intelligent monitoring lubricant

Real-Time and Online Lubricating Oil
Condition Monitoring Enabled by
Triboelectric Nanogenerator
Jun Zhao, Di Wang, Fan Zhang, Yuan Liu, Baodong Chen, Zhong Lin Wang,* Jinshan Pan,*
Roland Larsson, and Yijun Shi*
Cite This: ACS Nano 2021, 15, 1186911879 Read Online
ACCESS Metrics & More Article Recommendations *sı Supporting Information
ABSTRACT: An intelligent monitoring lubricant is essential
for the development of smart machines because unexpected and
fatal failures of critical dynamic components in the machines
happen every day, threatening the life and health of humans.
Inspired by the triboelectric nanogenerators (TENGs) work on
water, we present a feasible way to prepare a self-powered
triboelectric sensor for real-time monitoring of lubricating oils
via the contact electrification process of oilsolid contact (OS
TENG). Typical intruding contaminants in pure base oils can
be successfully monitored. The O
S TENG has very good
sensitivity, which even can respectively detect at least 1 mg
1 debris and 0.01 wt % water contaminants. Furthermore,
the real-time monitoring of formulated engine lubricating oil in
a real engine oil tank is achieved. Our results show that electron transfer is possible from an oil to solid surface during contact
fication. The electrical output characteristic depends on the screen effect from such as wear debris, deposited carbons,
and age-induced organic molecules in oils. Previous work only qualitatively identi
fied that the output ability of liquid can be
improved by leaving less liquid adsorbed on the TENG surface, but the adsorption mass and adsorption speed of liquid and its
consequences for the output performance were not studied. We quantitatively study the internal relationship between output
ability and adsorbing behavior of lubricating oils by quartz crystal microbalance with dissipation (QCM-D) for liquid
contact interfaces. This study provides a real-time, online, self-powered strategy for intelligent diagnosis of lubricating oils.
KEYWORDS: lubricating oils, condition monitoring, triboelectric nanogenerator, TENG, smart machines
H uman civilization and cultural communication bene greatly from the development of modern industry, which automobiles, trains, vessels, and aircrafts play 1 fiint
an important role in terms of convenience, e
fficiency, and
24 Unexpected and fatal failures of critical dynamic
components in machines happen every day, threatening the life
and health of humans. Hence, reliable condition monitoring
(CoMo) will thus be of importance in order to make sure that
the machine services can be reliably delivered.
48 With the
industry 4.0 automation increase, there is a need for smart
machines able to understand or sense the failures and make
decisions accordingly by arti
ficial intelligence and machine
Lubricants can extend machine lifetimes by orders of
magnitude, which is of great signi
ficance for energy
conservation and emission reduction.
1014 Using lubricants is
the most e
ffective way to control friction and wear, because
moving mechanical interfaces are commonly lubricated and
separated by
fluid lubricating films. Therefore, the lubricant is
an important source of information in the strategy to detect
machine failures, comparable to the role of human blood in the
detection and prevention of diseases.
15,16 The real-time
detection of lubricants can eliminate the need of costly
machine shutdowns for inspection, which would otherwise be
required to avoid the possibility of catastrophic component
failure during operation.
Intruding contaminants from thermal oxidation, wear debris,
carbon deposition, fuel, and moisture often exist in lubricating

Received: April 8, 2021
Accepted: June 14, 2021
Published: June 25, 2021

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oils and are mainly issues causing lubrication failure. For
example, the heat produced in the engine segment should
fluence the oil performance. Low oxidation stability of
lubricating oils may result in oil acidi
fication and carbon
deposition under high-temperature aging.
1719 Under operation, the fraction of wear debris in oils gradually increases
when frictional surfaces are worn (Fe and Cu debris),
10 and
the size of the debris is in the range of 10
100 μm.20 Fuel or
water may permeate engine lubricating oils
via the frictional
interface of piston/cylinder, when the piston is reciprocating
inside the cylinder.
2123 Fuel and water not only damage the
oil quality and lubrication performance, but also corrode the
There are many methods used as monitoring sensors for the
quality of lubricating oils. Some examples are optical methods,
acoustic emission detection methods and electromagneticinducted technologies.
15,20,24 Traditionally, some contaminant
ingressions can always be re
flected by a change of dielectric
constant of lubricant, therefore the contaminants can be
detected timely by monitoring the dielectric constant of the
used lubricant. However, these methods can only provide
limited information on the progression of ferrous wear debris
by o
ff-line monitoring and with relatively low accuracy. The
present sensors can only detect large particles with a diameter
of 100
300 μm at the lowest concentration of about 1 mg
1, and can only detect the water contaminant down to 0.33
wt %.
15,20 Most conventional detection sensors are quite large
and unwieldy, and need installation or attachment to
equipment systems, potentially causing interference with
monitoring systems. Due to reliance on external power
sources, the energy consumption is a challenge for their
miniaturization and weight reduction, and they have limitedservice life. It is much desired to develop a self-powered, highsensitivity, small, and even
flexible detection system for realtime, online monitoring of lubricating oils.
The triboelectric nanogenerator (TENG), based on the
conjunction of triboelectri
fication and electrostatic effects, was
developed for energy harvesting and self-powered monitoring
by Wang and co-workers in 2012.
25 TENG-based sensors have
successfully been used as mechanical sensors for detecting
water wave,
26,27 liquid flow rate,28 and organic29 and ion
30 based on liquidsolid contact electrification.
Inspired by the works on water-based systems, we propose a
method to develop TENG for oil condition monitoring. The
fication process of liquidsolid contact can produce a
surface charge on a large scale
via electron and ion transfer on
solid contact interfaces.8,3137 It has been found that
electron transfer is the dominating mechanism for the
fication process in solidsolid cases,38 so lubricating
oils with no ions can generate a certain amount of charges by
electron transfer between oil
solid interfaces.39,40
In this study, we present a feasible way to prepare and apply
a self-powered triboelectric sensor using oil
solid interacting
S TENG) for real-time, online monitoring of
lubricating oils. First, three kinds of pure base oils
fin 6 (PAO-6), paraffin and rapeseed oils) and
typical contaminant ingressions (i.e., thermal aging, wear
debris, carbon deposition, diesel oil, and water) are used.
the triboelectrification on liquidsolid interface, the electric
signals generated from the contact tribo-layers can detect
lubricating oil conditions. The working mechanism of the O
TENG is illustrated. On the basis of the model study, a sensor
is developed to be used as real-time and online monitoring of
an engine lubricating oil in actual operation. Results show that
this sensor has great potential in building a self-powered, realtime, and online monitoring system for lubricating oils.
Structure and Working Principle of the OS TENG.
The triboelectric sensor for detecting contaminants in oils is
developed by using a dropper covered with a copper foil as
shown in
Figure S1. Typically, polytetrafluoroethylene
(PTFE), low-density polyethylene (LDPE), or glass
(GLASS) tubes are used as substrates, and the copper layer
is deposited on the outside surface of the tubes, forming the
single electrode TENG sensor. In the case of liquid
flow sensor
Figure 1a, oil flows (PAO-6, paraffin, or rapeseed oils) are
squeezed or loosed from the grip tips of droppers, then the
flow proactively passes through the TENG surface, and the oil
flow motion will generate electric output signals. As shown in
Figure 1. Structure illustration and working principle of the OS TENG. (a) Structural schematic diagram of the developed OS TENG
sensor. (b) Typical output signal generated by the interfacial interaction between a pure lubricating oil and a Cu electrode. (c) Output
generation of the lubricating oil with contaminant ingressions.
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ACS Nano 2021, 15, 1186911879
Figure 1b, the output signal generation from the developed
S TENG is based on both triboelectrification and
electrostatic induction.
41,42 Because of the interaction with
Cu, the nonmetal surface layers of PTFE and LDPE retain a
layer of negative bound charges, while GLASS surface keeps
positive charges as shown in Supporting Information,
due to their different positions in triboelectric series.43,44
The initial voltage outputs for PTFE and LDPE-based TENGs
are mainly positive after a holding stage with weak signal drift.
Conversely, the GLASS-based TENG displays a relative
negative voltage signal shown in
Figure S3. The key to
monitor the oil condition is from the amplitude of output
values and the variation tendency of voltage signal in this
study, thus, the typical voltage outputs are uniformly processed
in one baseline (zero axis).
When the oil molecules initially approach a virgin surface
that has no pre-existing surface charges, initial electron transfer
occurs to make the solid surface be charged (
Figure S4). The
nonmetal surface will attract the charged oil molecules to form
an electric double layer (EDL) that will screen the electrostatic
inducted charges of the nonmetal layers. Therefore, electrons
flow from the ground to the Cu electrode under shortcircuit condition for an electric equilibrium. When the flow
leaves the nonmetal layer, the screen e
ffect will disappear and
electrons will
flow from Cu to the ground to reach a new
electric equilibrium. As illustrated in
Figure 1c, the
contaminants in a lubricating oil will change the electri
process performance of the oil, and can be re
flected from the
S TENG electric output. On the basis of the above working
mechanism, O
S TENG has the potential to monitor the
condition of lubricating oils. To clarify the role of
contaminants in altering the output signal of O
the in
fluence factors, that is, thermal aging, wear debris,
deposited carbons, fuel oil, and water are systematically studied
as summarized in
Table S1.
Base Lubricating Oil Condition Monitoring by the O
S TENG. During operation of the actual equipment, lubricating
oils inevitably su
ffer from thermal oxidation. Thermal aging is
therefore an important sign of oil deterioration.
17 To
demonstrate the applicability of the O
S TENG, pure base
oils with di
fferent aged degrees were first prepared. During the
test, the volume of each oil
flow in tubes was about 2 mL, and
the frequency of the oil
flowing through the electrode surface
was set at around 1
± 0.1 Hz by manually squeezing and
Figure 2 shows the voltage outputs of OS TENG
driven by pure base oils aged with various aging time periods
192 h). As shown in Figure 2ac, the output values of the
three pure base oils are about 0.1 V when the oil
flows in
contact with the PTFE tube. For aged PAO-6 oil
flow, the
output voltage increases from 0.1 V to 0.33 V with the increase
of aging time (up to 12 h). The maximum output value of
ffin oil is about 0.32 V when the aging time is 48 h,
whereas the output value of rapeseed oil does not increase after
aging for 3 h. As shown in
Figure 2di, the variation tendency
of voltage signals for LDPE and GLASS tubes resemble that for
PTFE tubes, which further con
firms that thermal aging greatly
ffects the output of OS TENG, and the OS TENG can
ffectively monitor the aging degree of lubricating oils.
Typical contaminant fractions as byproducts of wear process
and carbon deposition
15,20 increase gradually with time causing
machine deterioration and even failure. It is critical to monitor
the contaminants to avoid catastrophic failure. We dispersed
Figure 2. Typical signal curves in 5 s for thermal aging affecting on the output voltages of OS TENG monitoring pure base oils. PTFEbased OS TENG monitoring PAO-6 oil (a), paraffin oil (b), and rapeseed oil (c). LDPE-based OS TENG monitoring PAO-6 oil (d),
ffin oil (e), and rapeseed oil (f); GLASS-based OS TENG monitoring PAO-6 oil (g), paraffin oil, (h) and rapeseed oil (i).
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Fe/Cu particles as the wear debris and carbon blacks in base
oils to simulate lubricants that undergo the actual wear and
carbon deposition processes. The tested fractions and
morphologies of Fe/Cu debris and carbon blacks are
respectively summarized in
Table S1 and Figure S5. As
shown in
Figure S6, the output values initially increase and
then decrease with the increase of fraction, and the variation
tendency of voltage output for PTFE-based O
S TENG is in
good agreement with those for both LDPE-based and GLASSbased O
S TENGs. Figure 3 panels ac show the voltage
outputs of PTFE-based O
S TENG driven by debris-laden
flows with the fraction range from 0 mg mL1 to 20 mg mL1.
In contact with Fe debris-laden PAO-6
flow, the OS TENG
device has a maximum output voltage of 0.58 V at a debris
fraction of 4 mg mL
1. The maximum output voltages are 0.65
V (at 10 mg mL
1) and 0.37 V (at 4 mg mL1), respectively
for para
ffin and rapeseed flows. The voltage value of Cu debrisladen flow also increases with the increase of Cu debris
fraction, and then decreases at high fraction in
Figure S7.
When in contact with carbon black-laden
flows shown in
Figure 3df, the output voltages of base oils also have high
peaks with the increase of the fraction, but the critical fraction
value is much lower than that of Fe and Cu debris-laden
During actual equipment operation, fuel oil in an engine
combustor always enters into the engine lubricating oil from
the frictional contact area between the cylinder-wall and
The influence of fuel oil (diesel oil) is also studied (Figure
gi and Figure S8). It is clearly seen that the voltage value
gradually decreases at a high fraction of diesel oil. Oil/water
monitoring and separating is a worldwide concerned challenge
because of increasing industrial oily wastewater, as well as
correlatively frequent accidents.
4648 The monitoring of water
in oil is an interfacial challenge, and using TENG sensors is an
ffective way to address this challenge. As shown in Figure 3jl
Figure S9, the voltage output increases persistently with
the fraction of water increasing for all base oils, respectively,
passing PTFE-based, LDPE-based, and GLASS-based O
TEGNs. Especially, for the water-laden
flows of PAO-6 and
rapeseed oils, the output voltage obviously increases from 0.1
to 0.3 V when the water fraction increases only to 0.01 wt % as
shown in
Figure 3j and Figure 3l, which means this developed
Figure 3. Typical signal curves in 5 s for the output voltage of PTFE-based OS TENG monitoring pure base oils. Various fractions of Fe
debris (0
20 mg mL1) affecting the OS TENG output of PAO-6 oil (a), paraffin oil (b), and rapeseed oil (c). Various fractions of carbon
blacks (0
20 mg mL1) affecting the OS TENG output of PAO-6 oil (d), paraffin oil (e), and rapeseed oil (f). Various fractions of diesel
oils (0
30 wt %) affecting the OS TENG output of PAO-6 oil (g), paraffin oil (h), and rapeseed oil (i). Various fractions of water (01 wt
%) a
ffecting the OS TENG output of PAO-6 oil (j), paraffin oil (k), and rapeseed oil (l).
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OS TENG shows a high sensitivity for monitoring water in
oils in spite of a very low water fraction.
Electrical Output Mechanism of Lubricating Oils.
According to the output voltage of OS TENG above, it can
be found that the variation tendency of water-laden oil
flow is
fferent from those of Fe, Cu debris, and carbon black-laden
flows. When the water in the oil flow contacts the OS TENG
surface, the output voltage will increase because the output of
water/solid electri
fication is higher than that of oil/solid
fication as reported in the previous work.31 In addition,
as is known, the surface tension of water (62
72 mJ m2) is
much higher than that of oil (31
35 mJ m2),21,49 which
means the interfacial wettability of the oil
flow in contact with
the TENG surface is weaker when water is added to the oil,
that is, water has greater tendency to leave the surface, which
lessens water residues. On the contrary, it can be understood
that the output of diesel oil-laden
flow obviously decreases at a
high fraction, because the diesel oil with very low surface
tension (28 mJ m
2) is prone to spreading and adsorbing on
the TENG surface.
21 The output performances of aged base
oils (aged PAO-6 oil, aged para
ffin oil, and aged rapeseed oil)
are in good agreement with Fe/Cu debris-laden and carbon
black-laden oils, i.e., the output voltage slightly increases and
then decreases gradually with the increase of contaminant
fraction. With the increase of thermal aging, the color of the
oils gradually changes from colorless to orange for both PAO-6
Figure 4. Adsorption behavior and electrical output mechanism of lubricating oils. (a) QCM-D data showing the change in normalized
frequency with the increase in aging time. (b) Adsorption masses of oil molecules on Au substrate measured by QCM-D for base oils. (c)
Output voltages of base oils as a function of aging time for PTFE-based O
S TENG. (d) TAN values of base oils as a function of aging time.
(e) Critical fractions of incoming contaminants and adsorption masses of base oils. (f) Schematic diagram of the actual working principle of
S TENG in contact with contaminant-laden oils and the charge distribution in different stages.
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and paraffin oils shown in Figure S10. It means these two pure
oils have been severely oxidized after long aging time and they
are less stable than rapeseed oil. According to the results of
Fourier transform infrared spectroscopy (FTIR) in
Figure S11,
there are many oxygen-containing components (carboxylate,
carbonyl (1158 cm
1), and hydroxy (3470 cm1) groups)
especially of carboxylates generated in PAO-6 and para
ffin oils
after the long aging time (48
192 h).50,51 Therefore, these
aged oils display much higher polarity with a larger
concentration of ions than pure base oils, so the positive
relation between the output value and the thermal aging time is
due to the enhancement of the ion transfer process.
31 The
increased output signal from the Fe/Cu debris-laden and
carbon black-laden oils is considered to be caused by a higher
triboelectric generating capability of contaminant ingressions
(Fe, Cu, and carbon particles) than of organic components (oil
52,53 thereby enhancing contact electrification of
S TENG. However, the output voltage signal then
decreases at a higher fraction of contaminants, which is most
likely due to the unavoidable adhesion of oil on the tube after
flow passes. For example, obvious contaminant adsorbates
adsorbed on the inside walls of the nonmetal surfaces after
testing para
ffin oils with a high debris fraction (20 mg mL1)
as shown in
Figure S12, so the contaminants with some
charges remain on the surfaces, resulting in the partial
screening of the tribo-charges on the
films. Similarly, the
orange color organic components in aged oils easily adsorb on
the inner wall of O
S TENG after multiple-squeezing and
-loosing processes such as the para
ffin oil after the high aging
time (192 h) (
Figure S12).
The adsorbing behavior of oil
flows on surfaces play a critical
role in the output capacity of TENG. Therefore, we
quantitatively studied the adsorbing performance of lubricating
oils. We use a quartz crystal microbalance with dissipation
(QCM-D) to kinetically analyze the buildup of the adsorbed
layers on the inner wall of O
S TENG.54 In Figure 4a, a rapid
decrease in the frequency of pure base oils (0 h) is observed
due to the adsorption of oil molecules to the substrate. The
frequency values are very stable over time, and 300 s
adsorption time is allowed to ensure the saturated oils
adsorbed on the substrate. Then oils aged for a di
period (3
192 h) are investigated in sequence. For long aging
times (96
192 h), a significant decrease in the frequency is
observed, which means a longer time-aging oil adsorbs more
on the substrate. In addition, when base oils are injected into
the QCM cell, the increase in dissipation caused by adsorption
is obtained as shown in
Figure S13. It can be clearly seen that
the dissipation is small enough (
ΔD < 10Δf) (ΔD, dissipation;
Δf, resonance frequency), so the adsorbed film on the
substrate is veri
fied to be a rigid layer and the Sauerbrey
equation can be used to calculate the adsorbed mass from the
frequency change.
55,56 The results of the adsorption mass are
summarized in
Figure 4b. It is found that the adsorption mass
of these oils increases with the aging time. The aged para
ffin oil
has the lowest value of adsorption mass (11.5 mg cm
2) after a
long aging time (192 h), and the adsorption mass of aged
rapeseed oil is as large as 17.1 mg cm
2. During the whole
aging time (0
192 h), the adsorption mass of aged paraffin oil
is the smallest. The aged rapeseed oil always has a much larger
adsorption mass than the other two base oils, which further
firms the results in Figure 2 concerning the much lower
output values of aged rapeseed oil than those both of aged
ffin oil and aged PAO-6 oil.
Although the output voltages of all aged base oils increase at
first and then decrease (Figure 2), the critical aging time is
fferent as shown in Figure 4c. The critical aging time of aged
ffin oil is as high as about 48 h, while the critical values of
aged PAO-6 oil and rapeseed oil are 24 and 3 h, respectively. It
is because aged para
ffin oil is the most difficult to be adsorbed
on the surface (
Figure 4b), so that the screen effect is much
weaker, and the output signal can maintain a high value despite
the aging time increase. Comparatively, the corresponding
film of aged rapeseed oil with a strong adhesive effect
can be easily deposited on the surfaces, so the screen e
ffect is
irreversible, and the output signal decreases quickly. Apart
from the critical aging time, the output values at di
fferent aging
period are also di
fferent for these base oils. It can be seen that
the aged rapeseed oil has a little higher output value than both
aged PAO-6 oil and aged para
ffin oil during aging time from 0
to 5 h (
Figure 4c). The total acid number (TAN) of aged
rapeseed oil is much higher compared with that of the other
base oils (
Figure 4d) at the initial aging period, so the higher
output capacity is attributed to the enhancement of ion
transfer process for aged rapeseed oil in spite of the surface
adsorption e
ffect. With the aging time increasing (up to about
35 h), the TAN of aged PAO-6 oil sharply increases, meaning
ion transfer plays a prominent e
ffect for aged PAO-6 oil. For
ffin oil, when the aging time increases from 35 to 90 h, the
TAN of aged para
ffin oil is as large as that of aged PAO-6 oil,
and also there is a much smaller value of adsorption mass for
aged para
ffin oil (Figure 4b); thereby, aged paraffin oil displays
the highest output value. Furthermore, when the aging time is
over the critical value (24 h for PAO-6 oil; 48 h for para
oil), the TAN increases sharply and the output voltages of aged
PAO-6 oil and aged para
ffin oil decrease quickly, which
firms that the increase of the aging degree leads to the
excessive ions in the oil
flows and will interfere with electron
transfer process. For aged rapeseed oil, the TAN has no
evident change with the increase of aging time, but the
corresponding adsorption mass increases more obviously,
which results in the decrease of the output signal due to the
screen e
ffect from the adsorption layers around OS TENG.
The adsorption behaviors of lubricating oils not only play an
important role in the output signal of aged oil, but also directly
ffect the output performance of Fe/Cu debris-laden and
carbon black-laden oils. As shown in
Figure 4e, the adsorption
masses of pure rapeseed oil, pure PAO-6 oil, and pure para
oil are 12.7 mg cm
2, 11.6 mg cm2, and 9.7 mg cm2,
respectively. Meantime, the critical fractions of contaminants
(Fe debris, Cu debris, and carbon blacks) in turn increase for
each base oil. The lubricating oil with larger adsorption mass
can carry more contaminants to remain on the surface of O
TENG, which will lead to the quick saturation of screen e
and to the decrease of output capacity. Therefore, although
incoming contaminants can e
ffectively contact the nonmetal
dielectric surfaces and can provide a higher precharges of oil
flows, the corresponding strong adsorbed oil film is easily
deposited on the surfaces, so the screen e
ffect is irreversible,
and the output signal gradually decreases as shown in
Formulated Lubricating Oil Condition Monitoring by
the O
S TENG. To monitor full formulated lubricating oils
for industry application, a fresh formulated commercial engine
lubricating oil with various fractions of waste oil (instead of
pure base oils in the previous section) is used as a test object.
As shown in
Figure 5ac, the output voltages of the oil are
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much higher than those of pure base oils shown in Figure 2
and Figure 3. Pure base oils are composed of nonpolar
hydrocarbons and the signal output mainly results from a weak
electron transfer process, so the output value is low (
Figures 2
and 3). The commercial lubricating oils are composed of base
oils and other components, such as active/polar additives, that
is, antiwear, dispersing, and antirust additives,
5759 which can
improve signi
ficantly the interfacial transfer of electron and ion.
In addition, the charge trapping ability of the contact layer
increases with the increased dielectric constant.
60 The
dielectric constant of the employed commercial engine oil is
much higher than those of the base oils shown in
Figure S14.
Therefore, the output signals of commercial lubricating oils
(more than 1.0 V) are much higher than those of pure base oils
(about 0.1 V). The voltage values of engine lubricating oils
finally decrease at a high fraction of waste oils, which are in
good agreement with the output performance of contaminantladen base oils. This occurs because more and more incoming
contaminants such as wear debris are adsorbed on the
dielectric surfaces with the increase of waste oil fraction, and
also more age-induced ions with opposite charges are adsorbed
onto the contacted surfaces, thereby screening the tribocharges on the generation layer.
According to the above
findings, it is believed that the
designed O
S TENG could be applied in the actual field to
realize the real-time and online detection system in industrial
applications. To demonstrate this, a self-powered sensor is
developed for real-time and online monitoring of the engine
lubricating oil in an actual oil tank on a simulated test platform.
The mechanical motion of vehicles always involves accel
Figure 5. Output characterization of the OS TENG monitoring formulated lubricating oils. The output voltage of a commercial engine
lubricating oil in contact with PTFE-based (a), LDPE-based (b), and GLASS-based (c) O
S TENG as a function of the fraction of waste oil.
A sketch of a vehicle corresponding to a lubrication system area (d). A sketch of oil circuit and oil tank in the lubrication system (e). A
schematic diagram of a designed O
S TENG with a single electrode used in the industrial transportation system (f). Output monitor used
for O
S TENG in this study (g). Actual tested devices of output signals for the transportation lubrication system, which includes engine
lubricant oil, oil tank, and single electrode (h). Output voltage of the engine lubricating oil as a function of the fraction of waste oil (i,j).
Output characteristic after stretching 1500 reciprocating times compared with initial output (k).
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eration and deceleration on complex roads such as school
roads, highways, and road intersections.
61 Lubricating oils can
generate an oil wave in the con
fined space of the vehicle tank
because of the inertia. When the O
S TENG is attached to the
inner wall of the tank, an output signal can be obtained from
the contact-separation between the oil wave and the TENG
solid surface. A vehicle sketch with the oil circuit and oil tank
in the lubrication system is shown in
Figure 5d,e. The real-time
and online O
S TENG sensor comprises a short rectangular
Cu electrode fully covered by a PTFE
film that is attached to
an inner wall of the tank (
Figure 5f). The collected data is
transferred to a computer to realize real-time display by the
data capture device as displayed from the photography of
device in
Figure 5g. The tank shown in Figure 5h is driven by a
linear motor to generate the oil wave (Supporting
Movie 1,
Movie 2, and Movie 3, and Movie 4). Figure 5i shows the
output voltage of O
S TENG slightly decreases with a low
fraction of waste oil (0
2 mg mL1), and significantly
decreases with a fraction of waste oil from 3 mg mL
1 to 20
mg mL
1 (Movie 5, Movie 6, and Movie 7). This should be
due to the screen e
ffect of the incoming components, such as
wear debris, deposited carbon, and age-induced oxygencontaining groups, adsorbed on the TENG surfaces (
j). When the temperature of oil increases, the output voltage
decreases gradually (
Figure S15). This is because the high
temperature will cause thermionic emission which will lower
the TENG output performance.
62 The developed TENG is
suitable to be used in the oil tank and will have little
fire hazard
because the output values of electric current and charge are
only about
±0.4 nA and ±0.2 nC (Figure S16), which is safe
for such kind of application. In addition, there is not a distinct
change in the output values of the O
S TENG after the 1500
times cycles shown in
Figure 5k, which means the transferred
charges are also almost the same as that at the initial pristine
state. Hence, reciprocating cycles have little e
ffect on the
adsorption behavior indicating the designed O
exhibits good ductility and stability.
This study developed a self-powered triboelectric sensor for
monitoring lubricating oils based on the oil
solid interfacing
fication effect. The charge transfer between water
and solids is suggested to have both electrons and ions
exchange. But for the case of oil with solids, since there are no
ions in oil, the surface charges on the solid after contacting the
oil should be electron exchange from the oil molecules. Our
experimental results carried out here suggest that electrons are
transferred from oil to the glass surface, which is responsible
for the signals we have detected. This
finding is consistent with
our previous study for the water
oil interface, in which
electron transfer was also suggested.
63 For the base oils
α-olefin), paraffin, and rapeseed oils), the output voltage
finally decreases at long aging time and high debris fraction due
to the irreversible e
ffect of screen film adsorbed on the
electrode surface. We obtained the adsorption mass of these
base oils on the substrate further con
firming the screen effects
by QCM-D analysis. On the basis of the
findings, a selfpowered monitor is successfully developed for real-time and
online monitoring of the engine lubricating oil in an actual oil
tank. It is believed that this self-powered triboelectric sensor
has the great advantages of monitoring service performance of
lubricating oils for di
fferent mechanical systems in a costefficient way.
Characterization. The output voltage signal was measured using
a Keithley 6517 system electrometer. To measure the output
performance of the O
S TENG in a transportation system, a linear
motor from Wang
s group (LinMot, E1100, America) was used to
drive the system to move periodically with the amplitude of 100 mm
and cycle of 2.4 s, in which the position wait time of start or stop is 1
s. The micromorphology of this wear debris was taken by a scanning
electron microscope (Quanta 200 FEG, FEI, America). Oxygencontaining functional groups of aged oils were investigated by a
Fourier-transform infrared spectroscopy (Vertex, NETZSCH, Germany), and their total acid numbers (TAN) were obtained by a TAN
tester (Delit, China). A QCM-D instrument (Q-sense E4 system,
Biolin Scienti
fic, Sweden) was used to simultaneously measure the
changes of both resonance frequency (
Δf) and dissipation (ΔD) for
the adsorption of oil components on a normative Au substrate. The
dielectric constant of lubricating oil was measured by an automatic oil
dielectric loss and volume resistivity tester (Delit, China)
Experimental Section. Poly(α-olefin) and paraffin oils were
purchased from Shanghai Qicheng Industrial Co., Ltd., P. R. China.
Rapeseed oil was obtained from Arawan Co., Ltd., P.R. China. Before
S TENG test, we kept these base oils at 105 °C for 5 h in a
vacuum drying oven for dry processing. For dropper-based O
TENG tests, the velocity and acceleration of the oil are about 3.0 cm/
s and 6.0 cm
2/s, relatively. The flow rate and volume of the oil in the
dropper are about 0.6 mL/s and 2 mL at environment temperature
°C). Before QCM-D test, all these oils were fully diluted by
petroleum (30 wt %) for achieving low viscous tested oils and stable
adhesive data from QCM-D. The droppers and Cu electrodes were
bought from Beijing Jiashitao Technology Co. Ltd., P.R. China. The
full formulated commercial engine lubricant oil (0W-16) was achieved
from Autobacs Quality Co. Ltd., P.R. China. The waste oil was
acquired in a being-repaired motor vehicle from a dealership
(Tianyuanbao Road Auto Maintenance Center, China).
Fabrication of OS TENG. There are two types of OS TENG
fabricated in this study. They are manual dropper-based and single
electrode-based O
S TENGs. For the dropper-based OS TENG,
the width and area of the copper electrode covering the droppers are
15 mm and 263.8 mm
2. The inner diameters of all droppers are 5 mm.
The contact areas of the TENG in this study are all the same for
avoiding their in
fluence on the condition monitoring of oils. The Cu
electrode with a width of 15 mm was uniformly attached on a dropper
substrate. PTFE, LDPE, and GLASS are used as substrate materials as
shown in
Figure S1. The single electrode-based OS TENG was first
fabricated by preparing a long rectangular copper electrode with a
width of 3 mm and a length of 5 mm. Each surface of the electrode
was completely covered with an 80
μm thick PTFE film; meanwhile,
the edges of the electrode were sealed by the PTFE
film to prevent
contact with oils, and one of the O
S TENG surfaces was attached to
a double-sided foam tape substrate.
*sı Supporting Information
The Supporting Information is available free of charge at
Table of factors of the contaminant ingressions studied
for the di
fferent times in the atmospheric environment;
figures supporting the text (PDF)
Front view of oil tank (
Nonencapsulated oil tank1 (
Nonencapsulated oil tank2 (
Encapsulated oil tank (
Data view1 (
Data view2 (
Data view3 (
ACS Nano Article
ACS Nano 2021, 15, 1186911879
Corresponding Authors
Yijun Shi Division of Machine Elements, Luleå University of
Technology, Luleå SE-971 87, Sweden;
; Email: [email protected]
Jinshan Pan Division of Surface and Corrosion Science,
Department of Chemistry, KTH Royal Institute of
Technology, Stockholm SE-100 44, Sweden
[email protected]
Zhong Lin Wang CAS Center for Excellence in Nanoscience,
Beijing Key Laboratory of Micro-Nano Energy and Sensor,
Beijing Institute of Nanoenergy and Nanosystems, Chinese
Academy of Sciences, Beijing 101400, P. R. China;; Email: [email protected]
Jun Zhao Division of Machine Elements, Luleå University of
Technology, Luleå SE-971 87, Sweden; College of Mechanical
and Electrical Engineering, Beijing University of Chemical
Technology, Beijing 100029, P. R. China;
Di Wang Division of Machine Elements, Luleå University of
Technology, Luleå SE-971 87, Sweden
Fan Zhang Department of Engineering and Design, School of
Engineering and Information, University of Sussex, Brighton
BN1 9RH, United Kingdom;
Yuan Liu CAS Center for Excellence in Nanoscience, Beijing
Key Laboratory of Micro-Nano Energy and Sensor, Beijing
Institute of Nanoenergy and Nanosystems, Chinese Academy
of Sciences, Beijing 101400, P. R. China
Baodong Chen CAS Center for Excellence in Nanoscience,
Beijing Key Laboratory of Micro-Nano Energy and Sensor,
Beijing Institute of Nanoenergy and Nanosystems, Chinese
Academy of Sciences, Beijing 101400, P. R. China
Roland Larsson Division of Machine Elements, Luleå
University of Technology, Luleå SE-971 87, Sweden
Complete contact information is available at:
The authors declare no competing financial interest.
The authors want to thank Swedish Kempe Scholarship
Project (No. JCK-1903.1), the Swedish Research Council for
Environment, Agricultural Sciences and Spatial Planning
(Formas, No. 2019-00904), the Swedish Research Council
(No. 2019-04941), the Swedish Energy Agency (Energimyndigheten, Nos. 2017-008200, 2018-003910), and the National
Natural Science Foundation of China (Grant No. 51905027).
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