A soft lithium-titanium zinc ferrite is provided which has a resistivity of at least 10.sup.7 ohm-cm measured at 20.degree. C. and 100 volts, prepared from a mixture of oxides and/or carbonates in proportions, calculated as oxides, satisfying the formula EQU Li.sub.a Ti.sub.t Zn.sub.z Mn.sub.m Fe.sub.b O.sub.4 wherein PA1 a=0.5 (1+t-z) PA1 b=0.5 (5-3t-z-2m-5.epsilon.) PA1 0.ltoreq.t.ltoreq.0.08 PA1 0.50.ltoreq.z.ltoreq.0.60 PA1 0.005.ltoreq.m.ltoreq.0.035 PA1 0.02.ltoreq..epsilon..ltoreq.0.06 and .beta. from 0.0015 to 0.05 molar equivalent of Bi, wherein the Bi is present in the starting mixture or is added after calcining. A process for producing these ferrites is provided, together with magnetic deflectors incorporating them.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to soft lithium-titanium-zinc ferrites having 
a resistivity of at least 10.sup.7 ohm-cm measured at 20.degree. C. and 
100 volts, which are particularly well-suited to use in the manufacture of 
magnetic deflectors, especially deflection yokes for television receivers. 
2. Description of the Prior Art 
The ferrites of the present invention are particularly useful in the 
manufacture of electronic components for use in low-frequency 
applications. Materials currently in use for this kind of application are 
primarily manganese-zinc ferrites and magnesium-zinc ferrites. Each of 
these has disadvantages which render them less than ideal with respect to 
the combination of properties which are desirable in such applications. 
Manganese-zinc ferrites can be produced at low cost and have high initial 
permeabilities, but their resistivities are low, on the order of 1,000 
ohm-cm. As a consequence, they must be electrically insulated when used in 
the manufacture of magnetic coils, and this substantially increases the 
total cost price of devices in which they are used. 
Nickel-zinc ferrites may attain a resistivity as high as 10.sup.5 ohm-cm, 
but their cost is prohibitive and their magneto-striction is large, which 
generates background hum in television receivers. 
Magnesium-zinc ferrites having minor substitution of manganese and copper 
are capable of resistivities of 10.sup.6 ohm-cm, but they cannot be 
manufactured below a temperature of 1250.degree. C. 
A need therefore, continues to exist for a ferrite having high resistivity, 
high initial permeability, high saturation induction and high Curie point, 
and which can be manufactured at relatively low cost and at relatively low 
temperatures. 
SUMMARY OF THE INVENTION 
Accordingly, one object of the invention is to provide a soft 
lithium-titanium-zinc ferrite having a resistivity of at least 10.sup.7 
ohm-cm, and initial permeability of at least 300, a saturation induction 
of at least 1500, and a Curie point of at least 150.degree. C. 
Another object of the invention is to provide a ferrite which can be 
manufactured at a temperature of 1100.degree. C. or below. 
A further object of the invention is to provide a ferrite which can be 
manufactured from starting materials whose purity need not exceed 96%. 
Yet another object of the invention is to provide soft 
lithium-titanium-zinc ferrites which may be manufactured at a low cost. 
A still further object of the invention is to provide a method for 
manufacturing such soft lithium-titanium-zinc ferrites, and to provide 
magnetic deflectors incorporating these ferrites. 
Briefly, these objects and other objects of the invention as hereinafter 
will become more readily apparent can be attained by providing a soft 
lithium-titanium-zinc ferrite having a resistivity of at least 10.sup.7 
ohm-cm measured at 20.degree. C. and 100 volts, manufactured by forming a 
mixture of metal oxides and carbonates, crushing the formed mixture, 
calcining the crushed mixture in an air atmosphere at about 700.degree. C. 
for about 2 hours, and sintering the calcined mixture in an 
oxygen-containing atmosphere at from 950.degree. to 1100.degree. C. for 
from 1 to 18 hours; wherein said mixture consists essentially of metal 
oxides and carbonates in proportions, calculated as oxides, which satisfy 
the formula 
EQU Li.sub.a Ti.sub.t Zn.sub.z Mn.sub.m Fe.sub.b O.sub.4 
wherein 
a=0.5(1+t-z) 
b=0.5(5-3t-z-2m-5.epsilon.) 
0.ltoreq.t.ltoreq.0.08 
0.50.ltoreq.z.ltoreq.0.60 
0.005.ltoreq.m.ltoreq.0.035 
0.02.ltoreq..epsilon..ltoreq.0.06 
and .beta. from 0.0015 to 0.05 molar equivalent of Bi, as the oxide or 
carbonate, wherein said Bi is present in said mixture prior to crushing or 
is added after calcining but prior to sintering.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The ferrites of the present invention are prepared by a series of steps, 
beginning with a mixture of powdered metal oxides and other salts, which 
are crushed, calcined, optionally recrushed and compacted, and sintered, 
in a process similar to the type used for the manufacture of 
polycrystalline ferrites with spinel structure. The ferrites of the 
present invention are used in the manufacture of magnetic deflectors, 
especially deflection yokes for television receivers, wherein the magnetic 
deflection yoke comprises at least one of a ferrite core and a ferrit 
housing wherein the improvement comprises the ferrite being the ferrite of 
this invention. 
The components of the starting mixture are present in proportions which 
satisfy formula I 
EQU Li.sub.a Ti.sub.t Zn.sub.z Mn.sub.m Fe.sub.b O.sub.4 (I) 
wherein 
a=0.5(1+t-z) 
b=0.5(5-3t-z-2m-5.epsilon.) 
0.ltoreq.t.ltoreq.0.08 
0.50.ltoreq.z.ltoreq.0.60 
0.005.ltoreq.m.ltoreq.0.035 
0.02.ltoreq..epsilon..ltoreq.0.06. 
Bismuth is either included in the starting mixture in the proportion of 
from 0.0015 to 0.05 molar equivalent, or that amount of bismuth is added 
to the calcined material prior to sintering. The bismuth is a dopant which 
is essentially not a part of the composition of the principle crystalline 
phase, but constitutes a second phase located between the crystallites of 
the material. Bismuth is usually added in the form of its oxide or 
carbonate, or a mixture thereof. Preferably, the components of the 
starting mixture are present in proportions which satisfy the following 
formula 
EQU Li.sub.a Ti.sub.t Zn.sub.z Mn.sub.m Fe.sub.b O.sub.4 +.beta.Bi 
wherein 
0.ltoreq.t.ltoreq.0.06 
0.52.ltoreq.z.ltoreq.0.58 
0.03.ltoreq.m.ltoreq.0.035 
0.055.ltoreq..epsilon..ltoreq.0.060 
0.010.ltoreq..beta..ltoreq.0.015 
a=0.5(1+t-z) 
b=0.5(5-3t-z-2m-5.epsilon.) 
A small proportion of impurities such as carbon, silicon, calcium, 
magnesium and aluminum may be present without deleterious effect, up to a 
total of 5% by weight. Starting materials having impurity of at least 96% 
with respect to the critical elements are used, with the understanding 
that they may contain the aforementioned non-interfering impurities. 
It is preferable to use starting materials having a high surface area, 
since the manufacturing steps are facilitated thereby. It has been found 
that calcining may be accomplished more quickly and at a lower temperature 
when very fine, highly reactive powders are used. There is a high 
correlation between a high state of subdivision, high reactivity and high 
surface area. 
The preparation of materials according to the invention shown in the 
examples which follow used starting materials having approximately the 
following surface areas: 
TABLE I 
______________________________________ 
COMPOUND SURFACE AREA 
______________________________________ 
Li.sub.2 CO.sub.3 1.8 m.sup.2 /g 
TiO.sub.2 7.3 m.sup.2 /g 
ZnO 4.6 m.sup.2 /g 
MnCO.sub.3 11.7 m.sup.2 /g 
Fe.sub.2 O.sub.3 3.7 m.sup.2 /g 
Bi.sub.2 O.sub.2 CO.sub.3 
3.3 m.sup.2 /g 
______________________________________ 
Ferrites prepared according to the invention have resistivities at 
20.degree. C. and 100 volts which are at least 10.sup.7 ohm/cm, and 
preferably at least 10.sup.9 ohm/cm. Their initial relative permeability 
at 20.degree. C. is at least 300, and preferably, at least 500. Their 
saturation induction at a magnetic field of 5 oersteds and a temperature 
of 100.degree. C. is at least 1500, and preferably at least 2000. They 
have a Curie point of at least 150.degree. C. 
The manufacturing process for the ferrites of the invention will now be 
considered step by step, so that the details of the process may be better 
understood. 
FIRST STEP: PRODUCTION OF A HOMOGENEOUS POWDERED MIXTURE HAVING A HIGH 
SURFACE AREA 
The starting materials are weighed to a precision of 100 ppm, in the 
proportions indicated by formula (I). Heating losses and addition of iron 
during crushing are both taken into account. As will be seen in more 
detail in the examples, it is advantageous to have a slight deficiency in 
iron in the composition of the final material. In order to achieve a 
deficiency in iron .epsilon.' in the final product composition, a 
deficiency of iron .epsilon. is chosen in the starting material, such that 
the gain in iron attendant upon crushing with steel balls in a steel 
vessel is taken into account. This gain in iron during crushing depends in 
turn upon the type of crusher used, and the duration of the crushing 
operation, as well as upon the nature and grain size distribution of the 
materials being crushed. If .epsilon..sub.1 corresponds to the gain in 
iron attendant upon crushing, it is seen that 
EQU .epsilon.=.epsilon.'+.epsilon..sub.1. 
For examples 1-17 below, a crusher was used which operates by attrition in 
liquid medium. In order to avoid partial dissolution of lithium carbonate 
by water, an alcohol may be used, which is later eliminated by drying. In 
the examples, demineralized water was used, and the crushed material was 
then dried and carefully sieved. A crushing operation of about 30 minutes 
duration such as that described above, followed by drying, sieving and 
further mixing of the dry powders, produces a final powder having a 
surface area of 6 m.sup.2 /g, from starting materials with the surface 
areas specified above. The product has an even grain size, with an average 
grain diameter of 0.1 micrometer, with a small average deviation. 
SECOND STEP: PRODUCTION OF THE FERRITE PHASE AND SHAPING OF ARTICLES FOR 
SINTERING 
(a) Calcining: 
The powder obtained from the first step is calcined for about two hours at 
about 700.degree. C. in an air atmosphere. Calcining is carried out with 
the minimum possible compression of the powder in the oven. Loss of carbon 
dioxide is greatest at about 400.degree. C. and the rate of formation of 
the ferrite phase is greatest at about 600.degree. C. The ferrite phase is 
more than 95% formed at the conclusion of the calcining step. 
It has been shown that calcining in the absence of bismuth favors a rapid 
formation of the ferrite phase at a temperature not exceeding 700.degree. 
C., with a relatively small increase in grain size. This is desirable in 
subsequent steps of the operation, and contributes to the high quality of 
the final product. 
(b) Crushing with bismuth: 
Bismuth carbonate powder weighed in the first step according to the 
formula, is added to the calcined material obtained from the preceeding 
step. The bismuth is calculated on the basis of .beta. mole of Bi for each 
mole of formula (I). A further crushing of 30 minutes duration is effected 
under the same conditions used in the first step. 
(c) Sieving and shaping: 
The cake resulting from the second crushing is dried, and the powder 
obtained is sieved. The sieved powder is isostatically pressed, according 
to conventional techniques, under a pressure of 1.5 to 2 tons/cm.sup.2, 
preferably at about 1.6 ton/cm.sup.2. Using the starting materials 
specified, and under the operating conditions described above, compact 
discs having about 55% of the theoretical density may be obtained without 
the use of a binder. 
It is possible to use binders, which are added either during the second 
crushing operation or between drying and sieving, and to form the product 
by compression molding at a pressure comparable to that used above. This 
alternative is generally used for articles having irregular shapes. When a 
binder is used, it is subsequently driven off by reheating either before 
or at the start of the sintering step. 
THIRD STEP: SINTERING 
The formed articles are then heat-treated for from 1 to 18 hours, 
preferably for 16 hours, at a temperature of from 950.degree. C. to 
1100.degree. C., preferably at about 975.degree. C., for from 8 to 16 
hours, in an oxygen-containing atmosphere, preferably an oxygen 
atmosphere. Ferrites sintered in an air atmosphere have properties quite 
similar to those sintered in an oxygen atmosphere, except for their 
resistivities, which are lower by from 1 to 2 orders of magnitude. 
The following examples will further illustrate the effect of varying one or 
more parameters on the properties of the resulting product. The examples 
are provided herein for purposes of illustration only, and are not 
intended to be limiting unless otherwise specified. 
EXAMPLE 1: INFLUENCE OF DEFICIENCY OF IRON AND OF MANGANESE CONTENT 
The parameter .epsilon. was varied from 0.02 to 0.06, and the parameter 
.epsilon..sub.1 corresponding to the addition of iron during crushing was 
estimated at 0.02 from actual measurement. In the case where 
.epsilon.=0.02, the deficiency in iron in the final product, .epsilon.', 
will be seen to be approximately 0. 
Table 2 gives the results of variation of .epsilon. on the density d in 
g/cm.sup.3, the magnetic moment 4.pi.M.sub.S in gauss, the real component 
.mu.' and the imaginary component .mu." of the relative permeability and 
the resistivity in ohm-cm. 
TABLE 2 
______________________________________ 
Resistivity 
Sample No. 
.epsilon. 
d 4.pi.M.sub.S 
.mu.' 
.mu." 
in ohm-cm 
______________________________________ 
1 0.02 4,869 3910 300 4 6 .times. 10.sup.7 
2 0.04 4,885 3890 410 6 2 .times. 10.sup.9 
3 0.06 4,881 3810 440 5 1.4 .times. 10.sup.10 
______________________________________ 
In the three examples shown, the coefficients of formula (I) were as 
follows: 
t=0.05 
z=0.5 
m=0.035 
.beta.=0.005 
The sintering was carried out in an oxygen atmosphere at 975.degree. C. for 
16 hours. 
The permeability was measured at 50 KHz with an alternating magnetic field 
of 7.5 mOe. It may be seen from Table 2, that the resistivity reaches a 
very high value when the initial deficiency in iron exceeds 0.04. The 
resistivity is measured on a disc having a diameter of 10 mm and a 
thickness of 0.5 mm, with both faces silvered except for a guard ring of a 
few tenths of a mm. The measurements were made using a continuous voltage 
of 100 volts in an oil having a resistivity of 2.times.10.sup.13 ohm-cm at 
20.degree. C. 
A mixture of starting materials having the same composition as that used 
for Sample 2 was divided in three parts and sintered at three different 
temperatures. The results are shown in Table 3. 
TABLE 3 
______________________________________ 
Sintering Resistivity 
Sample No. 
Temperature in ohm-cm .mu.' .mu." 
______________________________________ 
2A 975.degree. C. 
2 .times. 10.sup.9 
410 6 
2B 1000.degree. C. 
7 .times. 10.sup.8 
430 8 
2C 1075.degree. C. 
1.4 .times. 10.sup.8 
530 26 
______________________________________ 
It may be seen that above 975.degree. C. the permeability increases but the 
resistivity decreases. 
The presence of manganese contributes to avoiding the formation of ferrous 
iron during the heat treatments. The Mn.sup.3+ ion interconverts more 
easily between the trivalent and the divalent states than the Fe.sup.3+ 
ion. It is known that the presence of divalent iron will considerably 
reduce the internal resistivity of the final material. Manganese, 
substituted for iron in the crystalline phase, has the two-fold advantage 
of neither reducing resistivity nor, in contrast to ferrous iron, adding 
to the magnetic drag, even in the form of the divalent Mn. However, 
manganese does have the disadvantage of reducing the Curie temperature. 
This disadvantage can be mitigated by holding the parameter m 
corresponding to the manganese content below or at most equal to 0.035. 
Excellent results can be obtained using the values 
m=0.035 
.epsilon.=0.06. 
These values are used in Sample 3 above and in other examples given below. 
EXAMPLE 2: INFLUENCE OF BISMUTH CONTENT 
Examination of mechanically polished or cleaved samples by scanning 
electron microscopy shows that bismuth forms a bismuth-rich second phase 
which forms at the grain boundaries. This second phase is much more 
resistive than the first crystallite phase and is liquid at the sintering 
temperature. 
In Table 4, the effect of increasing bismuth content is correlated with the 
same properties as in Table 2, with the other parameters and the sintering 
temperature being the same as for Sample 3. 
TABLE 4 
______________________________________ 
Resistivity 
Sample No. 
.beta. d 4.pi.M.sub.S 
.mu.' 
.mu." 
in ohm-cm 
______________________________________ 
4 0.0015 4.89 3830 440 8 4 .times. 10.sup.9 
5 0.005 4.88 3800 440 5 1.4 .times. 10.sup.10 
6 0.01 4.92 3840 450 5 3 .times. 10.sup.10 
7 0.02 4.93 3840 425 5 4 .times. 10.sup.10 
______________________________________ 
The results show that increasing bismuth content has the effect of 
increasing the overall resistivity of the material, measured on a disc 
having the aforementioned dimensions (10 mm diameter, 0.5 mm thickness). 
This is due to the fact that each crystallite is surrounded by a 
bismuth-containing phase. When the bismuth content exceeds 0.01, a certain 
saturation in the increase in resistivity is observed. It is not desirable 
to further increase the bismuth content on account of the difference in 
the coefficients of thermal expansion of the two phases, which may result 
in less complete cohesion between the crystallites and increased fragility 
of the final products. 
The increase in resistivity with increased bismuth content holds for the 
entire range of compositions according to the invention. 
The effect on this high resistivity due to bismuth of varying the voltage 
applied to the terminals of a sample disc, and of varying the temperature 
were studied. Measurements were carried out using a material according to 
the invention having the following parameters: 
t=0 
z=0.6 
.epsilon.=0.06 
m=0.035 
.beta.=0.01 
Table 5 shows the variation in resistivity as a function of the voltage 
applied to the terminals of a sample disc having a thickness of 0.5 mm, 
measured at 13.degree. C. 
TABLE 5 
______________________________________ 
Voltage (V) 
20 60 100 150 200 250 300 
______________________________________ 
Resistivity in 
ohm-cm .times. 10.sup.-10 
2.24 2.24 2.18 2.12 2.05 1.99 1.96 
______________________________________ 
Table 6 shows the variation in resistivity as a function of temperature at 
a voltage of 100 volts applied to the same disc. 
TABLE 6 
__________________________________________________________________________ 
Temperature (.degree.C.) 
-20 
0 20 
40 
60 80 100 120 140 
__________________________________________________________________________ 
Resistivity in 
ohm-cm .times. 10.sup.-10 
20. 
5. 
1.5 
0.4 
0.11 
0.03 
0.0085 
0.0025 
0.0007 
__________________________________________________________________________ 
An approximate formula for the variation of resistivity (.rho.) as a 
function of temperature is as follows: 
EQU .rho.=.rho..sub..infin. exp (W/kT) 
where T is the absolute temperature in degrees Kelvin, k is the Boltzmann 
constant, W is taken as 0.55 eV, and .rho..sub..infin. as 2 ohm-cm. 
In the range of the invention, the magnitude of variation of resistivity as 
a function of either applied voltage or temperature is roughly independent 
of the level of titanium and zinc contents. 
The presence of bismuth during the sintering step has the following 
additional advantages: 
(a) Effect on the temperature of maximum densification during sintering: 
A liquid phase appears at about 780.degree. C. As a consequence of this 
liquid phase, densification occurs at a much lower temperature while at 
the same time favoring the formation of crystallites of a desirable grain 
size (less than 30 microns). When the bismuth content reaches or exceeds 
0.001 atom of bismuth per ferrite molecule, sintering at 975.degree. C. is 
sufficient for the production of densities greater than 90% of the 
theoretical density. The increase in density shown in Table 4 does not 
only correspond to a systematic decrease in porosity, but also is due to 
the fact that bismuth is a heavy element which gives rise to a secondary 
phase heavier than the principle phase. 
(b) Effect on the average size of crystallites: 
Table 7 shows the effect on grain size for samples 4-7, all of which were 
sintered at 975.degree. C. for 16 hours in an oxygen atmosphere. 
TABLE 7 
______________________________________ 
Sample Bi content Grain size 
Number .beta. in microns 
______________________________________ 
4 0.0015 20 
5 0.005 9 
6 0.01 7 
7 0.02 6 
______________________________________ 
Correlated with the decrease in grain size with increase in bismuth content 
is a better distribution of porosity throughout the grain boundaries and 
fewer porosity defects in the interior of the crystallites. This is 
probably the reason for which the initial permeability remains essentially 
constant while the grain size decreases from 20 to 6 microns. In effect, 
the mean free path of the boundaries of the magnetic domains remains 
essentially constant. 
EXAMPLE 3: INFLUENCE OF ZINC AND TITANIUM CONTENTS 
The effect of varying the contents of zinc, represented by the parameter z, 
and of titanium, represented by the parameter t, were studied by measuring 
various characteristics of the final products. The tests were carried out 
with the following values for other parameters: 
m=0.035 
.epsilon.=0.06 
.beta.=0.01 
The following characteristics were measured: 
4.pi.M.sub.S : in gauss, measured at 20.degree. C. 
T.sub.c : Curie temperature in degrees Celsius. 
.mu.': real component of the complex relative initial permeability measured 
at 20.degree. C. in a 10 kHz alternating magnetic field having an 
amplitude of 7 mOe. 
tg.delta./.mu.': loss factor 
.rho.: resistivity in ohm-cm 
TF: coefficient of variation of the permeability (TF=.DELTA. 
.mu./.mu..sup.2 .DELTA.T, between +25.degree. and +55.degree. C.). 
B: induction at 5 oersteds, at 25.degree. C. and at 100.degree. C. 
TABLE 8 
______________________________________ 
Sample No. 
8 9 6 10 11 12 13 14 
______________________________________ 
z 0.45 0.50 0.50 0.525 
0.55 0.55 0.55 0.60 
t 0 0 0.05 0.05 0 0.05 0.10 0 
4.pi.M.sub.S 
4445 3980 3900 3780 3500 3300 3000 3030 
T.sub.c (.degree.C.) 
322 279 264 248 238 220 200 196 
.mu.' (20.degree. C.) 
340 420 440 460 510 570 610 660 
tg .delta./.mu.' .times. 10.sup.6 
46 40 37 35 33 30 31 30 
.rho. .times. 10.sup.-8 
92 100 160 130 140 170 110 160 
TF .times. 10.sup.6 
7.4 6.4 6.6 5.6 5.5 5.2 3.3 
B at 25.degree. C. 
3340 3120 2940 2770 2750 2710 2400 2400 
B at 100.degree. C. 
2800 2510 2310 2120 2110 1990 1720 1570 
______________________________________ 
Table 9 shows certain characteristics obtained for 3 other samples (15-17). 
It is seen that these three samples show substantially lower inductions at 
100.degree. C. 
TABLE 9 
______________________________________ 
Sample No. 15 16 17 
______________________________________ 
z 0.60 0.60 0.60 
t 0.05 0.10 0.15 
4.pi.M.sub.S 
2790 2500 2230 
T.sub.C 172 150 127 
.mu.' 710 740 800 
.rho. .times. 10.sup.-8 
90 63 100 
B (100.degree. C.) 
1130 620 0 
______________________________________ 
It may be seen from Tables 8 and 9 that the magnetization (4.pi.M.sub.S) 
and the Curie temperature diminish rapidly as the permeability increases, 
that is, as the zinc and/or titanium contents increase. Samples 11 and 12 
represent the best balance of properties from the point of view of the 
manufacture of magnetic deflectors. 
In all the examples given above (1-17), .mu.' remains essentially constant 
for magnetic field frequencies up to about 500 KHz. 
FIG. 1 shows the range of zinc and titanium contents in the examples 
studied, as represented by the parameters z and t. A first region, 
represented by the solid line I, contains points representing examples 1-7 
and 9-12. This region represents the range of values giving the best 
results for the manufacture of deflectors. A second region, represented by 
dotted line II, contains points representing Examples 8, 13 and 14. 
FIG. 2 graphically illustrates the sensitivity of the relative initial 
permeability to variations in the magnetic field applied to the material, 
measured in milliOersteds, for materials having the compositions 15-17. 
These samples have an excellent permeability but nevertheless show a 
rather low induction B at 100.degree. C. 
Having now fully described this invention, it will be apparent to one of 
ordinary skill in the art that many changes and modifications can be made 
thereto without departing from the spirit or scope of the invention set 
forth herein.