiometric amounts of NaOH
were aggregated. Thereafter, the formation of a dark green
precipitate can be observed, if the experimental conditions
established (temperature, ions nature and concen, r a h n , etc.)
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aq.
suspens ion
of paper pulp
in si tu synthes is of
I
surface c leaned
Fig.2. Diagram of the process
are the optimal for the formation of Green Rust. After the
particle formation, the system is aged at the same
temperature for several hours
1 h- 16h).
Besides the precipitation from the liquid phase, the in
situ approach introduces an important aspect in this
analysis: the formation of particles directly in the cellulose
walls [9]. The pulping (a step in the obtainment of paper
pulp from its vegetal source) dissolves lignin and most
hemicelluloses in fibers, leaving their
walls with
an irregular
structure of micropores and chaps. In addition, the chemical
cooking provokes the combination of cellulose with anionic
functional groups that can capture metai ions (in
this
case
MP that prec ipitate as hydroxides when a n alkali is added to
the suspen sion. In that way, part of the
M(0W2
is formed
directly associated to the fiber in the bulk of its tvalls.
Another strong influence of cellulose fibers in the system is
that local pH inside the fiber wall is much lower than in the
volume of the liquid
[lo],
a fact to take into account during
pH-sensitive reactions, like the fem te synthesis [
1
1 o
151.
Impregnation. The suspension is strongly stirred (2000
rpm or more) during 1 hour vrith a teflon stirring paddle,
forcing the particles to ente r the lum en of fibers. Th is process
can readily be followed with an optical microscope. Particles
can only penetrate into the lu men of the fibers, if they have
diameters lower
th n
the dimensions
of
pits (little holes
distributed along the fiber walls which communicate the
lumen with the outside). The in-out
flus
caused by turbulent
agita tion durin g the im preg natio n step introd,Jc::s the
particles into the lumen.
The formation of ferrite in the fiber walls arri t.ie lumen
loading determine the level of impregnation
th-
t are
achieved during the process. Among the Iariabies that
control
this
aspect are the particle size and the effect- eness
of agitation during the impregnation step.
Also
the ;;Ked at
which precipitation occurs seems to br inq.ortant.
Appropriate control
of
the process permits impregn 1tion to
reach similar levels to those reported for other lum e,i
1
oading
processes. Nevertheless, a good qualih of partlcles formed
can overcome low impregnation levels, as show
:
alues
for paper
I
(see table I)
Table
I. a
b12+
n the starting suspension and their molar ratios; b synthesis temperature; c procedure of addition
of
NaOH to the solution; d c i q w Ilogaphic
phase according;yRD;
e
Hc Oe)of the magnetic paper at room temperature;
f
,g remanent
and
saturation magnetization at room temperature emu/g xfened to
ash content and, in parenthesis, referred to paper mass;
h
ash content (weight per cent
)
of papers obtained,
In the
case of paper
E,
the coiitributio.1 of .adin from
the white layersused, makes impossible the assignationof the ash content to the impregnated ferrite); indicates he presence
of
big p0.25
mm)
particks, gnerally
vitreous and poorly magnetic, contributing to ashes and reducing the specific magnetic parameters.; Q
i
ndicates the presence of a magnetic field during the
synthesis; + indicates that
the f i b
were lumen loaded with presynthesized particlzs, The fmal hvo rows correspond
to
PC
diskette
and he magnet.: band of a
subway ticket,
as
references.
I_ _ _____ _ _ II _ _ ___ _ _ __ _ __ _ _ _ ____LI
a
C
e 11
d
f g ,
Paper cations T
alkali
phase Hc hl r IS
ashes
x
Fe 65OC
solutionjstir.
y-FelO1 7.4 0.95 (.03) 51 (1.8)
3.5
B
Fe 90C
pekts/stU *pFezOl/Fe304
31 4.2 0.4) 59.4(5.7) 9.6
C 8
Fe 65C pellets/stir.
y-Fe~O]iFejO4 56 5.2 (0.2) 43.9(1.7) 3.9
107 17.9 (5.7)- 66.7 (21) 31.8
Fe
65OC pek Wh0 stir y-FezOl
E 8
Fe
65C pellasino stir y-Fe201 . 93 (3.74) (14.1) ?
F Fe/Co = 4/1
25C
pelletdstir
FeOOWCo00H
6
O.l(O.007) ll.l(O.73) 6:j
G Fe/Co
=
411
41C pelletdstir CoFezO4/FesOd 327 18.5(2.05) 62.7 (7.0) 11.1
H FdCo = 211
65OC
pelletdstir
CoFe2O~iFe104 393 19.1 (2.43) 65.1 (8.3) 12.7
I FdCo
=
4/1 90C
pelletdstir
CoFe204/Fej04 333 18.1 (1.34) 61.3 (4.5) 7.4
J Q Fe/Co = 211 65C pelletdnostir CoFel Od/Fe~04 520 4.36(0.66) 17.3 (2.6) 15.1
K FeNi
=
2/1 65C pelletdstir (Ni,Fe)Fet04 52 1.7
0.05) 10
0.32) 3.2
L Fe/TJi = 2/1
9OoC pelletdstir (Ni,Fe)Fez04 54 9.23 (0.5) 44.2 (2.4) 5.4
M
FelCoMi= 2/.5/.5
90C pelletsstir NiFe204 169 10.8
6 .5 )
40.2 (2.4) 6
N Q Fe/Ni
=
2/1
65OC
pelletsnostir NiFe204 83 9.98 (1.7) 42.1 (7.2) 17.4
Fe/Ba = 12/1
90C pelletsstir Fe3O4+BaCO3 83 11 (0.4) 62.9 (2.4) 3.7
P FeBa = 1/1 9OoC pzllstslstir d3a0.yFe20,
5
0.2 (0.01)
9 7
0.5) 5.2
Q Fe/Ba = 111
9oC
pelletsstir
BaFellOls+ 2722 3.3 (0.04) 6.2 (0.08) 1.2
addition
.........................................................................................................................................................................................................................
a-FezOl
PCDisk y-Fe203 717 45.5 76.2 4
7
Fe203 242 10.3 26.1
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(in m u ) at fixed extemal fieldwith temperature (in IC fw
ferrite particles corresponding to paper
I.
LEGEND: kt vs.T at 50 Oe (curve
I),
Washing. The excess particles remaining in the suspension
are filtered out through a 0 .25
mm
sieve. Those particles
adhered at the extemal surface of fibers are removed by
washing. Two procedures were implemented for this
purpose. In the first, discontinuous process, the suspension
of fibers is stirred
1000
rpm) and filtered alternately,
changing the water in every batch. In the continuous
procedure, the suspensi.on is stirred within
a
meshed
cylinder which is placed into
a
glass
nlth
an upper water
outlet. and a water flow from inside to outside of about 2
liters per minute. The particles collected are washed until
negative chloride test, repeatedly decanting the suspension
with the help of a magnet, and drying at 60C. The optical
photograph in fig.3 b shows a cellulose fiber after the
washing step. with its lumen filled with femte particles and
the outer su rface clean, rrxidy to form m agnetic paper.
Sheet formation. Tlhe sheets of paper were formed
pressing the pad of fiber:; formed during the filtration of the
washed suspension in a Buchner funnel, and then vacuum
drying them in an oven at 60C. The colors of magnetic
papers formed range from light brown to black. This is
irrelevant for many of its potential applications, however,
printing demands low colored surfaces, white
if
possible. We
have obtained bi- and tri-layer papers where the colored
magne tic fibers are. behi.nd white printable surfaces. These
multilayer papers were compressed and dried in a press at
90
kglcm', heating the paper on both sides. The
magnetic response of these papers
J
and
E
respectively) are not affected at all by this
inclusion between w ht e layers.
B.
Analysis
Techniques
Structural analysis has been performzd in a
Phdips automatic powder X-ray dffraction
system
(XRD)
goniometer
P W
1820, w3h Cu
K a
radiation, of wavelengths: a1=
1
54439
and
a2=
1.54060 a2/ul intexity ratio of
0.5)
. The obtained diffractograms were
matched with a data base (TAD D+oO) end were
also used for crystallite size estimations
Electron micrographs were take
by a
Phdips
CM
12 transmission electron
microscope with an accelerating voltage of
120 kV.
Paper ashes
were meas ured b;t drying the
sheet at
105C
overnight and the? tuning 4
hours a weighted sample in an oven
at
925C.
The ashes content of pulps were determined
sepa rately for correctlon purpose? g:ving the
values of: eucal?ytus kraft
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Cuba errite. Th e subs titu tion of som e Fe ions by
CO?,
ermits the formation of papers with hlgher values of
Hc. Experiments w ere performed from room temperature to
90C. The spinel phase (C oFe204a n d o r
Codoped
Fe304)
s
formed above 40OC. Below
this
temperature,
a
mix of iron
and cobalt oxyhydro xides, with bad m agnetic response, are
formed.
Typical values of the magnetization parameters of
the samples are reported in table
I
(papers
F
to
J)
Notice
that the Hc = 520 Oe for paper
J
was attained with the
reaction vessel that was placed over a 3000 Oe magnet (like
those marked with n table I) and by adding NaOH pellets
without stirring.
TGA measurements in presence of a magnetic field for
particles charged in paper I, show a sharp fall in the sample
weight above 500C, and the fitted value lies between the
tabulated [16]Tc for magnetite (585C) and cobalt femte
(520C). This seem s
to
indicate the presence of a mixed or
non stoichiometric magnetic phase as c an be expected from
the cationic composition of the starting solution, in this
case: Fe
/CO=
4 1 . Measureme nts of the magnetization
at fixed external field in
this
temperature interval, with a
step of
2
degrees, were performed
wth
DC-VSM at constant
100
Oe and
50 Oe
field. Figure
4
shows the
M
Vs. T
depen dence s obtained and their first derivatives (computed
with finite increments) looking for an estimation for the Tc
[17] by the inflection point in the M
Vs.
T curve. There is
a minimum in the first derivatives at approximatelly 547C
which can
be
interpreted as the presence of a cobaltite with
some defect in its iron conten t.
Blagev and Him [18] have reported a surface doping of
magnetite
wth
cobalt, thanks to an ionic interchange of
solids particles with a CO solution. Following the
suggestion, we redisperse the paper A (formed with femte
charged fibers) in a 0.055 m o m solution of CoC12.6H20at
9OoC for 15 hours. The magnetic values for the paper
formed later: Hc =
5.85
Oe,
Mr
= 0.04 emu/g, Ms = 1.3
emu/g (referred to paper weight) and ashes 3.02
,
do not
seem to jus* Co dop ing in
this
case. Similar results were
obtained with paper
B.
Other ferrites. Look ing for a diversifica tion of the
magnetic charge installed in paper fibers, we used as well
Ni and Ba as substitute cations in the starting solution.
The results in table
I
show that the ferrimagnetic spinel
phase is formed in the first case, with Ni, but not with Ba.
Then, for this last element, we tried another way. The
particles were synthesized without fibers and once washed,
were heated 16 hours at 9OOOC in air and in argon
atmosphere. Finally the fibers were impregnated in a
traditional lumen-loading procedure. The
XRD
of thermal
treated particles shows the presence of barium hexaferrite
phase (13aFe12019) nd hysteresis curves reveal a very high
coercive field, greater than 2700 Oe. However, high
amounts of a-F e2 03 are also formed, and the specific
saturation and remanent magnetization are quite small
(paper
Q).
Other handicaps are that the advantages of in
situ synthesis are lost, and that during the thermal
treatment, sintering occurs,
so
that the fraction of suitable
particles for lumen loading
is
very small. This effect can be
observed in the low ash co ntent of paper Q .
IV. CONCLUSIONS
Magnetic paper with useful range values of Hc,
F r
and
Ms, can be prepared combining the in situ syr-thesis of
femtes and lumen loading of cellulose fibers. The cobalt
femte or Codoped magnetite formed in a we- w q in the
presence of paper fibers, allows the obta inmen t of mhgnetic
papers with coercive fields as high as
500 Oe.
Other
substitute cations (Ni, Ni+Co) can be used in the s m e way
but the values of Hc achleved are smaller. It nas been
observed that the precipitation rate is
an
important miable
in the control of the m agnetic properties of particles formed.
The method developed in this work permits the obtainment
of papers loaded to high levels, with a very good retention of
the magnetic filler and without damage to :heir paper-
making properties. The inclusion of the colored magnetic
layer b e h w x white surfaces improve its sptical
cha rac teristic s and does not affect the magneti,: re;pmse of
the paper.
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Slarchessauit, S Ricard
P.
Riouu;
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224 pp 133-1 39, : Ic2
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H.
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Fig.
3
Optical photogaphs of celhlose fibers,a: at the begining of the process,
b:after the
washmg
step,
with its
lumen loadedwith ferrite and the outer surface clean