CN105633511A - Na-CO<2> room-temperature secondary battery and manufacture method thereof - Google Patents

Abstract

The invention provides a Na-CO<2> room-temperature secondary battery, which is a button battery formed by assembling a negative shell, a spring sheet, a gasket, a sodium sheet, a membrane, an electrolyte, a carbon dioxide-air electrode and a porous positive shell, wherein the sodium sheet is a negative electrode; the carbon dioxide-air electrode is a positive electrode and comprises a positive catalyst with a porous structure and a current collector; the spring sheet is a stainless steel sheet and is used for filling an internal space of the battery to improve the air-tightness of the battery; the gasket is a stainless steel sheet; the membrane is located between the negative sodium sheet and the positive current collector and is used for blocking electron transportation; and the electrolyte is an NaClO<4>/tetraethylene glycol dimethyl ether solution and infiltrates the membrane. The secondary battery has the advantages of being ultra-high in capacity, rechargeable at room temperature, friendly to environment, low in cost, high in magnification and long in service life; and a preparation method is simple to operate, high in controllability, good in consistency and beneficial to large-scale production.

Description

A kind of Na-CO2Room temperature secondary cell and its preparation method

Technical field

The present invention relates to the preparation of Novel Room Temperature secondary cell, particularly a kind of Na-CO₂Room temperature secondary cell and its preparation method.

Background technology

Since the Industrial Revolution, the greenhouse gases that the heat absorptivities such as the carbonic acid gas that the mankind enter in air are strong increase year by year, and the Greenhouse effect of air also strengthen thereupon, and it has caused the concern that a series of problem has caused whole world various countries. Therefore, the seizure of carbonic acid gas and recycling cause people pay close attention to greatly and pay attention to. Nowadays, the application scale in the fields such as consumer electronics, power truck and intelligent grid constantly expands, the accumulator system that active demand energy density is higher. Therefore, develop more high-energy-density, environmental friendliness, cheap novel battery imperative.

Carbonic acid gas produces primarily of combustion of fossil fuel, photosynthesis of plant. Being a kind of colorless and odorless, combustion-supporting, not non-combustible gas under normal temperature, this makes it have good security. But its high chemical stability, also makes the reduction of carbonic acid gas utilize very difficult. Novel Na-CO₂Battery discharge procedure can become carbon by reducing carbon dioxide efficiently, discharges electric energy simultaneously. So far, Na-CO₂Battery has no report.

Summary of the invention

It is an object of the invention to for above-mentioned existing problems, it is provided that a kind of Na-CO₂Room temperature secondary cell, this secondary cell has vast capacity, room temperature can be filled, environmental friendliness, low cost, greatly multiplying power and long-life feature; Its preparation method is simple to operate, controllability is strong, consistence is good, and the method can effectively reduce the contact angle of active substance and electrolytic solution, it is to increase ion is in the transmission of phase interface, it is possible to effectively reduces battery interface impedance, promotes coulomb efficiency, be conducive to scale operation.

The technical scheme of the present invention:

A kind of Na-CO₂Room temperature secondary cell, the button battery assembled by negative electrode casing, shell fragment, pad, sodium sheet, barrier film, electrolytic solution, carbonic acid gas-air electrode and porous anode shell, wherein sodium sheet is negative pole, carbonic acid gas-air electrode is positive pole, and carbonic acid gas-air electrode is made up of the anode catalyst and collector with vesicular structure; Shell fragment is stainless steel substrates, fills for inside battery space to improve battery stopping property; Pad is stainless steel substrates; Barrier film between negative pole sodium sheet and plus plate current-collecting body, for intercepting electric transmission; Electrolytic solution is NaClO₄/ tetraethyleneglycol dimethyl ether (TEGDME) solution, electrolytic solution is infiltrated in barrier film.

Described anode catalyst is carbon nanotube, carbon 60 ball alkene or reduction graphite; Collector is nickel foam, 400-1000 object nickel net or aluminium net, and collector diameter is 14mm; Barrier film is polyethylene-polypropylene-polyethylene sandwich diaphragm (Celgard series membranes) that superposition is formed successively or porous gap glass fiber paper, and wherein the diameter of porous gap glass fiber filter paper is 16mm, and thickness is 0.3mm, and porosity is 92-98%; NaClO₄The concentration of/tetraethyleneglycol dimethyl ether (TEGDME) solution is 1molL^-1; Button battery is of a size of diameter 20mm, thick 3.2mm, and the diameter of porous anode shell is 20mm, and on porous anode shell, uniform diameter is 19 holes of 1mm.

The preparation method of described carbonic acid gas-air electrode, comprises the following steps:

1) by carbon nanotube, carbon 60 ball alkene or reduction graphite at 120 DEG C, under-0.1Mpa dry 2 hours;

2) dried carbon nanotube, carbon 60 ball alkene or reduction graphite are mixed with tetraethyleneglycol dimethyl ether (TEGDME), ultrasonic reaction 30min under 40KHz, the anode catalyst of obtained vesicular structure;

3) in the anode catalyst-TEGDME dispersion liquid of vesicular structure, polyvinylidene difluoride (PVDF) (PVdF) is added as the binding agent of anode catalyst and collector, the addition of vinylidene is the 5% of dispersion liquid quality, then dispersion liquid is sprayed on the collector of temperature 300-600 DEG C, in dispersion liquid, the anode catalyst of vesicular structure and the amount ratio of TEGDME are 20mg:10mL, obtained carbonic acid gas-air electrode.

A kind of described Na-CO₂The preparation method of room temperature secondary cell, puts into negative electrode casing successively by shell fragment, pad, sodium sheet, barrier film, drips the �� L electrolytic solution that adds 30, places the carbonic acid gas-air electrode prepared, and then installs porous anode shell, finally with sealing machine with 5MPa pressure compaction.

The technical Analysis of the present invention:

This Na-CO₂Room temperature anode of secondary battery is half open battery of carbonic acid gas-air electrode, air electrode is the battery device of anode catalyst and the collector composition with vesicular structure, its effect is for carbonic acid gas provides diffusion admittance, and the discharging product sodium carbonate offer avtive spot for generating in discharge process. The preparation of binder free air electrode is become on the porous collector of 300-600 DEG C by previously prepared catalyzer dispersion liquid atomizing spraying in temperature.

$dA=-SdT+pdV+{\mathrm{\&gamma;dA}}_S+\underset{B}{\&Sigma;}{\mathrm{\&mu;}}_B{\mathrm{dn}}_B$

$T{\left(\frac{\&part;S}{\&part;A_s}\right)}_{T,p,n_B}>0$

$\left(\frac{\&part;\mathrm{\&gamma;}}{\&part;T}\right)A_{s,p,n_B}<0$

By to the total differential of basic thermodynamics formula it will be seen that reduce along with the rising collection liquid surface tension force of temperature. When collector low surface tension, the catalyzer in attachment can have the bonding force stronger with collector when it recovers room temperature. The method avoid using binding agent just can make active substance be thin and uniform load on porous collector. And simple to operate, controllability is strong, consistence good.

The present invention provides Na-CO₂The surface modifying method of carbon-based material used in battery air electrode catalyst, the method can effectively reduce the contact angle of active substance and electrolytic solution, it is to increase ion is in the transmission of phase interface, it is possible to effectively reduces battery interface impedance, promotes coulomb efficiency.

It is an advantage of the invention that:

This Na-CO₂Room temperature secondary cell can efficiently reduce and utilize atmospheric carbon dioxide, possesses high energy density, room temperature can be filled, Large Copacity, the feature such as have extended cycle life; The present invention provides the anode catalyst preparation method being applicable to Semi-open system battery, and the method is simple and easy to do, can make active substance uniform fold collection liquid surface, it is to increase the utilization ratio of catalyzer; The present invention provides for Na-CO₂The surface modification method of modifying of battery positive pole carbon material catalyzer, by the reaction with electrolytic solution so that it is has less contact angle, better infiltrate electrolytic solution, it is to increase ion is in the transmission of phase interface. The designed button cell made has reversible specific capacity height (60000mAhg^-1), fill a current potential low (< 3.7V), good rate capability (4Ag^-1Time have 4000mAhg^-1), have extended cycle life (by capacity 2000mAhg^-1Can obtain 200 circles circulation) performance. This Na-CO₂Secondary cell possesses that room temperature can be filled, Large Copacity (1100Whkg^-1), low overpotential (0.6V), big multiplying power (4Ag^-1Time have 4000mAhg^-1Reversible capacity) and long-life feature (by capacity 2000mAhg^-1200 circle circulations can be obtained). The present invention has important using value in energy storage and environmental protection field.

Accompanying drawing explanation

Fig. 1 is Na-CO₂Battery air electrode carbon nanotube SEM.

Fig. 2 is Na-CO₂Battery air electrode, collector load carbon nanotube side SEM.

Fig. 3 is the ionic conductivity of different ionogen.

Fig. 4 is the battery impedance after carbon nanotube carries out different condition surface treatment.

Fig. 5 is the contact angle of carbon pipe after 2 hours of carbon nanotube and EtOH Sonicate and TEGDME.

Fig. 6 is the contact angle of carbon nanotube after ultrasonic 5 minutes of carbon nanotube and TEGDME and TEGDME.

Fig. 7 is the contact angle of carbon nanotube after ultrasonic 2 hours of carbon nanotube and TEGDME and TEGDME.

Fig. 8 is this battery system 1MNaClO₄/ TEGMDE voltage window (Ar atmosphere encloses middle test).

Fig. 9, for carbon nanotube is carried out surface modification by TEGDME, does not add the discharge and recharge figure of binding agent.

Figure 10, for carbon nanotube being carried out surface modification by TEGDME, adds the discharge and recharge figure of 5wt% binding agent.

Figure 11, for carbon nanotube is carried out surface modification by TEGDME, does not add the discharge and recharge figure of binding agent.

Figure 12 is that reduced graphene is as Na-CO₂The discharge and recharge figure of battery anode catalyst.

Figure 13 is that carbon 60 ball alkene is as Na-CO₂The discharge and recharge figure of battery anode catalyst.

Figure 14 is intercepting capacity 1000mAhg^-1Discharge and recharge figure.

Figure 15 is intercepting capacity 2000mAhg^-1Discharge and recharge figure.

Figure 16 is Na-CO₂Battery high rate performance.

Figure 17 is that discharge and recharge electrode slice XRD characterizes, detection discharging product sodium carbonate.

Figure 18 is that graceful sign is drawn in discharge and recharge, checking discharging product sodium carbonate and carbon.

Figure 19 is the impact getting rid of the carbon that carbon material electrode pair detection battery discharge produces, and assembles Na-CO by nano silver wire₂Battery positive pole, loses power spectrum (EELS) by electronics and discharging product carbon detected.

Figure 20 is the precipitation test of carbonic acid gas in process of charging.

Figure 21 is Na-CO₂Secondary chamber's temperature battery structure schematic diagram.

In figure: 1. negative electrode casing, 2. shell fragment, 3. pad, 4. sodium sheet, 5. barrier film, 6. anode catalyst 7. collector, 8. porous anode shell.

Embodiment

Below in conjunction with specific embodiment, elaborate the present invention further.

Embodiment 1:

A kind of Na-CO₂Room temperature secondary cell, as shown in figure 21, the button battery assembled by negative electrode casing 1, shell fragment 2, pad 3, sodium sheet 4, barrier film 5, electrolytic solution, carbonic acid gas-air electrode and porous anode shell 8, wherein sodium sheet 4 is negative pole, carbonic acid gas-air electrode is positive pole, and carbonic acid gas-air electrode is made up of with collector 7 anode catalyst 6 with vesicular structure; Shell fragment 2 is stainless steel substrates, fills for inside battery space to improve battery stopping property; Pad 3 is stainless steel substrates; Barrier film 5 between negative pole sodium sheet 4 and plus plate current-collecting body 7, for intercepting electric transmission; Electrolytic solution is NaClO₄/ tetraethyleneglycol dimethyl ether (TEGDME) solution, electrolytic solution is infiltrated in barrier film 5; Described anode catalyst is 6 carbon nanotubes; Collector 7 is 400 object nickel nets, and collector diameter is 14mm; Barrier film 5 is porous gap glass fiber paper, and the diameter of porous gap glass fiber filter paper is 16mm, and thickness is 0.3mm, and porosity is 92-98%; NaClO₄The concentration of/tetraethyleneglycol dimethyl ether (TEGDME) solution is 1molL^-1; Button battery is of a size of diameter 20mm, thick 3.2mm, and the diameter of porous anode shell 8 is 20mm, and on porous anode shell, uniform diameter is 19 holes of 1mm.

A kind of described Na-CO₂The preparation method of room temperature secondary cell, step is as follows:

1) screening of electrolytic solution

The electrolytic solution of common electrochemical stability comprises DMSO, TEGDME and DME. Wherein DMSO and Na metal reaction, it is clear that be not suitable for Na-CO₂Battery; And TEGDME (275 DEG C) has higher boiling point compared with DME (85 DEG C), volatility is lower, is more suitable for Na-CO₂Open battery system. Then, we test TEGDME is solvent, NaClO₄With NaTFSI be solute two kinds of electrolytic solution ionic conductivity as shown in Figure 3, figure shows: NaClO4/TEGDME has higher ionic conductivity (0.178Sm^-1), so being chosen to be final electrolytic solution. It has wide voltage window (1.2V 4.5V).

2) preparation of the anode catalyst of vesicular structure

Take commercialization carbon nanotube 20mg in vacuum drying oven 120 DEG C, under-0.1Mpa dry 2 hours, then the carbon pipe of drying is put into the beaker taking advantage of 20mLTEGDME, ultrasonic reaction 30min under 40KHz, the anode catalyst of obtained vesicular structure.

Fig. 4 is the battery impedance after carbon nanotube carries out different condition surface treatment. Figure shows: the carbon nanotube using TEGDME after ultrasonic 2 hours has the highest specific conductivity.

Fig. 5 is the contact angle of carbon pipe after 2 hours of carbon nanotube and EtOH Sonicate and TEGDME. Figure shows: be the contact angle of 18.1 �� with the carbon pipe of EtOH Sonicate after 2 hours and TEGDME, have relatively general contact angle.

Fig. 6 is the contact angle of carbon nanotube after ultrasonic 5 minutes of carbon nanotube and TEGDME and TEGDME. Figure shows: be the contact angle of 18.0 �� with the carbon pipe of TEGDME after ultrasonic 5 minutes and TEGDME, have contact angle relatively preferably.

Fig. 7 is the contact angle of carbon nanotube after ultrasonic 2 hours of carbon nanotube and TEGDME and TEGDME. Figure shows: be the contact angle of 13.8 �� with the carbon pipe of TEGDME after ultrasonic 2 hours and TEGDME, have relatively optimum contact angle.

3) preparation of carbonic acid gas-air electrode

Anode catalyst-TEGDME the dispersion liquid of vesicular structure adds polyvinylidene difluoride (PVDF) (PVdF) as the binding agent of anode catalyst and collector, the addition of vinylidene is the 5% of dispersion liquid quality, then dispersion liquid is sprayed on temperature 400 DEG C, 400 object collectors, in dispersion liquid, the anode catalyst of vesicular structure and the amount ratio of TEGDME are 20mg:10mL, carbon nanotube is attached to nickel net surface by intermolecular forces, and obtained charge capacity is the carbonic acid gas-air electrode of 5 �� g.

Fig. 1 is Na-CO₂Battery air electrode carbon nanotube SEM. Figure shows: carbon nanotube anode catalyst has abundant accumulation hole and large specific surface, it is beneficial to discharging product growth.

Fig. 2 is Na-CO₂Battery air electrode, collector load carbon nanotube side SEM. Figure shows: carbon tube layer with the uniform load of thickness of about 0.5 micron at collection liquid surface.

Figure 10, for carbon nanotube being carried out surface modification by TEGDME, adds the discharge and recharge figure of 5wt% binding agent. Figure 11, for carbon nanotube is carried out surface modification by TEGDME, does not add the discharge and recharge figure of binding agent. Figure 10 and Figure 11 is visible in contrast: first week discharge platform occurs at about 1.6V, and 2.8V starts charging platform occur. Owing to adding of binding agent sodium phenol makes cell resistance increase, low 0.7V when causing its sparking voltage more not add binding agent.

4)Na-CO₂The preparation of room temperature secondary cell

Shell fragment, pad, sodium sheet, barrier film are put into negative electrode casing successively, drips the �� L electrolytic solution that adds 30, place the carbonic acid gas-air electrode prepared, then porous anode shell is installed, finally with sealing machine with 5MPa pressure compaction. Series of cells process of assembling all carries out in the glove box of full Ar gas.

Na-CO₂The charge-discharge test of room temperature secondary cell

Test voltage scope is 1.5-4.0V.

Fig. 8 is this battery system 1MNaClO₄/ TEGMDE voltage window (Ar atmosphere encloses middle test). Figure shows: it is electrochemical stability that TEGDME electrolyte ties up to 1V to 4.5V.

Fig. 9 is button Na-CO₂Battery is at 1Ag^-1Under electric current, intercept 500mAhg^-1Charging and discharging curve. It may be seen that first week discharge platform occurs at 2.3V from figure line, there is charging platform in 3.2V. It is 4Ag in current density^-1Under obtain 4000mAhg^-1Specific storage (voltage > 2V).

Embodiment 2:

A kind of described Na-CO₂The preparation method of room temperature secondary cell, step is substantially the same manner as Example 1, and difference is: replace carbon nanotube with Graphene.

Figure 12 is Na-CO₂Battery is at 1Ag^-1Under electric current, intercept 500mAhg^-1Charging and discharging curve. It may be seen that first week discharge platform occurs at about 2.2V from figure line, there is charging platform in 3.1V.

Embodiment 3:

A kind of described Na-CO₂The preparation method of room temperature secondary cell, step is substantially the same manner as Example 1, and difference is: replace carbon nanotube with carbon 60 ball alkene.

Figure 13 is button Na-CO₂Battery is at 1Ag^-1Under electric current, intercept 500mAhg^-1Charging and discharging curve. It may be seen that first week discharge platform occurs at about 1.4V from figure line, there is charging platform in 3.5V.

Na-CO₂Battery is a kind of brand-new energy storage system, based on the quality Na-CO of active substance₂Battery has 12000Whkg^-1Power density. Based on the quality Na-CO of carbonic acid gas and sodium₂Battery has 1100Whkg^-1Theoretical(horse)power density. Nearly 3 times to the theoretical energy density of lithium ion battery. (Bruce, P.G.; Freunberger, S.A.; Hardwick, L.J.; Tarascon, J.-M., Li O₂andLi�CSbatterieswithhighenergystorage.NatureMaterials2011,11(1),19-29.)

Na-CO₂The detection of room temperature secondary cell:

Figure 14 is for using blue electrical testing system (Wuhan) to button Na-CO₂Battery carries out intercepting capacity 1000mAhg^-1Constant current charge-discharge test pattern. Figure shows: at intercepting specific storage 1000mAhg^-1Time, Na-CO₂Battery can stable circulation 200 cycle. At button Na-CO₂Before battery in 200 circle circulations, sparking voltage is greater than 2V, and charging voltage is less than 4V, all in the voltage window ranges of electrolytic solution.

Figure 15 is for using blue electrical testing system (Wuhan) to button Na-CO₂Battery carries out intercepting capacity 2000mAhg^-1Constant current charge-discharge test pattern. Figure shows: at intercepting specific storage 2000mAhg^-1Time, Na-CO₂Battery can stable circulation 200 cycle. In front 200 circle circulations, sparking voltage is greater than 2V, and charging voltage is less than 4V, all in the voltage window ranges of electrolytic solution.

Figure 16 is for using blue electrical testing system (Wuhan) to button Na-CO₂Battery carries out intercepting capacity 2000mAhg^-1Constant current charge-discharge test pattern. Figure shows: be 1Ag in charging and discharging currents density^-1To 4Ag^-1Time, battery charging and discharging overpotential increases to some extent, but sparking voltage is greater than 2V, and charging voltage is less than 4V, all in the voltage window ranges of electrolytic solution.

Figure 17 is that discharge and recharge electrode slice XRD characterizes, detection discharging product sodium carbonate. Figure shows: by the X-ray diffraction analysis to discharging product, obtain the diffracted signal of sodium carbonate. Prove Na-CO₂Battery creates sodium carbonate in discharge process, demonstrates this cell reaction

Figure 18 is the impact getting rid of the carbon that carbon material electrode pair detection battery discharge produces, and assembles Na-CO by nano silver wire₂Battery positive pole. Figure shows: by the Raman spectrum analysis to discharging product, obtain the Raman signal of sodium carbonate and carbon. Demonstrate Na-CO₂Battery creates sodium carbonate and carbon in discharge process, demonstrates this cell reaction

Figure 19 is the impact getting rid of the carbon that carbon material electrode pair detection battery discharge produces, and assembles Na-CO by nano silver wire₂Battery positive pole, loses power spectrum (EELS) by electronics and discharging product carbon detected. Figure shows: Na-CO₂Battery creates carbon in discharge process, demonstrates this cell reaction

Figure 20 is the precipitation test of carbonic acid gas in process of charging. Figure shows: in battery charging process, carbon dioxide production, with according toThe theoretical growing amount that stoichiometric number calculates is consistent. Finally confirm this cell reaction.

It is an object of the invention to provide a kind of novel, room temperature is reversible, the energy storage system of vast capacity, energy source in consumption GHG carbon dioxide. This energy storage system is meeting simultaneously preserving the ecological environment of people's energy demand, it is achieved Sustainable development. This secondary cell has vast capacity, room temperature can be filled, environmental friendliness, low cost, greatly multiplying power and long-life feature. It is emphasized that, compared with the lithium ion battery reported at present, it is convenient that the battery system of the present invention has preparation method, and raw material sources are extensive, with low cost. The present invention can be widely used in power cell and extensive energy storage field. Owing to the present invention can utilize GHG carbon dioxide efficiently. Therefore, in energy storage and environmental protection two fields, it all has great using value, is expected to become business-like new battery system in the future.

Claims (4)

1. a Na-CO₂Room temperature secondary cell, it is characterized in that: the button battery assembled by negative electrode casing, shell fragment, pad, sodium sheet, barrier film, electrolytic solution, carbonic acid gas-air electrode and porous anode shell, wherein sodium sheet is negative pole, carbonic acid gas-air electrode is positive pole, and carbonic acid gas-air electrode is made up of the anode catalyst and collector with vesicular structure; Shell fragment is stainless steel substrates, fills for inside battery space to improve battery stopping property; Pad is stainless steel substrates; Barrier film between negative pole sodium sheet and plus plate current-collecting body, for intercepting electric transmission; Electrolytic solution is NaClO₄/ tetraethyleneglycol dimethyl ether (TEGDME) solution, electrolytic solution is infiltrated in barrier film.

2. Na-CO according to claim 1₂Room temperature secondary cell, it is characterised in that: described anode catalyst is carbon nanotube, carbon 60 ball alkene or reduction graphite; Collector is nickel foam, 400-1000 object nickel net or aluminium net, and collector diameter is 14mm; Barrier film is polyethylene-polypropylene-polyethylene sandwich diaphragm (Celgard series membranes) that superposition is formed successively or porous gap glass fiber paper, and wherein the diameter of porous gap glass fiber filter paper is 16mm, and thickness is 0.3mm, and porosity is 92-98%; NaClO₄The concentration of/tetraethyleneglycol dimethyl ether (TEGDME) solution is 1molL^-1; Button battery is of a size of diameter 20mm, thick 3.2mm, and the diameter of porous anode shell is 20mm, and on porous anode shell, uniform diameter is 19 holes of 1mm.

3. Na-CO according to claim 1₂Room temperature secondary cell, it is characterised in that: the preparation method of described carbonic acid gas-air electrode, comprises the following steps:

1) by carbon nanotube, carbon 60 ball alkene or reduction graphite at 120 DEG C, under-0.1Mpa dry 2 hours;

2) dried carbon nanotube, carbon 60 ball alkene or reduction graphite are mixed with tetraethyleneglycol dimethyl ether (TEGDME), ultrasonic reaction 30min under 40KHz, the anode catalyst of obtained vesicular structure;

3) in the anode catalyst-TEGDME dispersion liquid of vesicular structure, polyvinylidene difluoride (PVDF) (PVdF) is added as the binding agent of anode catalyst and collector, the addition of vinylidene is the 5% of dispersion liquid quality, then dispersion liquid is sprayed on the collector of temperature 300-600 DEG C, in dispersion liquid, the anode catalyst of vesicular structure and the amount ratio of TEGDME are 20mg:10mL, obtained carbonic acid gas-air electrode.

4. a Na-CO as claimed in claim 1₂The preparation method of room temperature secondary cell, it is characterized in that: shell fragment, pad, sodium sheet, barrier film are put into negative electrode casing successively, drip the �� L electrolytic solution that adds 30, place the carbonic acid gas-air electrode prepared, then porous anode shell is installed, finally with sealing machine with 5MPa pressure compaction.