Note: Descriptions are shown in the official language in which they were submitted.
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Use of PCMs in heat sinks for electronic components
The present invention relates to the use of phase change materials in
cooling devices for electrical and electronic components.
In industrial processes, heat peaks or deficits often have to be avoided, i.e.
temperature control must be provided. This is usually achieved using heat
exchangers. In the simplest case, they may consist merely of a heat
conduction plate, which dissipates the heat and releases it to the ambient
air, or alternatively contain heat transfer media, which firstly transport the
heat from one location or medium to another.
The state of the art (Figure 1 ) for the cooling of electronic components,
such as, for example, microprocessors (central processing units = CPUs)
(2), are heat sinks made from extruded aluminium, which absorb the heat
from the electronic component, which is mounted on support (3), and
release it to the environment via cooling fins (1 ). The convection at the
cooling fins is almost always supported by fans.
Heat sinks of this type must always be designed for the most unfavourable
case of high outside temperatures and full load of the component in order
to avoid overheating, which would reduce the service life and reliability of
the components. The maximum working temperature for CPUs is between
60 and 90°C, depending on the design.
As the clock speed of CPUs becomes ever faster, the amount of heat they
emit jumps with each new generation. While hitherto peak outputs of a
maximum of 30 watts had to be dissipated, it is expected that in the next 8
to 12 months cooling capacities of up to 90 watts will be necessary. These
outputs can no longer be dissipated using conventional cooling systems.
For extreme ambient conditions, as occur, for example, in remote-
controlled missiles, heat sinks, in which the heat emitted by electronic
components is absorbed in phase change materials, for example in the
form of heat of melting, have been described (US 4673030A, EP 116503A,
US 4446916A). These PCM heat sinks serve for short-term replacement of
dissipation of the energy into the environment and cannot (and must not)
be re-used.
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Known storage media are, for example, water or stones/concrete for the
storage of sensible heat or phase change materials (PCMs), such as salts,
salt hydrates or mixtures thereof, or organic compounds (for example
paraffin) for the storage of heat in the form of heat of melting (latent
heat).
It is known that when a substance melts, i.e. is converted from the solid
phase into the liquid phase, heat is consumed, i.e. absorbed, and is stored
as latent heat so long as the substance remains in the liquid state, and that
this latent heat is liberated again on solidification, i.e. on conversion from
the liquid phase into the solid phase.
The charging of a heat storage system basically requires a higher temper-
ature than can be obtained during discharging, since a temperature
difference is necessary for the transport/flow of heat. The quality of the
heat is dependent on the temperature at which it is available: the higher
the temperature, the better the heat can be dissipated. For this reason, it is
desirable for the temperature level during storage to drop as little as
possible.
In the case of storage of sensible heat (for example by heating water), the
input of heat is associated with constant heating of the storage material
(and the opposite during discharging), while latent heat is stored and
discharged at the melting point of the PCM. Latent heat storage therefore
has the advantage over sensible heat storage that the temperature loss is
restricted to the loss during heat transport from and to the storage system.
The storage media employed hitherto in latent heat storage systems are
usually substances which have a solid-liquid phase transition in the tem-
perature range which is essential for the use, i.e. substances which melt
during use.
Thus, the literature discloses the use of paraffins as storage medium in
latent heat storage systems. International patent application WO 93/15625
describes shoe soles which contain PCM-containing microcapsules. The
PCMs proposed here are either paraffins or crystalline 2,2-dimethyl-1,3-
propanediol or 2-hydroxymethyl-2-methyl-1,3-propanediol. The application
Wp 83/24241 describes fabrics having a coating comprising microcap-
sules of this type and binders. Preference is given here to paraffinic
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hydrocarbons having from 13 to 28 carbon atoms. European Patent
EP-B-306 202 describes fibres having heat-storage properties in which the
storage medium is a paraffinic hydrocarbon or a crystalline plastic, and the
storage material is integrated into the basic fibre material in the form of
microcapsules.
US Patent 5,728,316 recommends salt mixtures based on magnesium
nitrate and lithium nitrate for the storage and utilisation of thermal energy.
The heat storage here is carried out in the melt at above the melting point
of 75°C.
In the said storage media in latent heat storage systems, a transition into
the liquid state takes place during use. This is accompanied by problems
in the case of industrial use of storage media in latent heat storage sys-
tems since sealing or encapsulation is always necessary in order to pre-
vent leakage of liquid resulting in loss of substance or contamination of the
environment. Especially in the case of use in or on flexible structures, such
as, for example, fibres, fabrics or foams, this generally requires micro-
encapsulation of the heat storage materials.
In addition, the vapour pressure of many potentially suitable compounds
increases greatly during melting, and consequently the volatility of the
melts often stands in the way of long-term use of the storage materials. On
industrial use of melting PCMs, problems frequently arise due to
considerable volume changes during melting of many substances.
A new area of phase change materials is therefore provided with a particu-
lar focus. These are solid-solid phase change materials. Since these sub-
stances remain solid during the entire use, there is no longer a require-
ment for encapsulation. Loss of the storage medium or contamination of
the environment by the melt of the storage medium in latent heat storage
systems can thus be excluded. This group of phase change materials is
finding many new areas of application.
US 5831831 A, J P 10135381 A and SU 570131 A describe the use of
similar PCM heat sinks in non-military applications. A common feature of
the inventions is the omission of conventional heat sinks (for example with
cooling fins and fans).
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The PCM heat sinks described above are not suitable for absorbing the
peak output of components having an irregular output profile since they do
not ensure optimised discharge of the PCM or also absorb the base load.
The object is to cool electronic and electrical components effectively and to
absorb temperature peaks.
The object is achieved by devices for cooling heat-generating electrical
and electronic components having an irregular output profile, essentially
consisting of a heat-conducting unit and a heat-absorbing unit which
contains a phase change material (PCM).
This invention relates to devices for cooling electrical and electronic
components (microprocessors in desktop and laptop computers both on
the motherboard and on the graphics card, power-supply parts and other
components which emit heat during operation) which have a non-uniform
output profile.
Cooling devices are, for example, heat sinks. Conventional heat sinks can
be improved by the use of PCMs if the heat flow from the electronic com-
ponent to the heat sink is not interrupted. An interruption in this sense
exists if the PCM, owing to the design of the heat sink, firstly has to absorb
the heat before the heat can be dissipated via the cooling fins - which
results in an impairment of the performance of the heat sink for a given
design.
There are various ways of ensuring that the PCM only absorbs the output
peaks.
Electrical and electronic components are usually cooled using heat sinks
(Figure 1 ) having cooling fins.
It has been found that it is advantageous to arrange the PCM in or on the
heat sink in such a way that a significant heat flow to the PCM only occurs
if the heat sink exceeds the phase change temperature TPC of the PCM
(Figure 2, Figure 3, Figure 4 and Figure 5).
It has been found that on reaching this temperature, the cooling capacity of
the cooling fins is supplemented by the heat absorption by the PCM. This
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causes a jump in the efficiency of the heat sink. It is thus achieved that the
electrical or electronic component is not overheated.
The use of PCMs in the manner according to the invention allows the use
of heat sinks of lower capacity since extreme heat peaks do not have to be
dissipated.
It has been found that particularly suitable phase change materials are
those whose phase change temperature TP~ is suitably below the critical
maximum temperature for the component.
15
Depending on the desired maximum temperature, all known PCMs are
suitable. Suitable for use of the PCMs in a heat transfer medium are
encapsulated materials or solid-solid PCMs which are insoluble in the heat
transfer medium.
A general example of the invention is explained in greater detail below.
The devices according to the invention are described with reference to an
example of the cooling of CPUs (central processing units) for computers.
In the device according to the invention (Figure 2), the PCM (4) is arranged
in or on the heat sink (1 ) in such a way that significant heat flow from the
CPU (2) on the support (3) to the PCM (4) only occurs if the heat sink
exceeds the phase change temperature TP~ of the PCM. It is thus ensured
that the PCM only absorbs the output peaks.
Various PCMs are available for this application. It is possible to use PCMs
whose phase change temperature is between -100°C and 150°C. For
use
in electrical and electronic components, PCMs in the range from 40°C to
95°C are preferred. In this case, the materials can be selected from
the
group consisting of the paraffins (C2o-C45), inorganic salts, salt hydrates
and mixtures thereof, carboxylic acids and sugar alcohols. A selection is
shown in Table 1.
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Material Melting pointMelting Group
C~ enthal J/
Heneicosane 40 213 Paraffins
Docosane 44 252 Paraffins
Tricosane 48 234 Paraffins
Sodium thiosulfate48 210 Salt hydrates
entah drate
M ristic acid 52 190 Carbox lic
acids
Tetracosane 53 255 Paraffins
Hexacosane 56 250 Paraffins
Sodium acetate 58 265 Salt hydrates
trih drate
Nonacosane 63 239 Paraffins
Sodium hydroxide64 272 Salt hydrates
monoh drate
Stearic acid 69 200 Carbox lic
acids
Mixture of lithium75 180 Salt hydrates
nitrate and
magnesium nitrate
hexah drate
Trisodium 75 216 Salt hydrates
phosphate
dodecah drate
Magnesium nitrate89 160 Salt hydrates
hexah drate
X litol 93-95 270 Su ar alcohols
Table 1
Also suitable are solid-solid PCMs selected from the group consisting of
diethylammonium chloride, dipropylammonium chloride, dibutylammonium
chloride, dipentylammonium chloride, dihexylammonium chloride, dioctyl-
ammonium chloride, didecylammonium chloride, didodecylammonium
chloride, dioctadecylammonium chloride, diethylammonium bromide,
dipropylammonium bromide, dibutylammonium bromide, dipentyl-
ammonium bromide, dihexylammonium bromide, dioctylammonium
30 bromide, didecylammonium bromide, didodecylammonium bromide,
dioctadecylammonium bromide, diethylammonium nitrate, dipropyl-
ammonium nitrate, dibutylammonium nitrate, dipentylammonium nitrate,
dihexylammonium nitrate, dioctylammonium nitrate, didecylammonium
nitrate, dioctylammonium chlorate, dioctylammonium acetate, dioctyl-
35 ammonium formate, didecylammonium chlorate, didecylammonium
acetate, didecylammonium formate, didodecylammonium chlorate,
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didodecylammonium formate, didodecylammonium hydrogensulfate,
didodecylammonium propionate, dibutylammonium 2-nitrobenzoate,
diundecylammonium nitrate and didodecylammonium nitrate.
Particularly suitable PCMs for use in electrical and electronic components
are those whose TPC is between 40°C and 95°C, such as, for
example,
didecylammonium chloride, didodecylammonium chloride, dioctadecyl-
ammonium chloride, diethylammonium bromide, didecylammonium
bromide, didodecylammonium bromide, dioctadecylammonium bromide,
diethylammonium nitrate, dioctylammonium nitrate, didecylammonium
nitrate and didodecylammonium nitrate.
Besides the actual heat storage material, the PCMs preferably comprise at
least one auxiliary. The at least one auxiliary is preferably a substance or
composition having good thermal conductivity, in particular a metal pow-
der, metal granules or graphite. The heat storage material is preferably in
the form of an intimate mixture with the auxiliary, the entire composition
preferably being in the form of either a loose bed or mouldings. The term
mouldings here is taken to mean, in particular, all structures which can be
produced by compaction methods, such as pelleting, tabletting, roll com-
paction or extrusion. The mouldings here can adopt a very wide variety of
spatial shapes, such as, for example, spherical, cubic or cuboid shapes. In
addition, the mixtures or mouldings described here may comprise paraffin
as an additional auxiliary. Paraffin is employed in particular if intimate
contact between the heat storage composition and a component is to be
established during use. For example, latent heat storage systems can be
installed with a precise fit in this way for the cooling of electronic compo-
nents. During installation of the heat storage system, the handling of, in
particular, a moulding described above is simple; the paraffin melts during
use, expels air at the contact surfaces and so ensures close contact
between the heat storage material and the component. Compositions of
this type are therefore preferably used in devices for cooling electronic
components.
In addition, binders, preferably a polymeric binder, may be present as
auxiliaries. In this case, the crystallites of the heat storage material are
preferably in finely divided form in the binder. The preferably polymeric
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binders which may be present can be the polymers which are suitable as
binder in accordance with the application. The polymeric binder is prefer-
ably selected from curable polymers or polymer precursors, which in turn
are preferably selected from the group consisting of polyurethanes, nitrite
rubber, chloroprene, polyvinyl chloride, silicones, ethylene-vinyl acetate
copolymers and polyacrylates. The suitable methods for incorporation of
the heat storage materials into these polymeric binders are well known to
the person skilled in the art in this area. He has no difficulties in finding,
where appropriate, the requisite additives, such as, for example, emulsi-
fiers, which stabilise a mixture of this type.
For liquid-solid PCMs, nucleating agents, such as, for example, borax or
various metal oxides, are preferably employed in addition.
Besides ensuring good heat transfer through metals (aluminium, copper,
etc.) or other heat conduction structures (metal powders, graphite, etc.),
the heat transfer in the heat sink may also be implemented in the form of a
heat pipe (for example US 5770903A for motor cooling incl. PCM).
In a heat sink with heat pipe (Figure 3), the interior of the heat sink (1 )
then
has, for example, a cavity (6), which is partially filled with a liquid and/or
gaseous medium. The liquid/gaseous heat transfer medium (5) is selected
from the group consisting of the halogenated hydrocarbons (for example
ethyl bromide, trichloroethylene or freons) and their equivalents. The
design of a heat pipe and the choice of a suitable medium presents no
problems to the person skilled in the art.
Besides this medium, the cavity also contains PCM particles (4), which
absorb heat as soon as the internal temperature in the heat pipe reaches
the phase change temperature TP~.
It has been found that encapsulated or microencapsulated PCMs and
solid-solid PCMs which are insoluble in the medium are particularly
suitable. All known PCMs can be used.
Surprisingly, it has been found that, due to the good mixing of the PCM/
medium suspension, the dynamics of the heat sink are particularly great.
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A further possibility has been found through a mixed form (Figure 4). The
CPU (2) is again mounted on a support (3). In order to improve the heat
conduction, cooling fins (7) are run through the cavity (6), which is in turn
partially filled with a liquid/gaseous heat transfer medium (5). Continuous
cooling fins are preferred. As in the previous variants, the cavity, besides
the liquid/gaseous heat transfer medium, here too contains PCM particles
(4), which absorb heat as soon as the internal temperature in the heat pipe
reaches the phase change temperature TPC.
The PCM can be compression moulded into any desired shapes. The
material can be compression moulded in pure form, compression moulded
after comminution (for example grinding), or compression moulded in
mixtures with other binders and/or auxiliaries. The mouldings can be
stored, transported and employed in a variety of ways without problems.
For example, the mouldings can be inserted directly into electronic
components (Figure 5). Here too, the CPU (2) is mounted on a support (3).
The mouldings are installed between the cooling fins in such a way that
they are in intimate contact with the surfaces of the cooling fins. The
thickness of the mouldings is selected so that a frictional connection is
formed between the fins and the moulding. The mouldings can also be
inserted between cooling fins/heat exchangers before the latter are
connected to form a stack.
However, these types of cooling with the aid of PCMs for absorbing heat
peaks are not restricted to use in computers. These systems can be used
in power switches and power circuits for mobile communications, trans-
mission circuits for mobile telephones and fixed transmitters, control
circuits for electromechanical actuators in industrial electronics and in
motor vehicles, high-frequency circuits for satellite communications and
radar applications, single-board computers, and for actuators and control
units for domestic appliances and industrial electronics.
These cooling devices can be applied to all applications in which heat
peaks are to be absorbed (for example motors for elevators, in electrical
substations and in internal-combustion engines).
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Symbol Explanation
1 Cooling ribs
2 Central processing unit (CPU)
3 Support
4 Phase change material (PCM)
5 Liquid/gaseous heat exchange medium
6 Cavity
~ Cooling fins in cavity
Z Entire component
Table 2: Explanation of the symbols in the figures
Examples
Example 1
A heat sink as shown in Figure 2 is designed for a processor whose
maximum operating temperature is 75°C. A phase change material having
a TP~ of between 60°C and 65°C is selected in the cavities in
the heat sink.
Sodium hydroxide monohydrate having a TP~ of 64°C was used here.
Example 2
A heat sink as shown in Figure 3 is designed for a processor having a
maximum operating temperature of 75°C. The cavities of the heat sink
contain trichloroethylene as heat transfer fluid. The PCM used is an
encapsulated paraffin. Nonacosane, which has a TPC of 63°C, is used.
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However, solid-solid PCMs are also suitable as phase change material
here. Didoceylammonium nitrate is suitable for this processor as it has a
TP~ of 66°C.
10
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