Conversion of a commercial microwave oven to a sintering
furnace controlled by computer
Gonçalo Leonel
Secção de Tecnologia Mecânica, Instituto Superior Técnico, Avenida Rovisco Pais, 1049-001 Lisboa, Portugal
Abstract
In Tecnologia mecância Laboratory Sinterization of several ceramics have been done in conventional furnaces that
take too much time to do the process and often the result samples does not meet the desirable mechanical and
chemical properties, so microwave heating was introduced. This method can deliver enhanced properties with shorter
cycles.
The aim of the present work is to take apart a commercial microwave oven and understand what is needed to chance
inside it in order to transform it in an efficient ceramics furnace and with enough space for multiple samples that will
be used in another studies and controlled by software so the control interface which needs to be built too.
To start to understand how this process undergoes, the temperature rise was studied not only inside the heating
chamber but also outside and near the heating source, the magnetron. The influence of the insulation box size and
from the power delivered by the magnetron was also studied.
The capability of this microwave oven was proved by sintering five cubic samples of hydroxyapatite obtained by 3D
printing that showed reasonable properties even without being sintered with optimal sintering parameters. The
software interface exhibited being capable of monitoring and control the process giving some useful graphical
information to the user, it is important to refer that this interface is independent of the microwave oven that it is working
with, meaning that there is not needed to change any command when the hardware is upgraded.
Keywords: Commercial microwave oven, Microwave heating, Microwave hybrid heating, Microwave sintering of
ceramics, Sintering Cycles, Microwave Control software, Samples Temperature control.
interaction of the materials with microwaves and is
function of the absorbed energy which in turn is
quantified by two important parameters: absorbed
power (P) and depth of microwave penetration (D) [4],
they can be defined by the following equations (1) and
(5) [5] :
𝑃 = 2𝜋𝑓𝜀0 𝜀"𝑒𝑓𝑓 |𝐸|2
(1)
Where:
𝑓 – microwave frequency (Hz)
Introduction
Microwave technology has been widely used in
communication like radar, television and satellite
applications, but Sixty years ago “Percy L. Spencer” [1]
invented the home microwave oven. This accessory has
been a reliable and economical source of energy to heat
our food. Only forty years after its invention, researchers
started to open borders to new materials, more
specifically heating, synthetizing and sintering ceramic
materials. [2]
𝜀0 – permittivity of free space (F/m)
Principles of microwave heating
𝜀"𝑒𝑓𝑓 – relative effective dielectric loss factor
Microwave energy is a nonionizing electromagnetic
radiation with frequencies in the range of 300 MHz to
300GHz and a wavelength ranging from 1mm and 1m in
open space. Currently, 2450 MHz is the most commonly
used frequency for the home microwave oven, this
frequency is designated as an industrial, scientific and
medical (ISM) radio band set aside for noncommunication purposes. [3]
Microwaves cause molecular motion by migration of
ionic species and/or rotation of dipolar species, the most
common case is the water molecule a dipole present in
food that due to the changes in the electromagnetic field
rotates, friction between molecules start to occur and
consequently generates heat. That is called dielectric
E – magnitude of the internal electrical field (V/m)
Introducing now two another important factors, the loss
factor, ε” corresponding to the ability of the material to
retard microwave energy as it passes through, and the
loss tangent, that indicates the capacity of the material
to be polarized and heated, assuming that the magnetic
loss is negligible (µ”=0):
𝜀"
(2)
𝑡𝑎𝑛𝛿 =
𝜀′
According to Lakshmanan in [4] “the time required for
polarization appears as a phase retardation of the
charging current. Instead of advancing by 90º, it
advances by some angle, δ, other than 90º. This phase
shift, which corresponds to a time lag between an
1
applied voltage and induced current, causes a loss of
current and dissipation of energy in the material”.
Absorbed power can now be rearranged:
𝑃 = 2𝜋𝑓𝜀0 𝜀"𝑒𝑓𝑓 |𝐸|2
(3)
= 2𝜋𝑓𝜀0 𝜀′𝑟 𝑡𝑎𝑛𝛿|𝐸|2 (𝑊
/𝑚3 )
Where:
σ – material electrical conductivity (Ω-1m-1)
𝜀" – loss factor
E – magnitude of the internal electrical field (V/m)
Figure 1 - Relationship between the dielectric loss factor and
the power absorbed per unit volume [7]
𝜀0 – permittivity of free space (F/m)
𝜀′𝑟 – relative dielectric constant
It is important to refer that almost all of this parameters
are dependent of the temperature, which means that
above a certain temperature, an insulator material could
start to absorb power and become absorber.
In the case of ceramic materials, at room temperature in
a 2.45 GHz microwave field, do not absorb microwaves,
but increasing the temperature to a critical temperature
makes their loss factor increase dramatically. This
temperature is usually around 0.4-0.5 of the melting
temperature of the material [4]. At this temperature
bonds between ions in the crystals begin to break, and
since they are free, they start to couple with the
microwave field, beginning the heating process.
Knowing the Absorbed power P it is now easy to
compute the temperature increase since all power
absorbed in the material is converted to heat [6]:
∆𝑇
𝑃
2𝜋𝑓𝜀0 𝜀′𝑟 𝑡𝑎𝑛𝛿|𝐸|2
=
=
(𝐾/𝑠)
∆𝑡 𝜌𝐶𝑝
𝜌𝐶𝑝
(4)
Where:
𝜌 – material density (kg/m3)
𝐶𝑝 – heat capacity (J/kg K)
The last important parameter is the penetration depth,
skin depth or attenuation distance (D) that is the
distance which the field strength decays 1/e (0.368) of
its original value [4]:
𝐷=
𝐶
1/2
2𝜋𝑓√2𝜀′(√1 + 𝑡𝑎𝑛2 𝛿 − 1)
(𝑚)
Comparison with conventional heating
Faced with conventional heating, Microwave heating
offers a great set of advantages [1], [4] like:
Energy saving – Microwave heating eliminates the need
for spending energy to heat the walls or resistances of
furnaces or ovens, this is extremely helpful in processes
where high temperature is needed since there is a
substantial reduction in heat losses in all the
components.
Energy transfer instead of Heat Transfer – In a common
process energy need to be transform in heat in some
kind of source and then transferred to the material to be
heated, this mean that some energy would be loss in
this transportation. In microwave heating energy is
transferred directly to the body and then the body itself
generate the heat, which means heating starts from
interior of the material body.
Rapid Heating and Quick start-up and stopping – Since
microwaves change rapidly the existing electric fields in
the medium, the dipoles present in the material change
their orientation rapidly in response to the changing
fields, producing a rapid increase of temperature.
Volumetric heating – Microwaves penetrate evenly into
the body, making the heating along all the volume, not
just locally in the surface, which lead to a much uniform
heating.
(5)
Where C is the speed of light (m/s).
Evaluating this two parameters, the absorbed power
and penetration depth, there are three main groups that
divide the behavior of certain materials towards a
microwave field.
Materials with a high loss factor ε” and penetration depth
will absorb more power and consequently convert more
energy in heat, so this materials are called absorbers or
dielectrics.
A low penetration depth normally are associated with
conductors or opaque materials where microwaves are
reflected and do not penetrate, in this case the absorbed
power is negligible.
The last ones are the invisible materials or insulator
materials which have a great penetration depth but no
power absorbed because these materials have a little
loss factor. Example of this behaviors is shown in Figure
1.
Power source
To produce microwaves or electromagnetic waves, a
vacuum tube is needed, this device have a cathode,
usually a metal wire and an anode, the resonant cavity.
2
In this case where a home microwave oven is used, the
vacuum tube is a magnetron, which is the most used in
all industry because of its low cost. The principal
disadvantage of this device is the fact that it only
generates one fixed frequency electromagnetic field [7].
The magnetic field is generated by two strong magnets
that are outside of a copper resonant cavity, inside of
this is the cathode that is a wire of a high emissive
material.
The exact operation point of the magnetron is partially
determined by the impedance of the power feed system,
the conditions of the oven cavity and the load inside of it
[8]. The conditions around the magnetron are important
too, because the power is affected by the heat, more
specifically the permanent magnets with the increase of
temperature tend to reduce the magnetic field within the
magnetron, causing the operation voltage to reduce.
The transformer suffer with the heat increase, the iron
will start to create more resistance in the transformer
windings, lowering its efficiency, according to [8] the
transformer could reach 165ºC without creating a big
impact in performance.
Since the magnetron is always operated at full power
due to the lack of control of the input anode current [8],
an alternative was found to control the power output.
The solution has been to turn on the power supply
(transformer) for a certain time and turn it off for another
certain time, making a cycle, named time base which
means that the time that the magnetron is on is
proportional to the power output. An example is given in
Figure 2 where 30% and 50% of the total output power
is achieved.
processing different materials as an alternative route to
conventional processing techniques.
The application of microwave heating for sintering some
ceramics has resulted in low-temperature processing
when compared with conventional sintering methods,
because the process of sintering materials in the
conventional methods involve indirect heating of green
parts to 0.6-0.8 of the melting temperature [5] in a
refractory type electrical/fossil fuel/induction furnace.
The problem with this is the use of a large number of
expensive heating elements and refractory materials to
achieve and maintain the high temperature for a long
time. Furthermore, it consumes more electricity or fuel
and need much more time to achieve the desirable
temperature.
Microwave processing has gained worldwide
acceptance as a method for sintering a variety of
materials, as it offers many advantages in terms of: [5]

Enhanced diffusion processes;

Reduced energy consumption and processing
cost;

Rapid heating rates and consequently reduced
processing times;

Decreased sintering temperatures;

Improved physical and mechanical properties.
Microwave Hybrid Heating
During microwave heating, the absorption of
electromagnetic energy raises the temperature from the
entire body of the piece, but heat loss from the surface
causes its temperature to become lower from the
internal temperature. This is called the inverse gradient
of temperatures [4], eliminating these gradients would
reduce internal stresses, which contribute to cracking of
parts and create a more uniform microstructure, which
may improve mechanical properties and reliability. So to
achieve this this, researchers have created the hybrid
heating methods [9] which consists in mixing
conventional heat sources to heat the surface of the
pieces and a microwave power source to heat internally
the piece, as it is show in Figure 3.
Figure 2 - Variable power, function of the time that the power
source is on and off [8]
This way of lowering the power brings an issue to the
filament heating, because it takes some time to reach
the operating temperature, usually it takes 1,5 seconds
to reach that temperature with a “self-start” system
mentioned in [8], from the same source it is said that
time bases shorter than 12 seconds can reduce the
useful life of the magnetron.
Figure 3 - Comparison between conventional method,
microwave alone and microwave hybrid heating [4]
Microwave sintering
To achieve hybrid heating, gas or an electric heat source
can be used, but to take advantage of the microwaves
Microwave technology has proven to be useful in a
number of applications and is currently used for
3
field, an external susceptor material can be used. A
susceptor is a high-loss material that couples with
microwaves and starts to produce heat.
second one to above that. These materials are very
porous and lightweight, have a very low thermal
conductivity and are nearly transparent to microwaves.
Since the sintering temperatures of conventional
ceramics are about 1000ºC to 1400ºC, Alumina
insulation type is the best choice.
Thermal Runaway
With this increase in loss factor of materials, many
researchers have shown that thermal runaway in
microwave-heated materials often occurs when Tc is
reached [10]. The sharp increase in microwave
absorption of some spots of the piece may cause local
overheating consequentially causing thermal instability,
this gradient of temperature inside the piece could lead
to cracks or abnormal grain growth.
This instability develops when microwaves power
exceeds some threshold value [11]. Below a “critical
power level”, the material will heat in a stable manner to
a steady value on the lower of the response curve, the
S-type dynamic curve of temperature VS power.
This curve is analyzed in [12] and where it is said that
depending on the microwave power, bistable steadystate temperatures may be expected.
Microwave Oven Project
To put into practice the concepts introduced previously,
some pieces of hardware were acquired, projected a
small insulation system, the electronic system modified
and a temperature control system implanted.
The hardware used in this project were:
A Candy CMW 20D S Microwave Oven with a power
output of 800W; a type S thermocouple inside the
insulation system and 2 type K thermocouples, one in
the metal cavity and another near the magnetron; an
adam-4019+ Data acquisition module; the insulation
made of Morgan FireBlocks JM28 and Morgan
Ceraboard 100 were used and the susceptors are
Silicon carbide Pieces.
Insulation System
Temperature monitoring system
In order to create an efficient insulation system, two
layers of insulation material were built, one from the
fireblocks and one from the ceraboards. This blocks
dimensions are 230x114x64mm and are made from
nearly 70% of alumina and 30% of Silica, both of this
components are invisible to microwaves inside the
spectrum of the working temperatures, it also has some
impurities that might reflect or absorb some energy but
without many importance, making this blocks the
cheapest solution for the insulation but it has higher
thermal conductivity than the other more expensive
boards made from fibrous alumina and silica, to
compensate this small disadvantage an extra layer of
insulations is added, this made from boards of 10mm
thick of fibrous ceramics that has half of the thermal
conductivity. According to what was said regarding the
optimization of the insulation system, inside of the
bottom block was created a box with 80mm x 80mm,
leaving the walls of the cavity with an optimized
dimension.
The main reason to have a square cavity with 80 mm of
length and 30 mm of elevation is the power limitation
since there is only 800W of output power available, the
cavity inside the insulation system cannot be too large
because it would take more time to heat via the
susceptors and if more silicon carbide slabs were
introduced, probably all the electromagnetic energy
would be absorbed by these susceptors, making
impossible the process of hybrid microwave sintering of
the samples.
Several tests were done with four different insulation
systems (Figure 4), the 1st and the 2nd one despite of
showing much better results, they just have a workable
space of 45x45x10 mm and 65x55x40 mm respectively,
which is not enough for the samples that this oven is
supposed to sinter. The 3rd one is the biggest one with a
Temperature monitoring is crucial in this process in
order to control the properties of the pieces that are
being worked and to prevent possible overheating.
Thermocouples are the most common way of measuring
temperature inside a furnace, but conducting
temperature measurements in a microwave furnace is
not a simple task due microwave interaction with
metallic elements of thermocouples.
It has been observed that the thermocouples shown
simultaneous arching and melting of leads in relatively
short periods of time inside microwave furnaces. [1]
Other means for measuring temperature like infrared
sensors were used in some researches but the lack of
reliable data on emissivity of different material and the
fact that these devices need to be calibrated with
thermocouples led to the researchers to investigate the
best way to put a thermocouple inside the microwaves.
As electric fields cannot penetrate the metallic sheets,
thermocouples with metallic sheaths were preferred for
temperature measurements. It was found too that by
placing thermocouples in well-grounded metallic
sheaths
perpendicular
to the
electric field,
measurements were much more accurate [13].
Insulation System
To achieve the best performance in microwave heating,
a good insulator material must be chosen, not only to
prevent heat loss from the susceptor, but also to
improve temperature uniformity. The design of the
thermal insulation is crucial too, a better design
improves the power distribution within the pieces to be
heated.
The two leading materials used in the insulation are
alumina and zirconia fiber – based materials, being the
first one used to temperatures below 1600ºC and the
4
1.
Temperature inside cavity (ºC)
workable area of 100x80x35, the silicon carbide slabs
cannot heat the cavity with a satisfactory heating rate,
for this reason, using this cavity might lead to thermal
runaway in the samples. The last one, the 4th,
notwithstanding the modest heating rate, it is
satisfactory and It has enough space for several
samples in its 80x80x30 mm heating cavity.
First layer of insulation made of ceraform boards –
Walls 10mm thick;
2.
Second layer of insulation made of Firebrick JM28 –
Walls 20mm thick and two of them with a special
shape to lose less heat;
3.
Susceptor
Slabs
–
Rectangular
Slabs
80mmx30mmx15mm;
1200
4.
900
ceramic that endure high temperatures;
1st Cavity
5.
600
2nd Cavity
introduce this blocks with 30mm height;
Last Cavity
0
0
5
10
15
20
25
Support blocks – To elevate the chamber to the
height of the magnetron with was required to
3rd Cavity
300
Pieces Plate – Plate 10mm thick made from a rigid
30
6.
Thermocouple – Type S thermocouple;
7.
PVC Thermocouple extension wire – Wire with 1m of
extension that connects the thermocouple to the data
Time (minutes)
acquisition module;
8.
Figure 4 - Comparison between several insulations boxes
From several tests done, it was concluded that the slabs
of silicon carbide (susceptors) have a bigger heating
effect when placed transversally to the wall where the
magnetron. The reason behind this is the fact that the
cavity is a commercial multimode cavity. This means
that the waves that come out of the waveguide are fired
to the cavity at free will and then reflected, forming a
non-uniform electromagnetic field as it is explained and
experimentally tested in [14]. In the same article it is
studied the effect of a mode stirrer inside the metal
cavity, this accessory is a metal fan that makes the
electromagnetic field more uniform. As this microwave
oven does not come with this accessory from factory, it
was decided not to add one.
ADAM 4019+ Data acquisition module.
Figure 6 - Insulation System inside the microwave oven
Temperature Control
The data acquisition module is connected via its usb port
to a computer that is prepared to receive the data and
evaluate it in order to turn on or off the magnetron. The
output of the computer is an usb to RS232 converter that
sends signals to a relay. In Figure 7Error! Reference
source not found. it is shown the wiring of the AC circuit
inside the microwave oven.
Project sketches
Figure 5 - Sketch of the insulation system and temperature
measurement cut in half
Figure 7 - Diagram of the wiring of the AC Circuit
The insulation system is shown in Figure 5 and the
components are:
As it can be seen there are two relays with the intention
of maintaining the cooling fan working while the
5
magnetron is turned on and off, promoting a better
cooling of the magnetron, this configuration was
imported from the stock circuit, adding only a security
switch and changing the control circuit board.
This circuit board was built to answer to the impulses
that are sent from the computer, to do this, a simple
circuit with a transistor, a NPN BC337 was used, this
transistor acts like a small relay that opens and closes
the current to the actual relay, but to do this only need a
small continuous signal [15] provided by the rs232 port.
A more simple circuit is shown in Figure 8.
insulation system. To know the system behavior, a
methodology was established.
Calibration
Temperature inside
cavity (ºC)
First, to know the maximum heating rate, the magnetron
was turned on at full power until the insulation box
reaches the sintering temperature, assumed 1100ºC.
1000
800
600
400
200
0
0
4
8
12
16
20
24
28
Time (min)
Inside the cavity
Near Magnetron
Outside metal cavity
Figure 9 - Trial run with the magnetron always turned on
Figure 8 - Scheme of connections between the Data
Acquisition Module, the computer and the Electronic Control
Unit
Analyzing the result, it is clear that the magnetron
temperature influences its performance, as it reaches
180ºC the heating capacity starts to drop and
consequently the heating rate drops too.
To solve this issue without making any change in the
cooling system of the magnetron, the power value was
lowered, several tests are shown in Figure 10 to several
power values.
Control Program
Temperature (ºC)
The control program needs to fulfill certain requirements
such as: Show the temperature inside the insulation, in
the metal cavity and near the magnetron; Show a
graphic from the Temperature inside the insulation vs
time; Start and Stop the magnetron; be able to change
and save some inputs like the maximum temperature,
the step temperature, the desired thermal gradient, the
time to both occasions and the cooling time, also to give
stable heating rates, it also needs to control the power
input.
The best program to design this interface is the National
Instruments LabVIEW® 2013 because it is intuitive and
have all the tools needed to make a laboratory interface,
from the capability of receive information from serial
ports and send information, save information in logs to
an end graphical and appellative user interface, but what
make it really easy to work it is the blocks programming
because it does not need code lines what makes the
construction of the background program and the
interface very instinctive.
The final program/Interface consists of five main parts,
the indicator lights and numeric indicators, the main
buttons, the live chart, the control inputs and the
advanced controls.
HR = 14 ºC/min
1000
800
600
HR = 25,4 ºC/min
400
HR = 27,6 ºC/min
200
0
0
5 10 15 20 25 30 35 40 45 50 55 60
Time (minutes)
Temp Cavity
Temp Magnetron
Figure 10 - Trial run with the power levels defined
Again, after the temperature inside the insulation box
reaches 800ºC the magnetron temperature stepped up
to 180ºC which resulted in a drop of the heating rate, but
not so big, compared with the test of Figure 9.
A curious fact discovered when a temperature is
maintain during a certain amount of time was that the
magnetron temperature drops considerably. That fact
was explored in order to maintain the magnetron
temperature below 180ºC. To achieve this, small steps
were introduced where a target temperature value is
maintain for a while. This process takes more time but
Calibrations and adjustments
Since the power absorbed by the susceptors is unknown
and hard to determine analytically, some investigation is
done in [16] and [17] about the energy absorption of
silicon carbide but to heat glass. To this project, that
analytical solution is not essential, so some trials runs
were made to see how the temperature rises inside the
6
ensures that the heating rate is kept constant, preserves
the magnetron at a lower temperature and keep the
samples from overheating, maintaining its temperature.
1180
Adjustments
Temp (ºC)
PTC Rings - To make the final adjustments and to see if
the thermocouple is measuring right the temperature,
there is a reliable method: the introduction of PTCR –
Process Temperature Control Rings in some trial runs.
This way, it was possible not only to see the real
temperature near the thermocouple, but also to make a
profile temperature of the heating chamber.
According to the product brochure [18], a PTC Ring is a
ceramic piece that registers permanently the total
amount of heat transferred to it, being it by conduction,
convection or radiation. It works by shrinking with the
increase of the temperature, the degree of shrinkage is
converted into a ring temperature with the help of a chart
that comes with the rings. This ring temperature is an
effective temperature for the total process, which means
that this temperature is influenced by the soak time that
it is imposed, that means that at a constant temperature
the ring will continue to shrink and give a higher value of
temperature when taken out.
1160
8
1150
4
1140
8
6
4
3st to 4th Stack (cm)
1150-1160
1160-1170
2
0
0
1170-1180
3rd to 2nd Stack (cm)
1170
Figure 12 - Temperature displacement recorded by the PTCR
Analyzing the results it is possible to see the presence
of some small cold and hot spots, consequent from the
non-uniformity of the electro-magnetic field around the
silicon carbide slabs but it is possible too to see that not
only that the cavity is at the desired temperature but also
over it, with only small variations. It is also concluded
that the overall temperature near this slabs is higher
10ºC to 30ºC than near the thermocouple as it was
expected. This is a fact that must be considered when
precision is needed in the process.
Mapping Process
As it is shown in Figure 11, 12 rings were used, they
were stacked in groups of 3 and distributed for the 4
corners of the heating chamber. They were heated to a
temperature (in the thermocouple) of 1150ºC for 1 hour.
The mean results are presented in Figure 12.
Final Tests
After the trial runs and the adjustments done, it time to
put some material inside the insulation box. In this
particular case, Hydroxyapatite samples.
This material, Hydroxyapatite is the principal inorganic
constituent of bones and teeth [19] and its mechanical
and chemical properties benefit from microwave hybrid
sintering as it will sinter at lower temperatures
comparing with the conventional method which prevents
the formation of tricalcium phosphate [20] that is
considered an impurity that degrades the structure and
purity of the material.
Unfortunately, at the time of this tests there was not
enough powder to make a fair share of samples to test
and conclude anything quantitative, therefore the results
of this tests are only qualitative, which means, only to
see if the microwave oven is capable of sintering.
Figure 11 - PTCR disposition in the heating cavity
First Test
For this first test a previously sintered sample was
placed touching the thermocouple, this way, the
thermocouple will register what is happening inside the
piece, in other words, it will register if the sample absorb
microwaves or not and if so, it will register too the
heating rate.
The result of this test is shown in Figure 13. When this
run is compared with one in which the heating cavity is
empty, it is clear that the environment heats at a much
slower pace than the sample (that heats 6 times faster
7
Temperature (ºC)
than the environment). This could make some problems,
because it will generate a thermal gradient in the
sample. Since the material is the same, it is assumed
that the behavior of the green samples in the presence
of a microwave field is similar.
1300
1200
1100
1000
900
800
700
600
500
400
300
200
100
0
HR =
248,16 ºC/min
Figure 14 - 4 Green Samples
Table 1 - Dimension Comparison between green and
sintered samples
HR = 40,263 ºC/min
Sample
Number
1
2
3
4
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
Time (minuntes)
Sample in Thermocouple
Dry run
Dimensions (mm) (Green | Sintered)
X
Y
Z
6,17 5,75 6,09 5,55 6,17 5,75
5,95 5,16 5,72 4,98 5,91 5,35
5,78 5,32 5,95 5,40 5,63 5,18
6,08 5,15 6,08 5,15 6,03 5,05
Figure 13 - Heating of a sample in contact with the
thermocouple
Sintering tests
Figure 15 - Green sample vs Sintered sample at 1200ºC
In order to test the sintering capabilities of this system 4
samples where printed in a 3D powder printer with a
small portion of a leftover powder of another project,
these samples have a cubic shape and a nominal edge
dimension of 6 mm.
To check the hybrid microwave principle several
parameters were tested, from the maximum
temperature to the sintering time. After the sintering
process complete the samples were measured.
Temperatures from 1000ºC to 1200ºC and times from 1
hour to 2 hours were tested, this values are based in
some other studies done with this material like [21], [20]
and [22]. The connecting polymer burning temperature
and time are the same as those used in the conventional
system.
The pieces to be sintered at 1000ºC (sample 1) and at
1200ºC (sample 2) will be in contact with the
thermocouple, in order to better control the heating rate
of the piece, to the higher temperature steps will also be
used for the same reason.
There are two pieces marked to be sintered at 1050ºC
(sample 2 and 3) to see if the results are similar in the
both pieces. They will be sintered in the same run. The
controlled temperature will be the environment
temperature to see if there is any interference of the
heating of the samples with this temperature.
5
3,95 MPa
Stress (MPa)
4
3
2,24 MPa
2
1,99 MPa
1
1,16 MPa
0
0
0,05
0,1
Compressive Strain (mm/mm)
Nº1 at 1000ºC
Nº3 at 1050ºC
Nº2 at 1050ºC
nº4 at 1200ºC
Figure 16 - Compressive Stress-Strain Curve of the 4
sintered samples with the maximum stress marked
From Table 1 it is visible that all the pieces suffered a
volume reduction, which is a trace of sinterization.
Evaluating the curves in Figure 16, there is a noticeable
difference in the behavior of the samples under
compression, even the samples sintered at the same
temperature and at the same time show some difference
in the curve and at the rupture stress, because of that,
this analysis can only be qualitative. The curve of the
sample sintered at 1200ºC stands out from the rest and
it is evident the behavior of a sintered sample, which is
a rise in stress and a brittle fracture in the end. The other
8
samples shown too traces of being sintered but with a
lower degree.
Although this samples were sintered, they do not have
the best mechanical properties. That would only be
achieved with more samples and optimization of the
sintering parameters so, as it was mentioned before,
these samples and these results only have a qualitative
meaning, which is evaluate if the samples are sintered
or not.
possible to introduce this interface in an industrial or
laboratorial environment.
This system is good to sinter small samples and only few
each time, it is ideal too research purpose only, in order
to make the physical system broader, it needs to be
upgraded.
Conclusions
The sintering capability of this microwave oven is proved
by the tests done with small samples but there are some
pending matters, like:
Future Work
Questions still unanswered
In this study one commercial microwave oven was taken
apart, studied and then reassembled with some
additional features, measuring temperature and receive
commands from a software interface, also an insulation
system was built so that the process of sintering could
be achieve inside the microwaves cavity, the software
interface was too built from square one for this purpose,
finally the whole system were tested in several
conditions.
As it was said before, this microwaves oven is somehow
limited by the power that it produces and by the
magnetron temperature /cool down system as it is later
concluded in the calibration phase. The insulation
system is not the best for the application because it has
some impurities that with the rise of temperature starts
to absorb microwaves, also the thermal conductivity of
the internal insulation blocks is not ideal, being higher
than the normal fibrous alumina insulation and this
limited the size of the work chamber. Several
dimensions for the insulation box were studied,
concluding that the smaller one have the best
performance but do not have enough space for several
samples, then an average performance box were
chosen but that can hold several samples at the same
time.
Even with all this setbacks the equipment revealed quite
acceptable results when sintering samples of
hydroxyapatite, with sintering levels and material
properties close to pieces sintered in more advanced
and powerful microwaves oven used in another studies,
only needing some tweaking in the sintering parameters
to avoid thermal runaway, to maintain the chemical
structure and to optimize the result properties.
The software interface can fill the gap that the
magnetron refrigeration system left when increasing the
temperature with the introduced steps, it can also control
the power output in order to maintain a constant heat
rate to avoid thermal runaway in the sintering samples.
Finally, maintaining the data acquisition module and the
controlling circuit, with the 2 relays, this interface can be
transported to a new microwaves ovens, just being
necessary to run some trial runs in the new system to
know the behavior and the heat rates. From the user
perspective this interface is very intuitive and graphical,
it not only generates a simple preview of the sintering
process but also generates a live chart with the
temperature inside the work area and the one from
magnetron and metal cavity, for this reason, it could be

The effect of having several samples dispersed by the
heating cavity

Discover the maximum allowed samples in the heating
cavity in order to maintain the process quality.

The samples size and geometry effect in the heating
process.

The heating rate effect in the sintering process of the
samples.

Tune the parameters to achieve the best sintering
process.

Sintering of another materials to test the veracity of the
“activation temperature”.
This questions can only be answered when more
samples are produced.
Improvements in the present apparatus
The physical system described in this research was
design to be cheap and to do only small jobs. To boost
the performance of the same microwaves oven the
insulation system must be changed to purer alumina
fibrous blocks with optimized thickness. To make the
magnetic field more uniform through the metal cavity, a
mode stirrer should be annexed to the wall in front of the
waveguide of the magnetron. Finally the magnetron
refrigeration system could be upgraded with one more
fan, this way the small steps were no longer necessary.
In this microwave oven will always exist a power
limitation which causes the work space to sinter to be
small and reserved to a few samples for each run also
the heating rates will be average, since the microwave
power available will be limited to each sample inside the
work space which lead to bad quality sintering.
Building of a new microwave oven
The best option is build a new microwave kiln from
scratch, taking into account the examples in [23] and
[24] where on one there is two magnetrons to have more
power available and dispersed and in the another one
there is a mode stirrer but with a powerful magnetron.
9
The metal cavity should be design in a way that the
electromagnetic field is as uniform as possible without
taking maneuverability which means that the insulation
system must be reached without any difficulties. The
outside of this cavity needs to be cooled because some
reading in the actual system showed that this cavity can
reach 100ºC.
[15]
[16]
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