BATTERIES
PROFILE
Perfecting the purity
of nickel
Hpulcas GmbH introduces super pure nickel grade for high-performance
electronic applications
IT
has long been known that the parts per million range
quantity of trace elements has a tremendous influence on
the properties of otherwise pure metals (AE van Ankel,
Reine Metalle, Berlin 1939) which is also true for nickel. The norm
for the purest nickel grade – Ni 270 – was established decades ago
for vacuum tube parts. Since then it has become possible to reduce
the content of trace elements in nickel drastically.
For measurement and control devices, specific properties and their
stability over time and different production lots is decisive. Both can
be achieved by a strict limitation of trace elements.
Current technologies for producing nickel
Starting material
Nickel is initially produced in the form of cathode plates or powder,
both having a purity of up to 99.98%. As cathode plates have a
number of undesirable properties, they are cut into squares and
either dissolved (by a galvanising process) or melted, to be
eventually used in the production of pure nickel, nickel alloys or
stainless steel.
Melt metallurgy
It is extremely difficult to reach the highest purity of the starting
material if nickel is processed by means of melt metallurgy. If nickel
is melted under an ambient air, carbon, aluminium, silicon and
titanium are added to deoxidise the melt as well as manganese and
magnesium to globularise sulphur. These elements are supposed
to be slagged, but remain partially in the melt and thus deteriorate
the degree of purity. When melting by industrial vacuum metallurgy
processes, the vacuum is not high enough, which is why the addition
of deoxidisers cannot be completely avoided. Vacuum-melted slabs
have either high oxygen or carbon content. To achieve high grades
of purity, nickel made by melt metallurgy must be purified. This can
be done by electroslag remelting (ESR) and zone refining (ZR).
nickel production. These trials have always failed because the
negative characteristics of cathode plates could not be completely
overcome. These are:
n The plates are covered with an orange peel effect, dotted with
occasional warts. Direct rolling of cathode plates results in
surface cracks;
n
As a result of the electrolytic winning process, cathode plates
show a columnar grain structure, which resists deformation
against the axis of the grains;
n
Cathode plates are produced by inserting a thin starter sheet
into the electrolytic bath. Both sides of a sheet are receiving
nickel layers built up by electrolytic deposition. The cathode plate
therefore consists of three layers. Upon rolling, the surface of the
plate, which is in contact with the rolls, flows faster than the
material inside the plate. The strain thereby induced may split
the plate horizontally during rolling, or at least loosen the
cohesion of the layers, also resulting in splitting or the
appearance of bubbles;
n
The starter sheet is usually corrugated to hinder bending in the
hot bath. As a result, a cathode plate takes this form as well.
Corrugation cannot completely hinder twisting, thus the plates
get out of plane. To use them in an industrial process, plates must
be flattened;
n
If the corrugation extends over the whole length of a plate, upon
flattening, three-dimensional stresses are introduced into the
plates, resulting in stress cracks upon hot rolling;
n
There could be substantial thickness differences of up to seven
millimetres cross- and longwise throughout the plate. Depending
on the distribution of thickness differences, upon rolling, plates
can take different shapes, e.g. a wedge shape of a cathode plate
results in a sabre shape of a rolled plate. Randomly dispersed
thickness differences result in different elongation upon rolling,
resulting at best in a fish-tail form of the end of a plate. Greater
Powder metallurgy
Powder can be produced by the Mond process, which allows one to
achieve high degrees of purity. The Ni 270 norm presupposes that
such qualities are to be produced by means of powder metallurgy.
Strip can be directly rolled from powder or pressed into slabs, which
are sintered and hot-rolled. Large slabs are difficult to produce by
powder metallurgy, however, resulting in density variations. Products
made by means of powder metallurgy are remaining porous to some
extent; fully dense products require massive subsequent reduction.
Production of high purity nickel directly from
cathode plates
Physical characteristics of cathode plates
Because of the high degree of purity of cathode plates, it has
frequently been tried to use them as starting materials for industrial
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PROFILE
BATTERIES
PROPERTIES OF HIGH PURITY NICKEL AND THEIR APPLICATIONS
The special properties of high purity nickel are used in the following applications:
PROPERTY
VALUE/ CHARACTERISTICS
Ni 99.98
Degree of purity (Ni-content)
SIGNIFICANCE
APPLICATION
Ni 99.2
Decreased amount of impurities drastically influences the
mechanical properties of pure metal.
99.98%
99.2%
Sputtering targets.
Segregations of impurities facilitate stress corrosion fracture
and intergranular corrosion.
C-Content
S-content
<20 ppm
1,000 ppm
Carbon results in hot shortness and increases hardness and
electrical resistance.
Surface segregation of sulfur results into a sulfur induced
breakdown of the passive film on nickel facilitating corrosion.
<2 ppm
1,000 ppm
Segregation to boundaries results into a hot shortness and
reduced mechanical stress.
Si-content
0.2 ppm
1,000 ppm
In the presence of silica (as well as Al and Ti oxides) thin products
are exposed to breaking (wire) or developing holes (strip).
Deep drawn products as electrode shells.
Products relying on catalytically
active surfaces.
Foil, thin wire. Glass molds for optical
quality glass.
Oxides increase die wear.
Expanded metal.
Tensile strength, MPa
300-350
450
High purity nickel (HPN) can be deformed by up to 98% without
intermediate annealing.
Deep drawn parts (as CCFL-elektrodes);
brazing spacer taking up
compressive stress.
Recrystallisation temperature, °C
350
690
Starting from about 500°C, nickel tends to sticking, when
bell-annealed. HPN annealing temperatures are below the
temperature, where sticking starts.
HPN can be annealed after cladding to
metals with low melting points as Al and Mg.
Curie-point, °C
360±1
354 - 358
Consistent Curie temperature.
HPN can be used for temperature sensing.
Electrical resistance, μΩ*cm
7.1
9.0
HPN nickel component can be reduced in size. Less Joule
heating during charging and discharging of batteries.
Current collectors in batteries, battery tabs;
Litz wire for use in aggressive climate and
high temperature.
+6,600
+4,700 to
+5,800
The electrical temperature coefficient of resistance describes
the change of resistance in dependence of a temperature
change. Nickel shows one of the highest co-efficients of
all metals.
Temperature coefficient of electrical
resistance 10-6K
Sensors: Resistance thermometers,
e.g. in e-cigarettes.
Controls: Regulator coil in glow plugs for
Diesel engines.
Table 1: Comparison of the properties of high purity and less pure nickel, significance and use
thickness differences may lead to stress cracks developing on
the surface during rolling. In the worst cases, holes appear in the
plate; and
n
Plates are loaded with hydrogen, which may create bubbles
during hot processing and render the welding of plates into
strip impossible.
Technology to produce strip and wire directly from
cathode plates
Hpulcas has developed a range of technologies to overcome the
deficiencies of cathode plates:
n Plates are scanned and data are transformed into computer-aided
design processable data. By means of CAD, a new surface
lying 0.2mm below the current surface is created. Extra surface
is then removed by milling, and orange skin effect and warts are
also removed;
n
By the same technique, thickness differences surpassing one
millimetre are also removed;
n
The declining edges of the plates are trimmed in order to hinder
the edges tearing during hot rolling;
n
Hydrogen is driven out by heat treatment;
n
A columnar grain structure is converted into a globular structure
by heat treatment; and
n
The three-layer structure is dissolved by promoting grain growth
over the layer boundaries.
Thereby the plates are prepared for processing. They are heated to
the hot rolling temperature, hot rolled, levelled, brushed to remove
the oxide scale, cut into rectangles of uniform width and welded
into a strip. By a sequence of reducing and annealing steps, grain
structure of the weld seam and the heat-affected zone are
approximated with the grain structure of the plates.
For the production of wire, the plates are cut into sticks, frontally
welded and drawn to a desired size.
Dieter Wittmann
hpulcas GmbH
+49 172 7492530
d.wittmann@hpulcas.com
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