Experimental Study on Decomposition and Evaporation Characteristics of N,N,N ',N '-Tetramethylethylenediamine and 1,2,4-Triazole

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energies

Article

Experimental Study on Decomposition and

Evaporation Characteristics of

N,N,N

0 ,N

0 -Tetramethylethylenediamine and

1,2,4-Triazole

Jungmin Yu1, Seung Wook Baek1,* and Sung June Cho2

1 _{Department of Aerospace Engineering, Korea Advanced Institute of Science and Technology (KAIST),}

291 Daehak-ro, Yuseong-gu, Daejeon 34141, Korea

2 _{Department of Applied Chemical Engineering, Chonnam National University, 77 Yongbong-ro,}

Gwangju 61186, Korea

* Correspondence: swbaek@kaist.ac.kr; Tel.:+82-42-350-3714

Received: 14 May 2019; Accepted: 18 August 2019; Published: 21 August 2019 

Abstract:N,N,N0,N0-Tetramethylethylenediamine (TMEDA) is one of the candidate substances of hydrazine for hypergolic bipropellant applications. In this study, two experimental analyses were performed to investigate the characteristics of TMEDA and TMEDA mixed with 1,2,4-triazole. First of all, thermogravimetric analysis was performed to analyze thermal decomposition process of the candidate materials. During heating, TMEDA started to evaporate at a temperature below its boiling point and 1,2,4-triazole evaporated subsequently. In addition, evaporation behaviors of TMEDA droplets with two concentrations of 1,2,4-triazole (10% and 20% by weight) were studied at various temperatures. A single droplet experimentation was accomplished in a high-pressure chamber using a heating system under atmospheric nitrogen gas condition. Its droplet behavior was recorded with a high-speed camera and the data were post-processed using Visual Basic software to trace the droplet diameter variation. Bubbling and puffing were observed in the case of the TMEDA containing 1,2,4-triazole above 400◦C. As a result, the rate of evaporation was substantially enhanced by increasing the concentration of 1,2,4-triazole at same temperature. However, the evaporation rates after TMEDA evaporation were the same irrespective of the concentration of 1,2,4-triazole.

Keywords: N,N,N0,N0-Tetramethylethylenediamine (TMEDA); 1,2,4-triazole; droplet; decomposition; evaporation

1. Introduction

In a liquid rocket engine using hypergolic bipropellant, ignition occurs spontaneously when the fuel interacts with oxidizer [1]. If the fuel and oxidizer do not react immediately in the interior of the combustion chamber, an explosion can occur due to an over-pressurized state by accumulated unburned fuel and oxidizer [2,3]. Therefore, ignition delay time is very important factor for a stable propulsion system of a liquid rocket engine. Normally, hypergolic propellants have a very short ignition delay time of less than tens of milliseconds. In the past few decades, hydrazine, monomethylhydrazine (MMH) and unsymmetrical dimethylhydrazine (UDMH) have been exploited as hypergolic bipropellants. Hydrazine and its derivatives have the characteristics of high specific impulse (Isp), short ignition delay, and high density. However, it is difficult to control them due to its toxicity; LD50(Lethal Dose 50)= 60 mg/kg for oral ingestion of hydrazine by mice [4].

Various amine azides have been prepared as alternative fuels and tested by many research groups. A suitable substitute for MMH must be less toxic while having a comparative density

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Energies 2019, 12, 3208 2 of 13

impulse under similar operating conditions. Thomson [5] suggested 2-N,N-dimethylaminoethylazide (DMAZ), bis (ethyl azide) methylamine (BAZ) and pyrollidinylethylazide (PYAZ). Stevenson et al. [6] tested the fuel mixtures with tertiary diamine, tri-amine or tetra-amine compounds as substitutes for fuels containing toxic MMH. The fuel mixtures comprise N,N,N0,N0-Tetramethylethylenediamine (TMEDA) with 2-N,N-dimethylaminoethylazide (DMAZ), TMEDA with tris (2-azidoethyl) amine (TAEA), and TMEDA with other cyclic amine azides. Each hypergolic fuel mixture was found to yield a reduced ignition delay in propellant systems, compared to that for each unmixed component. Wang et al. [7] examined the hypergolic characteristics between TMEDA and 90% HNO3using a drop test. They measured the ignition delay times by counting the video frames starting from the contact of two liquids to the onset of luminosity. McQuaid et al. [8] compared TMEDA, DMAZ and their mixtures with hydrazine in terms of ignition delay and specific impulse. Reddy et al. [9] conducted genetic toxicity studies on TMEDA and DMAZ, and proved that these substances are relatively safe compared to MMH through experiments on mice with the LD50TMEDA at 268 mg/kg. In 1905, saturated, tertiary, alkyl multi amines (STAMs) were extensively examined by the Phillips Petroleum Company [10]. Among the amines, TMEDA was considered to be most promising because it is close to MMH in many aspects. TMEDA has a longer ignition delay and a lower density than MMH. Some important properties and performance data of TMEDA and MMH are given in Table1[6,8,11,12] when using red fuming nitric acid (RFNA) as the oxidizer. Density impulse (DIsp) is basically the propellant specific impulse multiplied by its density. Even if there is not much difference in specific impulse between TMEDA and MMH, it is evident that each density impulse is significantly distinguished by their own density. Therefore, we considered several additives to increase the density of TMEDA-based fuel and are in the process of searching for a better additive that has high performance. In the present study, 1,2,4-triazole was selected among the candidate substances because it has high density and low toxicity. Some physical properties of 1,2,4-triazole are listed in Table2. The density of 1,2,4-triazole was generated using Percepta Platform by the ACD/Labs [13].

Table 1. Physical properties and performance parameters of monomethylhydrazine (MMH) and N,N,N0,N0-Tetramethylethylenediamine (TMEDA).

MMH TMEDA

Chemical formula CH3NHNH2 (CH3)2NCH2CH2N(CH3)2

Liquid density (g/cm3₎ _0.87 _0.78

Atmospheric melting point (◦_C) ₋₅₂ ₋₅₅

Atmospheric boiling point (◦C) 88 121

Ignition delay time with RFNA (ms) 8 14

Maximum specific impulse with RFNA (lbf·s/lbm) 284 281

Maximum density impulse with RFNA (103

lbf·s/ft3)

15.7 13.7

Table 2.Physical properties of 1,2,4-triazole.

Name Chemical Formula Density (g/cm3) at 25◦C Melting Point (◦C) at 1 atm Boiling Point (◦C) at 1 atm LD50(mg/kg) 1,2,4-triazole C2H3N3 1.3 ± 0.1 120 260 1648

A droplet of propellant sprayed from an injector is placed in a sudden situation in a particular temperature. Therefore, it is essential to realize the evaporation performance of an individual droplet of propellant. The research described above [5–8,11] did not include the momentary variation of a single droplet. In this study, an experimentation was implemented to examine the evaporation and thermal decomposition characteristics of TMEDA mixed with 1,2,4-triazole for hypergolic bi-propellant applications. A droplet experimental apparatus was adopted to account for an abrupt temperature change. In addition, evaporation rates of droplets according to temperature change were calculated.

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2. Experiments

2.1. TMEDA-Based Fuel Preparation

We used three kinds of TMEDA-based propellant with different concentrations. TMEDA and two different samples of TMEDA mixed with 1,2,4-triazole were prepared to characterize thermal decomposition and the droplet evaporation. The two different concentrations were a 90 wt.% TMEDA with 10 wt.% 1,2,4-triazole (blend type 1) and an 80 wt.% TMEDA with 20 wt.% 1,2,4-triazole (blend type 2). 1,2,4-triazole is a crystal at room temperature and is easily soluble in TMEDA. Other substances were not mixed. Each mixture concentration was listed in Table3.

Table 3.Concentration of TMEDA-based fuel.

Concentration (mol/m3)

TMEDA 1,2,4-Triazole

TMEDA 100 0

Blend type 1 TMEDA+ (10% 1,2,4-triazole) 90 10 Blend type 2 TMEDA+ (20% 1,2,4-triazole) 80 20 2.2. Experimental Methods

2.2.1. Thermal Analysis

In this study, the two different experimental methods performed were distinguished by their heating process. The first method is thermal analysis by a gradual increase in temperature. Courthéoux et al. [14] studied the decomposition behavior of hydroxylammonium nitrate (HAN) and hydrazinium nitroformate (HNF). The decomposition temperature of the solutions was observed to increase with the water content. The decomposition of these ionic solutions happened faster when the water was eliminated, for the ionic reagents could make interaction. In our previous study, Hwang et al. [15] performed thermogravimetric analysis to analyze the pyrolysis characteristics of the HAN-based materials.

In this study, we used two kinds of fuels, TMEDA and 1,2,4-triazole without water. Therefore, we investigate the effect of 1,2,4-triazole in the mixed material. Using a Setsys 16/18 differential scanning calorimeter, the thermogravimetric test was examined. It is possible to set the heating rate from 0.01 to 50 degrees Celsius per minute in the manual. The heating rate was arranged to 5 degrees Celsius per minute, empirically, while the final heating temperature was set to 300◦C. Actually, the variation of remaining fuel was not observed above 200◦C. While atmospheric pressure condition was maintained, the flow rate of nitrogen gas was set at 30 mL/min to avoid interference with the sample temperature distribution. The amount of each TMEDA-based fuel was about 20 mL and the analyses were carried out three times in each case.

2.2.2. Droplet Evaporation

In previous references of TMEDA-related propellant [5–8,11], the ambient temperature of the droplet was not precisely controlled enough to observe the phenomena of evaporation. Therefore, we used droplet experimental instrumentation to control the temperature and pressure [16,17]. Droplet evaporation was investigated by setting up a cylindrical test chamber equipped with a heating system and a droplet suspender. These data were post-processed to record the temporal variation of the droplet size using a high-speed charge-coupled device (CCD) camera. The current research work investigated the effects of additions of 1,2,4-triazole on evaporation characteristics of TMEDA droplet at various temperatures. The temporal behavior of a single TMEDA droplet evaporation with various concentrations (10%, 20% by weight) of 1,2,4-triazole was experimentally examined for various ambient temperatures (100–600◦C) under normal gravity. The droplet evaporation experimentation was organized under an atmospheric nitrogen gas environment.

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A schematic drawing of our experimental test is shown in Figure1. The droplet evaporation test was achieved using a cylindrical setup. A high-speed CCD camera recorded a droplet size variation. An electric furnace with two guide rails was implemented inside the chamber. The bottom of the electric furnace had an opening for smooth introduction of a droplet. Electric heaters were placed and insulated at both sides of the furnace. The electric coil was protected by a ceramic shield to avoid direct radiation. The two quartz windows were installed at the other two walls for an observation of the droplet behavior. A support made of silicon carbide fiber was located at the chamber bottom. And it is easily observed through the quartz windows of the furnace. More details of the experimental apparatus are available in [16,17]. The ambient temperature in the chamber was changed from 100◦C to 600◦

C by 100◦

C. The initial droplet diameter was in the range of 1.0 ± 0.1 mm. The recorded droplet images were post-processed using the Visual Basic program. First of all, the number of pixels inside droplet was counted, while the droplet diameter is estimated by a comparison with the number of pixels according to the fiber width. Figure2shows an example of image processing at a single droplet.

Energies 2019, 12, x FOR PEER REVIEW 4 of 13

examined for various ambient temperatures (100–600 °C) under normal gravity. The droplet evaporation experimentation was organized under an atmospheric nitrogen gas environment.

A schematic drawing of our experimental test is shown in Figure 1. The droplet evaporation test was achieved using a cylindrical setup. A high-speed CCD camera recorded a droplet size variation. An electric furnace with two guide rails was implemented inside the chamber. The bottom of the electric furnace had an opening for smooth introduction of a droplet. Electric heaters were placed and insulated at both sides of the furnace. The electric coil was protected by a ceramic shield to avoid direct radiation. The two quartz windows were installed at the other two walls for an observation of the droplet behavior. A support made of silicon carbide fiber was located at the chamber bottom. And it is easily observed through the quartz windows of the furnace. More details of the experimental apparatus are available in [16,17]. The ambient temperature in the chamber was changed from 100 °C to 600 °C by 100 °C. The initial droplet diameter was in the range of 1.0 ± 0.1 mm. The recorded droplet images were post-processed using the Visual Basic program. First of all, the number of pixels inside droplet was counted, while the droplet diameter is estimated by a comparison with the number of pixels according to the fiber width. Figure 2 shows an example of image processing at a single droplet.

① Lever ② Temperature controller ③ Quartz window ④ Electric furnace ⑤ Furnace bottom hole ⑥ Guide bar ⑦ Cylindrical vessel ⑧ High-speed CCD camera ⑨ Shock absorber ⑩ Micro-pump ⑪ Droplet lever ⑫ Droplet generator ⑬ Droplet ⑭ Silicon carbide fiber ⑮ Backlight bulb ⑯ Quartz glass window ○17 Nitrogen tank

Figure 1. Schematic drawing of experimentation.

Figure 2. A sample image of the droplet. Figure 1.Schematic drawing of experimentation.

examined for various ambient temperatures (100–600 °C) under normal gravity. The droplet evaporation experimentation was organized under an atmospheric nitrogen gas environment.

A schematic drawing of our experimental test is shown in Figure 1. The droplet evaporation test was achieved using a cylindrical setup. A high-speed CCD camera recorded a droplet size variation. An electric furnace with two guide rails was implemented inside the chamber. The bottom of the electric furnace had an opening for smooth introduction of a droplet. Electric heaters were placed and insulated at both sides of the furnace. The electric coil was protected by a ceramic shield to avoid direct radiation. The two quartz windows were installed at the other two walls for an observation of the droplet behavior. A support made of silicon carbide fiber was located at the chamber bottom. And it is easily observed through the quartz windows of the furnace. More details of the experimental apparatus are available in [16,17]. The ambient temperature in the chamber was changed from 100 °C to 600 °C by 100 °C. The initial droplet diameter was in the range of 1.0 ± 0.1 mm. The recorded droplet images were post-processed using the Visual Basic program. First of all, the number of pixels inside droplet was counted, while the droplet diameter is estimated by a comparison with the number of pixels according to the fiber width. Figure 2 shows an example of image processing at a single droplet.

① Lever ② Temperature controller ③ Quartz window ④ Electric furnace ⑤ Furnace bottom hole ⑥ Guide bar ⑦ Cylindrical vessel ⑧ High-speed CCD camera ⑨ Shock absorber ⑩ Micro-pump ⑪ Droplet lever ⑫ Droplet generator ⑬ Droplet ⑭ Silicon carbide fiber ⑮ Backlight bulb ⑯ Quartz glass window ○17 Nitrogen tank

Figure 1. Schematic drawing of experimentation.

Figure 2. A sample image of the droplet. Figure 2.A sample image of the droplet.

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3. Results

3.1. Thermal Decomposition of TMEDA-Based Fuels

As mentioned in Section2.2.1, the fuel on an open pan inside the sample was heated at 5◦C/min, after an initial stabilization. Figures3–5show the results for thermal decomposition of TMEDA-based fuels. In Figure3, the Y-axis shows the normalized weight percent by the initial mass. During heating, TMEDA-based fuels started to evaporate at temperatures below its boiling point and 1,2,4-triazole evaporated subsequently. The thermogravimetric results showed a two-step process of weight loss because the incorporation of a component having a higher boiling point. The thermal decomposition of a mixture finished before reaching the boiling temperature of 1,2,4-triazole and there was no residue after the complete decomposition. In Figure4, as 1,2,4-triazole increased, there was a clear change on mass curve in the vicinity the boiling temperature of TMEDA. Meanwhile, in Figure5, the thermal decomposition temperature was identified by a positive peak heat release rate at 114.1◦C, 110.0◦C, and 109.7◦C, respectively, for the TMEDA and other blend type 1 and 2. The evaporation duration of TMEDA-based fuels can be recognized by a negative heat release. The thermal decomposition of TMEDA-based fuels takes place while creating a heat release peak. In the experimental results, the weight of propellant was as follows. (1) TMEDA: 19.48 mg, (2) TMEDA+ (10% 1,2,4-triazole): 19.87 mg, (3) TMEDA+ (20% 1,2,4-triazole): 21.12 mg.

3. Results

3.1. Thermal Decomposition of TMEDA-Based Fuels

As mentioned in Section 2.2.1, the fuel on an open pan inside the sample was heated at 5 °C/min, after an initial stabilization. Figures 3–5 show the results for thermal decomposition of TMEDA-based fuels. In Figure 3, the Y-axis shows the normalized weight percent by the initial mass. During heating, TMEDA-based fuels started to evaporate at temperatures below its boiling point and 1,2,4-triazole evaporated subsequently. The thermogravimetric results showed a two-step process of weight loss because the incorporation of a component having a higher boiling point. The thermal decomposition of a mixture finished before reaching the boiling temperature of 1,2,4-triazole and there was no residue after the complete decomposition. In Figure 4, as 1,2,4-triazole increased, there was a clear change on mass curve in the vicinity the boiling temperature of TMEDA. Meanwhile, in Figure 5, the thermal decomposition temperature was identified by a positive peak heat release rate at 114.1 °C, 110.0 °C, and 109.7 °C, respectively, for the TMEDA and other blend type 1 and 2. The evaporation duration of TMEDA-based fuels can be recognized by a negative heat release. The thermal decomposition of TMEDA-based fuels takes place while creating a heat release peak. In the experimental results, the weight of propellant was as follows. (1) TMEDA: 19.48 mg, (2) TMEDA + (10% 1,2,4-triazole): 19.87 mg, (3) TMEDA + (20% 1,2,4-triazole): 21.12 mg. 0 50 100 150 200 250 300 0 20 40 60 80 100

Weight (%)

Temperature (

_°C)

TMEDA TMEDA + (10% 1,2,4-triazole) TMEDA + (20% 1,2,4-triazole)

Figure 3. Temporal change of reduced fuel mass with temperature profile.

0 50 100 150 200 250 300 0 5 10 15 20 25

M

ass decreasing rat

e (%/min)

Temperature (°C)

TMEDA

TMEDA + (10% 1,2,4-triazole) TMEDA + (20% 1,2,4-triazole)

Figure 4. Temporal change of decreasing rate of fuel mass. Figure 3.Temporal change of reduced fuel mass with temperature profile.

3. Results

3.1. Thermal Decomposition of TMEDA-Based Fuels

As mentioned in Section 2.2.1, the fuel on an open pan inside the sample was heated at 5 °C/min, after an initial stabilization. Figures 3–5 show the results for thermal decomposition of TMEDA-based fuels. In Figure 3, the Y-axis shows the normalized weight percent by the initial mass. During heating, TMEDA-based fuels started to evaporate at temperatures below its boiling point and 1,2,4-triazole evaporated subsequently. The thermogravimetric results showed a two-step process of weight loss because the incorporation of a component having a higher boiling point. The thermal decomposition of a mixture finished before reaching the boiling temperature of 1,2,4-triazole and there was no residue after the complete decomposition. In Figure 4, as 1,2,4-triazole increased, there was a clear change on mass curve in the vicinity the boiling temperature of TMEDA. Meanwhile, in Figure 5, the thermal decomposition temperature was identified by a positive peak heat release rate at 114.1 °C, 110.0 °C, and 109.7 °C, respectively, for the TMEDA and other blend type 1 and 2. The evaporation duration of TMEDA-based fuels can be recognized by a negative heat release. The thermal decomposition of TMEDA-based fuels takes place while creating a heat release peak. In the experimental results, the weight of propellant was as follows. (1) TMEDA: 19.48 mg, (2) TMEDA + (10% 1,2,4-triazole): 19.87 mg, (3) TMEDA + (20% 1,2,4-triazole): 21.12 mg. 0 50 100 150 200 250 300 0 20 40 60 80 100

Weight (%)

Temperature (

°C)

Figure 3. Temporal change of reduced fuel mass with temperature profile.

0 50 100 150 200 250 300 0 5 10 15 20 25

M

ass decreasing rat

e (%/min)

Temperature (°C)

TMEDA

TMEDA + (10% 1,2,4-triazole) TMEDA + (20% 1,2,4-triazole)

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0 50 100 150 200 250 300 -40 -20 0 20 40

Temperature (

°

C)

He

at releas

e rate

(

m

W

/min)

-50 0 50 100 150 200

Rel

eased heat (mW)

Figure 5. Temporal change of heat release rate and total released heat.

3.2. Evaporation Behavior of TMEDA-Based Fuels

After an initial heating up duration, a temporal change in droplet diameter squared becomes

almost linear while following the D2_{-law at the final stage of evaporation. In the case of TMEDA, the}

puffing during the evaporation inside the droplet did not occur as the ambient temperature increased. However, the micro-explosion process was found in the TMEDA droplet with the addition of 1,2,4-triazole. The nonappearance of a micro-explosion in the original TMEDA droplets shows that this phenomenon was just based on the existence of 1,2,4-triazole. The intensity of micro-explosions was substantially enhanced by increasing the concentration of 1,2,4-triazole at the same temperature.

As the droplet surface temperature increases, the evaporation process is initiated even before reaching boiling point. The boiling point of TMEDA is known as 121 °C. The droplet evaporation was measured in terms of the droplet diameter squared as a function of time. To easily compare droplet behavior, the droplet diameter squared as well as the evaporation time was normalized using the initial droplet diameter squared. When the droplet diameter squared is linearly reduced

with time, it is described by the following D2_-law:

𝐷 𝐷 𝐶 𝑡 (1)

where

𝐶 𝑑 𝐷

𝑑𝑡 (2)

is the evaporation coefficient. In this study, the diameter of TMEDA-based fuel droplets was measured, while the evaporation rate was estimated based on the linear variation of the droplet diameter squared. The evaporation coefficient is dependent not only on the thermophysical properties of the droplet, but also on the environmental conditions [18,19].

3.2.1. Variation of Droplet at Various Ambient Temperature

The droplet evaporation was experimentally accomplished in the range of 100–600 °C. In general, at lower ambient temperature, it is evident that the initial droplet diameter does not have an influence on the evaporation coefficient. However, at a higher temperature environment, the effect of the initial droplet diameter is no longer negligible so that the evaporation coefficient is observed to increase as the initial droplet diameter increases [20,21]. In this study, the temporal variations of droplet size were measured and plotted with the effective droplet diameter of 1.0 ± 0.1 mm.

Figure 5.Temporal change of heat release rate and total released heat. 3.2. Evaporation Behavior of TMEDA-Based Fuels

After an initial heating up duration, a temporal change in droplet diameter squared becomes almost linear while following the D2-law at the final stage of evaporation. In the case of TMEDA, the puffing during the evaporation inside the droplet did not occur as the ambient temperature increased. However, the micro-explosion process was found in the TMEDA droplet with the addition of 1,2,4-triazole. The nonappearance of a micro-explosion in the original TMEDA droplets shows that this phenomenon was just based on the existence of 1,2,4-triazole. The intensity of micro-explosions was substantially enhanced by increasing the concentration of 1,2,4-triazole at the same temperature. As the droplet surface temperature increases, the evaporation process is initiated even before reaching boiling point. The boiling point of TMEDA is known as 121◦C. The droplet evaporation was measured in terms of the droplet diameter squared as a function of time. To easily compare droplet behavior, the droplet diameter squared as well as the evaporation time was normalized using the initial droplet diameter squared. When the droplet diameter squared is linearly reduced with time, it is described by the following D2_-law:

D2 = D2_i −_C_v_t ₍₁₎

where

Cv = − dD2

dt (2)

is the evaporation coefficient. In this study, the diameter of TMEDA-based fuel droplets was measured, while the evaporation rate was estimated based on the linear variation of the droplet diameter squared. The evaporation coefficient is dependent not only on the thermophysical properties of the droplet, but also on the environmental conditions [18,19].

3.2.1. Variation of Droplet at Various Ambient Temperature

The droplet evaporation was experimentally accomplished in the range of 100–600◦C. In general, at lower ambient temperature, it is evident that the initial droplet diameter does not have an influence on the evaporation coefficient. However, at a higher temperature environment, the effect of the initial droplet diameter is no longer negligible so that the evaporation coefficient is observed to increase as the initial droplet diameter increases [20,21]. In this study, the temporal variations of droplet size were measured and plotted with the effective droplet diameter of 1.0 ± 0.1 mm.

Figure6illustrates the normalized squared diameter change (D2/D2_i) with normalized time

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These results display a steady state evaporating period without any heavy fluctuations. After an initial heating up duration, the droplet diameter squared reaches an almost linear variation with time (D2-law). In Figure6, after approximately 20 s/mm2, the slopes of TMEDA+ (10% 1,2,4-triazole) and TMEDA+ (20% 1,2,4-triazole) were changed slightly. In addition, in Figures8and9, the variations of evaporation rates were observed, too. These changes of gradient were attributed to the amount of 1,2,4-triazole in the blended fuel. Therefore, the evaporation history of TMEDA-based fuels specifies two different stages: vaporization of TMEDA before puffing and vaporization of the remaining 1,2,4-triazole fuel.

Figure 6 illustrates the normalized squared diameter change (𝐷 /𝐷 ) with normalized time 𝑡/𝐷 at 100 °C. Its sequential images of the evaporation TMEDA droplet were indicated in Figure 7. These results display a steady state evaporating period without any heavy fluctuations. After an initial heating up duration, the droplet diameter squared reaches an almost linear variation

with time (D2_{-law). In Figure 6, after approximately 20 s/mm}2_{, the slopes of TMEDA + (10%}

1,2,4-triazole) and TMEDA + (20% 1,2,4-triazole) were changed slightly. In addition, in Figures 8 and 9, the variations of evaporation rates were observed, too. These changes of gradient were attributed to the amount of 1,2,4-triazole in the blended fuel. Therefore, the evaporation history of TMEDA-based fuels specifies two different stages: vaporization of TMEDA before puffing and vaporization of the remaining 1,2,4-triazole fuel.

0 5 10 15 20 25 30 0.0 0.2 0.4 0.6 0.8

1.0 TMEDA_TMEDA₊_{(10 % 1,2,4-triazole)}

TMEDA + (20 % 1,2,4-triazole)

D

2

/D

2

[m

i

m

2

/mm

2

]

t/D

2_i

[s/mm

2

]

Figure 6. Normalized variation of evaporating TMEDA-based fuel droplet diameter squared with

normalized time at 100 °C.

t = 0.0 s t = 2.0 s t = 4.0 s t = 6.0 s t = 8.0 s

t = 10.0 s t = 12.0 s t = 14.0 s t = 16.0 s t = 18.0 s

Figure 7. Sequential images of the evaporating TMEDA droplet at 100 °C.

Figure 6. Normalized variation of evaporating TMEDA-based fuel droplet diameter squared with normalized time at 100◦C.

Figure 6 illustrates the normalized squared diameter change (𝐷 /𝐷 ) with normalized time 𝑡/𝐷 at 100 °C. Its sequential images of the evaporation TMEDA droplet were indicated in Figure 7. These results display a steady state evaporating period without any heavy fluctuations. After an initial heating up duration, the droplet diameter squared reaches an almost linear variation

with time (D2_{-law). In Figure 6, after approximately 20 s/mm}2_{, the slopes of TMEDA + (10%}

1,2,4-triazole) and TMEDA + (20% 1,2,4-triazole) were changed slightly. In addition, in Figures 8 and 9, the variations of evaporation rates were observed, too. These changes of gradient were attributed to the amount of 1,2,4-triazole in the blended fuel. Therefore, the evaporation history of TMEDA-based fuels specifies two different stages: vaporization of TMEDA before puffing and vaporization of the remaining 1,2,4-triazole fuel.

0 5 10 15 20 25 30 0.0 0.2 0.4 0.6 0.8

TMEDA + (20 % 1,2,4-triazole)

D

2

/D

2

[m

i

m

2

/mm

2

]

t/D

2_i

[s/mm

2

]

t = 0.0 s t = 2.0 s t = 4.0 s t = 6.0 s t = 8.0 s

t = 10.0 s t = 12.0 s t = 14.0 s t = 16.0 s t = 18.0 s

Figure 7. Sequential images of the evaporating TMEDA droplet at 100 °C. Figure 7.Sequential images of the evaporating TMEDA droplet at 100◦C.

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0 2 4 6 8 10 12 14 16 0.0 0.2 0.4 0.6 0.8

TMEDA + (20 % 1,2,4-triazole)

D

2

/D

2

[mm

i 2

/mm

2

]

t/D

2_i

[s/mm

2

]

normalized time at 200 °C. 0 2 4 6 8 10 12 0.0 0.2 0.4 0.6 0.8

TMEDA + (20 % 1,2,4-triazole)

D

2

/D

2

[m

i

m

2

/m

m

2

]

t/D

2_i

[s/mm

2

]

As the droplet surface temperature increases, the evaporation process begins even before reaching boiling point. At 400 °C above the boiling temperature of TMEDA, the expansion and fluctuation phenomena in the droplet size appeared. Simultaneously, the puffing process happens for TMEDA + (20% 1,2,4-triazole), as shown in Figure 10. These droplet puffing processes can be described using a diffusion-limit model [22,23]. Figure 11 shows sequential images of evaporating TMEDA + (20% 1,2,4-triazole) droplet and puffing phenomena is obviously identified here. In Figure 11, the droplet is shown to expand because of bubble formation. This behavior is typically found in the evaporation behavior of the multicomponent droplet as in our previous studies [15,21]. The post-puffing behavior for TMEDA + (20% 1,2,4-triazole) indicates that the droplet diameter

gradually decreases in the same way as for TMEDA + (10% 1,2,4-triazole). Similar to 400 °C, the

droplet was not completely evaporated at 500 and 600 °C as in the Figures 12 and 13. As shown in

the Figure 14, in TMEDA + (20% 1,2,4-triazole) droplet, the largest bubble was formed and a violent fluctuation was displayed inside the droplet. The values of diameter squared were changed 1.55, 0.74 and 1.54 at 1.63, 1.66 and 1.74 second, respectively. Therefore, it is evident that these sudden changes result from the micro-explosion phenomena. On the other hand, the droplets of TMEDA + (10% 1,2,4-triazole) did not show a strong puffing phenomenon as for TMEDA + (20% 1,2,4-triazole).

The normalized squared diameters for TMEDA + (20% 1,2,4-triazole) at 400 °C, 500 °C and 600 °C

Figure 8. Normalized variation of evaporating TMEDA-based fuel droplet diameter squared with normalized time at 200◦_C.

0 2 4 6 8 10 12 14 16 0.0 0.2 0.4 0.6 0.8

TMEDA + (20 % 1,2,4-triazole)

D

2

/D

2

[mm

i 2

/mm

2

]

t/D

2_i

[s/mm

2

]

normalized time at 200 °C. 0 2 4 6 8 10 12 0.0 0.2 0.4 0.6 0.8

TMEDA + (20 % 1,2,4-triazole)

D

2

/D

2

[m

i

m

2

/m

m

2

]

t/D

2_i

[s/mm

2

]

As the droplet surface temperature increases, the evaporation process begins even before

reaching boiling point. At 400 °C above the boiling temperature of TMEDA, the expansion and

fluctuation phenomena in the droplet size appeared. Simultaneously, the puffing process happens for TMEDA + (20% 1,2,4-triazole), as shown in Figure 10. These droplet puffing processes can be described using a diffusion-limit model [22,23]. Figure 11 shows sequential images of evaporating TMEDA + (20% 1,2,4-triazole) droplet and puffing phenomena is obviously identified here. In Figure 11, the droplet is shown to expand because of bubble formation. This behavior is typically found in the evaporation behavior of the multicomponent droplet as in our previous studies [15,21]. The post-puffing behavior for TMEDA + (20% 1,2,4-triazole) indicates that the droplet diameter

gradually decreases in the same way as for TMEDA + (10% 1,2,4-triazole). Similar to 400 °C, the

droplet was not completely evaporated at 500 and 600 °C as in the Figures 12 and 13. As shown in

the Figure 14, in TMEDA + (20% 1,2,4-triazole) droplet, the largest bubble was formed and a violent fluctuation was displayed inside the droplet. The values of diameter squared were changed 1.55, 0.74 and 1.54 at 1.63, 1.66 and 1.74 second, respectively. Therefore, it is evident that these sudden changes result from the micro-explosion phenomena. On the other hand, the droplets of TMEDA + (10% 1,2,4-triazole) did not show a strong puffing phenomenon as for TMEDA + (20% 1,2,4-triazole). The normalized squared diameters for TMEDA + (20% 1,2,4-triazole) at 400 °C, 500 °C and 600 °C

Figure 9. Normalized variation of evaporating TMEDA-based fuel droplet diameter squared with normalized time at 300◦C.

As the droplet surface temperature increases, the evaporation process begins even before reaching boiling point. At 400◦C above the boiling temperature of TMEDA, the expansion and fluctuation phenomena in the droplet size appeared. Simultaneously, the puffing process happens for TMEDA+ (20% 1,2,4-triazole), as shown in Figure10. These droplet puffing processes can be

described using a diffusion-limit model [22,23]. Figure11shows sequential images of evaporating TMEDA+ (20% 1,2,4-triazole) droplet and puffing phenomena is obviously identified here. In Figure11, the droplet is shown to expand because of bubble formation. This behavior is typically found in the evaporation behavior of the multicomponent droplet as in our previous studies [15,21]. The post-puffing

behavior for TMEDA+ (20% 1,2,4-triazole) indicates that the droplet diameter gradually decreases in the same way as for TMEDA + (10% 1,2,4-triazole). Similar to 400 ◦

C, the droplet was not completely evaporated at 500 and 600◦C as in the Figures12and13. As shown in the Figure14, in TMEDA+ (20% 1,2,4-triazole) droplet, the largest bubble was formed and a violent fluctuation was displayed inside the droplet. The values of diameter squared were changed 1.55, 0.74 and 1.54 at 1.63, 1.66 and 1.74 second, respectively. Therefore, it is evident that these sudden changes result from the micro-explosion phenomena. On the other hand, the droplets of TMEDA+ (10% 1,2,4-triazole) did not show a strong puffing phenomenon as for TMEDA + (20% 1,2,4-triazole). The normalized squared diameters for TMEDA+ (20% 1,2,4-triazole) at 400◦C, 500◦C and 600◦C were calculated as 0.85, 1.2 and 1.6, respectively. Consequently, the amount of 1,2,4-triazole plays a significant role in the bubble formation inside a droplet.

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Energies 2019, 12, 3208 9 of 13

were calculated as 0.85, 1.2 and 1.6, respectively. Consequently, the amount of 1,2,4-triazole plays a significant role in the bubble formation inside a droplet.

0 2 4 6 8 0.0 0.2 0.4 0.6 0.8

1.0 TMEDA_TMEDA₊_{(10 % 1,2,4-triazole)} TMEDA + (20 % 1,2,4-triazole) D 2 /D 2 [m i m 2 /m m 2 ] t/D2_i [s/mm2]

t = 0.0 s t = 0.8 s t = 1.6 s t = 2.4 s t = 3.2 s

t = 4.0 s t = 4.8 s t = 5.6 s t = 6.4 s t = 7.2 s

Figure 11. Sequential images of the evaporating TMEDA + (20% 1,2,4-triazole) droplet at 400 °C.

0 1 2 3 4 5 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 TMEDA TMEDA + (10 % 1,2,4-triazole) TMEDA + (20 % 1,2,4-triazole) D 2 /D 2 [m i m 2 /m m 2 ] t/D2_i [s/mm2]

Figure 10.Normalized variation of evaporating TMEDA-based fuel droplet diameter squared with normalized time at 400◦C.

0 2 4 6 8 0.0 0.2 0.4 0.6 0.8

1.0 TMEDA_TMEDA₊_{(10 % 1,2,4-triazole)} TMEDA + (20 % 1,2,4-triazole) D 2 /D 2 [m i m 2 /m m 2 ] t/D2 i [s/mm 2_]

t = 0.0 s t = 0.8 s t = 1.6 s t = 2.4 s t = 3.2 s

t = 4.0 s t = 4.8 s t = 5.6 s t = 6.4 s t = 7.2 s

Figure 11.Sequential images of the evaporating TMEDA+ (20% 1,2,4-triazole) droplet at 400◦_C.

0 2 4 6 8 0.0 0.2 0.4 0.6 0.8

1.0 TMEDA_TMEDA₊_{(10 % 1,2,4-triazole)} TMEDA + (20 % 1,2,4-triazole) D 2 /D 2 [m i m 2 /m m 2 ] t/D2 i [s/mm 2_]

t = 0.0 s t = 0.8 s t = 1.6 s t = 2.4 s t = 3.2 s

t = 4.0 s t = 4.8 s t = 5.6 s t = 6.4 s t = 7.2 s

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Energies 2019, 12, 3208 10 of 13

0 1 2 3 4 0.0 0.4 0.8 1.2 1.6

TMEDA + (20 % 1,2,4-triazole)

D

2

/D

2

[

i

mm

2

/m

m

2

]

t/D

2_i

[s/mm

2

]

t = 0.0 s t = 0.33 s t = 0.67 s t = 1.0 s t = 1.33 s

t = 1.67 s t = 2.0 s t = 2.33 s t = 2.67 s t = 3.0 s

3.2.2. Evaporation Rates of the TMEDA-Based Droplets

To predict the droplet evaporation rate of TMEDA-based fuels, a linear variation part of the evaporation was used except for the initial heating up period. The calculated evaporation rates using the lease square mean method were plotted in Figures 6–13. Two slope data are found for two stages of TMEDA-based fuel and the rate coefficient of each fuel is plotted as shown in the Figures 15 and 16. The presence of 1,2,4-triazole have improved the evaporation rate of mixed fuels before the evaporation of TMEDA. Therefore, the presence of 1,2,4-triazole have improved the evaporation rate of mixed fuels before evaporation of TMEDA. In the 2nd stage, the evaporation rates are almost same for both TMEDA + (10% 1,2,4-triazole) and TMEDA + (20% 1,2,4-triazole) droplet. This means that most of the remaining fuel droplets consist of 1,2,4-triazole.

0 1 2 3 4 0.0 0.4 0.8 1.2 1.6

TMEDA + (20 % 1,2,4-triazole)

D

2

/D

2

[

i

mm

2

/m

m

2

]

t/D

2_i

[s/mm

2

]

t = 0.0 s t = 0.33 s t = 0.67 s t = 1.0 s t = 1.33 s

t = 1.67 s t = 2.0 s t = 2.33 s t = 2.67 s t = 3.0 s

3.2.2. Evaporation Rates of the TMEDA-Based Droplets

To predict the droplet evaporation rate of TMEDA-based fuels, a linear variation part of the evaporation was used except for the initial heating up period. The calculated evaporation rates using the lease square mean method were plotted in Figures 6–13. Two slope data are found for two stages of TMEDA-based fuel and the rate coefficient of each fuel is plotted as shown in the Figures 15 and 16. The presence of 1,2,4-triazole have improved the evaporation rate of mixed fuels before the evaporation of TMEDA. Therefore, the presence of 1,2,4-triazole have improved the evaporation rate of mixed fuels before evaporation of TMEDA. In the 2nd stage, the evaporation rates are almost same for both TMEDA + (10% 1,2,4-triazole) and TMEDA + (20% 1,2,4-triazole) droplet. This means that most of the remaining fuel droplets consist of 1,2,4-triazole.

Figure 14.Sequential images of the evaporating TMEDA+ (20% 1,2,4-triazole) droplet at 600◦C. 3.2.2. Evaporation Rates of the TMEDA-Based Droplets

To predict the droplet evaporation rate of TMEDA-based fuels, a linear variation part of the evaporation was used except for the initial heating up period. The calculated evaporation rates using the lease square mean method were plotted in Figures6–13. Two slope data are found for two stages of TMEDA-based fuel and the rate coefficient of each fuel is plotted as shown in the Figures15and16. The presence of 1,2,4-triazole have improved the evaporation rate of mixed fuels before the evaporation of TMEDA. Therefore, the presence of 1,2,4-triazole have improved the evaporation rate of mixed fuels before evaporation of TMEDA. In the 2nd stage, the evaporation rates are almost same for both TMEDA+ (10% 1,2,4-triazole) and TMEDA + (20% 1,2,4-triazole) droplet. This means that most of the remaining fuel droplets consist of 1,2,4-triazole.

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Energies 2019, 12, 3208 11 of 13

0 100 200 300 400 500 600 0.0 0.2 0.4 0.6 0.8 TMEDA TMEDA + (10 % 1,2,4-triazole) TMEDA + (20 % 1,2,4-triazole)

C

V

(

mm

2

/s

)

Temperature (°C)

Figure 15. Evaporation coefficients of the TMEDA-based fuels at temperatures of 100–600 °C for the

first stage evaporation period.

C

V

(

mm

2

/s

)

Temperature (°C)

second stage evaporation period.

4. Conclusions

In this study, two experimental analyses were accomplished to examine the decomposition and evaporation behaviors of TMEDA-based fuels. In order to increase the fuel density, 1,2,4-triazole which have high density was added to TMEDA. The evaporation behaviors of TMEDA droplet blended with 10% and 20% 1,2,4-triazole were investigated using suspending droplet surrounded by electric furnace in cylindrical vessel. Evaporation coefficients were estimated at atmospheric pressure conditions and in the temperature range 100–600 °C. Except for an initial heating up duration, a change in the droplet diameter squared reaches an almost linear variation

with time, which is D2_{-law. The main findings in this study are as follows.}

(1) In the thermal decomposition period, TMEDA began to evaporate at temperature before reaching its boiling point and 1,2,4-triazole evaporate subsequently. The thermal

Figure 15.Evaporation coefficients of the TMEDA-based fuels at temperatures of 100–600◦C for the first stage evaporation period.

C

V

(

mm

2

/s

)

Temperature (°C)

first stage evaporation period.

C

V

(

mm

2

/s

)

Temperature (°C)

second stage evaporation period.

4. Conclusions

In this study, two experimental analyses were accomplished to examine the decomposition and evaporation behaviors of TMEDA-based fuels. In order to increase the fuel density, 1,2,4-triazole which have high density was added to TMEDA. The evaporation behaviors of TMEDA droplet blended with 10% and 20% 1,2,4-triazole were investigated using suspending droplet surrounded by electric furnace in cylindrical vessel. Evaporation coefficients were estimated at atmospheric pressure conditions and in the temperature range 100–600 °C. Except for an initial heating up duration, a change in the droplet diameter squared reaches an almost linear variation

with time, which is D2_{-law. The main findings in this study are as follows.}

(1) In the thermal decomposition period, TMEDA began to evaporate at temperature before reaching its boiling point and 1,2,4-triazole evaporate subsequently. The thermal

Figure 16.Evaporation coefficients of the TMEDA-based fuels at temperatures of 100–600◦C for the second stage evaporation period.

4. Conclusions

In this study, two experimental analyses were accomplished to examine the decomposition and evaporation behaviors of TMEDA-based fuels. In order to increase the fuel density, 1,2,4-triazole which have high density was added to TMEDA. The evaporation behaviors of TMEDA droplet blended with 10% and 20% 1,2,4-triazole were investigated using suspending droplet surrounded by electric furnace in cylindrical vessel. Evaporation coefficients were estimated at atmospheric pressure conditions and in the temperature range 100–600◦

C. Except for an initial heating up duration, a change in the droplet diameter squared reaches an almost linear variation with time, which is D2-law. The main findings in this study are as follows.

(1) In the thermal decomposition period, TMEDA began to evaporate at temperature before reaching its boiling point and 1,2,4-triazole evaporate subsequently. The thermal decomposition temperatures of TMEDA, TMEDA+ (10% 1,2,4-triazole) and TMEDA + (20% 1,2,4-triazole) were 114.1◦C, 110.0◦C, and 109.7◦C, respectively.

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Energies 2019, 12, 3208 12 of 13

(2) Droplet behavior such as bubble formation and strong puffing phenomena were observed in the case of the TMEDA containing 1,2,4-triazole above 400◦C.

(3) The evaporation rate was substantially enhanced by increasing the concentration of 1,2,4-triazole at the same temperature. However, the evaporation rates after evaporation of TMEDA-based fuels were same irrespective of the concentration of 1,2,4-triazole.

(4) There was no residue or solidified deposit after the complete depletion of the fuel droplet.

Author Contributions: Conceptualization, J.Y.; Validation, S.W.B.; Investigation, J.Y.; Writing—original draft preparation, J.Y.; Writing—review and editing, J.Y., S.W.B. and S.J.C.; Supervision, S.W.B.

Funding:This article was supported by BK21 Plus Program.

Conflicts of Interest:The authors declare no conflict of interest.

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